U.S. patent application number 09/884998 was filed with the patent office on 2001-12-13 for bushing for crawler belt and method of manufacture.
This patent application is currently assigned to KOMATSU LTD.. Invention is credited to Hamasaka, Naoji, Okayama, Chigo, Takayama, Takemori.
Application Number | 20010050121 09/884998 |
Document ID | / |
Family ID | 27461438 |
Filed Date | 2001-12-13 |
United States Patent
Application |
20010050121 |
Kind Code |
A1 |
Takayama, Takemori ; et
al. |
December 13, 2001 |
Bushing for crawler belt and method of manufacture
Abstract
Crawler belt bushings and their producing methods, with which
productivity and cost performance can be improved over the
carburization treatment and the induction hardening treatment.
After a workpiece made from steel is heated from its outer
circumferential surface such that at least the surface temperature
of the inner circumferential surface of the workpiece is raised to
a quenching temperature, a series of quenching operation is
performed to form quench hardened layers which extend toward the
core from the inner and outer circumferential surfaces respectively
and a soft, imperfectly hardened layer between these quench
hardened layers. The quenching operation comprises: cooling the
workpiece from the inner circumferential surface; cooling the
workpiece from the inner circumferential surface while heating from
the outer circumferential surface; and cooling the workpiece from
the outer circumferential surface.
Inventors: |
Takayama, Takemori; (Osaka,
JP) ; Okayama, Chigo; (Osaka, JP) ; Hamasaka,
Naoji; (Osaka, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
KOMATSU LTD.
|
Family ID: |
27461438 |
Appl. No.: |
09/884998 |
Filed: |
June 21, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09884998 |
Jun 21, 2001 |
|
|
|
09137845 |
Aug 21, 1998 |
|
|
|
6270595 |
|
|
|
|
Current U.S.
Class: |
148/570 |
Current CPC
Class: |
C21D 1/18 20130101; C21D
9/14 20130101; C21D 9/085 20130101 |
Class at
Publication: |
148/570 |
International
Class: |
C21D 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 1997 |
JP |
HEI.9-227876 |
Oct 31, 1997 |
JP |
HEI.9-300636 |
Feb 25, 1998 |
JP |
HEI.10-43903 |
Claims
1. A crawler belt bushing producing method, wherein a workpiece of
a crawler belt bushing made of a medium or high carbon steel or a
medium or high carbon low alloy steel is heated to a quenching
temperature or more; by use of a hardening system which can
independently start outer circumferential surface cooling and inner
circumferential surface cooling, the workpiece is first cooled from
either one of its outer circumferential surface and inner
circumferential surface and then cooling is carried out from the
other circumferential surface so that the workpiece is hardened
through its entire thickness; and then the workpiece is
tempered.
2. A crawler belt bushing producing method according to claim 1,
wherein said hardening system is designed to have a partition
between a cooling medium for the inner circumferential surface and
a cooling medium for the outer circumferential surface, taking into
account the flows of the cooling media, such that the cooling media
do not interfere with each other during cooling of the
workpiece.
3. A crawler belt bushing producing method according to claim 1,
wherein after the workpiece is substantially uniformly and entirely
heated to a quenching temperature by furnace heating and/or
induction heating, the workpiece is first cooled from either one of
the inner circumferential surface and outer circumferential
surface, and one or more seconds later, the workpiece is cooled
from the other circumferential surface.
4. A crawler belt bushing producing method according to claim 1,
wherein while scan induction heating in the axial direction of the
workpiece being carried out from either one of the inner
circumferential surface and outer circumferential surface, cooling
of the heated surface with a spray is started from either one of
the inner circumferential surface and outer circumferential
surface, and one or more seconds later, cooling of the heated
surface with a spray is started from the other circumferential
surface.
5. A crawler belt bushing producing method according to claim 1,
wherein said tempering process is carried out at 140 to 300.degree.
C.
6. A crawler belt bushing producing method according to claim 1,
wherein the hardness of the inner circumferential surface is
adjusted to Rockwell hardness (C type) HRC 45 to 55 by said
tempering process.
7. A crawler belt bushing producing method according to claim 6,
wherein said adjustment of the hardness of the inner
circumferential surface is carried out by stopping the induction
tempering from the inner circumferential surface and/or the cooling
from the inner circumferential surface during quenching operation,
earlier than the cooling from the outer circumferential surface,
thereby allowing self-tempering of the inner circumferential
surface.
8. A crawler belt bushing producing method wherein, after a
workpiece of a crawler belt bushing made of steel is heated to a
quenching temperature, (a) the cooling rate of the outer
circumferential surface of the workpiece is increased by first
cooling of the workpiece from its inner circumferential surface in
order to reduce heat capacity at the core of the workpiece and by
second cooling of the workpiece from its outer circumferential
surface which is started a certain time after the first cooling
and/or (b) the cooling rate of the outer circumferential surface of
the workpiece is increased by first cooling of the workpiece from
its inner circumferential surface in order to partially make the
core of the workpiece unhardenable by utilizing the mass effect of
the wall of the workpiece and by second cooling of the workpiece
from its outer circumferential surface which is started a certain
time after the first cooling, whereby a soft layer is formed within
the core of the workpiece at a cross-sectional position closer to
the inner circumferential surface and the hardened depth of the
outer circumferential surface is made to be greater than the
hardened depth of the inner circumferential surface, said processes
(a) and (b) being carried out within one cycle of quenching
operation, using a hardening system capable of performing inner
circumferential surface cooling and outer circumferential surface
cooling.
9. A crawler belt bushing producing method according to claim 8,
wherein said hardening system is designed to have a partition
between a cooling medium for the inner circumferential surface and
a cooling medium for the outer circumferential surface, taking into
account the flows of the cooling media, such that the cooling
medium for the inner circumferential surface does not interfere
with the outer circumferential surface during the first cooling
from the inner circumferential surface.
10. A crawler belt bushing producing method according to claim 9,
wherein said cooling media are quenching oil, water, water-soluble
quenching liquid or water fog and wherein said cooling from the
inner circumferential surface is jet cooling which uses a spray for
substantially uniformly cooling the inner circumferential
surface.
11. A crawler belt bushing producing method according to claim 8,
wherein after the workpiece is substantially uniformly and entirely
heated to a quenching temperature by furnace heating and/or
induction heating, quenching is carried out using said hardening
system.
12. A crawler belt bushing producing method according to claim 8,
wherein while scan induction heating in the axial direction of the
workpiece being carried out from either one of the inner and outer
circumferential surfaces, scan quenching is carried out by first
cooling the workpiece from the inner circumferential surface and
then cooling the workpiece from the outer circumferential surface,
under the condition that the temperatures of the inner and outer
circumferential surfaces are quenching temperatures equal to or
higher than the transformation temperatures of A1, A3 and/or
Acm.
13. A crawler belt bushing producing method according to claim 12,
wherein said scan quenching during the induction heating is carried
out such that the workpiece, an induction heating coil, an inner
circumferential surface cooling nozzle and an outer circumferential
surface cooling nozzle are relatively moved in the axial direction
of the workpiece and the workpiece is rotated substantially about
its mean axis.
14. A crawler belt bushing producing method according to claim 8 or
9, wherein cooling from the inner circumferential surface and
cooling from the outer circumferential surface are carried out at
substantially the same time at least within specified regions close
to the upper end face and lower end face, respectively, of the
workpiece, so that the upper and lower end faces are through
hardened.
15. A crawler belt bushing producing method wherein the workpiece
hardened by the bushing producing method set forth in claim 8 is
tempered at 140.degree. C. to 350.degree. C.
16. A crawler belt bushing producing method according to claim 8,
wherein said workpiece is made of a steel having a carbon content
equal to those of medium carbon steels and/or eutectoid steels,
which is 0.35 wt % or more, and having an alloy content within the
range of DI values with which the workpiece is through hardened by
simultaneous cooling of the inner and outer circumferential
surfaces and with which the hardened depth obtained by cooling from
the inner circumferential surface only is about one half the
thickness of the workpiece.
17. A crawler belt bushing producing method, wherein the inner
circumferential surface of the workpiece hardened by the bushing
producing method set forth in claim 8 is tempered by induction
tempering so as to have a surface hardness of Vickers hardness Hv
450 to 650.
18. A crawler belt bushing producing method wherein, after a
workpiece of a crawler belt bushing made of steel is induction
heated from its outer circumferential surface such that at least
the temperature of the inner circumferential surface of the
workpiece is raised to a quenching temperature, a series of
quenching operation comprising: (a) firstly cooling the workpiece
from the inner circumferential surface; (b) cooing the workpiece
from the inner circumferential surface while heating the workpiece
from the outer circumferential surface; and (c) cooling the
workpiece from the outer circumferential surface, is performed so
as to form quench hardened layers which extend toward the wall core
of the workpiece from the outer circumferential surface and from
the inner circumferential surface respectively and form a soft,
imperfectly hardened layer between said quench hardened layers.
19. A crawler belt bushing producing method wherein, while a
workpiece of a crawler belt bushing made of steel being heated from
its outer circumferential surface by scan induction heating in the
axial direction of the workpiece, using at least two vertically
aligned, induction coils, (a) the temperature of the inner
circumferential surface of the workpiece is raised to a quenching
temperature equal to the transformation temperatures of A1, A3
and/or Acm or more; (b) the workpiece is partially heated from the
outer circumferential surface by the induction coils while carrying
out first cooling from the inner circumferential surface; and (c)
the workpiece is then cooled from the outer circumferential
surface; whereby the inner and outer circumferential surfaces are
quench hardened so as to be substantially martensitic, said
processes (a), (b) and (c) being carried out at a certain position
of the workpiece.
20. A crawler belt bushing producing method according to claim 18
or 19, wherein cooling from the inner circumferential surface and
cooling from the outer circumferential surface are carried out at
substantially the same time at least within specified regions close
to the upper end face and lower end face, respectively, of the
workpiece, so that the upper and lower end faces are through
hardened.
21. A crawler belt bushing producing method according to claim 18
or 19, which uses a hardening system designed to have a partition
between a cooling medium for the inner circumferential surface and
a cooling medium for the outer circumferential surface, taking into
account the flows of the cooling media, such that the cooling
medium for the inner circumferential surface does not interfere
with the outer circumferential surface during the first cooling
from the inner circumferential surface.
22. A crawler belt bushing producing method according to claim 19,
wherein said scan quenching by induction heating is carried out
such that the workpiece, an induction heating coil, an inner
circumferential surface cooling nozzle and an outer circumferential
surface cooling nozzle are relatively moved and the workpiece is
rotated substantially about its mean axis.
23. A crawler belt bushing producing method according to claim 21,
wherein said cooling media are quenching oil, water, water-soluble
quenching liquid or water fog and wherein said cooling from the
inner circumferential surface is jet cooling which uses a spray for
substantially uniformly cooling the inner circumferential
surface.
24. A crawler belt bushing producing method, wherein the workpiece
hardened by the bushing producing method according to claim 18 or
19 is tempered at 140 to 350.degree. C.
25. A crawler belt bushing wherein quench hardened layers are
formed so as to extend toward its wall core from its outer
circumferential surface and from its inner circumferential surface
respectively and a soft, imperfectly hardened layer is formed
between said quench hardened layers, said quench hardened layers
and said soft layer being formed such that the quench hardened
layer of the outer circumferential surface has a depth greater than
the depth of the quench hardened layer of the inner circumferential
surface, by: (a) increasing the cooling rate of the outer
circumferential surface by first cooling of the workpiece from its
inner circumferential surface in order to reduce heat capacity at
the core and by second cooling of the workpiece from its outer
circumferential surface which is started a certain time after the
first cooling and/or (b) increasing the cooling rate of the outer
circumferential surface by first cooling of the workpiece from its
inner circumferential surface in order to partially make the core
unhardenable by utilizing the mass effect of the wall of the
workpiece and by second cooling of the workpiece from its outer
circumferential surface which is started a certain time after the
first cooling, the structure between said quench hardened layers
being composed of one or more structures selected from ferrite,
pearlite, bainite and martensite which are precipitated during
cooling from the quenching temperature, said bushing being low
temperature tempered.
26. A crawler belt bushing according to claim 25, wherein the
hardened depth of the outer circumferential surface is not less
than 1.1 times the hardened depth of the inner circumferential
surface.
27. A crawler belt bushing according to claim 25 or 26, which is
made of a steel having a carbon content equal to those of medium
carbon steels and/or eutectoid steels, which is 0.35 wt % or more
and having an alloy content within the range of DI values with
which the bushing is through hardened by simultaneous cooling of
the inner and outer circumferential surfaces and with which the
hardened depth obtained by cooling from the inner circumferential
surface only is about one half the thickness of the bushing.
28. A crawler belt bushing according to claim 25, which is tempered
at high temperature such that the quench hardened layer of the
inner circumferential surface has higher hardness than the quench
hardened layer of the outer circumferential surface and wherein the
surface hardness of the quench hardened layer of the inner
circumferential surface is adjusted to Vickers hardness Hv 450 to
650.
29. A crawler belt bushing according to claim 25, which is through
hardened at its upper and lower ends.
30. A crawler belt bushing having a carbon content of 0.35 to 2.0
wt %, containing at least one of the alloying elements of Mn, Si,
Cr, Mo and Ni, and made by a method in which a bushing workpiece
made of steel, which is through hardened by simultaneous cooling
from the outer and inner circumferential surfaces of the workpiece,
is induction heated from the outer circumferential surface so as to
raise at least the surface temperature of the inner circumferential
surface to a quenching temperature, and thereafter, a series of
quenching operation comprising: (a) firstly cooling the workpiece
from the inner circumferential surface; (b) heating the workpiece
from the outer circumferential surface while cooing the workpiece
from the inner circumferential surface; and (c) then, cooling the
workpiece from the outer circumferential surface, is performed so
as to form quench hardened layers which extend toward the wall core
of the workpiece from the outer circumferential surface and from
the inner circumferential surface respectively and form a soft,
imperfectly hardened layer between said quench hardened layers,
said soft layer between the quench hardened layers being composed
of one or more structures selected from ferrite, pearlite, bainite
and martensite which are precipitated during cooling from the
quenching temperature and which contain or do not contain granular
cementite dispersed therein.
31. A crawler belt bushing according to claim 30, wherein the
hardened depth of the outer circumferential surface is not less
than 1.1 times the hardened depth of the circumferential
surface.
32. A crawler belt bushing according to claim 30, which is tempered
at 140 to 350.degree. C. after quenching.
33. A crawler belt bushing according to claim 30, which is through
hardened at its upper and lower ends.
Description
TECHNICAL FIELD
[0001] The present invention relates to bushings for use in the
crawler belts of construction vehicles such as bulldozers and their
producing methods. The invention more particularly relates to
bushings for crawler belts excellent in wear resistance and impact
fatigue resistance and simple methods for producing them at low
cost.
BACKGROUND ART
[0002] As shown in FIG. 36, a crawler belt 51 for construction
vehicles etc. is composed of various parts. A crawler belt bushing
52 meshes with sprocket teeth for transmitting rotating movement
from final reduction gears and functions to rotate the crawler belt
51. As construction vehicles are operated in soil and rock, crawler
belt bushings need wear resistance at the inner and outer
circumferential surfaces. Vehicles also travel, running over and
colliding against soil and rock, crawler belt bushings need
tremendously high strength and toughness. To meet these
requirements, there have been proposed the following producing
methods for crawler belt bushings.
[0003] (1) A case hardening steel is carburized to form very hard
martensite on its inner and outer circumferential surface layers,
thereby ensuring wear resistance and strength (e.g. Japanese Patent
Publication (KOKOKU) No. 52-34806 (1977)).
[0004] (2) A medium carbon steel is used as a bushing material. The
bushing material is thermally refined and its inner and outer
circumferential surfaces are respectively induction hardened to
form very hard martensite thereon. After hardened by induction
hardening from the outer circumferential surface, the bushing
material is induction hardened from the inner circumferential
surface, so that a V-shaped hardened layer comprising tempered
martensite is formed between the inner and outer hardened layers,
thereby ensuring wear resistance and strength (Japanese Patent
Publication (KOKOKU) No. 63-16314).
[0005] (3) A medium carbon steel, whose hardenability is carefully
controlled by precise adjustment on its chemical composition, is
used for the bushing. Such a steel is heated in a furnace at a
temperature of 800.degree. C. or more and then rapidly cooled
thereby controlling the hardened depths of the inner and outer
circumferential surfaces to ensure wear resistance and
strength.
[0006] FIGS. 37(a), (b) and (c) schematically show typical
hardening patterns for the bushings manufactured by the above
conventional methods, and FIG. 37(d) shows the distributions of
hardness in the cross sections of these bushings. All of the
distributions indicate that there is formed a soft layer at the
wall core of the bushing.
[0007] These methods, however, reveal their own drawbacks. The
carburization method (1) consumes considerable carburization time
and has the economical problem of using large amounts of
carburizing gas. When producing large-sized crawler belt bushings
having great thickness for example, a great hardened depth is
required in order to ensure strength and wear resistance, which
gives rise to a decrease in productivity and to increased cost.
Further, since it takes long time to carburize and heat the inner
and outer circumferential surfaces, there will be formed a grain
boundary oxidized layer and imperfectly hardened layer having a
thickness of several tens of .mu.m, which causes a decrease in
fatigue strength and impairs impact resistance properties.
[0008] The induction hardening method (2) is improved over the
carburization method (1) in terms of cost, but involves thermal
refining treatment in order to assure hardness prior to the
induction hardening and double quenching of the inner and outer
circumferential surfaces, so that this method has proved to remain
a costly heating treatment. Further, when induction hardening the
inner circumferential surface of a small-diameter tubular part, an
inner circumference heating coil is needed and such a heating coil
is difficult to manufacture. Therefore, in many cases, the inner
circumferential surface of a tubular part is hardened by the above
carburization treatment, resulting in high cost.
[0009] The outer circumferential surface of a bushing in use is
subjected to severe wearing conditions due to soil and rock. To
increase the wear life of the bushing, the quench hardened layer at
the outer circumferential surface is preferably more deepened. To
this end, there has been made an attempt in Japanese Patent
Publication (KOKOKU) No. 63-16314 (1988). According to this
publication, after the outer circumferential surface of a steel is
once hardened deeply by induction hardening from the outer
circumferential surface, the inner circumferential surface is
shallowly hardened by induction hardening from the inner
circumferential surface, and a soft layer is formed between these
hardened layers by high temperature tempering. In any case, double
induction hardening is involved, which is disadvantageous in terms
of productivity and economy. In the method of this publication, it
is necessary to control the induction hardening from the outer
circumferential so as to restrict the hardness of the inner
circumferential surface to H.sub.RC 40, thereby preventing
quenching crack during the later induction hardening of the inner
circumferential surface. For applying this method to a
comparatively thin tubular part (e.g., crawler belt bushing), it is
necessary to control the temperature of the inner circumferential
surface with high precision, during the induction heating from the
outer circumferential surface and/or to control the hardenability
(DI value) of the steel material to be used. As a result, the
technical difficulty in deepening the outer circumferential surface
hardened layer and increased material cost are inevitable.
[0010] Japanese Patent Publication (KOKOKU) No. 1-37453 discloses a
quite economical heating process for producing through hardened
crawler bushings. This process uses a medium carbon steel for the
bushing. While scan induction heating is carried out from the outer
circumferential surface of the bushing, the bushing is cooled from
the outer circumferential surface, so that the bushing is hardened
across its entire thickness. For through hardening the bushing
across its entire thickness by cooling from the outer
circumferential surface only, high hardenability is required when
this process is applied to the production of thick, large-sized
crawler belt bushings, which inevitably entails an increase in
cost. In addition, in view of susceptibility to quenching crack
during cooling, the steel material that can be used in this process
is limited to medium carbon low alloy steels having carbon contents
of 0.5 wt % or less. As a result, it becomes difficult to improve
the wear resistance of the outer circumferential surface of a
crawler belt bushing.
[0011] The hardening process (3) has overcome the cost problems
presented by the processes (1) and (2), but presents other
problems. Specifically, it is necessary for this process to
accurately and narrowly control the hardenability (DI value) of the
steel material used, by correctly grasping the relationship between
the thickness of the steel and the cooling rate. This process has a
problem in the availability of materials to be used. In addition,
as the thickness of the busing becomes smaller, the bushing is more
through hardened across its entire thickness, causing higher
tensile residual stress at the inner and outer circumferential
surfaces. This could be a cause for quenching crack and a
considerable decrease in fatigue strength. To avoid this problem,
the DI value should be decreased and as a result, the steel
materials which meet this requirement will not be commercially
available. Further, if steels having small DI values are used, the
resultant bushings will have variations in hardness.
[0012] The present invention has been directed to overcoming the
above-described problems and therefore the prime object of the
invention is to provide a crawler belt bushing and its producing
method, improved over the products and processes of the above
conventional carburization and induction hardening in terms of
productivity and cost. This crawler belt busing is produced by
heating a tubular bushing workpiece made of steel to a quenching
temperature and then quenching the workpiece in a series of
quenching operation, without causing quenching crack, so that the
workpiece is quench hardened across its entire thickness.
[0013] The invention also aims to provide a long wear life, tough
crawler belt bushing and its producing method, improved over the
products and processes of the above conventional carburization and
induction hardening in terms of productivity and cost. This busing
is produced by heating a tubular bushing workpiece made of steel to
a quenching temperature and then applying a series of quenching
operation to this workpiece, the operation comprising: advance
cooling of the workpiece from the inner circumferential surface and
cooling of the workpiece from the outer circumferential surface
after waiting a certain time. With this process, quench hardened
layers are formed at the inner and outer circumferential surfaces
respectively, such that at least the depth of the outer
circumferential surface hardened layer is greater than that of the
inner circumferential surface hardened layer.
[0014] The invention provides a crawler belt bushing producing
method applicable to inexpensive steel materials having higher
commercial availability than the above-described through-hardening
process from the outer circumferential surface only, by achieving
through hardening by cooling from both inner and outer
circumferential surfaces although there is a difference between the
starting time for the inner circumferential surface cooling and the
starting time for the outer circumferential surface cooling.
[0015] The invention also aims to provide a crawler belt bushing
and its producing method, improved over the products and processes
of the above conventional carburization and induction hardening in
terms of productivity and cost, this busing being produced by the
following process. A tubular bushing workpiece made of steel is
heated by scan induction heating from its outer circumferential
surface such that at least the temperature of the inner
circumferential surface of the workpiece is raised to a quenching
temperature. Then, a series of quenching operation is carried out
as follows. While firstly starting cooling from the inner
circumferential surface, the outer circumferential surface is
heated by induction heating to restrict the cooling of the outer
circumferential surface occurring from the inner circumferential
surface. After waiting a certain time in order to prevent the core
of the workpiece from being fully hardened, cooling from the outer
circumferential surface is started. Through these steps, quench
hardened layers are formed at the inner and outer circumferential
surfaces respectively.
[0016] For ensuring formation of a quench hardened layer at the
inner circumferential surface, the advance cooling from the inner
circumferential surface, the induction heating from the outer
circumferential surface and the later cooling from the outer
circumferential surface are carried out as described earlier,
thereby forming a soft layer within the core of the bushing at a
cross-sectional position closer to the inner circumferential
surface. With this arrangement, a soft layer can be formed in the
core, even when using, as a bushing material, a steel having such a
great DI value that the steel is through hardened by cooling from
the inner circumferential surface only. In consequence, quenching
crack can be prevented and the hardened depth of the outer
circumferential surface can be greater than the hardened dept of
the inner circumferential surface, so that a crawler belt bushing
improved in wear resistance and fatigue strength and its producing
method can be provided.
[0017] The above induction heating/hardening method of the
invention is applicable to tubular parts similar to crawler belt
bushings. Since this method does not need to carry out heat
hardening from the inner circumferential surface and therefore does
not use an induction heating coil for an inner circumferential
surface, small-diameter tubular parts (e.g., small-sized crawler
belt bushings) and thin tubular parts (e.g., crawler belt bushings)
can be produced at low cost.
[0018] Further, since a soft layer can be formed within the wall of
the crawler belt bushing and quenching crack can be prevented even
when using a steel having high hardenability as has been noted
above, the invention can provide a crawler belt bushing having high
wear resistance at its outer circumferential surface and its
producing method, by hardening a steel material which has
comparatively high hardenability and a carbon content of about 2.0
wt % and is composed of austenite containing cementite grains
dispersed therein. It is preferable to substantially uniformly
disperse cementite in the workpiece by thermal refining or the
like, prior to starting of the hardening operation.
DISCLOSURE OF THE INVENTION
[0019] According to a first aspect of the invention, there is
provided a crawler belt bushing producing method, wherein a
workpiece of a crawler belt bushing made of a medium or high carbon
steel or a medium or high carbon low alloy steel is heated to a
quenching temperature or more; by use of a hardening system which
can independently start outer circumferential surface cooling and
inner circumferential surface cooling, the workpiece is first
cooled from either one of its outer circumferential surface and
inner circumferential surface and then cooling is carried out from
the other circumferential surface so that the workpiece is hardened
through its entire thickness; and then the workpiece is
tempered.
[0020] In the above method, after the bushing workpiece is heated
to a quenching temperature, a series of quenching operation is
carried out using a hardening system which can independently start
outer circumferential surface cooling and inner circumferential
surface cooling and using a cooling medium such as water,
water-soluble quenching liquid or oil. The quenching operation
comprises the steps of (i) advance cooling from either one of the
inner and outer circumferential surfaces to reduce heat capacity at
the core of the workpiece to provide a heat gradient and (ii)
cooling from the other circumferential surface which is started
after waiting for a certain time to reduce tensile stress due to
possible heat and deformation stresses generated during quenching
and therefore to reduce susceptibility to quenching crack caused by
through hardening. With this method, possible quenching crack due
to through hardening can be prevented even when using a steel which
has a high carbon content and high susceptibility to quenching
crack and is usually through hardened by simultaneous cooling from
the inner and outer circumferential surfaces. Accordingly, a
bushing having improved wear life at the outer circumferential
surface can be economically produced with this method.
[0021] By using a medium or high carbon steel having a carbon
content of 0.35 wt % to 1.5 wt % as a steel material for the
bushing and increasing the hardness of the quench hardened layer at
the outer circumferential surface to be equal or more than those of
carburization hardened bushings, the crawler belt bushing improved
in wear resistance and wear life can be produced at low cost. The
alloy composition of a steel is a factor that determines the
hardenability of the steel. The alloy compositions of the steels
used in the invention are determined by the lower limit of DI value
with which the bushing is through hardened by simultaneous cooling
from the inner and outer circumferential surfaces. As has been
noted above, the invention is basically directed to cooling from
the inner and outer circumferential surfaces and therefore it can
use steel materials less expensive than medium carbon low alloy
steels which can be through hardened by cooling from the outer
circumferential surface only. This advantage leads to considerable
cost reduction and makes the invention applicable to the production
of large-sized, thick crawler belt bushings.
[0022] While the wear resistance of the outer circumferential
surface of the bushing is ensured by use of medium and high carbon
steels, the impact resistance (toughness) of the bushing is
obtained by allowing self-tempering of the inner circumference by
finishing the cooling from the inner circumferential surface at an
early time. The impact resistance of the bushing may be obtained by
induction tempering the workpiece from the inner circumferential
surface subsequently to the quenching operation so that the
hardened depth of the inner circumferential surface is adjusted to
450 to 600 Hv. Accordingly, the crawler belt bushing having wear
resistance and impact resistance as high as those of carburization
hardened layers while keeping high hardness at the quench hardened
layer of the outer circumferential surface can be produced at low
cost by the invention.
[0023] The invention is based on the thermal operation in which
after substantially uniform, entire heating of the bushing, cooling
from either one of the inner or outer circumferential surfaces is
first started and then cooling from the other circumferential
surface is stared, so that hardening can be completed within one
cycle of operation. Unlike the conventional induction quenching,
the invention does not need to do adjustment twice, that is,
hardened depth adjustment for the inner circumferential surface and
for the outer circumferential surface, and does not need to
heat/quench the workpiece from the inner and outer circumferential
surfaces separately, so that the invention provides high
productivity. The heating method is not limited to induction
heating and furnace heating, but induction heating is preferred
when taking into account productivity, system cost and energy
efficiency.
[0024] Further, the hardening process of the invention employs a
quenching system in which starting time for inner circumferential
surface cooling and starting time for outer circumferential surface
cooling can be independently determined. Uneven cooling is likely
to occur when cooling the inner circumferential surface of a
tubular body, and therefore the invention preferably employs jet
cooling such as water spraying or oil spraying. In order to prevent
the cooling medium for the inner circumferential surface from
touching the outer circumference during the advance cooling from
the inner circumferential surface, it is preferable to set the
spray at an appropriate angle in consideration of the flows of the
cooling media as shown in FIG. 1 or to provide a partition
(shielding plate) as indicated by A in FIG. 1.
[0025] In the case incorporating furnace heating, when a
multiplicity of bushings is cooled by employing the above-described
advance cooling from the inner or outer circumferential surface, it
is preferable that the bushings be aligned with their adjacent end
faces in contact with each other, like one steel pipe as shown in
FIGS. 2(a) and 2(b), and then their inner circumferential surfaces
and outer circumferential surfaces be respectively cooled by
independently controlling inner circumferential surface cooling
water 2 and outer circumferential surface cooling water 3. It
should be noted that the cooling water 2 and the cooling water 3
are shielded from each other with a shielding plate 4. In the case
shown in FIGS. 2(b) and 2(c), there is disposed an inner
circumferential surface cooling nozzle 5.
[0026] The time difference hardening method, in which while heating
a part of the bushing by scan heating with an induction coil,
cooling from, for instance, the inner circumferential surface
starts in advance of cooling from the outer circumferential
surface, does not involve a large hardening system and has a high
degree of freedom in production. A preferred arrangement to effect
this method is shown in FIG. 3 in which shielding plates 4, 4' are
positioned at the upper and lower end faces of the bushing 1
respectively and the inner circumference surface cooling nozzle 5
and the outer circumferential surface cooling nozzle 6 are designed
such that the nozzle 5 firstly cools an induction heated zone and a
specified timer later, the nozzle 6 starts cooling. Preferably, the
scan hardening is carried out by the relative movement of an
induction heating coil 7, the nozzle 5 and the nozzle 6 along the
axis of the bushing 1 and such a relative movement is preferably
carried out while the bushing 1 being rotated. It is a matter of
course that when cooling the outer circumferential surface first,
the cooling nozzles should be arranged oppositely to the above
arrangement.
[0027] As has been described above, the invention is designed such
that (1) a bushing is substantially uniformly heated by induction
heating or furnace heating and (2) cooling from the inner or outer
circumferential surface of the bushing is started in advance of (3)
cooling from the outer or inner circumferential surface to
eliminate susceptibility to quenching crack, by use of a cooling
medium such as oil or water. With this arrangement, the crawler
belt bushing made of inexpensive, low hardenability, medium or high
carbon steel can be quench-hardened throughout its entire thickness
within one cycle of operation and the bushing thus hardened is
improved in the wear life of the outer circumferential surface and
production cost.
[0028] The formation of the deeply hardened layer having high
hardness, high wear resistance and carbon content equal to or
higher than those of carburized bushings leads to considerable
improvements in the wear resistance and wear life of the resultant
bushing. Further, the formation of the soft layer in the core
ensures toughness as high as that of the conventional bushings,
whereas tempering of the inner circumferential surface at higher
temperatures increases the toughness of the inner circumferential
surface layer. These all lead to improvements in the impact
strength and functions of the bushing.
[0029] According to a second aspect of the invention, there is
provided a crawler belt bushing producing method wherein, after a
workpiece of a crawler belt bushing made of steel is heated to a
quenching temperature,
[0030] (a) the cooling rate of the outer circumferential surface of
the workpiece is increased by first cooling of the workpiece from
its inner circumferential surface in order to reduce heat capacity
at the core of the workpiece and by second cooling of the workpiece
from its outer circumferential surface which is started a certain
time after the first cooling and/or
[0031] (b) the cooling rate of the outer circumferential surface of
the workpiece is increased by first cooling of the workpiece from
its inner circumferential surface in order to partially make the
core of the workpiece unhardenable by utilizing the mass effect of
the wall of the workpiece and by second cooling of the workpiece
from its outer circumferential surface which is started a certain
time after the first cooling,
[0032] whereby a soft layer is formed within the core of the
workpiece at a cross-sectional position closer to the inner
circumferential surface and the hardened depth of the outer
circumferential surface is made to be greater than the hardened
depth of the inner circumferential surface,
[0033] these processes (a) and (b) being carried out within one
cycle of quenching operation, using a hardening system capable of
performing inner circumferential surface cooling and outer
circumferential surface cooling.
[0034] In the invention having the above feature, a soft layer is
formed within the wall core at a cross-sectional position closer to
the inner circumferential surface, so that the bushing has a
U-shaped hardness distribution. With this arrangement, when using,
as the material of the bushing, a steel which is usually through
hardened by simultaneous cooling from the inner and outer
circumferential surfaces, the bushing can be prevented from
quenching crack. Further, the hardened depth of the outer
circumferential surface is made to be greater than the hardened
depth of the inner circumferential surface, which entails an
improvement in the wear life of the outer circumferential surface
of the bushing and enables economical manufacture.
[0035] The hardenabilities of the steels to which the invention is
applicable are dependent on their alloy compositions. Steels having
a wider variety of alloy compositions can be used by employing a
wider range of DI values with which the hardened depth achieved by
cooling from the inner circumferential surface only is about one
half the thickness of the bushing or less, even though the bushing
is through hardened by simultaneous cooling from the inner and
outer circumferential surfaces. With this arrangement, commercially
available, inexpensive steel materials can be used in the invention
and a hardened depth one-half the thickness of the bushing or more
can be easily ensured for the outer circumferential surface thereby
highly improving the wear life of the outer circumference of the
bushing.
[0036] The wear resistance of the outer circumferential surface of
the bushing is improved by the arrangement in which after heating
the workpiece to a quenching temperature, quenching is carried out
by advance cooling from the inner circumferential surface and later
cooling from the outer circumferential surface and in which
induction tempering is carried out from the inner circumferential
surface while the hardened depth of the outer circumferential
surface being kept high, thereby increasing particularly the
toughness of the inner circumference surface hardened layer. This
enables the economical manufacture of the bushing having wear
resistance and impact resistance equal to or higher than those of
carburization hardened layers.
[0037] According to the invention, in one cycle of operation, (1) a
workpiece is substantially uniformly heated by induction heating or
furnace heating and then, (2) cooling from the inner
circumferential surface of the workpiece is started in advance of
(3) cooling from the outer circumferential surface, by use of a
cooling medium such as oil or water, whereby the hardened depth of
the inner circumferential surface is made to be smaller than that
of the outer circumferential surface and whereby the cooling of the
outer circumferential surface can be expedited by the advance
cooling from the inner circumferential surface to further deepen
the hardened layer. With this arrangement, a soft layer can be
formed at the core thereby preventing possible quenching crack,
even when using a steel that is usually through hardened by
simultaneous cooling from the inner and outer circumferential
surfaces. Further, the outer circumferential surface hardened layer
can be made to be deeper than the inner circumferential surface
hardened layer, to improve the wear life of the bushing. This, in
consequence, brings about lots of benefits in economy.
[0038] In addition, the formation of the deeply hardened layer on
the outer circumferential surface entails improvements in the wear
resistance and wear life of the bushing, the hardened layer having
high wear resistance, high hardness and carbon content
substantially equal to those of carburized bushings. Further, the
formation of the soft layer in the core assures toughness equal to
that of the conventional bushings, and the high temperature
tempering of the inner circumferential surface toughens the inner
circumferential surface layer. These all contribute to improvements
in the impact strength and functions of the bushing.
[0039] According to a third aspect of the invention, there is
provided a crawler belt bushing producing method wherein, after a
workpiece of a crawler belt bushing made of steel is induction
heated from its outer circumferential surface such that at least
the temperature of the inner circumferential surface of the
workpiece is raised to a quenching temperature, a series of
quenching operation comprising:
[0040] (a) firstly cooling the workpiece from the inner
circumferential surface;
[0041] (b) cooing the workpiece from the inner circumferential
surface while heating the workpiece from the outer circumferential
surface; and
[0042] (c) cooling the workpiece from the outer circumferential
surface,
[0043] is performed so as to form quench hardened layers which
extend toward the wall core of the workpiece from the outer
circumferential surface and from the inner circumferential surface
respectively and form a soft, imperfectly hardened layer between
these quench hardened layers.
[0044] According to a forth aspect of the invention, there is
provided a crawler belt bushing producing method wherein, while a
workpiece of a crawler belt bushing made of steel being heated from
its outer circumferential surface by scan induction heating, using
at least two vertically aligned, induction coils,
[0045] (a) the temperature of the inner circumferential surface of
the workpiece is raised to a quenching temperature equal to the
transformation temperature of A1, A3 or Acm or more;
[0046] (b) the workpiece is partially heated from the outer
circumferential surface by the induction coils while carrying out
first cooling from the inner circumferential surface; and
[0047] (c) the workpiece is then cooled from the outer
circumferential surface;
[0048] whereby the inner and outer circumferential surfaces are
quench hardened so as to be substantially martensitic, these
processes (a), (b) and (c) being carried out at a certain position
of the workpiece.
[0049] In the invention having the above feature, after the
workpiece is heated from the outer circumferential surface by
induction heating so that the temperature of the inner
circumferential surface is raised to a quenching temperature, a
series of quenching operation is carried out in the following way,
using a cooling medium such as water, water-soluble quenching
liquid or quenching oil. In the quenching operation, cooling from
the inner circumferential surface is carried out while partially
heating the workpiece from the outer circumferential surface by
induction heating, thereby restricting the cooling of the outer
circumference from the inside. Then, cooling from the outer
circumferential surface is carried out after waiting a certain time
which is long enough to disallow the inside of the wall to be fully
quenched by the cooling from the outer circumferential surface.
With this arrangement, a soft, imperfectly hardened structure can
be formed within the core and a satisfactorily hardened depth can
be obtained at the outer circumferential surface by the later
cooling from the outer circumferential surface that starts after
waiting a certain time. This, in consequence, enables the
economical producing method for the crawler belt bushing having
quench hardened layers at the inner and outer circumferential
surfaces.
[0050] When using a steel which is usually through hardened by the
simultaneous cooling from the inner and outer circumferential
surfaces or by cooling from the inner circumferential surface only,
the invention can form a soft layer within the core at a
cross-sectional position closer to the inner circumferential
surface, because of the induction heating from the outer
circumference that is carried out during the advance cooling from
the inner circumferential surface. Therefore, in many cases, there
is substantially no need to control the hardenability of the steel
to be used in the invention, which enables use of inexpensive,
commercially available steel materials. This leads to cost
reduction.
[0051] It should be noted that the advance cooling from the inner
circumferential surface in the invention is intended for reducing
heat capacity in the core of the bushing. The reduction in heat
capacity, in turn, expedites the later cooling from the outer
circumferential surface so that the outer circumferential surface
hardened layer can be more deepened than the inner circumferential
surface hardened layer. The producing method incorporating the
above principle is particularly suited for the production of
crawler belt bushings having excellent wear life at their outer
circumferential surfaces.
[0052] For further deepening the quench hardened layer of the outer
circumferential surface, a steel, which has at least such
hardenability (DI value) that the hardened depth obtained by
hardening from the outer circumferential surface only is one half
the thickness of the bushing, is used for the bushing; the bushing
is hardened to a depth equal to the wear critical point (about one
half the thickness of the bushing); and the above-described soft
hardened layer is formed in the wall core at a position closer to
the inner circumferential surface. The resultant bushing is
superior in strength, toughness, and wear life.
[0053] As the method of induction heating the bushing from the
outer circumferential surface, an entire heating method or a scan
heating method may be employed. As shown in FIG. 4, in the entire
heating method, when heating the outer circumferential surface with
high frequency coil 2 during cooling from the inner circumferential
surface with an inner circumferential surface cooling nozzle 4, the
hardened depths of the inner and outer circumferential surfaces can
be controlled as required by controlling the electric power of the
high frequency coil 2. The cooling from the outer circumferential
surface may be carried out, for example, by moving outer
circumferential surface cooling nozzle 5 from underneath after
moving the high frequency coil 2 upward. Another arrangement is
such that the bushing is cooled by a coolant jetted through the
clearances between the inductors of the high frequency coil 2.
[0054] As shown in FIG. 5, in the scan heating method, a wide
induction coil or, more preferably, two or more vertically aligned
induction coils (high frequency coils) 8, 9 are used. These
induction heating coils are arranged to perform induction heating
so as to prevent the outer circumferential surface from being
cooled by the cooling from the inner circumferential surface. In
this way, the core of the wall will not be fully hardened.
[0055] The hardened depths of the inner and outer circumferential
surfaces can be easily controlled in the following way. Taking into
account the relative moving speed of the induction coils and the
bushing, the induction heating of the outer circumferential surface
during the cooling from the inner circumferential surface is mainly
performed by the second high frequency coil 9 and the distance
between the cooling position of the outer circumference of the
bushing cooled by the outer circumferential surface cooling nozzle
11 and the position of the second high frequency coil 9 is adjusted
to control the time to be taken for induction heating, that is, the
time after the cooling from the inner circumferential surface is
started until the cooling from the outer circumferential surface is
started.
[0056] These induction heating/hardening methods do not need to
heat and quench the bushing from the inner circumferential surface
and therefore enables the economical production of small-diameter
tubular parts (e.g., small-sized crawler belt bushings) and
extremely thin tubular parts (crawler belt bushings), for which
induction heating coils for an inner circumferential surface are
difficult to produce.
[0057] In view of wear resistance and strength, the carbon contents
of the steels used in this example are preferably 0.35 to 2.0 wt %,
so that the hardened bushing has a hardness of H.sub.RC 50 or more
and is improved in the hardness of the outer circumferential
surface hardened layer.
[0058] It is generally known that increasing of the carbon content
of a steel to be used is effective in economically manufacturing
crawler belt bushings having superior wear resistance and wear
life. The conventional induction hardening methods are not applied
to steels having carbon contents of 0.55 wt % or more because there
is a high risk of quenching crack. Thanks to the foregoing
heating/cooling principle, the invention can prevent quenching
crack and, therefore, can economically produce a crawler belt
bushing having superior wear resistance at the outer
circumferential surface by hardening the bushing in an austenite
state in which cementite grains are dispersed, even when steel
materials having comparatively good hardenability and carbon
contents as high as 2.0 wt % are used. Note that it is preferable
to treat the bushing, for example, by thermal refining, prior to
the hardening operation, in order that cementite grains are
substantially evenly dispersed in the structure of the bushing.
[0059] For particularly improving the wear resistance of the outer
circumferential surface, after the bushing workpiece is heated to a
quenching temperature, quenching is carried out by the above method
wherein the inner circumferential surface is first cooled, and
while the hardened depth of the outer circumferential surface being
kept high, the inner circumferential surface is induction tempered,
thereby particularly increasing the toughness of the inner
circumferential surface hardened layer. This makes it possible to
economically produce a crawler belt bushing having wear resistance
and impact resistance equal to or higher than those of
carburization hardened layers.
[0060] One of the features of the invention resides in the thermal
operation in which while the scan induction heating being carried
out, cooling from the inner circumferential surface is first
started and then, the outer circumferential surface is cooled, so
that heating/quenching is completed within one cycle of operation.
Therefore, there is no need to separately perform hardened depth
adjustment and separately perform heating/quenching for the inner
and outer circumferential surfaces, which results in high
productivity, savings in equipment investment cost, and improved
energy efficiency.
[0061] In the above hardening method, the hardened depth of the
inner circumferential surface can be made to be greater than the
hardened depth of the outer circumferential surface, by controlling
the output of the outer circumferential surface heating nozzle
during the advance cooling from the inner circumferential surface
and then carrying out the outer circumferential surface cooling.
Therefore, this method is suitably applied to the production of
high strength steel pipes used for delivering slurry etc., which
require high wear resistance at their inner circumferences.
[0062] In view of possible uneven cooling, the suitable cooling
method for the inner circumferential surface is jet cooling such as
water spraying or oil spraying. It is preferable to set the spray
at an angle in consideration of the flows of the cooling media as
shown in FIGS. 4 and 5 or to provide a partition such as the
shielding plate 1 shown in FIG. 4, in order that the cooling medium
for cooling the inner circumferential surface does not interfere
with the outer circumference during the advance cooling from the
inner circumferential surface.
[0063] A multiplicity of bushings can be treated by applying the
above hardening method to the bushings aligned with their adjacent
end faces in contact with each other as described earlier.
[0064] The time difference hardening method, in which the bushing
is partially heated by scan heating with the induction coil; the
inner circumferential surface is first cooled while heating is
carried out with the high frequency coils so as to prevent the
outer circumferential surface from being cooled by the cooling from
the inner circumferential surface; and then, cooling from the outer
circumferential surface is started, does not involve a large
hardening system and has a high degree of freedom in the
production. In this case, there may be provided the shielding plate
1 and shielding cap 6 near the lower and upper end faces of the
bushing respectively, as shown in FIG. 4. Preferably, the inner
circumferential surface cooling nozzle 4 first heats the induction
heating zone and a certain time later, cooling from the outer
circumferential surface is started, and while the bushing 3 being
rotated, the induction heating coil 2, the inner and outer
circumferential surface cooling nozzles 4, 5 are relatively moved
along the axis of the bushing, thereby performing the scan
hardening.
[0065] According to the invention, the scan induction heating,
cooling from the inner circumferential surface and cooling from the
outer circumferential surface are subsequently started with time
differences, whereby the bushing having quenched hardened layers at
the inner and outer circumferential surfaces and the soft layer at
the core, or the bushing having the outer circumferential surface
hardened layer that is deeper than the inner circumferential
surface hardened layer can be manufactured by one cycle of
operation. With this arrangement, quenching crack can be prevented
even when using a steel which is usually through hardened by the
simultaneous cooling from the inner and outer circumferential
surfaces. Further, the carbon contents of the steels to be used in
the invention can be increased. This leads to improvement in wear
life and enables an economical production method for long-life
bushings.
[0066] In the invention, it is preferable to simultaneously perform
cooling from the inner circumferential surface and from the outer
circumferential surface at least within a specified zone close to
the upper end face and the lower end face, respectively, of the
bushing. By controlling the striking position of the cooling medium
for the inner circumferential surface and the striking position of
the cooling medium for the outer circumferential surface to be
equal to each other at least within a specified zone of the bushing
during the advance cooling from the inner circumferential surface,
a hardened layer can be easily formed on the end face, so that the
resulting bushing has an ideal profile in which the imperfectly
hardened layer is enclosed by the hardened layers of the inner and
outer circumferential surfaces and the hardened layer of the end
face.
BRIEF EXPLANATION OF THE DRAWINGS
[0067] FIG. 1 is a sectional view of a quenching system.
[0068] FIGS. 2(a) and 2(b) are sectional views each showing a
quenching system for treating a multiplicity of bushings and
[0069] FIG. 2(c) is a longitudinal section of the system shown in
FIG. 2(b).
[0070] FIG. 3 is a sectional view of a hardening system employing
induction heating coils.
[0071] FIG. 4 is a schematic general view of an entire heating
induction hardening system.
[0072] FIG. 5 is a schematic view of a scan induction hardening
system.
[0073] FIG. 6 is a sectional view showing the configuration of a
bushing sample.
[0074] FIG. 7 is a graph showing the relationship between the
degree of through hardening of bushings having the dimension D and
the frequency of quenching crack in bushings.
[0075] FIG. 8 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 1 and
treated by the time difference hardening.
[0076] FIG. 9 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 2 and
treated by the time difference hardening.
[0077] FIG. 10 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 4 and
treated by the time difference hardening.
[0078] FIG. 11 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 5 and
treated by the time difference hardening.
[0079] FIG. 12 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 6 and
treated by the time difference hardening.
[0080] FIG. 13 is a graph showing the distributions of hardness in
bushings having the dimension A and the composition No. 7 and
treated by the time difference hardening.
[0081] FIG. 14 is a graph showing the distributions of hardness in
bushings having the dimension B and the composition No. 8 and
treated by the time difference hardening.
[0082] FIG. 15 is a graph showing the distributions of hardness in
bushings having the dimension C and the composition Nos. 7, 8 and
treated by the time difference hardening.
[0083] FIG. 16 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 2 and
treated by the time difference hardening.
[0084] FIG. 17 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 3 and
treated by the time difference hardening.
[0085] FIG. 18 is a graph showing the distributions of hardness in
bushings having the dimension D and the composition No. 4 and
treated by the time difference hardening.
[0086] FIG. 19 is a graph showing the relationship between the
hardened depths of the inner and outer circumferential surfaces of
a bushing having the dimension C and the lead time of advance
cooling from the inner circumferential surface.
[0087] FIG. 20 is a graph showing the relationship between the
hardened depths of the inner and outer circumferential surfaces of
a bushing having the dimension C and the lead time of advance
cooling from the outer circumferential surface.
[0088] FIG. 21 is a graph showing test results relating to the wear
of the outer circumferences of bushings in service.
[0089] FIG. 22 shows a method for testing collapse fatigue.
[0090] FIG. 23 is a graph showing the results of collapse fatigue
tests.
[0091] FIG. 24 shows a method for testing impact fatigue.
[0092] FIG. 25 is a graph (1) showing the results of impact fatigue
tests.
[0093] FIG. 26 is a graph (2) showing the results of impact fatigue
tests.
[0094] FIG. 27 is a graph showing the distribution of hardness in
the wall cross section of a bushing tempered at 140.degree. C. for
one hour after the scan induction heating/time difference
quenching.
[0095] FIG. 28 is a graph showing the results of the scan induction
hardening of bushings having the composition No. 4.
[0096] FIG. 29 is a graph showing the results of the scan induction
hardening of bushings having the composition No. 11.
[0097] FIG. 30 is a graph showing the results of the scan induction
hardening of bushings having the composition No. 12.
[0098] FIG. 31 is a graph (1) showing the result of the entire
induction hardening of a bushing having the composition No. 11.
[0099] FIG. 32 is a graph (2) showing the result of the entire
induction hardening of a bushing having the composition No. 11.
[0100] FIG. 33 is a graph showing the results of impact fatigue
tests.
[0101] FIGS. 34(a) to 34(f) show the steps of the scan induction
hardening in Example 4.
[0102] FIGS. 35(a) to 35(c) show the effects of Example 4.
[0103] FIG. 36 is an exploded perspective view of a crawler belt
bushing.
[0104] FIGS. 37(a), 37(b) and 37(c) are diagrams showing typical
hardening patterns for bushings produced by the conventional
methods and
[0105] FIG. 37(d) is a graph showing the distributions of hardness
in the cross sections of these bushings.
BEST MODE FOR CARRYING OUT THE INVENTION
[0106] Referring now to the drawings, crawler belt bushings and
their producing methods will be explained according to preferred
embodiments of the invention.
EXAMPLE 1
[0107] TABLE 1 shows the steel compositions of the bushings used in
Example 1. FIG. 6 shows the configuration of the bushings used in
this example and TABLE 2 shows the dimension of each bushing.
Heating for hardening was carried out in a furnace in a neutral
atmosphere and the spray quenching system shown in FIG. 1 was used.
The spray quenching system comprises a spray for cooling the inner
circumferential surface of a bushing and another spray for cooing
the outer circumferential surface. These sprays are independently
controlled so as to start cooling operation at different times. The
spray for the inner circumferential surface is designed to have an
adequate jetting angle with respect to a normal line of the inner
circumferential surface, so as to allow water present in the inner
circumference to flow toward the lower part of the bushing without
being trapped. Disposed near the lower end of the bushing is a
shield plate for dividing a flow of cooling water used for inner
circumferential surface cooling from a flow of cooling used for
outer circumferential surface cooling. Disposed near the upper end
is a cap for dividing a flow of cooling water used for inner
circumferential surface cooling from a flow of cooling water used
for outer circumferential surface cooling.
1TABLE 1 STEEL COMPOSITION (WT %) C Si Mn Cr P S Al Dl No 1 0.58
0.18 0.72 -- 0.015 0.015 0.03 0.91 No 2 0.54 0.23 0.81 -- 0.016
0.016 0.042 0.98 No 3 0.62 0.28 0.98 -- 0.017 0.018 0.041 1.25 No 4
0.61 0.19 1.17 -- 0.018 0.018 0.037 1.35 No 5 0.64 0.24 1.32 --
0.019 0.018 0.037 1.63 No 6 1.34 0.18 0.68 0.21 0.017 0.015 0.032
No 7 0.47 0.09 0.34 -- -- -- -- 0.49 No 8 0.53 0.23 0.48 -- 0.008
0.008 -- 0.69 No 9 0.57 0.23 0.81 -- 0.021 0.017 0.038 1.01 No 10
0.72 0.21 0.47 -- 0.025 0.02 0.014 0.81
[0108]
2 TABLE 2 D1 D2 t L A 41 24.4 8.3 81 B 47 28.2 9.4 94 C 59 38 10.5
138 D 79 50 14.5 202
[0109] Hardening operation was basically carried out under the
above conditions. Specifically, after intensely heating at
850.degree. C. for 30 minutes in a furnace, each crawler belt
bushing was quickly loaded in the quenching system shown in FIG. 1.
Quenching was carried out by inner circumferential surface cooling
and outer circumferential surface cooling under their respective
specified conditions and then, followed by low temperature
tempering at 140.degree. C. for 3 hours. It should be noted that
"entire induction heating" from the outer circumferential surface
of the bushing was incorporated as a part of the heating
process.
[0110] FIG. 7 shows the relationship between the degree of through
hardening and the frequency of quenching crack when the crawler
belt bushings (Dimension D) manufactured from the steel materials
Nos. 1 to 4 were quenched by simultaneous cooling from the inner
and outer circumferential surfaces. Surface residual stress is
plotted as the ordinate while the gradient of hardness in the outer
circumferential surface as the abscissa. FIG. 7 also indicates the
number of bushings out of 10 bushings, in which quenching crack
occurred. As seen from this figure, no quenching crack occurred in
the bushing made of the steel No. 1 and having a soft layer at its
core whereas quenching crack occurred in the completely
through-hardened bushings made of the steels Nos. 3 and 4. It is
understood from these results that the frequency of quenching crack
increases with the degree of through hardening.
[0111] FIGS. 8 to 12 show the distributions of hardness in the
cross sections of the crawler belt bushings (Dimension D) produced
from the steel materials Nos. 1, 2, 4, 5 and 6 when inner
circumferential surface cooling and outer circumferential surface
cooling were started at the same time and when inner
circumferential surface cooling was started in advance of outer
circumferential surface cooling. In the figures, the number of
bushings having quenching cracks per 10 bushings (crack ratio) is
indicated, from which it is understood that quenching crack can be
perfectly prevented by starting inner circumferential surface
cooling 2 seconds before outer circumferential surface cooling. The
lead time for preventing quenching crack is thought to be dependent
on the thickness of a crawler belt bushing to be treated. For
instance, it has been found that quenching crack can be prevented
in a small-sized crawler belt bushing (Dimension A) having a
thickness of 8.3 mm, by setting the lead time to about 1
second.
[0112] The bushing made of the steel No. 6 containing 1.34 wt %
carbon was through hardened and perfectly prevented from quenching
crack by starting inner circumferential surface cooling 8 seconds
before outer circumferential surface cooling. As understood from
FIGS. 10 to 12, the hardness of the outer circumference surface
hardened layer is 700 to 850 Hv, which is equal to or more than
those of crawler belt bushings treated by carburization heating and
therefore it is obvious that the wear resistance of the outer
circumference of the bushing has been significantly improved. Also,
quenching crack was checked when the crawler belt bushing
(Dimension D) manufactured from the steel No. 4 was quenched by
starting outer circumferential surface cooling in advance of inner
circumferential surface cooling. In this case, quenching crack was
prevented substantially similarly to the case of quenching of the
bushing No. 6 by firstly starting inner circumferential surface
cooling.
[0113] FIGS. 13 to 18 show the distributions of hardness in the
cross sections of bushings having the respective dimensions when
inner circumferential surface cooling and outer circumferential
surface cooling were started at the same time and when inner
circumferential surface cooling was started in advance of outer
circumferential surface cooling.
[0114] The following is understood from the results of the above
tests.
[0115] (1) Even when treating a steel which is usually fully
hardened at its core by simultaneously starting inner
circumferential surface cooling and outer circumferential surface
cooling, a soft layer can be formed within the core region at a
cross-sectional position closer to the inner circumferential
surface so as to form a U-shaped distribution of hardness, by
utilizing the mass effect of the wall of the bushing obtained by
employing different starting times for inner circumferential
surface cooling and outer circumferential surface cooling.
Therefore, the same effect can be expected by making the cooling
power of the inner circumferential surface different from the
cooling power of the outer circumferential surface, more
specifically, by making the cooling power of the inner
circumferential surface lower than that of the outer
circumferential surface.
[0116] (2) It is understood, from the relationship between the
degree of through hardening and the frequency of quenching crack
with respect to the bushings having the dimension D, that bushings
which are through hardened by the simultaneous cooling can be
perfectly prevented from quenching crack by imparting hardness to
the bushings so as to have a U-shaped distribution of hardness.
[0117] (3) Further, a steel, which is not usually through hardened
by the simultaneous cooling, can be more deeply hardened by the
effect of the increased cooling rate of the outer circumferential
surface. The increase in the cooling rate can be achieved by
reducing heat capacity in the core by the advance cooling from the
inner circumferential surface and achieved by the later cooling
from the outer circumferential surface.
[0118] FIG. 19 shows the relationship between the lead time of
advance cooling from the inner circumferential surface and the
hardened depths of the inner and outer circumferential surfaces of
the bushing having the dimension C. It is understood from this
figure that when the lead time is in a certain range, the maximum
hardened depth of the outer circumferential surface is obtained. In
view of wear life, it is preferable that the hardened depth of the
outer circumferential surface be at least 1.1 times the hardened
depth of the inner circumferential surface or more. As seen from
the data shown in FIGS. 13 to 18, the maximum hardened depth of the
outer circumferential surface in the invention is about twice the
hardened depth of the inner circumferential surface. This means
that excellent wear life can be achieved by the invention. FIG. 20
shows the relationship between the lead time of advance cooling
from the outer circumferential surface and the hardened depths of
the inner and outer circumferential surfaces of the bushing having
the dimension C. Oppositely to the result shown in FIG. 19, the
hardened depth of the inner circumferential surface can be
increased by starting outer circumferential surface cooling in
advance of inner circumferential surface cooling. This process is
suitably used in the production of wear-resistant, strong pipe
products having a bore for delivering earth/sand and slurry
therethrough.
[0119] Although the hardened depth of the outer circumferential
surface can be increased by the conventional heat treatment
employing thermal refining and induction hardening in combination,
this method needs to carry out thermal refining and induction
hardening for each of the inner and outer circumferential surfaces,
which is economically disadvantageous.
[0120] (Wear Resistance Test on Crawler Belt Bushings)
[0121] A crawler belt bushing (Dimension C; Composition No. 8; the
hardened depth of the outer circumferential surface 5.3 mm) treated
by the process according to Example 1 and a conventional carburized
bushing (Dimension C; quality=SCR420H; hardened depth=2.4 mm) were
respectively mounted on a crawler belt (D50, produced by Komatsu)
of a bulldozer and used in soil dressing in a rice field. FIG. 21
shows the results of the wear tests. After 2,200 hour operation,
the conventional bushing was worn by 5 mm, whereas the bushing
according to the invention was worn by 2.8 mm.
[0122] It took about 3,600 operating hours for the bushing of the
invention to reach the critical wear amount of 5 mm, which is a
considerable improvement in wear life. When considering the fact
that the critical wear amount is about one half the thickness of a
bushing and that the hardened layer of the outer circumferential
surface formed by the thermal treatment of the invention reaches
the substantial core region as seen from FIG. 13 to 18, the
invention has brought about a considerable improvement in wear
life. The test specimen of the invention has high hardness in the
outer circumferential surface hardened layer (see FIG. 15) and
therefore a low wear rate in the hardened layer, compared to the
conventional carburized bushing, so that it has proved to be
excellent in wear resistance.
[0123] (Collapse Fatigue Test on Crawler Belt Bushings)
[0124] FIG. 22 illustrates a method for testing collapse fatigue. A
bushing having the configuration shown in FIG. 6 was forced into a
crawler belt link 8 and a load F that was about twice the weight of
the vehicle was repeatedly imposed on a specified position (20 mm
away from the end face of the link in this example) to check the
number of repetitions, that is, how many times the load was imposed
until the bushing was brought to breakage. FIG. 23 shows the number
of repetitions which brought the bushing to breakage when loads of
2 to 37.5 tons were applied to the bushings having the dimension C
prepared according to this example. Three bushings, that is, the
above conventional carburized bushing, the bushing treated by the
time difference quenching of the invention, and the bushing treated
by the simultaneous quenching were compared in terms of fatigue
strength. It is apparent from the comparison that the bushing
according to the invention exhibits higher fatigue strength than
the conventional carburized bushing. Similar comparison was made
using large-sized specimens having the dimension D, from which it
was found that the bushing treated by the time difference quenching
of the invention was superior in fatigue strength to the
conventional carburized bushing and to the non-through-hardened
bushing treated by the simultaneous quenching.
[0125] (Impact Fatigue Test)
[0126] FIG. 24 shows a method for testing impact fatigue strength.
The crawler belt bushing treated by the thermal treatment according
to Example 1 was forced into a crawler belt link and repeatedly
struck with a hammer, causing impact load so that the stresses
exerted on the inner circumferential surface of the bushing were to
two, three, and four times the weight of the vehicle. The number of
repetitions which brought the bushing to breakage was checked to
measure its impact fatigue properties. For comparison, a specimen
(Vickers Hardness Hv=about 280) was prepared in the following way:
A bushing manufactured from a SCrB440H boron steel was thermally
refined (oil quenching at 850.degree. C. and tempering at
500.degree. C. for 3 hours) and after induction hardening, the
bushing was tempered at 180 C for 3 hours, thereby obtaining a
hardened depth of about 3.5 mm at the inner and outer
circumferential surfaces.
[0127] FIG. 25 shows the results of the measurements. It is
apparent from this figure that the bushings according to the
invention are improved in impact strength over the conventional
carburized bushings. Probably, this is attributable to the facts
that a grain boundary layer or imperfectly hardened layer exists in
the inner circumferential surface of the conventional carburized
bushings and that the carburized bushings have higher surface
carbon content (about 0.8 wt %) and higher surface hardness. This
means that the impact fatigue strength of the bushing of the
invention can be increased by adjusting the hardness of the inner
circumferential surface to increase toughness. FIG. 26 shows the
relationship between the hardness of the inner circumferential
surface and the number of impact ruptures when the bushing of the
invention was induction tempered from the inner circumferential
surface. As apparent from the figure, when the surface hardness Hv
is 550 to 600, the optimum strength can be obtained. For example,
when the bushing of the invention has a surface hardness Hv of 400,
it has higher strength than the conventional carburized bushing,
but excessive distortion occurred at the inner circumference of the
bushing after the test. This distortion may cause interference with
crawler belt pins, resulting in galling and abrasion. Therefore,
the hardness Hv for the invention is preferably 450 or more. The
upper limit of hardness for the invention is not particularly
specified by the comparison with the conventional carburized
bushing, but may be as high as the surface hardness of the
carburized bushing (Hv=up to 750). However, in order to achieve the
optimum impact strength properties, the hardness Hv of the inner
circumferential surface of the invention is preferably limited to
about 650.
Example 2
[0128] A bushing was hardened under the hardening conditions shown
in TABLE 3, using the hardening system shown in FIG. 3. The
dimension and composition of the bushing used in this example were
D, and No. 3 (the steel of No. 3 is usually through hardened by
simultaneous quenching from the inner and outer circumferential
surfaces), respectively. The difference between the position of the
inner circumferential surface of the bushing upon which cooling
water from the inner circumferential surface cooling nozzle 5
strikes and the position of the outer circumferential surface upon
which cooling water from the outer circumferential surface cooling
nozzle 6 strikes was adjusted to 30 mm. When the moving speed was 5
mm/sec., the lead time of inner circumferential surface cooling was
about 6 sec. Further, the temperature of the induction heating was
adjusted so as to obtain a temperature of about 920.degree. C. at
the outer circumferential surface and a temperature of about
850.degree. C. at the inner circumferential surface.
3TABLE 3 INDUCTION HEATING SCAN QUENCHING CONDITIONS HEATING
.multidot. QUENCHING CONDITIONS FREQUENCY (KHz) 1 OUTPUT (KW) 85
FEED RATE (mm/sec) 5.0 COOLING METHOD (INNER, OUTER) WATER
SPRAY
[0129] FIG. 27 shows the distribution of hardness in the cross
section of the bushing tempered at 140.degree. C. for one hour
after quenching. As seen from this figure, the hardened depth of
the outer circumferential surface of the bushing according to this
example is significant like Example 1 in which the time difference
quenching is carried out after heating in a furnace and it is
therefore understood that the bushing of this example has been
improved in wear life.
[0130] Although the induction heating coil is disposed on the side
of the outer circumferential surface of the bushing in this
example, it may be disposed on the side of the inner
circumferential surface of the bushing. However, it is preferable
to carry out induction heating from the outer circumferential
surface side, taking the operating performance of the hardening
system into account.
EXAMPLE 3
[0131] TABLE 4 shows the steel compositions of the bushings used in
Example 3. The bushings used in this example have the dimension C
(see TABLE 2). The scan induction hardening system shown in FIG. 5
was employed in this example. This hardening system comprises two
high frequency coils 8, 9 aligned vertically for heating the
bushing from the outer circumferential surface side; a nozzle 10
for cooling the inner circumferential surface of the bushing; and a
nozzle 11 for cooling the outer circumferential surface of the
bushing. The scan heating/quenching is carried out by the relative
movement of the bushing, coils 8, 9 and nozzles 10, 11 so that the
treatment proceeds upwardly, starting from the lower part of the
bushing. The inner circumferential surface nozzle 10 is designed to
have an adequate jetting angle with respect a normal line to the
inner circumferential surface in order to allow water present in
the inner circumference to flow toward the lower portion of the
bushing without being trapped. There is provided, near the lower
end of the bushing, a shield plate for dividing a flow of cooling
water used for inner circumferential surface cooling and a flow of
cooling water used for outer circumferential surface cooling from
each other. Disposed near the upper end of the bushing is a cap for
dividing a flow of cooling water used for inner circumferential
surface cooling and a flow of cooling used for outer
circumferential surface cooling from each other. A high frequency
power source of 6 KHz, giving an output of 50 KW was used and
hardening tests were conducted with the output of about 27 to 32
KW. Some bushing specimens were subjected to low-temperature
tempering at 140.degree. C. for 3 hours after quenching. Some were
subjected to entire induction heating from the outer circumference
surface, using the same high frequency power source, and when the
temperature of the inner circumferential surface has reached
850.degree. C., inner circumferential surface cooling was started
while continuing the induction heating. 6 seconds later, the
heating was finished to start outer circumferential surface
cooling.
4TABLE 4 No. C Si Mn Cr Mo P S Al 4 0.61 0.19 1.17 -- -- 0.018
0.018 0.037 11 0.75 0.18 0.61 1.02 0.16 0.010 0.012 0.028 12 1.45
0.28 0.58 0.82 -- 0.009 0.008 0.031
[0132] FIGS. 28 to 30 show the distribution of hardness in the
cross sections of the crawler belt bushings manufactured from the
steels Nos. 4, 11 and 12 (see TABLE 4) treated by the scan
hardening, in the case where inner circumferential surface cooling
and outer circumferential surface cooling are started at the same
time and in the case where the inner circumferential surface is
first cooled with the outer circumferential surface cooling nozzle
being shifted downward, and 6 to 10 seconds later, outer
circumferential surface cooling is started. It should be noted that
the crawler belt bushings associated with FIG. 30 were thermally
refined, by heating at 1,020.degree. C. for 30 minutes,
oil-quenching and then tempering at 600.degree. C. for 1 hour.
[0133] The following facts are understood from the results of the
above tests.
[0134] (1) Even when treating a steel which is usually hardened
through its cross section by simultaneous quenching from the inner
and outer circumferential surfaces, a soft layer can be formed
within the core at a cross-sectional position closer to the inner
circumferential surface so as to form a U-shaped distribution of
hardness, by carrying out induction heating from the outer
circumferential surface during the advance cooling, that is, the
cooling from the inner circumferential surface.
[0135] (2) Steels, which are usually hardened through their cross
sections by simultaneous quenching from the inner and outer
circumferential surfaces, can be perfectly prevented from quenching
crack by the hardening process of the invention so that steels
having very high carbon content can be used for the bushings.
[0136] (3) Even when treating a steel, which is not usually
hardened through its cross section by simultaneous quenching from
the inner and outer circumferential surfaces, hardened depth can be
increased by the effect of the increased cooling rate of the outer
circumferential surface. The cooling rate of the outer
circumferential surface is increased by reducing heat capacity in
the core region through the advance cooling from the inner
circumferential surface and by the later cooling from the outer
circumferential surface.
[0137] (4) Since this example uses steels having high carbon
content for the bushings, the hardness of the quench hardened layer
of each bushing is substantially as high as the hardness of the
carburized bushing. Further, the hardened layer of each bushing is
deeper than that of the carburized bushing, and therefore it is
understood that the bushings of this example have been considerably
improved in wear life (the wear life of a bushing is evaluated by
the time taken until about half thickness of the bushing has been
worn away).
[0138] FIG. 31 shows the distribution of hardness in the cross
section of the crawler belt bushing having the composition No. 11
when after entire induction heating, the inner circumferential
surface was first cooled and 10 seconds later, the outer
circumferential surface was cooled. The result shown in this figure
is substantially similar to the data obtained from FIG. 29 and it
is understood that quench hardened layers are formed in this
bushing by the same heating/cooling mechanism as that of the scan
hardening described earlier.
[0139] FIG. 32 shows the distribution of hardness in the cross
section of the crawler belt bushing having the composition No. 11
when after entire induction heating, the inner circumferential
surface was first cooled while heating the outer circumferential
surface with heating power (13 KW) which was about one-third the
heating power of the entire heating and 10 seconds later, the outer
circumferential surface was cooled. In this case, the hardened
depth of the inner circumferential surface could be increased, on
the contrary to the result shown in FIG. 31. Such a bushing is
suited for use in the production of wear-resistant, strong pipe
products with a bore for delivering earth/sand and slurry
therethrough.
[0140] (Impact Fatigue Test)
[0141] The crawler belt bushings were treated by the scan induction
hardening of this example and then tempered at 180.degree. C. for 3
hours. Then, these bushings were respectively forced into a crawler
belt link and tested, using the impact fatigue tester shown in FIG.
24. Specifically, the bushings were repeatedly struck with a
hammer, causing impact load so that the stresses exerted on the
respective inner circumferential surfaces were equivalent to two,
three, and four times the weight of the vehicle. The number of
repetitions which brought each bushing to breakage was checked to
measure the impact fatigue properties. For comparison, specimens
were prepared in the following way: For preparing a specimen
(Vickers Hardness Hv=about 280), a bushing manufactured from a
SCrB440H boron steel was thermally refined (oil quenching at
850.degree. C. and tempering at 500.degree. C. for 3 hours) and
then induction hardened to obtain a hardened depth of about 3.5 mm
at the inner and outer circumferential surfaces. Another specimen
was prepared by carburization-hardening a bushing made of SCr420H
steel at 930.degree. C., and then tempering it at 180.degree. C.
for 3 hours so as to obtain a hardened depth of 2.5 mm.
[0142] The results of the measurements are shown in FIG. 33. As
seen from this figure, the bushings prepared according to the
invention are improved in impact strength over the conventional
carburized bushings. This is presumably attributable to the facts
that a grain boundary layer or imperfectly hardened layer exists in
the inner circumferential surface of the conventional carburized
bushings and that the carburized bushings have higher surface
carbon content (about 0.8 wt %) and higher surface hardness. This
means that the impact fatigue strength of the bushings of the
invention can be increased by adjusting the hardness of the inner
circumferential surface to increase toughness. It is conventionally
known that when the surface hardness Hv of the inner
circumferential surface is 500 to 600, the optimum strength can be
obtained. For example, when the bushing of the invention has a
surface hardness Hv of 400, it has higher strength than the
conventional carburized bushings, but distortion occurred at the
inner circumference after the test. This distortion is a cause of
interference with crawler belt pins. Therefore, the hardness for
the invention is preferably 450 or more. The upper limit of
hardness Hv for the invention is not particularly specified by the
comparison with the conventional carburized bushing, but may be as
high as the surface hardness of the carburized bushing (Hv=up to
750). However, in order to achieve the optimum impact strength
properties, the hardness Hv of the inner circumferential surface of
the invention is preferably limited to about 650. It is important
particularly for the busing having the composition No. 12 and
manufactured according to the invention in which cementite grains
are dispersed, that cementite does not precipitate in the prior
austenite grain boundary and that the soft layer formed in the core
is mostly composed of a bainitic structure having granular
cementite dispersed therein.
EXAMPLE 4
[0143] In Example 4, scan hardening is carried out along the
bushing from the bottom to the top, using the scan induction
hardening system (shown in FIG. 5) employed in Example 3. Since the
heat input at the lower end of the bushing in the initial stage is
small, the high frequency coils 8, 9 are interrupted for a
specified length of time. When scan quenching is carried out by use
of the inner circumferential surface cooling nozzle 10 and the
outer circumferential surface cooling nozzle 11, the level at which
water jetted from the nozzle 10 strikes and the level at which
water jetted from the nozzle 11 strikes are made to be equal to
each other within specified zones close to the lower and upper end
faces, respectively, of the bushing. With this arrangement, the
bushing is hardened through its entire cross section at the lower
and upper ends thereof.
[0144] Reference is now made to FIG. 34 to describe the steps of
the hardening operation according to Example 4in detail.
[0145] In the initial stage shown in FIG. 34(a), the high frequency
coils 8, 9 are stationary and heating is carried out until the
inner circumferential surface of the bushing reaches the austenitic
temperature range. Then, the high frequency coils 8, 9 are moved
upward at constant speed, being followed by the inner and outer
circumferential surface cooling nozzles 10, 11. At this stage, the
level at which the coolant jetted from the nozzle 10 strikes upon
the bushing surface and the level at which the coolant jetted from
the nozzle 11 strikes upon the bushing surface are equal to each
other. At that time, the inner circumferential surface and outer
circumferential surface are simultaneously cooled within the region
having a specified height h.sub.1 from the lower end of the
bushing, as shown in FIG. 34(b). Since the inner and outer
circumferential surfaces are simultaneously cooled in the lower
region of the bushing, the bushing can be hardened through its
cross section in this region, if the steel of the bushing has
enough hardenability.
[0146] As shown in FIG. 34(c), after the striking level of the
coolants from both nozzles 10, 11 has reached a specified level,
the upward movement of the outer circumferential surface cooling
nozzle 11 is stopped, while the inner circumferential surface
cooling nozzle 10 being continuously moved upward at a specified
speed in synchronous relation with the high frequency coils 8, 9.
At the time when the difference between the striking level of the
coolant from the nozzle 10 and the striking level of the coolant
from the nozzle 11 becomes equal to a specified distance h.sub.2,
the nozzle 11 is moved upward synchronously with the nozzle 10 at
the same speed as that of the nozzle 10. In this way, the cooling
from the inner circumferential surface advances by the time that is
obtained by dividing the level difference h.sub.2 by the moving
speed of the nozzles 10, 11. Thanks to this advance cooling from
the inner circumferential surface, the temperature of the bushing
at the center in its cross section decreases to such a temperature
range in which hardening does not occur, and as a result, an
imperfectly hardened layer is created inside the wall of the
bushing.
[0147] At the time when the striking level of the coolant from the
nozzle 10 has reached a position having a specified distance
h.sub.3 (=h.sub.1) from the upper end of the bushing, the nozzle 10
is stopped, as shown in FIG. 34(d). Thereafter, the nozzle 11 is
continuously moved upward. When the striking level of the coolant
from the nozzle 10 and the striking level of the coolant from the
nozzle 11 become equal to each other as shown in FIG. 34(e), the
nozzles 10, 11 are synchronously moved toward the upper end of the
bushing while keeping the coolant striking levels of both nozzles
the same, as shown in FIG. 34(f). When reached the upper end, the
nozzles are stopped, while cooling is continued until the bushing
is completely cooled.
[0148] FIG. 35 shows three entire hardened patterns when a 15
mm-thick crawler belt bushing for bulldozers is hardened.
Specifically, (a) is the pattern obtained by carburization
hardening; (b) is the pattern obtained by the time difference
hardening incorporating induction heating, with a constant
positional relationship between the cooling nozzles; and (c) is the
pattern obtained by the hardening process of Example 4. The
bushings used herein are made of a S55C steel having a DI value
(indicating the hardenability of steel) of 1.5, and the time
difference (i.e., lead time) between the inner circumferential
surface cooling and the outer circumferential surface cooling is 6
seconds. As seen from FIG. 35, in the bushing treated by the
hardening process of Example 4, hardened layers are formed at both
ends, and the depth of the outer circumference is greater than the
depth of the inner circumference. In addition, the depth of the
outer circumference of Example 4 is greater than that of the
carburized bushing. In contrast with this, the bushing (b) treated
by the time difference hardening with a constant nozzle positional
relationship has a hardened layer at the upper end face only, with
the internal imperfectly hardened layer extending throughout the
lower end of the bushing.
* * * * *