U.S. patent application number 12/007445 was filed with the patent office on 2008-08-28 for ferrous seal sliding parts and producing method thereof.
Invention is credited to Chikara Nakao, Takemori Takayama.
Application Number | 20080202652 12/007445 |
Document ID | / |
Family ID | 34889418 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080202652 |
Kind Code |
A1 |
Takayama; Takemori ; et
al. |
August 28, 2008 |
Ferrous seal sliding parts and producing method thereof
Abstract
A ferrous seal sliding part excellent in heat crack resistance,
seizure resistance and abrasion resistance is provided. The ferrous
seal sliding part has a seal sliding surface, wherein the seal
sliding surface has a quench hardened layer having a structure in
which a martensite parent phase forms a solid solution with carbon
of 0.15 to 0.6 wt % and contains cementite dispersed therein in a
content of 3 to 50% by volume.
Inventors: |
Takayama; Takemori;
(Hirakata-shi, JP) ; Nakao; Chikara;
(Hirakata-shi, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W., SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
34889418 |
Appl. No.: |
12/007445 |
Filed: |
January 10, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11065105 |
Feb 25, 2005 |
|
|
|
12007445 |
|
|
|
|
Current U.S.
Class: |
148/612 ;
148/319; 148/321; 148/617; 148/654 |
Current CPC
Class: |
C21D 1/18 20130101; C22C
37/06 20130101; F16C 33/34 20130101; C22C 38/02 20130101; C22C
38/18 20130101; C21D 2211/003 20130101; C21D 9/0068 20130101; C21D
5/00 20130101; F16C 13/006 20130101; C22C 38/04 20130101; C22C
37/10 20130101 |
Class at
Publication: |
148/612 ;
148/319; 148/321; 148/654; 148/617 |
International
Class: |
C21D 5/00 20060101
C21D005/00; C21D 8/00 20060101 C21D008/00; C21D 5/04 20060101
C21D005/04; C22C 37/00 20060101 C22C037/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2004 |
JP |
2004-056416 |
Feb 10, 2005 |
JP |
2005-034801 |
Claims
1. A ferrous seal sliding part having a seal sliding surface,
wherein said seal sliding surface has a quench hardened layer
having a structure in which a martensite parent phase forms a solid
solution with carbon of 0.15 to 0.6 wt % and contains cementite
dispersed therein in a content of 3 to 50% by volume.
2. A ferrous seal sliding part having a seal sliding surface,
wherein said seal sliding surface has a quench hardened layer
having a structure in which a martensite parent phase forms a solid
solution with carbon of 0.15 to 0.7 wt % and contains cementite in
a content of 3 to 50% by volume and graphite in a content of 3 to
15% by volume dispersed therein.
3. A ferrous seal sliding part according to any one of claims 1 to
2, wherein said martensite parent phase has a layered structure
region comprising platy cementite and martensite phases dispersed
therein in an area ratio of 20% or more, whereby cementite
including said platy cementite is dispersed in the martensite
parent phase in a total content of 3 to 50% by volume.
4. A ferrous seal sliding part according to any one of claims 1 to
2, wherein said seal sliding surface is formed by a steel product
containing carbon of 0.5 to 1.8 wt % and one or more alloy element
selected from the group consisting of Cr of 0.3 to 3 wt %, V of 0.1
to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %.
5. A ferrous seal sliding part according to claim 2, wherein said
seal sliding surface is formed by using a cast iron selected from
the group consisting of a gray cast iron, a nodular graphite cast
iron, a vermicular graphite cast iron and a pearlite malleable cast
iron, in which said cast iron has a structure in which graphite is
dispersed in a parent phase having a pearlite structure containing
carbon of 2 to 4.5 wt % and further one or more alloy element
selected from the group consisting of Cr of 0.5 to 4 wt %, V of 0.1
to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %.
6. A ferrous seal sliding part according to claim 2, wherein said
seal sliding surface is formed by using a white cast iron
containing carbon 2 to 4.5 wt % and further one or more alloy
element selected from the group consisting of Cr of 0.5 to 4 wt %,
V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %,
or a cast iron in which cementite of said white cast iron is
partially graphitized.
7. A ferrous seal sliding part according to any one of claims 1 to
2, wherein an average concentration of Cr in said cementite is 2.5
to 15 wt %.
8. A ferrous seal sliding part according to claim 3, wherein
cementite dispersed in said martensite parent phase contains
granulated cementite having an average grain size of 0.1 to 10
.mu.m in addition to said platy cementite.
9. A ferrous seal sliding part according to any one of claims 1 to
2, wherein said seal sliding surface contains retained austenite in
a content of 10 to 50% by volume.
10. A ferrous seal sliding part according to claim 9, wherein said
seal sliding surface is formed by a steel product or a cast iron
containing either any one of Si of 0.5 to 3.5 wt % or Al of 0.25 to
2 wt %, or both Si and Al in a total amount of 0.5 to 3.5 wt %.
11. A ferrous seal sliding part according to claim 9, wherein said
seal sliding surface is formed by a steel product or a cast iron
containing both Mn and Ni in a total amount of 2 to 7 wt %.
12. A ferrous seal sliding part according to claims 1 to 2, wherein
said seal sliding surface contains one or more alloy element
selected from the group consisting of V, Ti, Zr, Nb, Ta and Hf in a
total amount of 0.05 to 5 wt %, whereby at least any one of
carbide, nitride and carbonitride of said alloy elements, having an
average grain size of 0.1 to 5 .mu.m, is dispersed in a total
content of 0.1 to 10% by volume.
13. A ferrous seal sliding part according to claim 5, wherein said
cast iron contains Cu of 5 to 15 wt %, whereby Cu alloy phase is
dispersed in 3 to 10% by volume.
14. A producing method for a ferrous seal sliding part comprising;
a preparing step for preparing a steel product containing carbon of
0.5 to 1.8 wt % and further one or more alloy element selected from
the group consisting of Cr of 0.3 to 3 wt %, V of 0.1 to 0.5 wt %,
Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %; and a quenching step
for heating said steel product at a heating rate such that
temperature rises from Al transformation temperature to a quenching
temperature in the range of 850 to 1100.degree. C. within 10
seconds and then rapidly cooling.
15. A producing method for a ferrous seal sliding part comprising;
a preparing step for preparing a cast iron selected from the group
consisting of a gray cast iron, a nodular graphite cast iron, a
vermicular graphite cast iron and a pearlite malleable cast iron,
in which graphite is dispersed in a parent phase having a pearlite
structure containing carbon of 2 to 4.5 wt % and further one or
more alloy element selected from the group consisting of Cr of 0.5
to 4 wt %, V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5
to 2 wt %; and a quenching step for heating said cast iron at a
heating rate such that temperature rises from Al transformation
temperature to a quenching temperature in the range of 850 to
1100.degree. C. within 10 seconds and then rapidly cooling.
16. A producing method for a ferrous seal sliding part comprising;
a preparing step for preparing a white cast iron containing carbon
of 2 to 4.5 wt % and further one or more alloy element selected
from the group consisting of Cr of 0.5 to 4 wt %, V of 0.1 to 0.5
wt %, Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %, or a cast iron in
which cementite of said white cast iron is partially graphitized;
and a quenching step for heating said cast iron at a heating rate
such that temperature rises from Al transformation temperature to a
quenching temperature in the range of 850 to 1100.degree. C. within
10 seconds and then rapidly cooling.
17. A producing method for a ferrous seal sliding part according to
any one of claims 14 to 16 further comprising; a heat treatment
step for concentrating one or more alloy element selected from the
group consisting of Cr, V, Mo and W in cementite in said ferrous
seal sliding part.
18. A producing method for a ferrous seal sliding part according to
any one of claims 14 to 16 further comprising; a heat treatment
step for dispersing a pearlite structure region comprising platy
cementite and ferrite in an area ratio of 20% or more in a steel
before quenching.
19. A producing method for a ferrous seal sliding part according to
any one of claims 14 to 16, wherein said preparing step for
preparing said steel product or said cast iron is a step for
preparing a steel product or a cast iron containing either any one
of Si of 0.5 to 3.5 wt % or Al of 0.25 to 2 wt %, or both Si and Al
in a total amount of 0.5 to 3.5 wt %.
20. A producing method for a ferrous seal sliding part according to
any one of claims 14 to 16, wherein said preparing step for
preparing said steel product or said cast iron is a step for
preparing a steel product or a cast iron containing Mn and Ni in a
total amount of 2 to 7 wt %.
21. A producing method for a ferrous seal sliding part according to
any one of claims 15 and 16, wherein said cast iron contains Cu of
5 to 15 wt % whereby Cu alloy phase is dispersed in 3 to 10% by
volume.
22. A producing method for a ferrous seal sliding part according to
any one of claims 14 to 16, wherein said heating rate is
150.degree. C./sec or more.
23. A producing method for a ferrous seal sliding part according to
any one of claims 14 to 16, wherein a pre-heating step for
preheating said steel product or said cast iron at 300.degree. C.
to Al transformation temperature after said preparing step and
before said quenching step.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to ferrous seal sliding parts
used for rollers, idlers and reduction gears for construction
machines and producing methods thereof.
BACKGROUND OF THE INVENTION
[0002] A track roller assembly and a reduction gear apparatus of a
construction machine are equipped with ferrous floating seal parts
for the purpose of preventing leakage of lubrication oil from
inside thereof as well as entering of earth and sand therein.
Accordingly, such floating seals are widely produced by applying
adequate treatment in such that a seal sliding surface thereof is
quenched to have a hard martensite structure, or a large amount of
hard cementite and Cr.sub.7C.sub.3 carbide are crystallized in 30%
by volume while transforming a parent phase to a martensite
structure by quenching in order to improve seizure resistance and
abrasion resistance. An exemplary floating seal is made by using a
high-carbon and high-Cr cast iron or a Ni-hard cast iron (for
example, as shown in Japanese Patent Publication (KOKAI) No.
S51-59007).
[0003] In addition, a ferrous floating seal in which
abrasion-resistant material is thermal-sprayed to a seal sliding
surface thereof may be used for some purposes.
[0004] In the ferrous floating seal used for the purpose of sealing
a lubrication oil in the reduction gears and the track rollers, a
seal sliding surface thereof is abraded as fine particles of earth
and sand are entered in the seal sliding surface by hulling motion
in the earth and sand, and is lubed with the lubrication oil sealed
therein. Accordingly, a ferrous floating seal capable of
withstanding a very severe lubrication condition is required. Even
if a most conventionally used hard ferrous floating seal part made
of a high-carbon and high-Cr cast iron is applied, when setting
pressure (press force) at assembling is high, considerable
quenching crack (heat crack), seizure and abnormal abrasion occur
on the seal sliding surface, resulting in leakage of oil.
[0005] And, even if various tool steels such as a cold work tool
steel and a high speed steel (SKH material) are employed to
increase the seizure resistance and the abrasion resistance,
seizing due to defect of seizure resistance easily occurs,
resulting in insufficient heat crack resistance and abrasion
resistance. In addition, since such steels are very expensive, they
have a problem that cost of material increases in view of material
yield before a product is finished.
[0006] And, in resent years, construction machines such as a
bulldozer are required to drive at a high speed for improvement in
working efficiency, and therefore, the ferrous floating seal
necessarily rotates at a high speed. This also causes quenching
crack, seizure and abnormal abrasion, resulting in leakage of
oil.
[0007] In order to solve the above-mentioned problems, an object of
the present invention is to provide ferrous seal sliding parts
excellent in heat crack resistance, seizure resistance and abrasion
resistance and a producing method thereof.
SUMMARY OF THE INVENTION
[0008] A ferrous seal sliding part according to the present
invention has a seal sliding surface, wherein the seal sliding
surface has a quench hardened layer having a structure in which a
martensite parent phase forms a solid solution with carbon of 0.15
to 0.6 wt % and contains cementite dispersed therein in a content
of 3 to 50% by volume.
[0009] Here, the seal sliding part includes a ferrous floating
seal.
[0010] A ferrous seal sliding part according to the present
invention has a seal sliding surface, wherein the seal sliding
surface has a quench hardened layer having a structure in which a
martensite parent phase forms a solid solution with carbon of 0.15
to 0.7 wt % and contains cementite in a content of 3 to 50% by
volume and graphite in a content of 3 to 15% by volume dispersed
therein.
[0011] A producing method of a ferrous seal sliding part according
to the present invention comprises a preparing step for preparing a
steel product containing carbon of 0.5 to 1.8 wt % and further one
or more alloy element selected from the group consisting of Cr of
0.3 to 3 wt %. V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W of
0.5 to 2 wt %; and a quenching step for heating the steel product
at a heating rate such that temperature rises from Al
transformation temperature to a quenching temperature in the range
of 850 to 1100.degree. C. within 10 seconds and then rapidly
cooling.
[0012] Here, the steel product may be formed in a shape of a
ferrous seal sliding part.
[0013] A producing method of a ferrous seal sliding part according
to the present invention comprises a preparing step for preparing a
cast iron selected from the group consisting of a gray cast iron, a
nodular graphite cast iron, a vermicular graphite cast iron and a
pearlite malleable cast iron, in which graphite is dispersed in a
parent phase having a pearlite structure containing carbon of 2 to
4.5 wt % and further one or more alloy element selected from the
group consisting of Cr of 0.5 to 4 wt %, V of 0.1 to 0.5 wt %, Mo
of 0.3 to 2 wt % and W of 0.5 to 2 wt %; and a quenching step for
heating the cast iron at a heating rate such that temperature rises
from Al transformation temperature to a quenching temperature in
the range of 850 to 1100.degree. C. within 10 seconds and then
rapidly cooling.
[0014] Here, the cast iron may be formed in a shape of a ferrous
seal sliding part.
[0015] A producing method of a ferrous seal sliding part according
to the present invention comprises a preparing step for preparing a
white cast iron containing carbon of 2 to 4.5 wt % and further one
or more alloy element selected from the group consisting of Cr of
0.5 to 4 wt %, V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W of
0.5 to 2 wt %, or a cast iron in which cementite in the white cast
iron is partially graphitized; and a quenching step for heating the
cast iron at a heating rate such that temperature rises from Al
transformation temperature to a quenching temperature in the range
of 850 to 1100.degree. C. within 10 seconds and then rapidly
cooling.
[0016] Here, the cast iron may be formed in a shape of a ferrous
seal sliding part.
[0017] As described above, the present invention will provide
ferrous seal sliding parts excellent in heat crack resistance,
seizure resistance and abrasion resistance and a producing method
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1(a) to FIG. 1(d) are photographs showing a structure
of each cast iron having a pearlite structure for a ferrous
floating seal.
[0019] FIG. 2 is a graph showing a solid-solution formation
mechanism of .gamma. phase by using a phase diagram of iron, carbon
and M and constant carbon activity lines.
[0020] FIG. 3 is a graph showing constant carbon activity lines of
iron, carbon and Cr (at 1000.degree. C.).
[0021] FIG. 4 is a phase diagram showing an effect of alloy element
to iron and Si of 3 wt %.
[0022] FIG. 5(a) is a graph showing a relation between induction
heating temperature and quenching hardness, FIG. 5(b) is a graph
showing a relation between induction heating temperature and carbon
concentration of martensite, and FIG. 5(c) is a graph showing a
relation between induction heating temperature and volume (% by
volume) of .theta. phase.
[0023] FIG. 6 is a photograph showing a structure of No. 4 steel to
which rapidly induction quenching is applied.
[0024] FIG. 7 is a graph showing a relation between heating
temperature and, quenching hardness and an amount of retained
.gamma..
[0025] FIG. 8 is a photograph showing a structure of No. 4 steel in
which pearlite cementite and granulated cementite are dispersed, to
which rapidly induction quenching is applied.
[0026] FIG. 9 is a cross section drawing showing a ferrous floating
seal.
[0027] FIG. 10 is a schematic drawing showing a floating seal
tester.
DETAILED DESCRIPTION OF EMBODIMENT OF THE INVENTION
[0028] In the present invention, rapid induction heating and
cooling results in that a quench hardened layer is formed at the
seal sliding surface. The quench hardened layer has a layered
structure region comprising hard platy cementite and a low carbon
martensite phase excellent in thermal shock resistance in a total
content of 20% or more by area (pearlitely structure region, call
the cementite in the pearlitely structure region as pearlitely
cementite) whereby cementite including the platy cementite is
dispersed therein in 3 to 50% by volume. This increases capability
for applying a lubricating oil onto a seal sliding surface and
achieves a ferrous seal sliding part excellent in
tempering-softening resistance and heat crack resistance. And,
since an unhardened layer remains in the ferrous sliding part
partially, a ferrous seal sliding part having toughness can be
obtained. Although radiant heating and salt bath heating may be
applicable as substitute for the rapidly induction heating, the
rapidly induction heating is preferable from the viewpoint of
productivity and economical efficiency.
[0029] A ferrous floating seal, which is one example of a ferrous
seal sliding part, capable of withstanding the severe condition
requires to have a seal sliding surface excellent in seizure
resistance, which can be improved by increasing heat crack
resistance. Therefore, the present invention provides a ferrous
floating seal, in which at least seal sliding surface thereof is
quench hardened (a quench hardened layer) by induction heating
quenching. As the result, the seal sliding surface has a structure
in which an amount of carbon which forms a solid solution with a
martensite parent phase is so adjusted to 0.15 to 0.6 wt % that
heat crack resistance of the seal sliding surface can be improved.
And, the structure has further hard cementite available for
increasing abrasion resistance and seizure resistance dispersed in
the martensite parent phase in a content of 3 to 50% by volume.
[0030] A ferrous floating seal part according to the present
invention has a seal sliding surface, wherein the sliding surface
has a martensite parent phase which forms a solid solution with
carbon of 0.15 to 0.7 wt %. And, in the martensite parent phase,
graphite in a content of 3 to 15% by volume, in addition to
cementite in a content of 3 to 50% by volume, are dispersed.
[0031] The reason that the lower limit of a content of the
cementite to be dispersed is adjusted to 3% by volume is such that
an effect to improve seizure resistance is sufficiently
demonstrated by dispersing hard particles under a conventional
sliding condition, and as a result, abrasion resistance is also
improved. In addition, in order to further improve abrasion
resistance to entering of earth and sand, it is effective that
harder carbide, nitride and carbonitride are dispersed in a larger
amount. For example, in a case of a high-carbon and high-Cr cast
iron, a content of Cr.sub.7C.sub.3 carbide to be dispersed is
adjusted in 35 to 50% by volume. Accordingly, it is preferable that
the upper limit of a content of cementite to be dispersed is set to
50% by volume and the lower limit thereof is set at 10% by volume.
Another reason that the upper limit of a content of cementite to be
dispersed is set at 50% by volume is that when a content of the
cementite exceeds 50% by volume, the ferrous floating seal becomes
brittle excessively.
[0032] In the ferrous floating seal made of the aforesaid
high-carbon and high-Cr cast iron, it is presumed that a martensite
phase excellent in heat crack resistance contains carbon of about
0.8 wt %. Thus, the upper limit of an amount of carbon which forms
a solid solution with a martensite parent phase is preferably set
at 0.7 wt %. However, in view of a concentrations of carbon
contained in a hot work tool steel (SKD6, SKD7, SKD61, SKD62, SKD8
and 3Ni-3Mo steel) and the like which requires high heat crack
resistance, it is preferable that the upper limit of an amount of
carbon which forms a solid solution with a martensite parent phase
is set at 0.6 wt % and the lower limit thereof is set at 0.15 wt
%.
[0033] Further, in order to improve abrasion resistance to earth
and sand, it is preferable that hardness of a martensite parent
phase is HRC50 or more. In addition, in order to ensure heat crack
resistance stable, it is more preferable that an amount of carbon
contained in a martensite parent phase is adjusted to 0.2 to 0.5 wt
%.
[0034] It is thought that abrasion resistance to earth and sand
necessary for a ferrous floating seal is improved as the dispersed
carbide is MC-type carbide, Cr.sub.7C.sub.3 or M.sub.6C special
carbide and has higher hardness. However, when the carbide is too
hard, scraping characteristic against the counterpart surface to
the seal sliding surface becomes large, resulting in heat
generation, therefore causing seizure and heat crack easily.
Accordingly, in the present invention, it is preferable that
comparatively soft cementite having hardness of about Hv1000 to
1300 is dispersed.
[0035] Graphite is used as a solid lubrication and an oil pocket to
improve lubricating property, therefore to improve seizure
resistance. A content of the graphite is preferably adjusted such
that the lower limit thereof is 3% by volume, at which improvement
in lubricating property is apparently demonstrated, and the upper
limit thereof is 15% by volume which is a maximum amount contained
in a conventional cast iron. And, the upper limit is more
preferably 10% by volume at which improvement in lubricating
property is saturated and toughness is obtained.
[0036] In order to improve lubricating property under the aforesaid
severe sliding condition of a ferrous floating seal, a ferrous
floating seal according to the present invention has a layered
structure region (colony) in which pearlitely (platy) cementite
showing an oil pocket effect for receiving an lubricating oil on a
sliding surface is dispersed in a martensite phase in layers. The
layered structure region is dispersed in a martensite parent phase
of the seal sliding surface of the ferrous floating seal in an area
ratio of 20% or more. This improves seizure resistance and heat
crack resistance of the seal sliding surface. In addition,
cementite, including the pearlitely cementite and the other
cementite, is dispersed in a hard martensite parent phase in a
total content of 3 to 50% by volume so that abrasion resistance to
earth and sand will be improved.
[0037] And, since the dispersed pearlitely cementite is harder than
the hard martensite phase which exists among the dispersed
pearlitely cementite, an area other than the dispersed pearlitely
cementite serves as an oil pocket for receiving a lubricating oil
so that lubricating property will be improved, and therefore
seizure resistance will be improved and occurrence of heat crack
will be prevented.
[0038] FIG. 1(a) is a photograph showing a structure (a pearlitely
structure) in which a steel product having a substantially
eutectoid carbon concentration (carbon concentration of 0.8 wt %)
is cooled from an austenite state to cause pearlite transformation.
In the figure, area with gray color shows cementite and area with
white color shows ferrite phases. FIG. 1(a) shows that a large
number of pearlite transformed regions (colonies) are aggregated,
resulting in almost all areas becoming to have a pearlite
transformed structure. In the present invention, the pearlitely
colonies comprising a cementite-martensite phase are formed such
that a steel product which has a pearlite transformed structure
comprising cementite and ferrite phase before induction quenching
(induction heating quenching) is rapidly heated and rapidly cooled
so that the ferrite will be transformed to martensite while the
cementite does not dissolve but remains. From the result, an area
ratio of the pearlitely colonies is larger than the eutectoid
concentration of the steel product and is almost 100% by area in
view of a phase diagram, at which a content of the cementite does
not exceed 12% by volume. Therefore, it is preferred that a larger
amount of granulated cementite should be dispersed from the
viewpoint of improvement in abrasion resistance. In the present
invention, it is possible that granulated cementite is dispersed in
the pearlitely cementite dispersed region in a content of as large
as 20% by volume. And, in the viewpoint of improvement in
lubricating property of a sliding surface, for example, an oil
groove of 15% by area of a sliding surface of a conventional
bearing apparently improves its lubricating property. From this
result, it is preferable that an area ratio of the pearlitely
cementite colony is 20% or more by area and it is more preferable
that a content of the pearlitely cementite colony is 50% or more by
volume.
[0039] And, FIG. 1(b) to FIG. 1(d) are photographs showing
structures of typical cast irons in which each parent phase thereof
is the aforesaid pearlitely structure. FIG. 1(b) is a photograph
showing a gray cast iron in which graphite flakes are dispersed,
FIG. 1(c) is a photograph showing a nodular graphite cast iron in
which spheroidal graphite is dispersed, FIG. 1(d) is a photograph
showing a white cast iron comprising cementite (white phases) and a
pearlite structure (black areas) in ledeburite. These cast irons
are also applicable for the present invention.
[0040] It is difficult from the viewpoint of an equilibrium phase
diagram to densely disperse cementite (granulated cementite and
pearlitely cementite) in a quenched martensite parent phase which
forms a solid solution with carbon of 0.15 to 0.7 wt % according to
the present invention. To disperse cementite in the quenched
martensite parent phase densely, a ferrous floating seal according
to the present invention has a seal sliding surface, which is
formed by a steel product containing carbon of 0.5 to 1.8 wt % and
one or more alloy element selected from the group consisting of Cr
of 0.3 to 3 wt %, V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W
of 0.5 to 2 wt %, which are alloy elements concentrated in
cementite remarkably. And, it is preferable that the steel product
is preheated under a heating condition in which at least either one
of ferrite phase or austenite phase coexists with cementite to
heighten a content of Cr contained in the cementite in the steel
product up to at least 2.5 wt % or more, preferably 3 wt % or more.
This prevents forming a solid solution of cementite with an
austenite phase during heating at induction hardening. The steel
product is substantially made of iron containing carbon and the
aforesaid alloy elements, and it is preferable to be added one or
more alloy element selected from the group consisting of Si, Al,
Mn, Ni, Cr. V, Mo, W. Cu. Co, B. Ti, Nb, Zr, P, S, Ca, Pb and Mg in
the viewpoint of improvement in hardenability, tempering-softening
resistance, corrosion resistance, machineability, abrasion
resistance and seizure resistance, in addition to improvement
achieved by the present invention. And, in some cases, the alloy
element and one or more impurity element selected from the group
consisting of S, P, O and N get mixed in the steel product to
produce the steel product. So, such alloy elements may get mixed as
far as they have little effect on the purpose of the present
invention. The steel product, as well as a later-described cast
iron, substantially made of iron means a steel product to which the
alloy elements and the impurity elements are suitably
contained.
[0041] A ferrous floating seal according to the present invention
has a seal sliding surface, wherein the seal sliding surface has a
martensite parent phase which forms a solid solution with carbon of
0.15 to 0.7 wt % and contains cementite in a content of 3 to 50% by
volume and graphite in a content of 3 to 15% by volume dispersed
therein.
[0042] In the ferrous floating seal part, it is possible that the
sliding surface is formed by using one of cast irons selected from
the group consisting of a gray cast iron, a nodular graphite cast
iron, a vermicular graphite cast iron (or a compacted vermicular
graphite cast iron) and a pearlite malleable cast iron, wherein the
cast iron has a structure in which graphite in a content of 3 to
15% by volume is dispersed in a parent phase having a pearlite
structure containing carbon of 2 to 4.5 wt % and further one or
more alloy element selected from the group consisting of Cr of 0.5
to 4 wt %, V of 0.1 to 0.5 wt %, Ho of 0.3 to 2 wt % and W of 0.5
to 2 wt %.
[0043] In the ferrous floating seal part, it is also preferable
that the seal sliding surface is formed by using a white cast iron
containing carbon of at least 2 to 4.5 wt % and one or more alloy
element selected from the group consisting of Cr of 0.5 to 4 wt %,
V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %,
or a cast iron in which cementite of the white cast iron is
partially graphitized so that cementite in a content of 15 to 50%
by volume and graphite in a content of 3 to 15% by volume will be
dispersed in a martensite parent phase or a parent phase composed
of pearlitely cementite and a martensite phase which forms a solid
solution with carbon of 0.15 to 0.7 wt %.
[0044] And, when a steel product in which a ferrite phase
coexistent with a cementite phase before quenching is equilibrium
heated at 700.degree. C., a distribution coefficient .alpha.KM of
an alloy element M is shown by dividing a concentration (wt %) of
the alloy element M in the cementite phase by a concentration (wt
%) of the alloy element M in the ferrite phase, for example, each
distribution coefficient is represented as follows; .alpha.KCr of
Cr=28, .alpha.KMn of Mn=10.5, .alpha.KV of V=9, .alpha.KMo of
Mo=7.5, .alpha.KW of W=2, .alpha.KNi of Ni=0.34, .alpha.KCo of
Co=0.23, .alpha.KSi of Si=approximately 0 and .alpha.KAl of
Al=approximately 0. From the distribution coefficients, it is
understood that Mn, Cr, Mo, V and W are concentrated in cementite;
Si, Al. Ni and Co are concentrated in ferrite. Therefore, for
example, when cementite is dispersed in a ferrite phase in 50% by
volume as described above, Si, Al. Ni and Co are concentrated in
the ferrite phases each having a concentration of about 2 times,
about 2 times, about 1.5 times and about 1.6 times that of the
added amount, respectively, while Cr and Mn are diluted in the
ferrite phases each having a concentration of about 0.07 times and
about 0.17 times that of the added amount, respectively.
[0045] As for a heat treatment method for concentrating Cr in
cementite before induction hardening, various treatment methods are
applicable, such as a method for heating a steel having a
martensite structure or a pearlite structure in a two-phase region
of ferrite and cementite, a method for cooling at a slow cooling
rate such as a air-cooling or a furnace-cooling at a stage of a
pearlite structure formation, a constant-temperature treatment in
the range of Al transformation temperature to 600.degree. C. and a
spheroidizing treatment of cementite (as described in Iron and
Steel Institute of Japan, "Heat Treatment of Steel: basis and
operation standard". MARUZEN Co. Ltd, p 44 to 46).
[0046] Furthermore, in a case of a steel having a carbon
concentration larger than a eutectoid carbon concentration, a
pre-heat treatment is also preferably applied, in which the steel
is maintained with being heating at a two-phase region of austenite
and cementite in the range of Al transformation temperature to Acm
transformation temperature to concentrate C.sub.r in the cementite
and then cooled so as to transform a parent phase to a martensite,
a bainite and/or a fine pearlite structures. Alternatively, in the
pre-heat treatment, it is also preferred that after maintaining
with being heating, the steel is slow-cooled to 600.degree. C. to
disperse granulated cementite and/or pearlite cementite. In
addition, Cr in the cementite in the two-phase region of austenite
and cementite, within a temperature range of 750 to 850.degree. C.,
is concentrated to have a concentration about 8 to 10 times a
concentration of Cr in the austenite. For example, when a steel
containing carbon of 0.9 wt % and Cr of 1.5 wt % is maintained with
being heated at 820.degree. C., the steel is transformed to have a
structure in which cementite containing Cr of 11 wt % is dispersed
in about 2% by volume. Accordingly, as described later, applying
induction hardening causes the dispersed cementite to remain
without forming a solid solution.
[0047] The process in which cementite is dispersed in a low-carbon
martensite parent phase containing carbon of 0.15 to 0.7 wt % can
be explained by using a solid solution formation mechanism
(velocity) of cementite which forms a solid solution with austenite
when a material composed of the aforesaid Cr concentrated cementite
and a ferrite phase is maintained with being heating at a quenching
temperature by rapidly heating such as induction hardening. The
solid solution formation mechanism can be understood by using a
relation between a ternary phase_diagram of iron, carbon and M (an
alloy element) at a heating temperature as shown in FIG. 2 and a
constant carbon activity line as shown in the figure.
[0048] FIG. 2 is a graph schematically showing a ternary phase
diagram of iron, carbon and an alloy element M which has almost the
same affinity with carbon as that of Cr. In the present invention,
a ferrous floating seal having a seal sliding surface is provided,
wherein the seal sliding surface is formed by a steel product which
contains carbon of 0.5 to 1.8 wt %, at least either one of Cr of
0.3 to 3 wt % or V of 0.1 to 0.5 wt %, one or more alloy elements
selected from the group consisting of Si, Al, Mn, Ni, Mo, W, Cu,
Co, B, Ti and P, impurity elements of S, O and N and a residue of
iron. A typical composition of the steel product is represented a
point A in FIG. 2. A carbon activity equal to a carbon activity of
the steel transfers as represented in a thin line passing through
the point A. Specifically, the constant carbon activity line
changes upward because carbon activity is decreased as an addition
amount of the alloy element M increases. Then, the constant carbon
activity line crosses with a line showing solid solubility of
cementite at a point B, and then linearly connected at a point C
showing a composition of cementite containing an alloy element M at
equilibrium with the crossing point (the point B).
[0049] Another constant carbon activity lines (as represented in
thin lines) in FIG. 2 are determined according to each carbon
activity. A carbon activity becomes higher as its carbon
concentration increases. Here, a carbon activity Ac is defined as 1
at a point D showing solid solubility of graphite along an
iron-carbon axis (a binary phase diagram of iron and carbon).
[0050] Each composition of ferrite and cementite in the steel
having the composition shown at the point A in FIG. 2 is shown in a
point E and a point F, respectively. On rapid heating to a
quenching heating temperature, the cementite having the composition
of the point F is transformed such that the alloy element M remains
but carbon having significant high diffusion ability forms a solid
solution with austenite rapidly. At this time, a composition of an
austenite boundary at local equilibrium with a cementite boundary
is shown in a point G. Here, since a carbon activity at the point G
is larger than the carbon activity of the steel of the point A,
carbon diffuses rapidly due to a gradient of chemical potential of
carbon. Then, at a region where cementite has formed a solid
solution and regions where were being originally ferrite, carbon is
first homogenized along a constant activity line passing through
the point B, as shown in the figure, and then the alloy element is
second homogenized.
[0051] However, when a large amount of the alloy element M is added
in the steel (a point H) and therefore a large amount of the alloy
element M concentrates in the cementite (a point J), a carbon
activity (a point K) of austenite equilibrium to the cementite,
which is transformed such that the alloy element M remains but
carbon forms a solid solution with cementite, becomes lower than a
carbon activity at the point H. So, while carbon rapidly diffuses
to a concentration as shown in a point L along a constant carbon
activity line passing through a point K for a very short period, a
solid solution formation proceeds more rapidly, as the result, the
cementite can not form a solid solution without diffusion of the
alloy element M along a solid solution line of cementite from the
point K to the point B while the cementite is completely forming a
solid solution. And, a solid solution formation of cementite is
suddenly suppressed by a rate-controlling of the diffusion of the
alloy element M.
[0052] And, a period in which the cementite completely forms a
solid solution becomes longer, as a concentration of the alloy
element M in the cementite becomes larger than a concentration of
the alloy element M of the point B at which a constant carbon
activity line passing through the point A and the point H is
crossed with a solid solubility line of the cementite. In order to
decrease the amount of the cementite which does not form a solid
solution at high-frequency heating (induction heating), it is
necessary that a concentration of the alloy element M in the
cementite is adjusted to a concentration smaller than that of the
alloy element M of the point B. And, since a carbon concentration
of the point L along a constant carbon activity line passing
through the point K is almost corresponding to a carbon
concentration of a martensite parent phase in which cementite is
dispersed without forming a solid solution, it is preferable that a
carbon concentration of the point L may be set at 0.15 to 0.7 wt %
according to the present invention. And, it is necessary to
regulate a J point of cementite, at which a carbon concentration of
the L point is 0.15 to 0.7 wt %.
[0053] A composition of regions where cementite forms a solid
solution around cementite which is diffused without forming a solid
solution has substantially the same composition as that at the
point K in FIG. 2. The alloy element concentration of the region is
significantly higher than those of the point L and the point H, and
therefore a carbon concentration becomes easily higher.
Accordingly, it is found that a martensite transformation
temperature Ms of the regions moves to lower so that it will be
likely to form a retained austenite phase excellent in toughness
and conformability around the cementite which does not form a solid
solution.
[0054] More specifically, induction hardening in which a quenching
treatment is carried out by rapidly heating at 1000.degree. C. will
be studied by using a ternary phase diagram of iron, carbon and Cr,
as shown in FIG. 3, and constant carbon activity lines (at
1000.degree. C.).
[0055] (1) A Case in which Cementite Rapidly Forms a Solid Solution
(a Case in which a Concentration of Cr in Cementite is Low)
[0056] When a steel shown as the point A of the figure (which has
carbon of 0.8 wt % and Cr of 0.4 wt %) is sufficiently heated at
700.degree. C. in the region in which cementite coexists with
ferrite, the steel is transformed to have a composition of the
point B (cementite, Cr of 2.6 wt %) and the point C (ferrite, Cr of
0.09 wt %). Then, when the steel having the transformed composition
is rapidly heated by induction heating to a temperature of
1000.degree. at which the steel becomes an austenite state, the
point B and the point C transfer toward the point A, along the
arrows in the figure, causing the ferrite and the cementite to be
homogenized. As described above, carbon rapidly diffuses, as shown
in arrows of the figure, in the austenite (the point C) which had
originally a ferrite structure via the point D while the alloy
element contained in the cementite of the point B is hardly being
diffused in the austenite. After the cementite has formed a solid
solution, Cr element is gently homogenized toward the point A along
the constant carbon activity line passing through the point A with
diffusion of Cr. And, at a point in which the cementite forms a
solid solution completely by more rapidly induction heating, a
carbon concentration of a martensite parent phase becomes equal to
that of the point A, so that martensite having higher hardness will
be obtained.
[0057] Accordingly, a concentration Cr in the cementite is about
4.5 wt %, when a concentration of carbon which forms a solid
solution with a martensite phase according to the present invention
is 0.7 wt %, whereby it appears that regulating a concentration of
Cr in the cementite to 4.5 wt % or less prevents remaining of
cementite which does not form a solid solution.
[0058] (2) A Case 1 in which a Solid Solution Formation of
Cementite Drastically Delays.
[0059] When a steel shown as the point E of the figure (which has
carbon of 0.8 wt % and Cr of 1 wt %) is sufficiently heated at
700.degree. C. in the region in which cementite coexists with
ferrite, the steel is transformed to have a composition of the
point G (ferrite, Cr of 0.24 wt %) and the point F (cementite, Cr
of 6.61 wt %). Then, when the steel having the transformed
composition is rapidly heated by induction heating to a temperature
of 1000.degree. C. at which the steel becomes an austenite state,
as similar to the aforesaid embodiment, the point F transfers
toward the point H, causing a solid solution formation of cementite
with ferrite. Since a carbon activity at the point H (an austenite
boundary having a carbon activity equal to that of the cementite
which forms a solid solution) becomes lower than that at the point
E, the cementite first forms a solid solution by the carbon
diffusion rate controlling mechanism to a constant carbon activity
line passing through the point H. And then, a composition (the
point H) of a .gamma. phase at equilibrium with the cementite
transfers along a solid solubility line of the cementite toward the
point I, having the same carbon activity as the point E, on a solid
solubility line of the cementite. This results that the cementite
forms a solid solution associated with diffusion of Cr, and then
the cementite completely forms a solid solution when a composition
of the .gamma. phase has a composition of the point I. And, a
concentration of carbon in the martensite parent phase after
quenching is about 0.5 wt %, and therefore, it is found that the
cementite of about 5% by volume is dispersed without forming a
solid solution in a very hard martensite phase.
[0060] And, when a concentration of carbon in the martensite parent
phase is 0.15 wt %, the upper limit of a concentration of Cr in a
cementite phase is about 12 wt %. Accordingly, under a quenching
condition in which after rapidly heating to a quenching temperature
of 1000.degree. C. and then cooling is applied, adjusting a
concentration of Cr in a cementite phase to 4.5 to 13 wt % makes it
possible to obtain a quench hardened layer in which cementite is
dispersed in a martensite parent phase having carbon of 0.15 to 0.7
wt % without forming a solid solution.
[0061] (3) A Case 2 in which the Solid Solution Formation of
Cementite Drastically Delays.
[0062] The point H in the case of (2) assumes that a two-phase
equilibrium in which Cr.sub.7C.sub.3 carbide different from
cementite is equilibrium to a .gamma. phase and the unequilibrated
cementite is equilibrium to the .gamma. phase is formed during a
solid solution formation of the cementite. In such a solid solution
formation process of cementite, cementite forms a solid solution by
the carbon diffusion rate controlling mechanism to a constant
carbon activity line (about 0.2) passing through the point J on a
solid solubility line of Cr.sub.7C.sub.3 carbide. In the subsequent
process of the solid solution formation of the cementite, a
restriction condition, in which a composition of a .gamma. phase
boundary is changed from at least the point J at which
Cr.sub.7C.sub.3 carbide does not precipitate, along a solid
solubility line of Cr.sub.7C.sub.3 carbide toward the point K in a
three-phase (.gamma. phase and cementite phase and Cr.sub.7C.sub.3
phase) coexistence region where the Cr.sub.7C.sub.3 carbide does
not need to precipitate, is added so that it is not necessary to
form Cr.sub.7C.sub.3 carbide before disappearance of the cementite.
The restriction condition causes delay in forming a solid solution
of cementite.
[0063] In such a case, a concentration of carbon in a martensite
parent phase obtained by the induction heating and quenching is
about 0.4 wt %, and cementite of about 6% by volume is dispersed
without forming a solid solution in the hard martensite parent
phase having hardness of HRC 57 to 61.
[0064] From the studied results, the limit point at which a
significant delay in a solid solution formation of cementite occurs
is a case where Cr is concentrated in cementite to have a
concentration of about 3.5 wt % or more under a heating condition
of 1000.degree. C. When under a heating condition of 900.degree.
C., a concentration of Cr in cementite is about 2.5 wt % (when
under a heating condition of 800.degree. C., it is about 2 wt %).
For example, when a steel containing carbon of 0.55 wt % and Cr of
0.3 wt % is heated at 700.degree. C., a concentration of Cr in the
cementite is 2.6 wt %, obtained by using the following
equation.
A concentration of Cr=.alpha.KCr.times.a concentration of Cr in a
steel/(1-(a concentration of carbon in a
steel/6.67).times.(1-.alpha.KCr)).
[0065] Accordingly, the lower limit of the addition amount of Cr is
about 0.3 wt %, and more preferably about 0.5 wt % or more.
[0066] In order to disperse cementite without forming a solid
solution stably, it is necessary that an average amount of Cr in
cementite is 2.5 to 15 wt %, preferably 3.5 to 13 wt %.
[0067] .alpha.KCr shows a distribution coefficient showing a degree
of concentrating of Cr between ferrite phase and cementite. Each
distribution coefficients at 600.degree. C. is represented as
follows; .alpha.KCr of Cr=52, .alpha.KMn of Mn=19 and .alpha.KMo of
Mo=12. It has been known that the higher the distribution
coefficient of an alloy element is, the higher the alloy element
tends to concentrate in the cementite.
[0068] When an amount of Cr in the cementite is adjusted to 2.5 wt
% or less, in a case in which rapidly induction heating is carried
out, a solid solution formation of cementite is delayed by a moving
resistance of a boundary between cementite phase and austenite
phase, in addition to the mechanism in which cementite forms a
solid solution depending to the carbon diffusion rate controlling,
whereby it is expected that cementite is dispersed without forming
a solid solution in a quench hardened layer. It will be achieved by
quenching just after rapidly heating from Al transformation
temperature to a quenching temperature at a heating rate of
500.degree. C./sec or more. However, since an amount of carbon
which forms a solid solution with martensite is unable to be
adjusted and a solid solution formation of Cr with cementite causes
hardness of cementite to be higher (up to Hv1300), addition of Cr
is indispensable.
[0069] And, Mn is an element which has a distribution coefficient
.alpha.KMn higher than distribution coefficients .alpha.KV of V and
.alpha.KMo of Mo and is easily to concentrate in cementite.
However, in an amount range (0.3 to 2 wt %) of Mn added to a
conventional steel, Mn does not form special carbide having an
austenite structure and has behavior to decrease carbon activity in
austenite less than about half of the behavior of Cr. Accordingly,
Mn effects less on delay in a solid solution formation of
cementite. However, since Mn significantly contributes to a
formation of retained austenite by the aforesaid mechanism and to
hardenability, it is preferable that Mn is added within an amount
range (0.1 to 2 wt %) of Mn added to a conventional steel.
[0070] A distribution coefficient .alpha.KM of an alloy element H
between cementite phase and ferrite was determined when
sufficiently heated at 700.degree. C. as described above. So, when
heated at 600.degree. C., a distribution coefficient each of Cr,
Mn, V and Mo becomes higher, and therefore Cr. Mn, V and Mo are
concentrated in cementite more densely. However, if a heating
period is too short, such an alloy element does not sufficiently
concentrate, whereby a pre-heat treatment at a eutectoid
temperature or less is preferably carried out.
[0071] A distribution coefficient .alpha.KM is constant, even if
the cementite is pearlitely cementite. Accordingly, it is preferred
that a steel having a pearlite transformed structure before
induction quenching is heat-treated at a temperature lower than a
eutectoid temperature of the steel.
[0072] And, in order to prepare a structure in which pearlitely
cementite and cementite particle will be dispersed, it is
preferable that cementite is precipitated and dispersed as
granulate state in a temperature range in which austenite and
cementite exist at equilibrium and then pearlite transformation is
caused while being cooled.
[0073] Alloy elements such as V, Cr, Mo and W, each having high
distribution coefficient between ferrite and cementite, not only
concentrate in cementite easily but also form carbide such as
Fe.sub.21Mo.sub.2C.sub.6, V.sub.4C.sub.3 and WC special carbide,
similar to Cr.sub.7C.sub.3 carbide described in the case (3),
whereby the same study as the Cr.sub.7C.sub.3 was carried out. As a
result, an addition of V of 0.1 wt % or more, Mo of 0.3 wt % or
more and W of 0.5 wt % or more causes delay in a solid solution
formation of cementite. Accordingly, in the present invention, in
the viewpoint of economical efficiency, it is preferable that V, Mo
and W are added by mixture, if necessary, in addition to Cr of 0.3
wt % or more.
[0074] And, when V of over 0.05 wt % is added, it is crystallized
as V.sub.4C.sub.3 carbide in a steel before induction quenching.
The V.sub.4C.sub.3 carbide remains in a martensite parent phase
with forming little solid solution, even if induction hardening is
applied thereto. Since the V.sub.4C.sub.3 is remarkable hard
carbide and demonstrates considerable improvement in abrasion
resistance when applied to the aforesaid high-speed steel, it is
preferred that the addition amount thereof is adjusted to 0.05 to 5
wt % (0.1 to about 10% by volume) in view of the high-speed
steel.
[0075] Accordingly, a ferrous floating seal according to the
present invention has a seal sliding surface, wherein the seal
sliding surface contains one or more alloy elements selected from
the group consisting of V, Ti, Zr, Nb, Ta and Hf in a total amount
of 0.05 to 5 wt % so that at least any one of carbide, nitride and
carbonitride of the alloy elements, having an average grain size of
0.1 to 5 .mu.m, will be dispersed in a total content of 0.1 to 10%
by volume.
[0076] And, for the purpose of more improvement in abrasion
resistance, it is preferable that an addition amount of V is
adjusted to 0.5 wt % or more, and more preferably to 1 wt % or
more.
[0077] Cementite other than the pearlitely cementite contains Cr of
an average concentration of 2.5 to 15 wt %. Such the cementite may
be crystallized at previous austenite grain boundaries, and
preferably may be granulated cementite having an average grain size
of 0.1 to 10 .mu.m from the viewpoint of strength. And, abrasion
resistances of a sliding material and a tool steel are well
improved when a grain size of carbide contained therein is large,
and frictional force caused by sliding affects mainly a region
within a depth of about 10 .mu.m below the sliding surface. From
the results, it is preferable that the upper limit of an average
grain size of granulated cementite is about 10 .mu.m for the
purpose of improvement in abrasion resistance.
[0078] And, when granulate cementite is made to grow so as to have
an average grain size of 3 .mu.m or more, it is preferable that a
heat-treatment in the two-phase region of austenite and cementite
is applied thereto at 900.degree. C. or more. And, it is preferable
that a steel or a cast iron with an addition of carbon of 1.2 wt %
or more is used.
[0079] In the heat-treatment method for rapid-cooling after rapid
induction heating as described above, alloy elements such as Cr,
which easily concentrates in cementite, and carbon are concentrated
at vicinity of cementite which does not form a solid solution, and
therefore retained austenite forms easily. In light of this fact,
in the present invention, a content of the retained austenite is
adjusted to 10 to 50% by volume of the parent phase thereof. As a
result, internal notch action which occurs at a large amount of
dispersed cementite will be defused, and conformability of a seal
sliding surface, heat crack resistance and seizure resistance will
be improved. The reason that the upper limit of a content of
retained austenite is 50% by volume is because retained austenite
of more than 50% by volume makes abrasion resistance smaller. In
addition, the upper limit thereof is preferably 35% by volume.
[0080] In the ferrous floating seal part to which the aforesaid
induction quenching is applied, since Cr, Mo, V W and Mn are
concentrated in cementite and then the induction quenching is
carried out, concentrations of such elements in a martensite parent
phase of the quench hardened layer is considerably small, resulting
in decreasing tempering-softening resistance of the martensite
phase. This raises a problem that abrasion resistance and seizure
resistance are decreased. Accordingly, in the present invention, a
steel or a cast iron containing either Si of 0.5 to 3.5 wt % or Al
of 0.25 to 2 wt %, or both Si and Al in a total amount of 0.5 to
3.5 wt % is induction heated and quenched to produce a ferrous
floating seal, since Si and Al hardly forms a solid solution with
cementite and improves tempering-softening resistance of a
martensite parent phase.
[0081] Since Si and Al demonstrate remarkable tempering-softening
resistance to 450.degree. C. and work more effectively and
economically than Cr even in a high-temperature range of 500 to
600.degree. C., it is preferable that Si and Al are positively
added.
[0082] In addition, Si, in reverse of Cr, significantly increases
carbon activity in an austenite phase, thereby to decreases an
amount of carbon which forms a solid solution with a martensite
phase, in relation to an equation of 0.1.times.an amount of Si (wt
%). And, Si moves Al transformation temperature and A3
transformation temperature to higher. Accordingly, Si works to
increase heat crack resistance of a seal sliding surface. And, in
the viewpoint in which the upper limit of an amount of carbon which
forms a solid solution with a martensite phase is adjusted to
preferable value of 0.5 wt %, it is preferable that Si is added in
a martensite phase to have a concentration of 3 wt %. Here, an
addition amount (wt %) of Si is calculated by an equation of (a
volume of a martensite phase (% by volume).times.3)/100. And, the
upper limit of an amount of SI in a martensite parent phase is 6.5
wt %. It is also preferable that Al having less effect on carbon
activity of an austenite phase is positively added to increase
tempering-softening resistance without decreasing an amount of
carbon in a martensite parent phase.
[0083] In the present invention, since ferrite stabilized elements
such as Si and Al are used in a large amount, it is necessary to
examine a problem in which a ferrite phase remains in a quench
hardened layer at induction hardening. As shown in a calculated
phase diagram of iron of 3 wt % and carbon in FIG. 4, in a steel to
which Si of 3 wt % is added, an addition of carbon of 0.35 wt % or
more, preferably 0.55 wt % or more, causes sufficiently
austenitizing at a heating temperature (850 to 1100.degree. C.) of
induction quenching. And, it is also preferable that austenite
stabilized elements such as Mn, Ni and Cu in a total amount of 2 wt
% or more are added. Alternatively, when Al substitute for Si is
added, since Al has ferrite stabilization ability two times of Si,
the upper limit of addition amount of Al is preferably set at 2 wt
% in the present invention.
[0084] And, when Al coexistent with Ni is added, toughness is well
improved and further age hardening is demonstrated. Accordingly, in
the present invention, Ni of 1 to 6 wt % coexistent with Al of 0.25
to 2 wt % are preferably added. Accordingly, in the present
invention, it is preferred that a ferrous floating seal part is
produced by using a steel or a cast iron containing both of Mn and
Ni in a total amount of 2 to 7 wt %. In a case in which cementite
is dispersed in a martensite parent phase, Ni, Al, Si, Co and Cu
are concentrated in the martensite parent phase. For example, in a
case in which cementite is dispersed in 50% by volume, a
concentration of Ni in the martensite phase is up to 1.5 to 9 wt %,
a concentration of Si or a mixture of Si and Al in the martensite
phase is up to 1 to 7 wt % and a concentration of Al in the
martensite phase is up to 0.5 to 4 wt %. Accordingly, as observed
in a precipitation hardening type hot work tool steel (5Ni-2Al tool
steel), remarkable age hardening is expected.
[0085] And, an alloy element increasing softening-tempering
resistance includes Mo and W. However, Mo and W have solid
solubility with cementite as small as about 2 wt %. And, an amount
of Mo which forms a solid solution with a martensite parent phase
thereby to affect on tempering-softening resistance is under 0.5 wt
%. Accordingly, it is preferable that the upper limit of an
addition amount of Mo is set at 2 wt %. And, since W has
substantially the same solid solubility with cementite as that of
Mo, the upper limit of a total addition amount of Mo and W is set
at 2 wt % or less.
[0086] When a steel has a structure composed of ferrite and
pearlite and contains coarse ferrite before induction hardening,
there will be a problem that induction heating for a short period
may not causes carbon to be sufficiently dispersed in austenite.
Accordingly, in the present invention, it is preferable that a seal
sliding surface is made such that one or more alloy element
selected from the group consisting of V, Ti, Zr, Nb, Ta and Hf in
an amount of 0.05 to 5 wt % is contained therein so that at least
any one of carbide, nitride and carbonitride of the alloy elements,
having an average grain size of 0.1 to 5 .mu.m, will be dispersed
in a total amount of 0.1 to 10% by volume. As the result, the
structure composed of ferrite and pearlite becomes finer thereby to
prevent occurrence of coarse ferrite. Alternatively, it is also
preferable that an amount of carbon in the steel is adjusted to 0.6
wt % or more.
[0087] Carbide, nitride and carbonitride of any one or more alloy
element selected from the group consisting of V, Ti, Zr, Nb, Ta and
Hf, each has less solid solubility with austenite during induction
heating compared with cementite, and is very hard, whereby they are
compounds excellent in seizure resistance to a steel. Accordingly,
an addition thereof in a small amount contributes to fining crystal
grain of austenite and to improvement in seizure resistance and
abrasion resistance. Accordingly, in the present invention, in
order to improve abrasion resistance by using such compounds, in
view of an amount of V.sub.4C.sub.3 and WC carbide in a high-speed
steel, the upper limit of an addition amount of the such compound
is adjusted to 5 wt % so that the such compound will be contained
in 10% by volume at the maximum. For example, in a case of TiC, by
using a specific gravity of TiC of 4.9 g/cm.sup.3, an addition of
Ti of 5 wt % forms TiC of 6.3 wt %, which is contained in 10% by
volume.
[0088] In order to efficiently improve seizure resistance and
abrasion resistance of a seal sliding surface, it is preferred that
at least any one of carbide, nitride and carbonitride, which are
precipitated at steel solution, has a relatively large grain size.
At this time, it is preferable that these compounds have an average
grain size of 0.1 .mu.m or more, as shown in size distribution of
high-speed steel carbide, and more preferably 5 .mu.m or less in
view of scraping characteristic against the counterpart surface to
the sliding surface.
[0089] And, in a ferrous floating seal part according to the
present invention, it is possible that a seal sliding surface
thereof is produced by using one of cast irons selected from the
group consisting of a gray cast iron, a nodular graphite cast iron,
a vermicular graphite cast iron and a pearlite malleable cast iron,
in which graphite is dispersed in a parent phase having a pearlite
structure containing carbon of 2 to 4.5 wt % and one or more alloy
element selected from the group consisting of Cr of 0.5 to 4 wt %,
V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %.
At this time, it is preferable that the cast iron contains Cu of 5
to 15 wt % so that Cu alloy phase will be dispersed in the seal
sliding surface in 3 to 10% by volume. This improves conformability
and lubricating property at sliding and prevents propagation of
crack when heat crack occurs. Here, the lower limit (a content of
3% by volume) of a content of Cu alloy phase is set such that oil
pocket effect having the same level as dispersed graphite is
apparently demonstrated, and the upper limit thereof is set at 10%
by volume in view of decreasing of abrasion resistance.
[0090] In a producing method for a ferrous floating seal according
to the present invention, a steel product or a cast iron is rapidly
heated from a preheated state at room temperature or under Al
transformation temperature to a quenching temperature of 850 to
1100.degree. C. at a heating rate of 6.degree. C./sec within 10
seconds by induction heating, and then rapidly cooled. Such
induction heating and quenching causes at least seal sliding
surface to be quench hardened.
[0091] For the aforesaid steel product and the aforesaid cast iron,
as described above, the following may be applied.
(1) Steel product containing carbon of 0.5 to 1.8 wt % and further
one or more alloy element selected from the group consisting of Cr
of 0.3 to 3 wt %, V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W
of 0.5 to 2 wt %. (2) Cast iron selected from the group consisting
of a gray cast iron, a nodular graphite cast iron, a vermicular
graphite cast iron and a pearlite malleable cast iron, in which
graphite is dispersed in a parent phase having a pearlite structure
containing carbon of 2 to 4.5 wt % and one or more alloy element
selected from the group consisting of Cr of 0.5 to 4 wt %, V of 0.1
to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5 to 2 wt %. (3) White
cast iron containing carbon of 2 to 4.5 wt % and further one or
more alloy element selected from the group consisting of Cr of 0.5
to 4 wt %, V of 0.1 to 0.5 wt %, Mo of 0.3 to 2 wt % and W of 0.5
to 2 wt %, or cast iron in which cementite of the white cast iron
is partially graphitized. (4) The steel product (1), the cast iron
(2) and the cast iron (3) each containing either any one of Si of
0.5 to 3.5 wt % or Al of 0.25 to 2 wt %, or both Si and Al in a
total amount of 0.5 to 3.5 wt %. (5) The steel product (1), the
cast iron (2) and the cast iron (3) each containing Mn and Ni in a
total amount of 2 to 7 wt %. (6) The cast irons (2) and (3) each
preferably containing Cu of 5 to 15 wt % so that Cu alloy phase
will be dispersed in 3 to 10% by volume.
[0092] And, it is preferable that one or more alloy element
selected from the group consisting of Si, Al, Mn, Ni. Cr, V, Mo, W,
Cu. Co, B, Ti, Nb, Zr, P, S, Ca, Pb, Mg, Sn, Ba and Re (rare earth)
is suitably added to the steel product and the cast irons of (1) to
(6), in addition to the aforesaid elements, in order to ensure
hardenability, tempering-softening resistance, corrosion
resistance, machineability, abrasion resistance and seizure
resistance and in order to regulate a structure of graphite to be
dispersed in the cast iron and a formation of a pearlite structure.
And, in some cases, it is not prevented that the alloy element and
one or more impurity element selected from the group consisting of
S, P, O and N get mixed in a steel product to produce the steel
product. So, such alloy elements may get mixed as far as they have
little effect on the purpose of the present invention. At this
time, a total amount of such alloy element does not exceed 5 wt %
and is preferably set at 2 wt % or less.
[0093] A producing method of a ferrous floating seal according to
the present invention comprises a preparing step for preparing the
aforesaid steel product or the aforesaid cast iron; and a quenching
step for heating the steel product or the cast iron at a heating
rate such that temperature rises from Al transformation temperature
to a quenching temperature in the range of 850 to 1000.degree. C.
within 10 seconds and then rapidly cooling. The quenching step
results in that the steel product has a structure in which
cementite in a content of 3 to 50% by volume is dispersed in a
martensite parent phase which forms a solid solution with carbon of
0.15 to 0.6 wt % and that the cast iron has a structure in which
cementite in a content of 3 to 50% by volume and graphite in a
content of 3 to 15% by volume are dispersed in a martensite parent
phase which forms a solid solution with carbon of 0.15 to 0.7 wt
%.
[0094] The producing method preferably further comprises a heat
treatment step for concentrating one or more alloy element selected
from the group consisting of Cr, V, Mo and W in cementite in the
ferrous floating seal.
[0095] And, the producing method preferably further comprises a
heat treatment step for dispersing a pearlite structure region
comprising platy cementite and ferrite in an area ratio of 20% or
more in a steel before quenching.
[0096] As described later, SUJ2 (containing carbon of 1.01 wt % and
Cr of 1.5 wt % and having hardness of Hv200), in which cementite is
sufficiently spheroidized is heated to each quenching temperature
at a heating rate of 6.degree. C./sec and then rapidly cooled, and
then hardness of a quench hardened layer, a residual amount of
cementite and an amount of carbon which forms a solid solution with
a martensite phase of the quench hardened layer are measured. As a
result, it is found that an organization in which cementite is
dispersed in the low-carbon martensite phase in a high-density of
5% or more by volume is formed, achieving the purpose of the
present invention. At this time, it is also found that a suitable
heating temperature is 900 to 1000.degree. C. When a concentration
of Cr decreases lower than that of SUJ2, a concentration of Cr in
cementite will decrease and the lower limit of suitable heating
temperature will decrease to about 850.degree. C. On the contrary,
when a concentration of Cr increases higher than SUJ2, the upper
limit thereof will increase to about 1100.degree. C. And, when
considering a heating rate of 6.degree. C./sec, it is expected that
a heating method using a salt-bath is suitably applied. However,
from the viewpoint of productively, environment of quenching work
and economical efficiency, an induction heating method is suitably
applied. At this time, a heating period is preferably within 10
second which is obtained by an induction heating rate.
[0097] And, a seal sliding surface of a ferrous floating seal part
is pre-heated within 300.degree. C. to Al transformation
temperature, and rapidly heated to a quenching temperature of 850
to 1100.degree. C. (preferably 900 to 1100.degree. C.) at a heating
rate of 150.degree. C./sec or more (within 3 seconds) by induction
heating using a high frequency of 60 kHz and below, and then
rapidly cooled. Such quenching treatment makes at least seal
sliding surface of the ferrous floating part quench hardened. This
enables producing a ferrous floating seal part, in which pearlite
cementite is dispersed in a martensite phase to provide higher
hardness and little strain.
[0098] Referring now to the drawings, there will be explained
floating seal parts and producing method thereof according to
preferred embodiment of the invention.
Example 1
[0099] In this example, in order to prove that a floating seal part
in which cementite is densely dispersed in a parent phase
comprising low carbon martensite has remarkably improves sliding
property, by using steel products which has various compositions as
shown in Table 1, a structure of each of the steel products
quenched by induction hardening under various conditions is
examined. "Volume of cementite (% by volume)" and "volume of
.gamma. phase (% by volume)" each shown in Table 1 are obtained by
observing a photograph of each structure, and "PV value" in Table 1
will be described later.
TABLE-US-00001 TABLE 1 COMPOSITION (wt %) AND PROPERTY OF TEST
STEELS kg/cm .times. m/ CONTENT OF CONTENT OF CEMENTITE RETAINED
.gamma. PHASE PV No. C Si Ai Mn Ni Cr Mo W Co (% BY VOLUME) (% BY
VOLUME) VALUE No. 1 0.98 0.27 0.024 0.48 1.47 7(GRAIN) 29 1.6 No. 2
0.53 0.21 0.83 1.01 0.16 2.6(GRAIN) 17 1.2 No. 3 0.84 1.12 0.019
0.4 0.91 5.8(PEARLITE) 33 2 No. 4 0.98 0.55 0.023 1.11 1.08
5.8(GRAIN) 35 1.3 No. 4 6.2(GRAIN + 28 1.9 PEARLITE) No. 5 1.25
0.81 0.031 1.06 1.42 12(GRAIN + 36 2.2 PEARLITE) No. 6 1.62 0.21
1.12 1.45 2.21 18(GRAIN + 38 2.7 PEARLITE) No. 7 1.61 2.45 0.029
1.5 1.02 20(GRAIN + 32 3.1 PEARLITE) No. 8 3.41 2.49 0.47 4.82 2.06
19(EUTECTIC .theta. + 26 3.9 GRAPHITE) No. 9 3.45 1.51 1.51 0.52
4.5 1.61 21(EUTECTIC .theta.) + 23 4.5 GRAPHITE No. 10 3.62 1.78
0.01 0.48 0.02 0.06 3(GRAPHITE FLAKE + 11 3.2 PEARLITE) COMPARATIVE
0.98 0.27 0.024 0.48 1.47 3(GLANULAR .theta.) 12 0.6 STEEL 1
COMPARATIVE 3.6 1.1 0.59 15.2 2.3 Cr.sub.7C.sub.3: 35% -- 1.8 STEEL
2 BY VOLUME COMPARATIVE 3.2 1.2 0.51 9.2 6.1 4.9 5
M.sub.6C,Cr.sub.7C.sub.3: 35% -- 2.5 STEEL 3 BY LOLUME COMPARATIVE
3.41 2.49 0.47 4.82 2.06 42(EUTECTIC .theta.) 16 2.3 STEEL 4
[0100] No. 1 steel (correspondent to SUJ2) in Table 1, to which a
cementite granulating treatment (slow cooling) for heating at
810.degree. C. for 2 hours and then slowly cooling to 600.degree.
C. was applied, was heated to various temperatures in the range of
800 to 1000.degree. C. at a heating rate of 6.degree. C./sec by
induction heating and then water quenched. Then, hardness of the
quenched layer of the steel was measured. FIG. 5(a), (b) and (c)
are graphs showing a relation between a concentration of carbon in
martensite and an amount of cementite which does not form a solid
solution, based on the hardness of the quenched layer. From the
graphs, it was found that concentrating of Cr (an amount of Cr of
about 9 wt %) in cementite caused a delay in a solid solution
formation of cementite with austenite during heating. And, in order
to obtain martensite having sufficient hardness enough for a
ferrous floating seal, it was necessary to set a heating
temperature at least 900.degree. C. or more, at which a
concentration of carbon in martensite was about 0.3 wt % and hard
cementite particles were dispersed in 12% by volume. Accordingly,
it was found that sufficient seizure resistance, heat crack
resistance and abrasion resistance required for a ferrous floating
seal were achieved.
[0101] And, in a case of in which a heating temperature was set at
1000.degree. C., a very hard quench hardened layer in which
cementite in a content of about 6% by volume was dispersed in a
martensite parent phase containing carbon of 0.7 wt % was obtained.
However, since a retained austenite phase increases thereby to
saturate hardness of the quenched layer, it was necessary for a
ferrous floating seal that at least one condition among the later
conditions was satisfied, in which an induction quenching
temperature was under 1000.degree. C. from the viewpoint of heat
crack resistance, an amount of carbon in martensite was under 0.7
wt % and cementite was dispersed in 3% or more by volume.
[0102] No. 4 test steel of Table 1 to which the aforesaid
granulating treatment (slow cooling) was applied and another No. 4
test steel which was maintained at 820.degree. C. for 1.5 hours and
then cooled to disperse pearlite cementite and granulated cementite
were prepared. Both of No. 4 test steels were heated to various
temperatures in the range of 900 to 1100.degree. C. at a heating
rate of 1000.degree. C./sec faster than a conventional heating
rate, and then quenched. Then, quenched sliding surfaces of the
both steels were studied.
[0103] As a result, as shown in FIG. 6, in the No. 4 test steel
which the granulating treatment (slow cooling) was applied to and
then quenched from a heating temperature of 1000.degree. C.,
granulated cementite was dispersed in a large amount. And, as shown
in FIG. 7, the quenched layer thereof was hardened to have hardness
of Hv830 at a maximum in spite of containing retained austenite in
30 to 45% by volume. And, if retained austenite would be contained
in as large as 50% by volume, abrasion resistance thereof could not
be damaged.
[0104] On the other hand, FIG. 8 shows a structure of a seal
sliding surface of the No. 4 test steel in which after dispersing
pearlitely cementite and granulated cementite, was heated to
1000.degree. C. and then quenched. From the figure, platy cementite
having a pearlite structure was dispersed in a martensite parent
phase. And, the test steel had hardness of Hv940 higher than the
hardness (Hv880) of the test steel of FIG. 6.
[0105] By using No. 4 steel containing a structure before pearlite
transformation, a relation between a heating rate and a heating
temperature when pearlite cementite was dispersed was studied. As a
result, even in a case of a heating rate of 150.degree. C./sec and
a heating temperature of 900.degree. C., pearlitely cementite was
dispersed and the quenched layer was hardened to have hardness of
Hv945. In order to disperse pearlitely cementite stably, a heating
rate of 100.degree. C./sec or more, more preferably 150.degree.
C./sec or more, was required when the lower limit of heating
temperature was 850.degree. C.
[0106] In FIG. 6 and FIG. 8, a concentration of Cr in cementite,
which were analyzed by using an energy dispersive X-ray analyzer
(EDAX) in an erector microscope, was shown. It was observed that a
concentration of Cr in pearlitely cementite was remarkably dense
but not as much as in granulated cementite, whereby pearlitely
cementite easily formed a solid solution. Accordingly, applying a
heat treatment in which Cr was concentrated in pearlitely cementite
before quenching allowed more stably dispersion of pearlitely
cementite.
[0107] A concentration of carbon in martensite of No. 1 steel to
which rapidly heating and quenching was applied was 0.5 wt %, which
was obtained by using a lattice parameter of the martensite phase
of the steel. As compared the result with the result (0.7 wt %) of
No. 1 steel, rapidly induction heating decreased a concentration of
carbon which formed a solid solution and increased an dispersion
amount of cementite. This was preferable in the viewpoint of
improvement in heat crack of a ferrous floating seal.
[0108] These results of the SUJ2 to which the aforesaid induction
hardening was applied were applicable to a white cast iron, a gray
cast iron, a nodular graphite cast iron, a vermicular graphite cast
iron, a pearlite malleable cast iron, each of which a parent phase
was a pearlite structure, and a cast iron in which white cast iron
was graphitized to disperse graphite and cementite finely.
Example 2
[0109] In this example, each of the test steels of Table 1 was
machined to have a shape of a floating seal, as shown in FIG. 9,
and each sliding surface of the test steels was heated to a
quenching temperature of 950.degree. C. at a heating rate of
1000.degree. C./sec by the aforesaid induction hardening, and after
tempering at 160.degree. C. for one hour, a lapping treatment was
applied to prepare floating seal specimens. And, for each of the
floating seal specimens, heat crack resistance and seizure
resistance was measured by using a sliding test apparatus (a
floating seal tester) of FIG. 10 in air.
[0110] The floating tester used a floating seal member, in which
each of the prepared floating seal specimens was used as a pair of
seal rings with the seal surface contacted each other. An O-ring
which pressed one of the seal rings was rotated around an axis of
the floating seal member with respect to a fixed O-ring which
pressed another seal ring with applying load to the O-ring to be
rotated. Here, the seizure resistance was evaluated by using a PV
value (kgf/cm:m/sec), which was obtained by product of P (pressure)
and V (revolution rate) when sliding resistance (sliding surface
temperature) rapidly increased while changing a rotating rate (a
revolution rate V) under a condition in which press load between
the seal surfaces was kept at 63 kg/cm (pressure P was 2 kg/cm, the
pressure was load per seal surface length) to enclose engine oil
(EO#30). And, as comparative specimens, were prepared a comparative
specimen 1: SUJ2 quenched at 840.degree. C., a comparative specimen
2: high-carbon and high-Cr cast iron, a comparative specimen 3:
high-carbon and high-Cr and Mo cast iron, and a comparative
specimen 4: martensite cast iron of Ni-Hard cast iron, the
comparative specimens 2, 3 and 4 being conventionally used for a
floating seal part.
[0111] Table 1 showed an amount (% by volume) of cementite in a
quenched structure, an amount (% by volume) of retained austenite
and a PV value of each of the test specimens. As compared the test
steels No. 1 to No. 7 with the comparative specimen 1, it was
understood that dispersing cementite in a low-carbon martensite
parent phase increased a PV value, and further dispersing pearlite
cementite increased a PV value as so much as improving in
tempering-softening resistance by Si and Al. And, as compared with
comparative specimens 2, 3 and 4, it was understood that the test
steels No. 1 to No. 7 were superior from an economical
viewpoint.
[0112] And, as compared the test steels No. 8 to No. 10 made of
high-carbon cast irons with the comparative specimen No. 4, it was
found that dispersing graphite improved seizure resistance
sufficiently and caused cementite to be dispersed in a high
density. Accordingly, the test steels No. 8 to No. 10 were suitable
for a ferrous floating part excellent in abrasion resistance (here,
the test steels No. 8 and No. 9 were made such that a white cast
iron were graphitized at 950.degree. C. for one hour and cooled to
transform a parent phase to a pearlite structure, and then
induction heated and quenched).
[0113] Since a ferrous floating seal, as shown in FIG. 9, often had
a sealing portion of a thickness of about 2 to 5 mm, large
deformation easily occurred when the hardened layer was too deeply
quenched by induction hardening. However, the rapidly induction
hardening according to the example 2 allowed a surface layer to be
quenched shallowly in a moment, and therefore was suitable for
preventing such deformation. Especially, rapidly induction
hardening after pre-heating at under Al transformation temperature
was preferred.
[0114] However, in the case of application of the rapidly induction
hardening for a large size floating seal part, since cost of
equipment became expensive, it was also possible that a steel in
which Cr was concentrated to have a concentration of 5 wt % or more
was heated by using a salt bath of 1000.degree. C. in view of FIG.
3.
[0115] The present invention is not limited to any of the
above-described constructions and embodiments, and various
modifications of the present invention can be made without
departing from the technical ideas.
* * * * *