U.S. patent application number 11/883793 was filed with the patent office on 2008-07-03 for high-concentration carburized/low-strain quenched member and process for producing the same.
Invention is credited to Hisashi Abe, Toshio Fukushima, Koji Horikiri, Isao Machida.
Application Number | 20080156399 11/883793 |
Document ID | / |
Family ID | 36793120 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080156399 |
Kind Code |
A1 |
Machida; Isao ; et
al. |
July 3, 2008 |
High-Concentration Carburized/Low-Strain Quenched Member and
Process for Producing the Same
Abstract
A super carburized, low-distortion quenched member with higher
performance and minimized heat-treatment distortion is provided. A
process for the production thereof includes a primary treatment and
a secondary treatment. The primary treatment includes heating a
steel member for a machine structure to a temperature within an
austenite region by vacuum carburizing (low-pressure carburizing)
to have carbon dissolved at least at a eutectoid carbon
concentration of a surface layer portion of the member and then
quenching the member to have at least one of ultrafine carbide and
nuclei of the carbide formed in the surface layer portion of the
member. The secondary treatment includes subsequently heating and
soaking the member to a temperature within the austenite region and
then conducting rapid quenching to have ultrafine carbide
precipitated in an outermost surface layer portion.
Inventors: |
Machida; Isao; (Tokyo,
JP) ; Abe; Hisashi; (Tokyo, JP) ; Fukushima;
Toshio; (Tokyo, JP) ; Horikiri; Koji; (Tokyo,
JP) |
Correspondence
Address: |
KUBOVCIK & KUBOVCIK
SUITE 1105, 1215 SOUTH CLARK STREET
ARLINGTON
VA
22202
US
|
Family ID: |
36793120 |
Appl. No.: |
11/883793 |
Filed: |
February 8, 2006 |
PCT Filed: |
February 8, 2006 |
PCT NO: |
PCT/JP2006/302161 |
371 Date: |
August 7, 2007 |
Current U.S.
Class: |
148/223 ;
148/319 |
Current CPC
Class: |
C21D 9/00 20130101; C22C
38/04 20130101; C21D 6/002 20130101; C23C 8/80 20130101; C23C 8/22
20130101; C21D 2211/003 20130101; C21D 1/78 20130101; C21D 1/06
20130101; C22C 38/02 20130101; C22C 38/22 20130101 |
Class at
Publication: |
148/223 ;
148/319 |
International
Class: |
C23C 8/22 20060101
C23C008/22 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 8, 2005 |
JP |
2005-032302 |
Claims
1. A process for producing a super carburized, low-distortion
quenched member, which comprises a primary treatment of heating a
steel member for a machine structure to a temperature within an
austenite region by vacuum carburizing (low-pressure carburizing)
to have carbon dissolved at least at a eutectoid carbon
concentration of a surface layer portion of said member and then
quenching said member at a cooling rate of from 3 to 15.degree.
C./sec from said temperature within said austenite region to a
temperature not higher than an A.sub.1 transformation point to have
at least one of ultrafine carbide and nuclei of said carbide formed
in said surface layer portion of the said member; and a secondary
treatment of subsequently heating and soaking said member to a
temperature within said austenite region and then conducting rapid
quenching to have ultrafine carbide precipitated in a range of from
10 to 30% in terms of effective case depth percentage in an
outermost surface layer portion.
2. A process according to claim 1, wherein in said secondary
treatment, additional carburizing treatment is applied to said
surface layer portion of said member.
3. A process according to claim 2, wherein in said secondary
treatment, said ultrafine carbide is caused to precipitate in said
surface layer portion of said member to form a structure composed
primarily of martensite and containing a mixed structure of
troostite, retained austenite and the like in parts thereof such
that said outermost layer portion (a portion A) of said layer, a
layer portion (a portion B) inner than said portion A and a layer
portion (a portion C) inner than said portion B are in an order of
A.gtoreq.C.gtoreq.B in terms of the fineness of austenite grain
size.
4. A super carburized, low-distortion quenched member comprising a
surface layer portion of a structure composed primarily of
martensite and containing a mixed structure of troostite and
retained austenite or the like in parts thereof, wherein in said
surface layer, an outermost surface layer (a portion A), a layer (a
portion B) inner than said portion A and a layer (a portion C)
inner than said portion B are in an order of A.gtoreq.C.gtoreq.B in
terms of the fineness of austenite grain size.
Description
DESCRIPTION
[0001] 1. Technical Field
[0002] This invention relates to carburizing and quenching
treatment widely used as a reinforcement method for machine
structural members, more specifically to a super carburized,
quenched member featuring temper softening resistance, high
strength, high contact pressure and the like, especially to a super
carburized, low-distortion quenched member (which may hereinafter
be referred to simply as "member") with mutually conflicting
properties, that is, higher performance and heat-treatment
distortion attained together and also to its production
process.
[0003] 2. Prior Art
[0004] Owing to excellent properties such as high fatigue strength
and wear resistance, carburized and quenched members (hereafter
referred to as "case hardened members") are widely used as various
members in transport equipment, industrial machines and the like.
From the viewpoint of dimensional reductions, weight reductions
and/or the like through further improvements in the performance of
such members, numerous developments have been made on case hardened
members. Recently, the vacuum carburizing (low-pressure
carburizing) process has been developed. Compared with the
conventional gas carburizing process, the vacuum carburizing
process has excellent characteristic features such as environmental
friendliness, the prevention of intergranular oxidation, the
feasibility of high-temperature carburizing treatment, and easy
control of carburizing and carbon diffusion, and therefore, is
expected to find still broader utility from the standpoints of
further improvements in the performance and quality of members and
further improvements in their productivity.
[0005] As a method for providing a machine structural member such
as a gear or axle member with improved pitting resistance by
applying carburizing and quenching to the member, there is
carbonitriding treatment. According to this treatment, carbon and
nitrogen are caused to concurrently diffuse into the matrix of a
member such that the member can be provided with improved temper
softening resistance. In addition, there has also been developed
super carburizing treatment to have carbide precipitated in a
surface layer portion of a member such that the member can be
provided with improved temper softening resistance. Keeping in step
with evolutions in low-pressure carburizing facilities, a great
deal of research has been conducted in recent years.
[0006] As a representative example of the super carburizing
treatment, Patent Document 1 discloses a carburizing treatment
process for a member. According to Patent Document 1, it is
proposed to form quasispheroidal or spheroidal carbide at a volume
percentage of 30% or higher within a range up to a depth of 0.4 mm
by conducting precarburizing to such a carbon content that
spheroidal carbide is caused to precipitate in a surface layer
portion of a steel member and the carbon concentration in the
surface layer portion becomes not higher than Acm but not lower
than a eutectoid concentration between steel and carbon, slowly
cooling or quenching the thus-treated member to convert the surface
layer portion into a bainite, pearlite or martensite structure, and
then heating the member at a ramp rate of not greater than
20.degree. C./min from the Ac1 point to a temperature in a range of
from 750 to 950.degree. C. to effect carburizing and quenching.
[0007] According to the above-described process, the member can be
improved in properties such as pitting properties owing to the
precipitation of the carbide in the surface layer portion of the
member. Nonetheless, the resulting member involves problems such as
a deformation and distortion by heat treatment, because the process
is super carburizing that causes the precipitation of the carbide
as much as 30% in the surface layer portion.
[0008] As a method for causing carbide to precipitate in an
ultrafine form in a surface layer portion of a member by super
carburizing, many heating and cooling methods have been
investigated. In Patent Document 1, it is described to be desirable
that subsequent to the precurburizing, air cooling (which forms a
bainite or pearlite structure) or quenching (which forms a
martensite structure) is conducted, and that in the carbide-forming
treatment as the next step, the member is heated at a slow ramp
rate of not greater than 20.degree. C./min from the Acl
transformation temperature to a temperature within the range of
from 750 to 950.degree. C., and after direct quenching or air
cooling, the member is again heated and quenched.
[0009] Further, Patent Document 2 and Patent Document 3 propose, as
an optimal method, to conduct slow cooling (or 30.degree. C./hr or
less) after precurburizing or primary carburizing.
[0010] When the quenching after the precarburizing or primary
carburizing is conducted by air cooling or slow cooling in the
method disclosed in Patent Document 1, 2 or 3, however, a network
of carbide tends to precipitate along grain boundaries in a surface
layer portion of a member. The next step, that is, the
carbide-forming treatment can hardly break up the network of
carbide in a short time to have the carbide distributed and
precipitated within the surface layer portion. To overcome this
shortcoming, heating and subsequent cooling may be conducted a
plurality of times in some instances.
[0011] On the other hand, Patent Document 1 also discloses
quenching with an aim directed toward forming a martensite
structure by increasing the cooling rate of a member subsequent to
its precarburizing. This technique, however, involves a potential
problem that carbide nuclei in a surface layer portion may dissolve
out. It is also concerned that the quenching may take place with
supersaturated carbon, and due to high-carbon martensitic
transformation, the member may develop a greater deformation or
distortion through an expansion, shrinkage or the like.
[0012] Patent Document 4 discloses a production process of a case
hardened member by low-pressure carburizing. There is a reference
to the conversion of carbide into an ultrafine form such as the
control of the carbon concentration at 0.5 to 0.7 wt. % in primary
carburizing and at 0.7 to 1 wt. % in secondary carburizing and the
control of primary cooling at a very slow rate of from 1 to
10.degree. C./min. Concerning deformation strain, however, this
production process is not expected to be preferred like the
above-mentioned Patent Documents 1, 2 and 3.
[0013] Just for readers' information, a description is now made of
some advantages of low-pressure carburizing, which is finding
wide-spread commercial utility in recent years, over conventional
gas carburizing. [0014] a) A change from a carburizing step to a
diffusion step can be readily and promptly modified. [0015] b)
High-temperature treatment is feasible so that prompt carburizing
can be conducted. [0016] c) No intergranular oxidation takes place
in a surface layer portion of a member, and in the member under
treatment, it is hence possible to inhibit the occurrence of cracks
which would otherwise begin to take place from such a defect.
[0017] d) No sooting takes place, thereby causing no uneven
carburizing which would otherwise take place as a result of
sooting.
Patent Document 1: JP-B-62-24499
Patent Document 2: JP-B-2787455
Patent Document 3: JP-B-2808621
Patent Document 4: JP-A-2002-348615
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0018] Even in super carburizing by the conventional low-pressure
carburizing, however, no optimal balance can be achieved between
the progress of formation of carbide within a surface layer portion
of a member under treatment and the microstructure of the surface
layer portion. The problem of a deformation or strain of the
treated member, therefore, still remains unresolved. As a
consequence, grinding, strain-correcting finishing or the like is
essential for the member after the carburizing step. Such
additional work has led to a reduction in the inherent ability of
super carburizing that permits use under higher contact pressure, a
reduction in productivity and an increase in manufacturing cost,
thereby preventing the popularization of super carburizing
treatment. Means for Resolving the Problem
[0019] The present invention has resolved the above-described
problem by developing an optimal process, which makes it possible
to use a member under a higher contact pressure and also to provide
the member with a lower strain while making use of low-pressure
carburizing facilities that permit a variety of control promptly
with higher accuracy as to the concentration of carbon in the
member, the repetition of carburizing treatment/diffusion
treatment, and diverse temperature conditions, heating conditions
and cooling rate (quenching) conditions for heating, soaking,
carburizing, quenching and the like of the member.
[0020] The above-described problem can be resolved by the present
invention as defined below: [0021] 1. A process for producing a
super carburized, low-distortion quenched member, which comprises a
primary treatment of heating a steel member for a machine structure
to a temperature within an austenite region by vacuum carburizing
(low-pressure carburizing) to have carbon dissolved at least at a
eutectoid carbon concentration of a surface layer portion of the
member and then quenching the member at a cooling rate of from 3 to
15.degree. C./sec from the temperature within the austenite region
to a temperature not higher than an A.sub.1 transformation point to
have at least one of ultrafine carbide and nuclei of the carbide
formed in the surface layer portion of the member; and a secondary
treatment of subsequently heating and soaking the member to a
temperature within the austenite region and then conducting rapid
quenching to have ultrafine carbide precipitated in a range of from
10 to 30% in terms of effective hardened depth percentage in an
outermost surface layer portion. [0022] 2. A production process as
described above, wherein in the secondary treatment, additional
carburizing treatment is applied to the surface layer portion of
the member. [0023] 3. A production process as described above,
wherein in the secondary treatment, the ultrafine carbide is caused
to precipitate in the surface layer portion of the member to form a
structure composed primarily of martensite and containing a mixed
structure of troostite and retained austenite or the like in parts
thereof such that the outermost layer portion (a portion A) of the
layer, a layer portion (a portion B) inner than the portion A and a
layer portion (a portion C) inner than the portion B are in an
order of A.gtoreq.C.gtoreq.B in terms of the fineness of austenite
grain size.
[0024] A super carburized, low-distortion quenched member
comprising a surface layer portion of a structure composed
primarily of martensite and containing a mixed structure of
troostite and retained austenite or the like in parts thereof,
wherein in the surface layer, an outermost surface layer (a portion
A), a layer (a portion B) inner than the portion A and a layer (a
portion C) inner than the portion B are in an order of
A.gtoreq.C.gtoreq.B in terms of the fineness of austenite grain
size.
Advantageous Effects of the Present Invention
[0025] The process according to the present invention performs the
treatment of a member in low-pressure carburizing facilities while
making the combined use of the primary treatment of conducting
adequate super carburizing and quenching at an optimal cooling rate
and the secondary treatment of subsequently causing a fine carbide
to simply and efficiently precipitate; and can minimize the
deformation and strain of the member treated through the heat
treatment. Owing to the adoption of this process, the greatest
concern about the conventional super carburizing, for example, the
cumbersome grinding, strain correction and the like of the member
after the treatment, such as the bending of an axle or the
deformation strain of a tooth profile, can be substantially
relieved, thereby bringing about advantageous effects that
significant improvements can be made in the productivity, quality
and cost of the case hardened member.
[0026] According to the process of the present invention,
additional carburizing treatment may be applied to the surface
layer portion of the member in the secondary treatment. This
additional carburizing treatment makes it possible to achieve a
high hardness of matrix and also to reduce the crystal grain size
of an outermost surface layer portion of the member to an ultrafine
grain size and, therefore, is also extremely effective for
providing the member with higher strength and higher toughness. By
the process of the present invention, it is possible to readily
achieve higher strength, higher toughness, higher contact pressure
and the like for members such as axles and gears to which super
carburizing has heretofore been hardly applicable. Therefore, the
process according to the present invention can be widely applied to
fields where there is a high need for such properties, and has an
advantageous effect that it can make significant contributions to
improvements in the performance of a member and also to reductions
in the size and weight of the member.
BEST MODES FOR CARRYING OUT THE INVENTION
[0027] Based on best modes for carrying out the invention, the
present invention will next be described in further detail. The
followings are the course of technical endeavors and the findings,
which have led to the present invention.
[0028] With a view to developing a super carburizing process for
causing ultrafine carbide to precipitate in a surface layer portion
of a member by using low-pressure carburizing facilities, the
present inventors carried out a thorough investigation on possible
relations between the concentration of carbon in the surface layer
portion and various heating and cooling conditions and the
precipitation form of the ultrafine carbide in the surface layer
portion and the microstructure of the matrix. Concerning
improvements or the like in strain by heat treatment while assuming
members such as gears and axles, research and development was also
conducted from many directions. An aim was then set at the
establishment of a novel process for super carburizing and
low-strain quenching, which can achieve both of
mutually-conflicting properties of providing a member with higher
performance by super carburizing and minimizing a deformation,
distortion or the like of the member while balancing them at high
levels.
[0029] Upon applying super carburizing to a surface layer portion
of steel (member) , the most important point is to have ultrafine
carbide precipitated as much as possible in a surface layer portion
of the member through the optimal combination of the primary
treatment and the secondary treatment. In the control of the
formation of the ultrafine carbide, carburizing and quenching
facilities also play an important role. In the present invention, a
variety of developments were conducted while using low-pressure
carburizing facilities that compared with conventional carburizing
facilities, permit a variety of control promptly with higher
accuracy as to the concentration of carbon in the member, the
repetition of carburizing treatment/diffusion treatment, and
diverse temperature conditions, heating conditions and cooling rate
(quenching) conditions for heating, soaking, carburizing, quenching
and the like of the member.
[0030] Described specifically, a variety of investigations were
conducted on the heating, soaking, super carburizing, diffusion and
cooling (quenching) conditions of a member during the primary
treatment to firstly reduce the deformation or strain of the member
at the stage of the primary treatment. In the secondary treatment
as the next step, carburizing and quenching (cooling) conditions
are important to permit adjustments or the like in the
precipitation of ultrafine carbide and the grain size of austenite
in the carburized layer. Specifically, it has been found that in
the secondary treatment, the deformation or strain of a member by
the heat treatment can be minimized by controlling a range, in
which the ultrafine carbide precipitate in a surface layer portion
of the member, to 10 to 30% in terms of effective case depth
percentage and further by converting an outermost surface layer
portion into an ultrafine crystalline structure.
[0031] The term "effective case depth percentage" as used herein
means a ratio (t/T) of a precipitated depth (t) of ultrafine
carbide existing in an outermost surface layer portion of a member
to an effective case depth (T) of the member after completion of
the secondary treatment (including the tempering treatment at
180.degree. C.). It is to be noted that the term "effective case
depth" means a distance from a surface of a hardened layer, which
is still in a quenched state or has been tempered at a temperature
not exceeding 200.degree. C., to the position of a critical depth
of a Vickers hardness (HV) of 550 as measured by the Method of
Measuring Case Depth Hardened by Carburizing Treatment for Steel
(JIS G0557).
[0032] Next, the term "precipitated depth of ultrafine carbide"
means the maximum depth, where the ultrafine carbide exists, from
the outermost surface layer portion of the member as determined by
an analysis under an optical microscope or an electron microscope.
To facilitate the discrimination of the ultrafine carbide, the
member is analyzed in a state of being etched with an etching
solution such as 5% nital etching reagent.
[0033] The vacuum carburizing (low-pressure carburizing) facilities
for use in the present invention are equipped with a carburizing
and heating chamber including a treatment furnace which is
sectionally controllable at different pressures of from 200 to
2,000 Pa, and are available on the market. Conventionally-available
vacuum carburizing facilities are all usable in the present
invention. As the primary treatment in the present invention, the
member is heated and soaked to a predetermined temperature in the
furnace of the facilities, and to raise the concentration of carbon
in the surface layer portion of the member to or higher than the
eutectoid carbon concentration, the member is then quenched at an
appropriate cooling rate. In the subsequent secondary treatment,
the carbide is caused to precipitate in an ultrafine form in the
surface layer portion of the member, optionally followed by
additional carburizing treatment as needed.
[0034] According to the primary treatment in the process of the
present invention, steel to be treated (member) is heated and
soaked to an austenite region of from 900 to 1,100.degree. C.,
carburizing is conducted such that the carbon concentration of a
surface layer portion becomes preferably 0.8 wt. % or higher, and
from the thus-carburized state, quenching is then conducted at an
optimal cooling rate. Optimal cooling conditions are to evenly cool
the member at a cooling rate of from 3 to 15.degree. C./sec over a
temperature range of from the carburizing temperature (the
temperature in the austenite region) to the A.sub.1 transformation
temperature or lower, preferably to 400.degree. C. or lower. By
this cooling, ultrafine carbide is caused to precipitate in the
surface layer portion of the member so that a structure composed
primarily of martensite is formed in the surface layer portion. The
term "ultrafine carbide" means an M.sub.23C.sub.6 type carbide
formed as a result of bonding of carbide-forming elements such as
Cr and Mo in Fe.sub.3C (cementite) or steel with carbon dissolved
in supersaturation.
[0035] In the secondary treatment, the non-carburized portion
(interior) of the member is heated and soaked to a range of from an
austenizing temperature to the austenizing temperature+80.degree.
C., preferably to a range of from 10 to 70.degree. C. above the
austenizing temperature, and is then rapidly quenched to effect
precipitation of ultrafine carbide such that the carbon
concentration of the surface layer portion becomes preferably 0.8
wt. % or higher, more preferably 1.0 to 2.0 wt. %. It is preferred
to apply, in parallel with the precipitation of the ultrafine
carbide in the surface layer portion, additional carburizing
treatment to the surface layer portion to promote the precipitation
of the ultrafine carbide in the surface layer portion, and from the
state that the carbon concentration of the matrix has been
adequately adjusted, to further conduct rapid quenching.
[0036] The temperature of the final quenching after the secondary
treatment varies depending on the pretreatment conditions, that is,
whether the final quenching is after the heating and soaking or
after the heating, soaking and additional carburizing. The rapid
quenching can be conducted at the temperature after the
pretreatment or at a temperature raised or lowered relative to the
temperature of the pretreatment. In other words, the temperature of
the final quenching after the secondary treatment can be set at a
level commensurate with the quality of heat treatment such as the
hardness and microstructure required for the member.
[0037] With a view to establishing optimal conditions for super
carburizing, the present inventors conducted a detailed
investigation on the carbon concentrations upon heating, soaking
and carburizing and diffusion and various cooling (quenching)
conditions with respect to the primary treatment in which super
carburizing is applied to a surface layer portion of a member in
low-pressure carburizing facilities and the secondary treatment in
which ultrafine grains of carbide are caused to precipitated in the
surface layer portion. As a result, it was succeeded in obtaining a
super carburized, quenched member having a carbon concentration of
preferably 0.8 wt. % or higher, more preferably from 1.0 to 2.0 wt.
% in a range of from 10 to 30% in terms of the percentage of an
effective case depth (t/T) in an outermost surface layer portion
and having a three-layer structure consisting of a superultrafine
grain layer of No. 10 or greater austenite grain size, a fine grain
layer and an ultrafine grain layer in this order from the outermost
surface layer. It has been found that the super carburized,
quenched member is minimized in deformation or distortion after the
treatment and that the correction of a strain, which has been
unavoidable in the conventional super carburizing, can be obviated
or can be readily conducted compared with the conventional
process.
EXAMPLES
[0038] Based on certain Examples, the present invention will next
be described in further detail.
[0039] Machine structural steels (materials) shown in Table 1 were
provided. Those materials were subjected beforehand to normalizing
treatment at 900.degree. C. and were then machined to prepare
stepped round-bar test pieces of .phi.30/.phi.25/.phi.20.times.L
300 mm, respectively. As carburizing and quenching of each test
piece, the primary treatment of the super carburizing step in the
present invention was conducted using facilities which permitted
heating and carburizing at a low pressure and also permitted oil
hardening and high pressure gas cooling.
[0040] It is to be noted that steel grades 1 and 2 are carburizing,
quenching steels as specified under the JIS, steel grade 1 is
SCM420, chromium-molybdenum steel, and steel grade 2 is SCr415,
chromium steel. MAC14 as steel grade 3 is a grade for a commercial
product developed by a steel maker, and is steel developed by
increasing the Cr content in comparison with the above-described
two steel grades and further adding Mo element with a view to
causing M.sub.23C.sub.6 type ultrafine carbide to precipitate upon
super carburizing (the primary and secondary treatments).
TABLE-US-00001 TABLE 1 Used Steels and Their Chemical Components
(wt. %) Steel grade C Si Mn P S Cr Mo 1 SCM420 0.20 0.30 0.75 0.019
0.025 1.10 0.20 2 SCr415 0.16 0.35 0.78 0.021 0.019 1.05 0.02 3
MAC14 0.15 0.27 0.53 0.020 0.022 2.50 0.38
[0041] Table 2 summarizes the results obtained by experimenting in
various ways effects of the cooling rate on the states of carbide
to be precipitated in surface layer portions of test pieces and the
deformations of the test pieces by heat treatment through the
primary treatment in the present invention. As conditions for the
primary treatment, super carburizing of each test piece was
conducted by the heat cycle shown in FIG. 1 such that subsequent to
heating and soaking, an effective case depth of 0.5 mm would be
achieved. Described specifically, super carburizing and diffusion
treatment of each test piece were alternately conducted at
950.degree. C. for about 70 minutes, respectively, such that the
carbon concentration of the surface layer portion of the test piece
in its final state would be controlled at about 1.5 wt. %. From a
state that the carbon concentration of the surface layer portion of
each test piece was in supersaturation, quenching of the test piece
was conducted under the corresponding cooling rate condition shown
in Table 2 to investigate the shape and size of the carbide in the
surface layer portion of the test piece and the microstructure of
the surface layer portion of the test piece.
[0042] To determine the deformations and strains of the
above-described steel grades by the primary treatment, stepped
round-bar test pieces (.phi.30/.RTM.25/.phi.20.times.L 300 mm) of
the respective steel grades were provided as test pieces. In a
state of being supported at opposite ends, each test piece was
analyzed for a runout at its axial central part to investigate a
relationship between the cooling rate and the axial of the test
piece.
TABLE-US-00002 TABLE 2 Relationships between Cooling Conditions for
Primary Treatment and Precipitation Form of Carbide and Runout
Cooling Microstructure Steel rate Shape and size of of surface
Runout TIR Ex./Comp. Ex. No. grade (.degree. C./sec) carbide layer
portion (mm) Comp. Ex. 1 SCM420 1 Flaky, 3-10 .mu.m F + P + B 0.45
Ex. 2 Same as 12 Granular, 0.5-5 .mu.m M + T 0.17 above Comp. Ex. 3
Same as 20 Granular, .ltoreq.2 .mu.m M + .gamma. 0.38 above Comp.
Ex. 4 SCr415 1 Flaky, 3-10 .mu.m F + P 0.40 Ex. 5 Same as 4
Granular, 0.5-5 .mu.m M + T 0.15 above Comp. Ex. 6 MAC14 1 Granular
+ flaky, 5 .mu.m F + P + B 0.38 Ex. 7 Same as 7 Flaky, 2-7 .mu.m M
+ T 0.20 above TIR: Total Indicating Reading
[0043] The signs shown in the table and analysis methods of the
properties shown there will now be described below. [0044] 1) The
cooling rate indicates an average cooling rate at the axial central
part of each test piece from the quenching temperature of
950.degree. C. after the completion of the carburizing and
diffusion for the test piece to 400.degree. C. [0045] 2) The shape
and size of carbide was observed under a scanning electron
microscope. [0046] 3) Abbreviations for microstructures [0047] F:
ferrite, P: pearlite, B: bainite, T: troostite, M: martensite, y:
retained austenite. [0048] 4) The radial runout indicates a runout
of a test piece, which was mounted on a both-end supporting, runout
measuring instrument, as measured at a central part of the test
piece by a dial gauge.
[0049] In each of the comparative examples shown as Test Piece Nos.
1, 4 and 6 in Table 2, the cooling rate during the cooling was as
low as 1.degree. C./sec so that the carbide precipitated in the
surface layer portion consisted primarily of a network of carbide
formed of carbide flakes bonded together and the matrix was in the
form of an slack quenching structure of ferrite, pearlite and
bainite. As a consequence, those comparative examples were all
large in radial runout and deformation. The comparative example
shown as Test Piece No. 3, on the other hand, was subjected to
rapid cooling equivalent to conventional oil quenching (20.degree.
C./sec) . Its surface layer portion contained a very small amount
of precipitated carbide, and had a structure quenched from a high
carbon state that carbon was in supersaturation. That comparative
example was large in radial runout and deformation.
[0050] When the cooling rate was 4 to 12.degree. C./sec as in each
of the examples as Test Piece Nos. 2, 5 and 7 (the present
invention), ultrafine carbide precipitated in a large amount, and
moreover, microstructures appeared as nuclei for the ultrafine
carbide, leading to improvements in the deformation and distortion
(runout) of the test piece as the outstanding serious problems of
super carburizing. Described specifically, compared with slow
cooling that cooling is slow or rapid quenching that cooling is
fast in contrast, the radial runout of each of the test pieces
according to the present invention was of approximately a half
level of the radial runouts in the rest of the examples, thereby
realizing a substantial reduction in radial runout. From these
results, the cooling rate during the quenching in the primary
treatment is optimally 3 to 15.degree. C./sec.
[0051] Table 3 shows the results obtained by using representative
ones of the test pieces subjected to the primary treatment shown in
Table 2, applying the secondary treatment in various ways to the
representative test pieces to cause ultrafine carbide to finally
precipitate in their surface layer portions, and investigating the
carbon concentrations, states of precipitated carbide,
microstructures, crystal grain sizes, etc. in their surface layer
portions and the radial runouts of the test pieces. As conditions
for the secondary treatment, the heat cycle shown in FIG. 2 was
followed, the soaking temperature was selectively set at three
levels of 800.degree. C., 850.degree. C. and 900.degree. C., all
above the A.sub.1 transformation temperature, and subsequent to the
heating and soaking, additional carburizing was also conducted at
the same time to achieve a carbon concentration higher than the
eutectoid carbon concentration as a technique for further raising
the carbon concentrations in the surface layer portions and also
increasing the amounts of precipitated ultrafine carbide through
the secondary treatment.
[0052] The subscript "n" in (carburizing/diffusion)n or (additional
carburizing/diffusion)n in FIGS. 1 through 3 means the number of
repetitions of carburizing or diffusion in the corresponding step,
and is set in commensurate with the quality required for each
member. In the case of Test Piece No. 2 shown as an example in
Table 2, for example, n was set at 8 (n=8), and in the case of Test
Piece No. 2-2 shown as an example in Table 3, on the other hand, n
was set at 5 (n=5).
TABLE-US-00003 TABLE 3 Relationships between Treatment Conditions
for Secondary Treatment and Precipitation Form of Carbide and
Runout Austenite grain size and three-layer Temperature of
structure secondary Precipitation of Outermost Ex./ Steel treatment
Additional carbide surface Three Comp. Ex. No. grade (.degree. C.)
carburizing Shape Amount layer layers Comp. Ex. 2-1 SCM420 900
Applied Ultrafine A little .gtoreq.10 Included Ex. 2-2 Same as 850
Applied Ultrafine Adequate .gtoreq.10 Included above Comp. Ex. 2-3
Same as 800 Applied Flaky Excessive .gtoreq.10 Included above Ex.
5-1 SCr415 850 Applied Ultrafine Adequate .gtoreq.10 Included Ex.
5-2 Same as 850 Not applied Ultrafine A little 6-8 Not above
included Ex. 7-1 MAC14 850 Applied Ultrafine Adequate .gtoreq.10
Included Ex. 7-2 Same as 850 Not applied Ultrafine A little 6-8 Not
above included Carbon Effective Percentage of Ex./ Steel
concentration case depth effective case Runout TIR Comp. Ex. No.
grade Microstructure (%) (mm) depth, t/T (%) (mm) Comp. Ex. 2-1
SCM420 M + .gamma. 1.6 0.52 5 0.35 Ex. 2-2 Same as M 1.5 0.48 25
0.14 above Comp. Ex. 2-3 Same as M + F 1.5 0.46 20 0.20 above Ex.
5-1 SCr415 M 1.6 0.50 18 0.16 Ex. 5-2 Same as M 1.2 0.46 10 0.23
above Ex. 7-1 MAC14 M 1.4 0.53 25 0.21 Ex. 7-2 Same as M + .gamma.
1.1 0.48 15 0.25 above TIR: Total Indicating Reading
[Analysis Method of Carbon Concentration Surface Layer Portion]
[0053] Using each of the test pieces
(.phi.30/.phi.25/.phi.20.times.L 300 mm), chips were collected by
lathe turning from the surface layer portion to the 0.05 mm depth
of its .phi.25 mm portion, and the carbon concentration of the
surface layer portion was determined by a chemical analysis.
[0054] From Table 3, the Test Piece No. 2 series indicate effects
on the precipitation form of carbide and others when the secondary
treatment temperature was varied, and the Test Pieces No. 5 and No.
7 series indicate effects on the precipitation of ultrafine carbide
and the final carbon concentrations in the surface layer portions
depending on whether or not the additional carburizing was applied
in the secondary treatment.
[0055] Concerning the secondary treatment temperature (which may
herein after be called "the additional carburizing temperature"),
the temperature of 900.degree. C. employed for Test Piece No. 2-1
involves a problem in that the carbide in a surface layer portion
dissolves to lead to a reduction in the overall precipitation of
carbide grains and also to an increase in the radial runout of the
test piece. With the secondary treatment temperature of 800.degree.
C. employed for Test Piece No. 2-3, carbide flakes precipitate at
grain boundaries in the surface layer portion, and the core portion
of the member is quenched incomplete. Test pieces, therefore,
develop variations in radial runout. From these results, the
optimal temperature for the treatment that causes ultrafine carbide
to precipitate in a surface layer portion by the secondary
treatment can preferably be a temperature equivalent to the A.sub.3
transformation temperature+10-70.degree. C., which is determined by
the composition of the member (before the carburizing
treatment).
[0056] As to whether or not the additional carburizing treatment is
applied in the secondary treatment, the application of the
additional carburizing treatment has been recognized, as evident
from the results of Test Piece Nos. 5-1 and 7-1, to bring about the
advantageous effect that carbide precipitates in an ultrafine form,
to say nothing of an improvement in the concentration of carbon in
the surface layer portion. As a reason for the advantageous effect,
it may be contemplated that, as the carbon in the surface layer
portion precipitate as carbide and the concentration of carbon in
the matrix becomes lean, the replenishment of carbon to the surface
layer portion by the additional carburizing could promote the new
formation of ultrafine carbide, such as Fe.sub.3C and
M.sub.23C.sub.6, and nuclei thereof.
[0057] As shown in FIG. 4, it has also been found that in the
member subjected to the additional carburizing treatment, the
austenite grain size of the outermost surface layer portion is
reduced to an ultrafine grain size. The term "ultrafine grain size"
corresponds to an austenite grain size of No. 10 or greater as
measured by the carburized grain-size testing method in JIS-G0551,
"Method of Testing Austenite Grain Size for Steel". A significant
characteristic feature has also been discovered in that a
three-layer structure formed of fine grains and ultrafine grains is
formed extending toward the inside. Paying attention to a
relationship between the austenite grain size and the carburized
layer, the grain sizes of the outermost surface layer portion
greatest in the amount of precipitated ultrafine carbide, the
carburized layer portion (fine grain portion) located inside the
outermost surface layer portion and the ultrafine grain portion
located still inside the fine grain portion are in a relationship
of A.gtoreq.C.gtoreq.B, in which "A", "C" and "B" stand for the
outermost surface layer portion, the ultrafine grain portion and
the fine grain portion, respectively. Incidentally, the austenite
grain size of a surface layer portion in conventional carburizing
is generally equivalent to No. 7 or 8. In the present invention,
the surface layer portion has a grain structure of the
characteristic three-layer structure which does not appear in the
conventional carburizing treatment.
[0058] As an advantageous effect of such an ultrafine grain layer,
it has a significant characteristic feature in that the toughness
of a hardened surface layer, said toughness having been a concern
about conventional carburized members, can be improved and high
toughness can also be imparted to the carburized layer itself in
addition to the feasibility of higher contact pressure as a
characteristic feature of the present invention, and therefore, is
extremely effective for providing carburized members with still
higher strength from now on.
[0059] Table 4 shows effects of the percentage of an effective case
depth of a carbide layer precipitated in super carburizing
according to the present invention on various properties. Various
test pieces were prepared by providing SCM420, JIS steel for
machine structure, as a material, subjecting the material to
normalizing treatment at 900.degree. C. beforehand, and then
machining the resultant material. The super carburizing of each
test piece was conducted by the heat cycle of primary treatment and
secondary treatment shown in FIG. 3. Each treated test piece was
analyzed and investigated for pitting life, impact strength,
distortion by heat treatment, etc. Concerning effects of the carbon
concentration of the outermost surface layer portion of each test
piece shown in Table 5, the test piece was treated by the heat
cycle shown in FIG. 3 in a similar manner as the various test
pieces in Table 4, and the carbon concentration and the like of the
treated test piece were investigated.
[0060] The adjustment of the precipitation depth of carbide in
Table 4 was effected primarily by the control or the like of the
carburizing time and carbon concentration, and the adjustment of
the carbon concentration of the outermost surface layer portion in
Table 5 was effected by controlling the process gas flow, treatment
time and the like upon repeating carburizing and diffusion in the
primary treatment and secondary treatment in accordance with a
program calculated beforehand. Process gases for low-pressure
carburizing include propane, acetylene, ethylene and the like.
Among these, most popular and economical propane was used. As an
inert gas upon diffusion, on the other hand, nitrogen gas was used.
Further, the rapid quenching in the secondary treatment was
conducted by oil. As an alternative, the rapid quenching can also
be conducted by high pressure gas which makes sole or mixed use of
gases such as N.sub.2, He and H.sub.2.
TABLE-US-00004 TABLE 4 Effects of the Percentage of Effective Case
Depth on Strength, Durability and Distortion by Heat Treatment.
Carbon Rolling Percentage concentration fatigue of effective of the
outermost life Impact case depth, Carburizing surface layer (number
of strength Roundness Ex./Comp. Ex. Sign t/T (%) time (min) portion
(%) rotations) (J) (.mu.) Comp. Ex. A 5 80 1.0 6.5 .times. 10.sup.6
118 29 Ex. B 10 104 1.5 1.1 .times. 10.sup.7 110 31 Ex. C 20 119
1.7 2.1 .times. 10.sup.7 105 39 Ex. D 30 134 1.9 2.3 .times.
10.sup.7 98 50 Comp. Ex. E 40 149 2.0 2.2 .times. 10.sup.7 67
65
TABLE-US-00005 TABLE 5 Effects of the Carbon Concentration of
Outermost Surface Layer Portion on Strength, Durability and
Distortion by Heat Treatment Carbon Rolling concentration of
fatigue life Impact Ex./ outermost surface Carburizing (number of
strength Roundness Additional Ref. Ex. Sign layer portion (%) time
(min) rotations) (J) (.mu.) carburizing Ref. Ex F <0.8 80 5.3
.times. 10.sup.6 56 30 Not applied Ex. G 1.0 80 1.5 .times.
10.sup.7 87 32 Same as above Ex. H 1.5 80 2.0 .times. 10.sup.7 69
37 Same as above Ex. I 1.0 96 1.9 .times. 10.sup.7 116 35 Applied
Ex. J 1.5 133 2.4 .times. 10.sup.7 111 39 Same as above Ex. K 2.0
130 2.6 .times. 10.sup.7 98 53 Same as above
[0061] 1) The percentage of effective case depth indicates the
ratio (t/T) of the depth (t) of an ultrafine carbide layer to a
case depth (T) of 550 HMV or greater in terms of micro-Vickers
hardness. [0062] 2) The rolling fatigue life indicates the number
of repetitions of rotation until occurrence of pitting under the
below-described conditions. [0063] Contact pressure: 3 GPa,
rotation speed: 1,500 rpm, slipping ratio: -40%, oil temperature:
80.degree. C. [0064] 3) The impact strength indicates destructive
energy as measured using a Charpy test piece. [0065] 4) The
roundness indicates the amount of a deformation of the inner
diameter of a ring in the X-Y direction as measured by a profile
measuring instrument while using as the ring a test piece in a ring
form of .phi.100(.phi.80).times.15 t.
[0066] A description will now be made about effects of the
percentage of effective case depth on the rolling fatigue life.
When an ultrafine carbide layer was as shallow as 5% in terms of
the percentage of effective case depth as in the comparative
example represented by the sign A, it is considered that the amount
of precipitated ultrafine carbide itself was small and therefore,
that the treated test piece did not have temper softening
resistance, which is characteristic to super carburizing, and was
low in pitting toughness. In the case of the comparative example
represented by the sign E in which the percentage of effective case
depth was 40%, the high hardness range was broadened, resulting in
a problem that the impact strength was reduced, and with respect to
a deformation by heat treatment as determined in terms of
roundness, there was also a tendency toward increased distortion.
From these results, the percentage of effective hardened depth in a
precipitated carbide layer is optimally in a range of from 10 to
30%.
[0067] A description will next be made about effects of the carbon
concentration of the outermost surface layer portion shown in Table
5 on the pitting life. It is considered that the signs H, J and K,
in each of which the carbon concentration of the outermost surface
layer portion was high, were superior in pitting life and that in
the cases of the signs G and I in each of which the carbon
concentration was 1%, that is, lower compared with the former
signs, they were somewhat inferior in pitting life. When the carbon
concentration of the outermost surface layer portion is lower than
0.8 wt. % as in the sign F shown as a referential example, the test
piece was significantly inferior in pitting toughness. Namely, the
greater the amount of ultrafine carbide precipitated in the
outermost surface layer portion and the higher the carbon
concentration of the outermost surface layer portion, the better
the pitting life. Accordingly, the carbon concentration of super
carburizing can be set preferably at 0.8 wt. % or higher in the
present invention.
[0068] Regarding the upper limit to the carbon concentration
through carburizing, no particular problem arose up to 2.0 wt. %.
An increase in carbon concentration to a still higher level in
excess of 2.0 wt. % involves a potential concern that precipitation
of carbide flakes may be facilitated and the impact strength and
deformation by heat treatment of the test piece may tend to become
disadvantageous. It is, therefore, necessary to set the carbon
concentration of the outermost surface layer portion at a level
commensurate with properties required for the member (test
piece).
[0069] A description will next be made about effects of the
additional carburizing treatment in the secondary treatment in the
signs I, J and K on the pitching life, impact strength and
deformation (strain) by heat treatment. Compared with the signs G
and H in each of which the carbon concentrations was similar but
the additional carburizing was not applied, the signs I, J and K
varied less in all the properties and were better. As a reason for
this advantage, it can be contemplated that the additional
carburizing treatment may stabilize the carbon concentration of the
matrix and may also promote the formation of ultrafine carbide in
the outermost surface layer portion, the carburized layer itself
may be converted into a dense and well-balanced structure, and the
quality available through the heat treatment may be thoroughly
stabilized.
[0070] From the above-described various analysis results, it is
desired, as optimal treatment conditions in the process of the
present invention, to employ machine structural steel as a member,
to conduct super carburizing as a combination of the primary
treatment and the secondary treatment in low-pressure carburizing
facilities to treat the member under optimal heating and cooling
conditions, and then to control the final step such that the depth
of precipitated carbide falls within the range of from 10 to 30% in
terms of the percentage of effective case depth and the carbon
concentration of the surface layer becomes 0.8 wt. % or higher.
INDUSTRIAL APPLICABILITY
[0071] As appreciated from the above-described series of results,
the present invention can provide an absolutely novel, super
carburized, low-distortion quenched member and its production
process. According to the present invention, machine structural
members such as gears and axle members can be provided with higher
strength and can be used under higher contact pressure, thereby
making it possible to materialize with low distortion the needs for
various members of higher strength, higher performance, lighter
weight and smaller size, such as members required to have low
distortion, rotary sliding or reciprocal sliding members equipped
with bearing structures, and members required to have high contact
fatigue resistance and high abrasion resistance under high contact
pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0072] [FIG. 1] Heat cycle of the primary treatment.
[0073] [FIG. 2] Heat cycle of the secondary treatment.
[0074] [FIG. 3] Heat cycle of the examples.
[0075] [FIG. 4] Optical micrograph (magnification: .times.100) of
Test Piece No. 2-2 in Table 3.
* * * * *