Note: Descriptions are shown in the official language in which they were submitted.
CA 02594838 2007-07-13
1
DESCRIPTION
HIGH-CONCENTRATION CARBURIZED/LOW-STRAIN QUENCHED
MEMBER AND PROCESS FOR PRODUCING THE SAME
Technical Field
[0001] 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 quenchedmember (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.
Prior Art
[0002] 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
=
CA 02594838 2007-07-13
2
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.
[0003] 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.
[0004] As a representative example of the super carburizing
treatment, Patent Document 1 discloses a carburizing
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3
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 C/min from the Ad l point to a temperature
in a range of from 750 to 950 C to effect carburizing and
quenching.
[0005] 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.
[0006] 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
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4
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 C/min from the Ad l transformation temperature to a
temperature within the range of from 750 to 950 C, and after
direct quenching or air cooling, the member is again heated
and quenched.
[0007] Further, Patent Document 2 and Patent Document 3
propose, as an optimal method, to conduct slow cooling (or
30 C/hr or less) after precurburizing or primary
carburizing.
[0008] When the quenching after the precarburi zing 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.
[0009] On the other hand, Patent Document 1 also discloses
CA 02594838 2007-07-13
quenching with an aim directed toward forming a martens ite
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.
[ono] 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 C/min. Concerning
deformation strain, however, this production process is not
expected to be preferred like the above-mentioned Patent
Documents I, 2 and 3.
[0on] 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.
a) A change from a carburizing step to a diffusion step can
be readily and promptly modified.
b) High-temperature treatment is feasible so that prompt
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carburizing can be conducted.
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.
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, published May 24, 1980.
Patent Document 2: JP-B-2787455, published June 15, 1990.
Patent Document 3: JP-B-2808621, published June 5, 1990.
Patent Document 4: JP-A-2002-348615, published December 4,
2002.
Disclosure of the Invention
Problem to be Solved by the Invention
[0012] 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
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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
[0013] 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.
[0014] The above-described problem can be resolved by the
present invention as defined below:
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 C/sec from the
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temperature within the austenite region to a temperature
not higher than an Al 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.
[0015] 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.
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>C>B in terms of the fineness of austenite grain size.
_ _
[0016] A super carburized, low-distortion quenched member
comprising a surface layer portion of a structure composed
primarily of martensite and containing a mixed structure
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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>C>B in terms of the fineness of austenite
_ _
grain size.
Advantageous Effects of the Present Invention
[0017] 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.
[0018] According to the process of the present invention,
additional carburizing treatment may be applied to the
CA 02594838 2011-01-27
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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.
According to an embodiment of the present invention,
there is provided a process for producing a .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 to dissolve carbon to at least
at an eutectoid carbon concentration of a surface layer
portion of said steel member and then quenching said steel
ak 02594838 2012-03-09
10a
member at a cooling rate of from 3 to 15 C/sec from said
temperature within said austenite region to a temperature
which, at a maximum, reaches an Al transformation point, to
form an ultrafine granular carbide or nuclei of said
carbide, or both, in said surface layer portion of the said
steel member; and
a secondary treatment of subsequently heating and
soaking said steel member to a temperature within said
austenite region and then conducting quenching to
precipitate ultrafine granular carbide which are from 0.5
to 5.0 pm in diameter, to an effective case depth
percentage range of from 10 to 30% in an outermost surface
layer portion, and applying an additional carburizing
treatment to said surface layer portion of said steel
member,
wherein said effective case depth percentage range is
a ratio of t/T, where t is a precipitated depth of the
ultrafine granular carbide in the outermost surface layer
portion of the steel member and T is an effective case
depth of the steel member after completion of the secondary
treatment.
A further embodiment provides a super carburized, low-
distortion quenched member produced by a process of the
present invention, wherein said member comprises a surface
layer portion of a structure comprising martensite, as a
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10b
main component, and containing a mixed structure of
troostite and retained austenite in parts thereof, wherein
in said surface layer, an outermost surface layer (A), an
inner layer (B) extending inwardly from portion A and an
interior layer (C) which extends further inward from layer
(B) are in an order of in terms of the fineness of
austenite grain size.
Best Modes for Carrying out the Invention
[0019] 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.
With a view to developing a super carburizing process
for causing ultrafine carbide to precipitate in a surface
CA 02594838 2007-07-13
11
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.
[0020] 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
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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,quenchingand
the like of the member.
[0021] 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
to 30% in terms of effective case depth percentage and
further by converting an outermost surface layer portion
into an ultrafine crystalline structure.
[0022] The term "effective case depth percentage" as used
herein means a ratio (t/T) of a precipitated depth (t) of
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ultrafine granular 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 treatinent
(including the tempering treatment at 180 C) . It is to be
noted that the term "effective case depth" means a distance
froma surface of a hardened layer, which is still in a quenched
state or has been tempered at a temperature not exceeding
200 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) .
Next, the term "precipitated depth of ultrafine
carbide" means the maximumdepth, 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.
[0023] 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
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14
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.
[0024] 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 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 C/sec over a temperature range of from the
carburizing temperature (the temperature in the austenite
region) to the A1 transformation temperature or lower,
preferably to 400 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 M23C6 type carbide formed as
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= 15
a result of bonding of carbide-forming elements such as Cr
and Mo in Fe3C (cementite) or steel with carbon dissolved
in supersaturation.
[0025] 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 C, preferably to a range of from 10 to 70 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.
[0026] 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
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16
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.
[0027] 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
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17
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
[0028] Based on certain Examples, the present invention will
next be described in further detail.
Machine structural steels (materials) shown in Table
1 were provided. Those materials were subjected beforehand
to normalizing treatment at 900 C and were then machined
to prepare stepped round-bar test pieces of 400/4)25/4)20 X
L300 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.
[0029] 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 M23C6 type
ultrafine carbide to precipitate upon super carburizing (the
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. = 18
primary and secondary treatments).
[0030]
Table 1: Used Steels and Their Chemical Components
(wt.%)
Steel
Si Mn P S Cr
Mo
grade
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
[0031] 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 =would
be achieved. Described specifically, super carburizing and
diffusion treatment of each test piece were alternately
conducted at 950 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 % Froma state that the carbon concentration
of the surface layer portion of each test piece was in
CA 02594838 2007-07-13
19
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.
[0032] To determine the deformations and strains of the
above-described steel grades by the primary treatment,
stepped round-bar test pieces (00/(1)25/(1)20 X L300 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.
.
20
A
[0033]
Table 2: Relationships between Cooling Conditions for Primary Treatment and
Precipitation
Form of Carbide and Runout
Cooling
Microstructure
Steel Shape and size of Runout.TIR
Ex./Comp.Ex. No. rate
of surface
grade( carbide
(mm)
( C/sec)
layer portion
Comp.Ex. 1 SCM420 1 Flaky, 3-10 m
F + P + B 0.45 n
0
Same as
I.)
Ex. 2 12 Granular, 0.5-5 m
M + T 0.17 in
above
ko
a,
co
w
Same as
co
Comp.Ex. 3 20 Granular, <2 m
M + y 0.38 I.)
above
0
0
-.3
1
Comp.Ex. 4 SCr415 1 Flaky, 3-10 m
F + P 0.40 0
-.3
I
H
w
Same as
Ex. 5 4 Granular, 0.5-5 m
M + T 0.15
above
_
Comp.Ex. 6 MAC14 1 Granular + flaky, 5 m
F + P + B 0.38
Same as
Ex. 7 7 Flaky, 2-7 m
M + T 0.20
above
TIR: Total Indicating Reading
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11
21
[0034] The signs shown in the table and analysis methods of
the properties shown there will now be described below.
1) The cooling rate indicates an average cooling rate at
the axial central part of each test piece from the
quenching temperature of 950 C after the completion of
the carburizing and diffusion for the test piece to 400 C.
2) The shape and size of carbide was observed under a scanning
electron microscope.
3) Abbreviations for microstructures
F: ferrite, P: pearlite, B: bainite, T: troostite, M:
martensite, y: retained austenite.
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.
[0035] 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 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 C/sec). Its
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=
22
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.
[0036] When the cooling rate was 4 to 12 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 C/sec.
[0037] 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
CA 02594838 2009-12-14
23
concentrations, states of precipitated carbide,
microstructures and crystal grain sizes, 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 C, 850 C and 900 C,
all above the A1 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.
The subscript "n" in (carburizing/diffusion),, 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) .
,
_
24
4
[0038]
Table 3: Relationships between Treatment Conditions for Secondary Treatment
and Precipitation
Form of Carbide and Runout
Austenite grain size
Precipitation of
Tempera-
and three-layer
carbide
ture of Additional
structure
Ex./ Steel
No. secondary carburizing
Comp.Ex. grade
treatment
Outermost
Three
n
( C) Shape
Amount surface
layers
0
layer
I.)
in
ko
a,
Comp.Ex. 2-1 SCM420 900 Applied
Ultrafine A little >10 Included co
w
co
I.)
Same as
0
Ex. 2-2 850
Applied Ultrafine Adequate >10 Included 0
above
--3
1
0
--3
1
Same as
H
Comp.Ex. 2-3 800 Applied Flaky
Excessive >10 Included w
above
Ex. 5-1 SCr415 850
Applied Ultrafine Adequate >10 Included
Same as
Not
Ex. 5-2 850 Not
applied Ultrafine A little 6-8
above
included
Ex. 7-1 MAC14 850
Applied Ultrafine Adequate 10 Included
Same as
Not
Ex. 7-2 850 Not
applied Ultrafine A little 6-8
above
included
25
.
.
Table 3 (Cont'd)
Carbon Effective
Percentage of Runout.TIR
Ex./ Steel Micro-
No. concent- case depth
effective case
Comp.Ex. grade structure
(mm)
ration (%) (mm)
depth, t/T (%)
Comp.Ex: 2-1 5CM420 M + 7 1.6 0.52
5 0.35
Same as
n
Ex. 2-2 M 1.5 0.48
25 0.14
above
0
I.)
in
ko
Same as
a,
Comp.Ex. 2-3 M + F 1.5 0.46
20 0.20 co
above
w
co
I.)
Ex. 5-1 SCr415 M 1.6 0.50
18 0.16 0
0
-.3
1
0
Same as
Ex. 5-2 M 1.2 0.46
10 0.23 1
above
H
w
Ex. 7-1 MAC14 M 1.4 0.53
25 0.21
_.
Same as
Ex. 7-2 M + 7 1.1 0.48
15 0.25
above
_
TIR: Total Indicating Reading
CA 02594838 2007-07-13
A I
26
[0039] [Analysis method of carbon concentration surface layer
portion]
Using each of the test pieces (00/4)25/(1)20X L300 mm) ,
chips were collected by lathe turning from the surface layer
portion to the 0.05 mm depth of its 425 mm portion, and the
carbon concentration of the surface layer portion was
determined by a chemical analysis.
[0040] 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.
[0041] Concerning the secondary treatment temperature
(which may hereinafter be called "the additional carburizing
temperature"), the temperature of 900 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 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,
CA 02594838 2007-07-13
=
27
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 A3 transformation temperature + 10-70 C,
which is determined by the composition of the member (before
the carburizing treatment).
[0042] 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 Fe3C and M23C6, and nuclei thereof.
[0043] As shown in FIG. 4, it has also been found that in
the member subj ected 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
CA 02594838 2007-07-13
,
28
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>C>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.
[0044]
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
CA 02594838 2007-07-13
, = < 29
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.
[0095] 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 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.
[0046] 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
CA 02594838 2009-12-14
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 and ethylene.
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 N2, He
and H2.
_
31
[0047]
Table 4: Effects of the Percentage of Effective Case Depth on Strength,
Durability and
Distortion by Heat Treatment
0
Carbon
Rolling
Percentage
0
concentration
fatigue Impact I.)
of effective Carburizing
Roundness ul
Ex./Comp.Ex. Sign of the outermost
life strength ko
case depth, time (min)
( ) a,
surface layer
(number of (J.) co
t/T (%)
w
co
portion (%)
rotations)
I.)
0
0
Comp.Ex. A 5 80 1.0
6.5X106 118 29 --3
1
0
--3
Ex. B 10 104 1.5
1.1X107 110 31 1H
w
Ex. C 20 119 1.7
2.1X107 105 39
Ex. D 30 134 1.9
2.3X107 98 50
Comp.Ex. E 40 149 2.0
2.2X107 67 65
r
32
A
[0048]
Table 5: Effects of the Carbon Concentration of Outermost Surface Layer
Portion on Strength,
Durability and Distortion by Heat Treatment
Carbon Rolling
n
Ex./ concentration of Carburizing fatigue life
ImpactRoundness Additional o
Sign
strength I.)
Ref.Ex. outermost surface time (min) (number of
( ) carburizing in
(LT)
ko
layer portion (%) rotations)
a,
op
Lo
op
Ref .Ex F <0.8 80 5.3 X 106
56 30 Not applied I.)
0
0
-A
I
Ex. G 1.0 80 1.5 X 107
87 32 Same as above 0
-A
I
H
Ex. H 1.5 80 2.0 X 107
69 37 Same as above Lo
Ex. I 1.0 96 1.9 X 107
116 35 Applied
Ex. J 1.5 133 2.4X107
111 39 Same as above
Ex. K 2.0 130 2.6X107
98 53 Same as above
CA 02594838 2007-07-13
(a 4
33
[0049] 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.
2) The rolling fatigue life indicates the number of
repetitions of rotation until occurrence of pitting under
the below-described conditions.
Contact pressure: 3 GPa, rotation speed: 1,500 rpm,
slipping ratio: -40%, oil temperature: 80 C.
3) The impact strength indicates destructive energy as
measured using a Charpy test piece.
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 000(4)80)
X 15t.
[0050] 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
CA 02594838 2007-07-13
=kl
34
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%.
[0051] 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
CA 02594838 2007-07-13
p
invention.
[0052] 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) .
[0053] 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.
CA 02594838 2007-07-13
* 4
36
[0054] 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
[0055] 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
CA 02594838 2007-07-13
= = 37
members required to have high contact fatigue resistance
and high abrasion resistance under high contact pressure.
Brief Description of the Drawings
[0056] [FIG. 1] Heat cycle of the primary treatment.
[FIG. 2] Heat cycle of the secondary treatment.
[FIG. 3] Heat cycle of the examples.
[FIG. 4] Optical micrograph (magnification: X100) of Test
Piece No. 2-2 in Table 3.
