U.S. patent application number 12/681762 was filed with the patent office on 2011-03-31 for heat-treatment process for a steel.
Invention is credited to Steven Lane, Peter Neuman, Ingemar Strandell, Mikael B. Sundqvist.
Application Number | 20110073222 12/681762 |
Document ID | / |
Family ID | 38739170 |
Filed Date | 2011-03-31 |
United States Patent
Application |
20110073222 |
Kind Code |
A1 |
Strandell; Ingemar ; et
al. |
March 31, 2011 |
Heat-Treatment Process for a Steel
Abstract
A process for inducing a compressive residual stress in a
surface region of a steel component, the process comprising a heat
treatment having the following steps: (i) providing a component
comprising a steel composition; (ii) induction heating at least a
part of the component followed by quenching said at least part,
wherein the hardness in a surface region of the component is
increased; and (iii) subsequently performing a martensite and/or
bainite through hardening step to obtain a microstructure
comprising martensite and/or bainite.
Inventors: |
Strandell; Ingemar;
(Savedalen, SE) ; Neuman; Peter; (Goteborg,
SE) ; Sundqvist; Mikael B.; (Goteborg, SE) ;
Lane; Steven; (Houten, NL) |
Family ID: |
38739170 |
Appl. No.: |
12/681762 |
Filed: |
October 3, 2008 |
PCT Filed: |
October 3, 2008 |
PCT NO: |
PCT/SE2008/000544 |
371 Date: |
December 14, 2010 |
Current U.S.
Class: |
148/575 |
Current CPC
Class: |
Y02P 10/25 20151101;
F16C 33/62 20130101; Y02P 10/253 20151101; F16C 2202/04 20130101;
C21D 1/10 20130101; C21D 1/18 20130101; F16C 2204/66 20130101 |
Class at
Publication: |
148/575 |
International
Class: |
C21D 1/10 20060101
C21D001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 4, 2007 |
GB |
0719457.4 |
Claims
1. A heat treatment process for inducing a compressive residual
stress in a surface region of a steel component, the process
comprising the steps of: providing a steel component; induction
heating at least a part of the component followed by quenching the
part of the component so as to increase the hardness in a surface
region of the component; and subsequently performing at least one
of a martensite through hardening of the component and a bainite
through hardening of the component to obtain a microstructure
comprising the at least one of martensite and bainite.
2. The process as claimed in claim 1, wherein the step of induction
heating includes heating the part of the component is heated to a
depth of from about 0.5 mm to about 3 mm.
3. The process as claimed in claim 1, wherein the step of induction
heating includes heating the surface of the at least part of the
component to a temperature within a range of from about
1000.degree. C. to about 1100.degree. C.
4. A process for inducing a compressive residual stress in a
surface region of a steel component, the process comprising a heat
treatment having the following steps: providing a steel component;
performing at least one of a martensite through hardening of the
component and a bainite through hardening of the component to
obtain a microstructure including at least one of martensite and
bainite; and induction heating at least a part of the component
followed by quenching the at least part of the component so as to
increase the hardness in a surface region of the component.
5. The process as claimed in claim 4, wherein the step of induction
heating includes heating the part of the component to a depth of
from about 1 mm to about 6 mm.
6. The process as claimed in claim 4, wherein the step of induction
heating includes heating the surface of the part of the component
to a temperature within a range of from about 900.degree. C. to
1000.degree. C.
7. The process as claimed in claim 4, further comprising the step
of tempering the component subsequent to quenching the
component.
8. The process as claimed in claim 4, wherein the steel component
is formed of one of a medium carbon steel and a high carbon
steel.
9. The process as claimed in claim 8, wherein the steel component
is formed of a high carbon chromium steel.
10. The process as claimed in any claim 4, wherein the step of
induction heating includes one of medium frequency induction
heating and high frequency induction heating.
11. The process as claimed in claim 10, wherein the induction
heating is performed at a frequency within a range of from about 2
kHz to about 100 kHz.
12. (canceled)
13. The process as claimed in claim 4, wherein the martensite
through hardening step includes austenitising the steel component
and subsequently quenching the steel component below the martensite
start temperature.
14. The process as claimed in claim 13, wherein the through
hardening step includes post-quenching the component to promote
further austenite to martensite transformation.
15. The process as claimed in claim 14, wherein the step of
induction heating includes tempering the component subsequent to
quenching.
16. The process as claimed in any claim 4, wherein the bainite
through hardening step includes austenitising the steel, quenching
the steel above the martensite start temperature, and subsequently
performing an isothermal bainite transformation.
17. The process as claimed in claim 16, wherein the isothermal
bainite transformation is performed at a temperature in the range
of from about 210.degree. C. to about 240.degree. C. for a duration
within a range of from about 2.5 hours to about 20 hours.
18. The process as claimed in claim 13, wherein the steel component
is austenitised at a temperature in the range of from about
790.degree. C. to about 890.degree. C. and for a duration within a
range from about 20 minutes to about 60 minutes.
19. The process as claimed in claim 1, further comprising the step
of a second induction heating of at least a part of the component
followed by quenching the at least part of the component, wherein
the hardness in a surface region of the component is increased.
20. The process as claimed in claim 4, wherein the process is a
non-thermochemical process.
Description
[0001] The present invention relates generally to the field of
metallurgy and a heat-treatment process for a steel component. The
process induces a compressive residual stress (CRS) in a surface
region of the component with the corollary of an improvement in
mechanical properties, for example fatigue performance.
[0002] Conventional techniques for manufacturing metal components
involve hot-rolling or hot-forging to form a bar, rod, tube or
ring, followed by a soft forming process to obtain the desired
component. Surface hardening processes are well known and are used
to locally increase the hardness of surfaces of finished components
so as to improve, for example, wear resistance and fatigue
resistance.
[0003] A number of surface hardening processes are known for
improving fatigue performance. Shot peening involves bombarding the
surface of the metal component with rounded shot to locally harden
surface layers. However, this process results in a rough surface
finish which can create other problems and therefore additional
steps need to be taken to improve the surface finish. This adds to
productions costs.
[0004] Case-hardening may also be achieved by heating the steel
component in a carbonaceous medium to increase the carbon content,
followed by quenching and tempering. This thermochemcial process is
known as carburizing and results in a surface chemistry that is
quite different from that of the core of the component.
Alternatively, the hard surface layer may be formed by rapidly
heating the surface of a medium/high carbon steel to above the
ferrite/austenite transformation temperature, followed by quenching
and tempering to result in a hard surface layer. Heating of the
surface has traditionally been achieved by flame hardening,
although laser surface-hardening and induction hardening are now
often used. Induction hardening involves heating the steel
component by exposing it to an alternating magnetic field to a
temperature within or above the transformation range, followed by
quenching. Heating occurs primarily in the surface of the
component, with the core of the component remaining essentially
unaffected. The penetration of the field is inversely proportional
to the frequency of the field and thus the depth of the hardening
can be adjusted in a simple manner. The penetration of the field
also depends on the power density and interaction time.
[0005] An alternative to case-hardening is through-hardening.
Through-hardened components differ from case-hardened components in
that the hardness is uniform or substantially uniform throughout
the component. Through-hardened components are also generally
cheaper to manufacture than case-hardened components because they
avoid the complex heat-treatments associated with carburizing, for
example. The steel grades that are used depend on the component
section thickness. For components having a wall thickness of up to
about 20 mm, DIN 100Cr6 is typically used. For larger section
sizes, higher alloyed grades are used such as for example, DIN
100CrMo7-3, DIN 100CrMnMo7, DIN 100CrMo7-4, or DIN 100CrMnMo8.
[0006] For through-hardened steel components, two heat-treating
methods are available: martensite hardening or austempering.
Component properties such as toughness, hardness, microstructure,
retained austenite content, and dimensional stability are
associated with or affected by the particular type of heat
treatment employed.
[0007] The martensite through-hardening process involves
austenitising the steel prior to quenching below the martensite
start temperature. The steel may then be low-temperature tempered
to stabilize the microstructure. The martensite through-hardening
process typically results in a compressive residual stress (CRS) of
from 0 to +100 MPa between the WCS (working contact surface) and
down to an approximately 1.5 mm depth below the WCS.
[0008] The bainite through-hardening process involves austenitising
the steel prior to quenching above the martensite start
temperature. Following quenching, an isothermal bainite
transformation is performed. Bainite through-hardening is sometimes
preferred in steels instead of martensite through-hardening. This
is because a bainitic structure may possess superior mechanical
properties, for example toughness and crack propagation resistance.
The bainite though-hardening process results in a CRS of from 0 to
-100 MPa between the WCS and down to an approximately 1.5 mm depth
below the WCS.
[0009] Numerous conventional heat-treatments are known for
achieving martensite through-hardening and bainite
through-hardening.
[0010] The present invention aims to address at least some of the
problems associated with the prior art.
[0011] Accordingly, in a first aspect the present invention
provides a process for inducing a compressive residual stress in a
surface region of a steel component, the process comprising a
heat-treatment having the following steps:
[0012] (i) providing a component comprising a steel
composition;
[0013] (ii) induction heating at least a part of the component
followed by quenching said at least part, wherein the hardness in a
surface region of the component is increased; and
[0014] (iii) subsequently performing a martensite and/or bainite
through-hardening step to obtain a microstructure comprising
martensite and/or bainite.
[0015] During the induction heating, said at least part of the
component is preferably heated to a depth of from 0.5 to 3 mm, more
preferably from 0.75 to 2.5 mm, still more preferably from 1 to 2
mm. That is the induction heating preferably penetrates to a depth
of at least about 0.5 mm and up to a maximum depth of up to about 3
mm. Induction heating to such depths, in conjunction with the other
steps of the process, has been found to induce a compressive
residual stress (CRS) in a surface region of the component with the
corollary of an improvement in mechanical properties, for example
fatigue performance.
[0016] During the induction heating, the surface of said at least
part of the component preferably reaches a temperature of from 1000
to 1100.degree. C., more preferably from 1020 to 1080.degree. C.
After quenching, the surface microstructure comprises martensite or
at least martensite as the predominant phase.
[0017] The process may further comprising, after step (iii):
[0018] (iv) induction heating at least a part of the component
followed by quenching said at least part of the component, wherein
the hardness in a surface region of the component is increased.
[0019] In a second aspect the present invention provides a process
for inducing a compressive residual stress in a surface region of a
steel component, the process comprising a heat-treatment having the
following steps:
[0020] (a) providing a component comprising a steel
composition;
[0021] (b) performing a martensite and/or bainite through-hardening
step to obtain a microstructure comprising martensite and/or
bainite.
[0022] (c) induction heating at least a part of the component
followed by quenching said at least part of the component, wherein
the hardness in a surface region of the component is increased.
[0023] In the second aspect, during the induction heating, said at
least part of the component is preferably heated to a depth of from
1 to 6 mm, more preferably from 2 to 5 mm.
[0024] In the second aspect, during the induction heating, the
surface of said at least part of the component preferably reaches a
temperature of from 900 to 1000.degree. C., more preferably from
920 to 980.degree. C. After quenching, the surface microstructure
comprises martensite or at least martensite as the predominant
phase.
[0025] In the second aspect, following the induction heating and
the quenching, the component is preferably subjected to tempering,
preferably low temperature tempering at a temperature of up to
about 250.degree. C.
[0026] The present invention will now be further described. In the
following passages different aspects/embodiments of the invention
are defined in more detail. Each aspect/embodiment so defined may
be combined with any other aspect/embodiment or aspects/embodiments
unless clearly indicated to the contrary. In particular, any
feature indicated as being preferred or advantageous may be
combined with any other feature or features indicated as being
preferred or advantageous.
[0027] The present invention involves either pre- or post-induction
processing in relation to a through-hardening heat-treatment
process in order to introduce thermal strains and/or phase
transformation strains such that a large compressive residual
stress (CRS) is achieved. In particular, the present invention
enables a steel product to be produced with a CRS in the range of
-200 to -900 MPa at the near surface, typically being maintained at
-300 to -500 MPa at 1 mm depth below the surface. The near surface
is typically less than 300 microns below the heat-treated
surface.
[0028] The process is applicable to all though-hardening steel
grades. The steel will typically be a medium (0.3 to 0.8% carbon)
or high carbon steel (>0.8% carbon) such as a high carbon
chromium steel or a low alloy bearing steel. For example, 0.65-1.20
wt. % C, 0.05-1.70 wt. % Si, 1.1-2.2 wt. % Cr, 0.10-0.1.10 wt. %
Mn, 0.02-1.0 wt. % Ni, 0.02-0.70 wt. % Mo, and the balance Fe,
together with any unavoidable impurities. Suitable commercial
examples include: DIN 100Cr6 (=SAE 52100), DIN 100CrMo7-3, DIN
100CrMnMo7, DIN 100CrMo7-4, and DIN 100CrMnMo8.
[0029] The induction heating is preferably medium and/or high
frequency induction heating and is advantageously performed at a
frequency of from 2-100 kHz. The interaction time and power level
may be varied having regard to the component size and desired
depth.
[0030] The inducting heating is preferably followed by quenching,
for example to room temperature (20 to 25.degree. C.) or even to
0.degree. C. or less.
[0031] In the first aspect, the induction heating step
advantageously achieves rapid surface heating using medium and/or
high frequency induction heating (preferably at a frequency of
2-100 kHz, more preferably 5 to 20 kHz) to a depth of typically 0.5
to 3 mm, more typically 1 to 2 mm. The surface preferably reaches a
temperature of from 1000 to 1100.degree. C., more preferably from
1020 to 1080.degree. C. As noted above, after the induction
heating, the component is preferably quenched using, for example,
oil or a polymer solution in order to `freeze` the effect of the
surface conditioning.
[0032] In the second aspect, the induction heating step
advantageously achieves rapid surface heating using medium or high
frequency induction heating (preferably at a frequency of 2-100
kHz, more preferably 40 to 130 kHz) to a depth of typically 1 to 6
mm, more typically 2 to 5 mm. The surface preferably reaches a
temperature of from 900 to 1000.degree. C., more preferably from
920 to 980.degree. C. As noted above, after the induction heating,
the component is preferably quenched using, for example, oil or a
polymer solution in order to `freeze` the effect of the surface
conditioning.
[0033] If the process of either the first or second aspects
involves a martensite through-hardening step, then conventional
processes may be relied on. For example, the martensite
through-hardening step will typically comprise austenitising the
steel and subsequently quenching the steel below the martensite
start temperature (Ms is typically 180 to 220.degree. C., more
typically 190 to 200.degree. C., still more typically approximately
200.degree. C.). Quenching may be performed using, for example,
molten salt. Following the martensite through-hardening step, the
component is preferably post-quenched in, for example, cold water
to promote further austenite to martensite transformation.
Following the post-quench, the component is preferably subjected to
low temperature tempering to stabilize the microstructure.
[0034] Similarly, if the process involves a bainite
through-hardening step, then conventional processes may be relied
on. For example, the bainite through-hardening step will typically
comprise austenitising the steel and quenching the steel above the
martensite start temperature (Ms is typically 180 to 220.degree.
C., more typically 190 to 200.degree. C., still more typically
approximately 200.degree. C.). Quenching may be performed using,
for example, oil or molten salt. This is followed by an isothermal
bainite transformation, which is preferably performed at a
temperature in the range of from 200 to 250.degree. C., more
preferably from 210 to 240.degree. C. The steel is preferably held
within this temperature range for from 1 to 30 hours, more
preferably from 2.5 to 20 hours depending on the steel grade and
section thickness.
[0035] Irrespective of whether one or both of martensite and/or
bainite are desired, the steel is preferably austenitised (prior to
the quench below/above the martensite start temperature).
Austenitising is well known in the art. However, the inventors have
found (particularly in relation to the first aspect) that applying
through-hardening using a 10-50.degree. C. lower hardening
temperature than what would normally be used (e.g. 840 to
890.degree. C.) further promotes the CRS build-up. This is believed
to be because the core is under-austenitised in relation to the
slightly over-austenitised surface portion. Therefore, the phase
transformation differences will be more pronounced. The benefit of
having a delayed phase transformation in the surface portion is
that it will take place on a fully or partially transformed core,
which will restrict the possibility for plastic deformation (phase
transformations usually involve a volume increase), and the final
surface stress state will therefore become compressive. For these
reasons, austenitising is preferably performed at a temperature in
the range of from 790 to 890.degree. C., more preferably from 790
to 880.degree. C., still more preferably from 790 to 840.degree. C.
The steel is preferably held within this temperature range for from
10 to 70 minutes, more preferably from 20 to 60 minutes.
[0036] The austenitisation is typically performed in an atmosphere
furnace where the component can reach a homogeneous temperature
throughout its cross-section. Consequently, a homogenous
austenitisation and cementite dissolution is advantageously
achieved.
[0037] In the process of the present invention the chemical
composition of the steel remains essentially unchanged. In other
words, the process does not need to involve a thermochemical
enrichment process. This is in contrast conventional case-hardening
treatments.
[0038] The final microstructure comprises either (tempered)
martensite or bainite as the major phase or a combination of the
two. Cementite may also be present. In general, the microstructure
appears to be essentially homogeneous from the surface to the core.
However, some inherent segregation of alloying elements (e.g. N, C,
Cr, Si, Mn) may be present.
[0039] The hardness within the surface is typically 50-75 HRC, more
typically 56-68 HRC. The retained austenite content is typically
0-30%.
[0040] The underlying core also comprises either martensite and/or
bainite or mixtures thereof. The hardness of the core
microstructure is typically greater than 50 HRC, more typically
greater than 56 HRC. The hardness of the core generally does not
exceed 67 HRC, more typically it does not exceed 64 HRC. The
retained austenite content is typically 0-20%.
[0041] In the second aspect of the present invention, the
heat-treatment steps result in a transition zone visible both in
hardness and in microstructure.
[0042] The component may be any type of steel component. For
example, component for a bearing such as a raceway or a rolling
element.
[0043] The present invention enables a product to be produced with
a CRS in the range of -200 to -900 MPa at the near surface, being
maintained at -300 to -500 MPa at 1 mm depth below the surface.
Such a CRS profile compares very favourably to conventional
components.
[0044] Thus, in a third aspect the present invention provides a
component formed from steel, wherein the component comprises
through-hardened martensite and/or through-hardened bainite and has
a substantially homogeneous chemical composition and
microstructure, at least a part of the component having a
compressive residual stress profile comprising -200 to -900
[0045] MPa at the near surface, and -300 to -500 MPa at 1 mm depth
below the surface.
[0046] In a fourth aspect, the present invention provides a process
involving a combination of the first and second aspects. Here, a
first induction heating step, corresponding to the first aspect,
introduces mainly a carbide dissolution gradient that affects the
phase transformation characteristics. This is followed by
martensite and/or bainite though-hardening. Next, a second
induction heating step, corresponding to the second aspect, is
performed to introduce thermal strains between the surface and the
core.
[0047] The present invention will now be described further with
reference to the following Examples and the accompanying drawings,
provided by way of example, in which:
[0048] FIG. 1 is a plot showing the compressive residual stress
profile for the component of Example 1;
[0049] FIGS. 2a and 2b are micrographs showing the surface (a) and
core (b) microstructures for the component of Example 1;
[0050] FIG. 3 is a plot showing the hardness profile for the
component of Example 1 after the induction heating step but before
the bainite through-hardening step;
[0051] FIG. 4 is a plot showing the hardness profile for the
component of Example 1 after the induction heating and bainite
through-hardening steps;
[0052] FIG. 5 is a plot showing the compressive residual stress
profile for the component of Example 2 after the heat-treatment and
compared to standard martensite and standard bainite;
[0053] FIGS. 6a, 6b and 6c are micrographs showing the surface (a),
transitional zone (b), and core (c) microstructures for the
component of Example 2 after the bainite through-hardening and
induction heating steps;
[0054] FIG. 7 is a plot showing the hardness profile for the
component of Example 2 after the bainite through-hardening and
induction heating steps; and
[0055] FIG. 8 is a plot showing the compressive residual stress
profile for the component of Example 3 after the martensite
through-hardening and induction heating steps.
EXAMPLE 1
Pre-Processing and Bainite Rehardening
[0056] Test component: Spherical roller bearing (SRB) outer ring
with CD 180 mm formed from 100Cr6 steel. [0057] Pre-processing:
Inductive surface heating using .about.10 kHz to reach a surface
temperature of .about.1050.degree. C. and a pre-processing depth of
.about.2 mm, followed by quenching using a 5% Aquaquench polymer
solution. [0058] Bainite-through hardening: Furnace rehardening
using 820.degree. C. and 20 minute soaking time, followed by
quenching and transformation in .about.230.degree. C. molten
Petrofer AS140 salt for 240 minutes, followed by cooling in still
air.
[0059] FIG. 1 is a plot showing the compressive residual stress
profile for the component of Example 1. The plot shows a near
surface CRS of -300 to -800 MPa. The CRS is maintained at -300 down
to at least 1.2 mm.
[0060] FIGS. 2a and 2b are micrographs showing the surface (a) and
core (b) microstructures for the component of Example 1. The
micrographs show a bainite microstructure. The surface
microstructure is slightly coarser with less residual carbides
(cementite) than the core.
[0061] FIG. 3 is a plot showing the hardness profile for the
component of Example 1 after pre-induction process only
[0062] FIG. 4 is a plot showing the hardness profile for the
component of Example 1 after the complete process.
EXAMPLE 2
Bainite Through Hardening and Post-Processing
[0063] Test component: Cylindrical roller bearing (CRE) inner ring
with OD 120 mm formed from 100Cr6 steel) [0064] Bainite
through-hardening: Furnace rehardening using 860.degree. C. and 20
min soaking time, followed by quenching and transformation in
.about.230.degree. C. molten Petrofer AS140 salt for 240 min,
followed by cooling in still air. [0065] Post-processing: Inductive
surface heating using .about.8 kHz to reach a surface temperature
of .about.940.degree. C. and a case depth of .about.1.8 mm,
followed by quenching using a 5% Aquatensid polymer quench solution
and tempering at 160.degree. C. for 60 min
[0066] FIG. 5 is a plot showing the compressive residual stress
profile for the component of Example 2 after the heat-treatment and
compared to standard martensite and standard bainite.
[0067] FIGS. 6a, 6b and 6c are micrographs showing the surface (a),
transitional zone (b), and core (c) microstructures for the
component of Example 2 after the bainite through-hardening and
induction heating steps. The micrographs show a martensite surface
microstructure, a tempered bainite microstructure in the transition
zone, and a bainite core microstructure.
[0068] FIG. 7 is a plot showing the hardness profile for the
component of Example 2 after the bainite through-hardening and
induction heating steps. The hardness profile shows a transition
zone.
EXAMPLE 3
Martensite Through Hardening and Post-Processing Under Interference
Fit
[0069] Test component: Deep groove ball bearing (DGBB) inner ring
with OD 62 mm formed from 100Cr6 steel [0070] Martensite through
Hardening: Furnace rehardening using 860.degree. C. and 20 min
soaking time, followed by oil quenching in 60.degree. C. oil and
tempering at 160.degree. C. for 60 min [0071] Post-processing:
Mounting on over-sized shaft resulting in hoop-stress. Inductive
surface heating using .about.90 kHz to reach a surface temperature
of .about.940.degree. C. and a case depth of .about.1.8 mm,
followed by quenching using a 5% Aquatensid polymer quench solution
and tempering at 160.degree. C. for 60 min. Removal of shaft.
[0072] FIG. 8 is a plot showing the CRS profile for the component
of Example 3 after the martensite through-hardening and induction
heating steps with different levels of hoop stress.
* * * * *