U.S. patent application number 11/718941 was filed with the patent office on 2008-02-28 for method for thermally treating a component consisting of a fully hardenable, heat-resistant steel and a component consisting of said steel.
This patent application is currently assigned to SCHAEFFLER KG. Invention is credited to Franz-Josef Ebert, Christian Schulte-Noelle, Werner Trojahn.
Application Number | 20080047632 11/718941 |
Document ID | / |
Family ID | 35674946 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080047632 |
Kind Code |
A1 |
Trojahn; Werner ; et
al. |
February 28, 2008 |
Method for Thermally Treating a Component Consisting of a Fully
Hardenable, Heat-Resistant Steel and a Component Consisting of Said
Steel
Abstract
The aim of the method is to achieve a higher penetration depth
of the diffusion element coupled with a greater depth of case
hardening and a higher case hardening degree, whilst preventing an
excessive enrichment of the case, thus obtaining an increased
fatigue limit for the component. To achieve this, the full
hardening of the component and the plasma-ion hardening of the case
of the component are carried out in a common working step. The
component is heated to a common hardening and diffusion temperature
T.sub.H+D lying above the upper transformation temperature
A.sub.C3, the component being held at the common hardening and
diffusion temperature T.sub.H+D until the carbon in the component
has fully austenitized and dissolved and until the case has been
enriched to the desired degree by the diffusion element. The
component is then quenched.
Inventors: |
Trojahn; Werner;
(Niederwerrn, DE) ; Schulte-Noelle; Christian;
(Lippstadt, DE) ; Ebert; Franz-Josef; (Hammelburg,
DE) |
Correspondence
Address: |
LUCAS & MERCANTI, LLP
475 PARK AVENUE SOUTH
15TH FLOOR
NEW YORK
NY
10016
US
|
Assignee: |
SCHAEFFLER KG
Industriestrasse 1-3
Herzogenaurach
DE
91074
|
Family ID: |
35674946 |
Appl. No.: |
11/718941 |
Filed: |
November 4, 2005 |
PCT Filed: |
November 4, 2005 |
PCT NO: |
PCT/DE05/01975 |
371 Date: |
July 9, 2007 |
Current U.S.
Class: |
148/222 ;
148/328; 148/622 |
Current CPC
Class: |
C23C 8/02 20130101; C21D
6/04 20130101; C21D 9/36 20130101; C23C 8/38 20130101; C21D 1/09
20130101; C23C 8/26 20130101; C23C 8/32 20130101; C23C 8/22
20130101; C21D 9/40 20130101 |
Class at
Publication: |
148/222 ;
148/328; 148/622 |
International
Class: |
C21D 6/02 20060101
C21D006/02; C22C 38/00 20060101 C22C038/00; C23C 8/22 20060101
C23C008/22; C23C 8/38 20060101 C23C008/38 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 9, 2004 |
DE |
10 2004 053 935.9 |
Claims
1. A method for thermally treating a component of a fully
hardenable heat-resistant steel, the thermal treatment comprising a
full hardening, a case hardening and an annealing of the component,
the full hardening involving a heating of the component to a
hardening temperature above the upper transformation temperature
A.sub.C3, a holding of the component at the hardening temperature
and a quenching of the component, in which the case hardening takes
place under the action of a diffusion element, involves a heating
of the component to a diffusion temperature, a holding of the
component at the diffusion temperature and a cooling of the
component and is carried out as plasma ion hardening, and in which
the annealing involves a once-only or multiple heating of the
component to an annealing temperature below the lower
transformation temperature A.sub.C1, a holding of the component at
the annealing temperature and a cooling of the component, wherein
the full hardening of the component and the plasma ion hardening of
the case of the component are carried out in a joint work step, in
that the component is heated to a joint hardening and diffusion
temperature T.sub.H+D above the upper transformation temperature
A.sub.C3, in that the component is held at the joint hardening and
diffusion temperature T.sub.H+D up to the complete austenitizing
and dissolving of the carbon contained and up to the desired
enrichment of the case with the diffusion element, and in that the
component is subsequently quenched and internal compressive
stresses are thereby formed in the outer case.
2. The method as claimed in claim 1, wherein the height of the
joint hardening and diffusion temperature T.sub.H+D is adapted
essentially to the required hardening temperature T.sub.H of the
steel grade of the component.
3. The method as claimed in claim 2, wherein the joint hardening
and diffusion temperature T.sub.H+D is set in a temperature range
of between 1070.degree. and 1150.degree. C.
4. The method as claimed in claim 2, wherein the duration of
holding .DELTA.t.sub.H+D at the joint hardening and diffusion
temperature T.sub.H+D is adapted to the longer of the two required
holding durations, the required hardening holding duration
.DELTA.t.sub.H or the required diffusion holding duration
.DELTA.t.sub.D.
5. The method as claimed in claim 4, wherein, in the event of a
longer required diffusion holding duration .DELTA.t.sub.D, the
joint hardening and diffusion temperature T.sub.H+D is lowered in
order to avoid a coarsening of the core structure of the
component.
6. The method as claimed in claim 5, wherein the lowering of the
joint hardening and diffusion temperature T.sub.H+D takes place by
about 20.degree. to 40.degree. C.
7. The method as claimed in claim 1, wherein carbon (C) and/or
nitrogen (N) is used as diffusion element for the plasma ion
hardening of the case of the component, and in that, for this
purpose, during the plasma ion hardening the component is subjected
to an ionizable gas emitting carbon (C) and/or nitrogen (N).
8. The method as claimed in claim 7, wherein during a subsequent
annealing of the component, the annealing temperature T.sub.A is
adapted to the fractions of the diffusion element which are
dissolved in the steel, in such a way that, after cooling, the
greatest hardness is established in the case of the component.
9. The method as claimed in claim 8, wherein the annealing
temperature T.sub.A is set at a value in the range of 500.degree.
to 600.degree. C.
10. The method as claimed in claim 1 wherein a heat-resistant steel
is used as the source material of the component.
11. A component consisting of a fully hardenable heat-resistant
steel which has undergone thermal treatment which comprises a full
hardening of the component, a case hardening of the component and
an annealing of the component, wherein the thermal treatment has
taken place as claimed in claim 1.
12. The component as claimed in claim 11, wherein the component
forms a bearing component of a rolling bearing.
13. The component as claimed in claim 12, wherein the rolling
bearing is designed for mounting a mechanically and thermally
highly loaded shaft of a thermal engine.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method for thermally treating a
component consisting of a fully hardenable heat-resistant steel,
the thermal treatment comprising a full hardening of the component,
a case hardening of the component and an annealing of the
component, the full hardening involving a heating of the component
to a hardening temperature above the upper transformation
temperature A.sub.C3, a holding of the component at the hardening
temperature and a quenching of the component, the case hardening
taking place under the action of at least one diffusion element,
involving a heating of the component to a diffusion temperature, a
holding of the component at the diffusion temperature and a cooling
of the component and being carried out as plasma ion hardening, and
the annealing involving a once-only or multiple heating of the
component to an annealing temperature below the lower
transformation temperature A.sub.C1, a holding of the component at
the annealing temperature and a cooling of the component and also
an optional refrigeration.
[0002] The invention relates, further, to a component consisting of
a fully hardenable heat-resistant steel which has undergone thermal
treatment which comprises a full hardening of the component, a case
hardening of the component and an annealing of the component.
BACKGROUND OF THE INVENTION
[0003] Thermally and mechanically highly loaded components, such
as, for example, the bearing components of rolling bearings which
are used for mounting the main shaft of a jet engine or of a gas
turbine, mostly consist of a fully hardenable heat-resistant steel
and, during production, are tailored to the subsequent intended use
by means of suitable thermal treatment. The respective workpieces,
hereafter called components, are to have, along with high strength,
both high toughness and high wear resistance. In order to achieve
this, the thermal treatment of components of this type normally
comprises a full hardening, a case hardening and a subsequent
annealing of the components, the order in which full hardening and
case hardening are carried out can be varying.
[0004] The full hardening of a component, generally designated as
hardening, is a purely thermal method. Hardening or full hardening
involves a heating of the component to a hardening temperature
above the upper transformation temperature A.sub.C3 of the steel of
911.degree. C., a holding of the component at this hardening
temperature and a subsequent quenching of the component. The
heating of the component is controlled, in terms of time, in such a
way that a temperature rise as uniform as possible is established
in the overall component and therefore a deformation of the
component is avoided.
[0005] The hardening temperature is what is known as the
austenitizing temperature at which an essentially complete
transformation of the cubic body-centered ferrite into the cubic
face-centered austerite as well as a dissolution of the carbon
bound in the initial material as carbide into atomic carbon take
place. In the case of high-alloy steels, the hardening temperature
normally lies between 1050.degree. and 1230.degree. C., and the
duration of holding at the hardening temperature may last from 0.5
to three hours.
[0006] The quenching of the component takes place at a rate which
lies above the critical cooling rate of the respective steel grade.
The overall component thereby assumes a martensitic structure, this
being associated with a marked increase in hardness to above 60 HRC
to normally a maximum of 64 HRC.
[0007] The hardening may, if appropriate, also be followed by
low-temperature treatment, for example in the form of a cooling of
the component to -190.degree. C., with the result that any residual
austenite present is transformed into martensite. Hardening gives
rise to internal stresses in the component, normally tensile
stresses at the margin and compressive stresses in the core of the
component. However, tensile stresses in the case of a component are
a disadvantage, since these are intensified by tensile forces
occurring during operation, this being conducive to crack formation
and to crack propagation, and therefore the fatigue strength of the
component, in particular under oscillating load, is reduced.
[0008] By contrast, the case hardening of a component is a
thermochemical method. In this, the respective component is
exposed, at the same time with heating to and holding at a
diffusion temperature, to a solid, liquid or gaseous medium or
plasma which contains a diffusion element, such as, for example,
carbon, nitrogen or a mixture of the two elements, which, under
these conditions, diffuses into the case of the component and, in
conjunction with subsequent cooling, leads to a hardening of the
case of the component.
[0009] In a use of carbon (carburizing) and of a mixture of carbon
and nitrogen with predominantly carbon (carbonitriding) as
diffusion element, the diffusion temperature lies in the range of
between 850.degree. and 980.degree. C., while in a use of nitrogen
(nitriding) and of a mixture of nitrogen and carbon with
predominantly nitrogen (nitrocarburizing) as a diffusion element
the diffusion temperature, by contrast, lies in the range of
between 500.degree. and 580.degree. C.
[0010] In case hardening in the form of plasma ion hardening, by an
electrical voltage being applied between the housing of the
treatment furnace and the component, in connection with a glow
discharge, a plasma consisting of positively charged ions of the
diffusion element is generated and is shot onto the surface of the
component. As a result, first, the surface of the component is
cleaned, subsequently the case of the component is additionally
heated, and the diffusing of the diffusion element into the case is
reinforced. By the electrical voltage of the glow discharge being
controlled, the enrichment of the case with the diffusion element
can be metered exactly. This is important inasmuch as an excessive
enrichment of the case leads to formation of foreign carbides or
foreign nitrides which result in a reduction in the strength and in
the corrosion resistance of the component.
[0011] In plasma-ion hardening with nitrogen (plasma nitriding),
the diffusion temperature typically lies between 350.degree. and
600.degree. C., while, in the use of carbon as a diffusion element,
the diffusion temperature, by contrast, lies between 700.degree.
and 1000.degree. C. The hardness achievable by the case hardening
is up to 66 HRC. Normally, after case hardening, internal
compressive stresses are present in the marginal region of the
component and internal tensile stresses are present in the core of
the component, thus resulting in a higher load-bearing capacity
under oscillating stress. However, the depth of the hitherto
achievable hardening of the case to a maximum of 0.2 mm is
relatively low, this being reduced even further by a mechanical
final machining which is usually carried out, such as, for example,
grinding. The duration of holding at the diffusion temperature can
amount to between 0.5 and 4 hours.
[0012] The annealing of the component mostly takes place as the
final work step after full hardening and case hardening and
involves an, if appropriate, multiple heating of the component to
an annealing temperature below the lower transformation temperature
A.sub.C1 of the steel, a holding of the component at this annealing
temperature and a subsequent cooling of the component. As a result,
variations in the martensitic structure are brought about, which
lead to a reduction in the brittleness and internal stresses which
have occurred essentially during the full hardening, and therefore
to an increase in the toughness of the component. In the case of
high-alloy steel, the annealing temperature lies in the range of
500.degree. to 600.degree. C. The duration of holding at the
annealing temperature amounts to about 1 to 2 hours. The reduction
in hardness caused by the annealing amounts to between 1 and 5 HRC,
depending on the steel grade.
[0013] Further information on thermal and thermochemical methods
for the thermal treatment of steel may be gathered from the
relevant DIN standards and the Kraftfahrtechnischen Taschenbuch
[Motor Vehicle Manual] of BOSCH, 24.sup.th edition, p. 304 ff.,
chapter "Thermal Treatment".
[0014] In DE 40 33 706 C2, the subject of which is, for an increase
in corrosion resistance, the replacement of carbon by nitrogen in
the case hardening of a component consisting of steel, a method for
thermal treatment is described, which involves a case hardening of
the case with nitrogen at a diffusion temperature above the lower
transformation temperature A.sub.C1, a subsequent direct hardening
and a final annealing. Direct hardening, in this context, means
that no cooling takes place between the case hardening and
hardening, but, instead, the treatment temperature is increased
directly from the diffusion temperature to the hardening
temperature. In a method variant, there is provision for carrying
out the case hardening as plasma ion hardening. This known method
has the disadvantage that the hardening of the case caused by the
case hardening is partially cancelled again by the subsequent
direct hardening, and that only a low penetration depth of the
diffusion element can be achieved by means of the case hardening
described.
[0015] By contrast, WO 98/01597 A1 presents a method for thermally
treating a rolling bearing component consisting of a high-alloy
steel, in which the case hardening, which is carried out as plasma
ion hardening with nitrogen as the diffusion element (plasma ion
nitriding), takes place only after the mechanical final machining
of the component, that is to say after hardening and annealing. The
diffusion temperature lies between 375.degree. and 592.degree. C.,
preferably at 460.degree. C. The diffusion holding duration amounts
to between 1 and 2 hours. The maximum depth reached in the hardened
case is around 0.5 mm. However, a uniformly hardened case can be
achieved only to a depth of about 0.15 mm, this being relatively
thin in a disadvantageous way.
[0016] In the method, disclosed in DE 697 19 046 T2, for the
production of casehardened bearing components, the case hardening
takes place in the form of plasma ion carburizing at a diffusion
temperature of above 482.degree. C. at the commencement of thermal
treatment. This is followed by hardening in the form of direct
hardening at a hardening temperature of between 982.degree. and
1200.degree. C. In this known method, too, the hardening of the
case caused by the case hardening is partially cancelled again by
the subsequent direct hardening, so that, as a result, a hardness
of the case of the component of a maximum of 60 HRC is
achieved.
[0017] In a similar method for the production of rolling bearing
components which is described in DE 197 07 033 A1, the respective
components are case hardened, at the commencement of thermal
treatment, by means of plasma ion nitriding or plasma ion
carbonitriding at a diffusion temperature of between 530.degree.
and a maximum of 780.degree. C., are thereupon hardened at a
hardening temperature of 1020.degree. to 1120.degree. C., are
subsequently treated with low temperature at a temperature of
-190.degree. C. and are finally annealed at an annealing
temperature of 180.degree. C. or 450.degree. to 520.degree. C. This
method, too, has the disadvantages already mentioned above, and the
maximum achievable hardness of the case of the component amounts to
62 HRC.
OBJECT OF THE INVENTION
[0018] The object on which the invention is based is to specify a
method of the type initially mentioned for thermally treating a
component consisting of a fully hardenable heat-resistant steel, by
means of which, while avoiding an excessive enrichment of the case
in the case hardening of the component, a higher penetration depth
of the diffusion element, along with a deeper hardening of the
case, and also a higher case hardness are achieved, and,
consequently, an increased fatigue strength of the component, in
particular under pulsating and alternating load, is achieved.
[0019] Further, a component consisting of a fully hardenable
heat-resistant steel and having an increased fatigue strength is to
be specified.
SUMMARY OF THE INVENTION
[0020] The finding on which the invention is based is that, by a
deeper and greater hardening of the marginal zone of a component,
higher and deeper-reaching internal compressive stresses are
generated which lead to a marked increase in the fatigue strength
of the component.
[0021] Consequently, the object regarding the method is achieved
according to the invention, in conjunction with the preamble of
claim 1, in that the full hardening of the component and the plasma
ion hardening of the case of the component are carried out in a
joint work step, in that the component is heated to a joint
hardening and diffusion temperature above the upper transformation
temperature A.sub.C3, in that the component is held at the joint
hardening and diffusion temperature up to the full hardening and up
to the desired enrichment of the marginal zone with the diffusion
element, and in that the component is subsequently quenched.
[0022] Advantageous embodiments of the method according to the
invention are the subject matter of subclaims 2 to 10.
[0023] By the case hardening being carried out in the form of
plasma ion hardening at the relatively high hardening temperature
above the upper transformation temperature A.sub.C3 of the steel, a
greater penetration depth of the diffusion element and consequently
a deeper hardening of the case of the component are achieved, as
compared with the known methods. Since case hardening takes place
simultaneously with the full hardening of the component, an
otherwise customary weakening, occurring during subsequent full
hardening in a separate work step, of the case hardening by the
diffusion element diffusing out is avoided. A greater hardness of
the case of up to 68 HRC is thereby achieved. In addition to an
increased wear resistance of the surface of the component, this
leads to an increased fatigue strength of the component thus
treated, which is particularly advantageous under oscillating load.
As a positive secondary effect of the largely simultaneously
executed full hardening of the component and case hardening of the
component, a time saving in the overall thermal treatment of more
than 2 hours is obtained.
[0024] Basically, the height of the joint hardening and diffusion
temperature and the duration of holding at the joint hardening and
diffusion temperature are determined by the respective steel grade
and the intended use of the relevant component. The height of the
joint hardening and diffusion temperature is therefore expediently
adapted essentially to the required hardening temperature of the
steel grade of the component, since insufficient full hardening
would occur at too low a temperature and undesirable structural
conditions would occur at too high a temperature. In experimental
investigations, a value of between 1050.degree. and 1150.degree. C.
has proven particularly suitable for the joint hardening and
diffusion temperature.
[0025] Depending on the steel grade and the desired properties of
the component, however, different durations of holding at the joint
hardening and diffusion temperature may be required for full
hardening and case hardening. So that both treatment methods can be
carried out completely, however, the duration of holding at the
joint hardening and diffusion temperature is expediently governed
by the longer of the two required holding durations, the required
hardening holding duration or the required diffusion holding
duration.
[0026] In the case where the required hardening holding duration is
greater than the required diffusion holding duration, the case
hardening carried out as plasma ion hardening may be terminated in
a simple way, before the end of the full hardening of the
component, by switching off the electrical voltage of the glow
discharge and by the suction-extraction of the plasma gas.
[0027] In the case, often arising, where the required hardening
holding duration is lower than the required diffusion holding
duration, the joint hardening and diffusion temperature is
advantageously lowered in order to avoid a coarsening of the core
structure of the component. This measure is based on the finding
that the dissolution of the carbon contained in the steel in the
form of carbides required for full hardening, is promoted
relatively highly with a rising temperature and relatively weakly
with a rising duration of holding at the hardening temperature, and
that a holding of the hardening temperature after the complete
dissolution of the carbides leads, however, to a coarsening of the
structure in the core region of the component which is associated
with undesirable embrittlement. To avoid these adverse effects, a
lowering of the joint hardening and diffusion temperature by about
20.degree. to 40.degree. C., as compared with the otherwise
conventional hardening temperature, has proven to be expedient.
[0028] The diffusion element predominantly considered for the
plasma ion hardening of the case of the component is carbon (C),
nitrogen (N) and a mixture of the two elements. Consequently,
during plasma ion hardening, the component is subjected to an
ionized gas emitting carbon and/or nitrogen.
[0029] Due to the enrichment of the case which is brought about
thereby, the steel in the case reacts differently to the subsequent
annealing treatment from the core region of the component.
Basically, with a rising annealing temperature of 520.degree. to
560.degree. C., the hardness reaches a maximum, in order to fall
again when the annealing temperature increases further. The exact
position of this maximum is dependent on the dissolved fractions of
carbon and/or nitrogen, the required annealing temperature rising
with an increasing solution fraction of the diffusion element.
[0030] To achieve as high a hardness as possible in the case,
therefore, the annealing temperature is adapted to the fractions of
the diffusion element which are dissolved in the steel, in such a
way that, after cooling, the greatest hardness is established in
the case of the component. For this purpose, it has proven
beneficial to set the annealing temperature at a value in the range
of 500.degree. to 600.degree. C. The case hardness achievable
thereby lies in the range of 60 to 66 HRC, whereas a hardness of 58
to 63 HRC is established in the core zone of the component.
[0031] To apply the method according to the invention, commercially
available heat-resistant rolling bearing steels, such as, for
example, the high-speed steel M50 according to AISI standard and
the high-speed steel S 18-0-1 according to DIN 17350, may be used
as source material.
[0032] The method according to the invention is preferably employed
in the production of bearing components, such as inner racing
rings, outer racing rings and rolling bodies of rolling bearings,
which are provided for mounting a mechanically and thermally highly
loaded shaft of a thermal engine, such as, for example, the rotor
shaft of a jet engine, a propeller turbine, a gas turbine or an
exhaust gas turbocharger of an internal combustion engine.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The invention is explained in more detail below, by way of
example, with reference to the accompanying drawings in which:
[0034] FIG. 1 shows a temperature/time graph of the method
according to the invention;
[0035] FIG. 2 shows an internal stress/depth graph;
[0036] FIG. 3 shows a hardness/depth graph determined by means of
measurements.
DETAILED DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 illustrates qualitatively the time sequence of a
thermal treatment according to the invention. In a first work step
1, the full hardening and the case hardening of the respective
component take place jointly. For this purpose, the component is
first heated uniformly to the joint hardening and diffusion
temperature T.sub.H+D in the range of between 1030.degree. and
1150.degree. C. above the upper transformation temperature
A.sub.C3, is then held at this temperature over the holding
duration .DELTA.t.sub.H+D under the action of a plasma emitting
carbon and/or nitrogen ions and is subsequently quenched. The
holding duration .DELTA.t.sub.H+D for the joint full hardening and
case hardening of the component is longer than the holding duration
.DELTA.t.sub.H, which would be required for a separate full
hardening 1' of the component, the temperature profile of which is
indicated by dashes.
[0038] To avoid a coarsening of the core structure of the component
brought about by the longer holding duration .DELTA.t.sub.H+D, the
joint hardening and diffusion temperature T.sub.H+D is lowered by
about 20.degree. to 40.degree. C., as compared with the hardening
temperature T.sub.H during separate full hardening 1'. After the
joint full hardening and case hardening, a low-temperature
treatment 2 of the component to about -190.degree. C. is carried
out. This is followed by the annealing 3 of the component at an
annealing temperature T.sub.A of the value of 500.degree. to
600.degree. C. below the lower transformation temperature
A.sub.C1.
[0039] Since the full hardening and the case hardening of the
component are carried out in the form of plasma ion hardening in a
joint work step at the relatively high joint hardening and
diffusion temperature T.sub.H+D above the upper transformation
temperature A.sub.C3, this results in greater hardening and,
because of the greater penetration depth of the diffusion element,
in a deeper hardening of the case of the component. As a result,
high internal compressive stresses are generated in the marginal
zone which greatly increase the fatigue strength of the component
advantageously.
[0040] In the graph of FIG. 2, the internal stresses in the case of
a component which consists of an AISI M50 high-speed steel are
compared with one another for two different thermal treatments. The
internal stress values have been determined experimentally by means
of X-ray diffractometry (XRD).
[0041] The internal stress profile of the upper curve 4 applies to
a generally conventional thermal treatment which involves full
hardening at 1100.degree. C. for 1 hour, triple annealing at
540.degree. C. for 2 hours and once-only annealing at 560.degree.
C. for 2 hours. This results in the case of the component in a
virtually constant internal tensile stress of 50 MPa which is
relatively unfavorable for the fatigue strength of the
component.
[0042] By contrast, the internal stress profile of the lower curve
5 applies to a thermal treatment according to the invention which
involves a simultaneous full hardening and case hardening in the
form of plasma carbonitriding at 1100.degree. C. for 3 hours,
triple annealing at 540.degree. C. for 2 hours and once-only
annealing at 560.degree. C. for 2 hours. This results in the case
of the component in an internal compressive stress of the order of
-100 MPa with peak values of about -130 MPa in a depth of 0.2 to
0.3 mm, which leads to a marked increase in the fatigue strength of
the component.
[0043] The corresponding profile of the hardness of the component
against its depth or its surface distance is depicted, for the
thermal treatment according to the invention, in the graph
according to FIG. 3 for three identical treatment tests. The
hardness has, at a depth of about 0.2 mm, a maximum value of 62 HRC
and falls continuously toward the core to a value of about 59 HRC.
This hardness profile ensures a high toughness and fatigue strength
of the component, simultaneous with a high wear resistance of the
surface.
List of Reference Symbols
[0044] 1 Joint full hardening and case hardening [0045] 1' Separate
full hardening [0046] 2 Lower-temperature treatment [0047] 3
Annealing [0048] 4 Internal stress profile (in conventional thermal
treatment) [0049] 5 Internal stress profile (in thermal treatment
according to the invention) [0050] A.sub.C1 Lower transformation
temperature [0051] A.sub.C3 Upper transformation temperature [0052]
t Time [0053] T.sub.A Annealing temperature [0054] T.sub.H
Hardening temperature [0055] T.sub.H+D Hardening and diffusion
temperature [0056] .DELTA..sub.tD Diffusion holding duration,
holding duration in separate case hardening [0057] .DELTA.t.sub.H
Hardening holding duration, holding duration in separate full
hardening [0058] .DELTA.t.sub.H+D Holding duration in joint full
hardening and case hardening
* * * * *