FIELD
The present disclosure relates to the field of material science. In particular, the
present disclosure relates to the field of process for hardening metallic elements.
DEFINITIONS
As used in the present disclosure, the following term is generally intended to ha5 ve
the meaning as set forth below, except to the extent that the context in which they
are used indicate otherwise.
The expression „quenching‟ used hereinafter in this specification refers to, but is
not limited to, rapid cooling of a heated component or workpiece in water, oil, gas
10 or any other suitable coolant, often but not always, intended to enhance certain
material properties such as hardness, crystal grain structure and the like.
The expression „shallow cryogenic treatment‟ used hereinafter in this
specification refers to, but is not limited to, cooling of a metallic element below
0oC, usually until the element attains a temperature of around -80oC to -145oC.
15 The time period required for which shallow cryogenic treatment is carried out
may vary depending upon the temperature of the metallic element, the type of
cryogen used, and so on. The most commonly used cryogen is liquid nitrogen.
The expression „L10‟ used hereinafter in this specification refers to, the hours in
service that 90% of identical bearings of a sufficiently large population, will
20 survive, when subject to identical dynamic load.
The expression „basic dynamic load rating (C)‟ used hereinafter in this
specification refers to, a constant load in magnitude and direction, which is radial
for radial bearings and axial and centrally acting for thrust bearings, that will
result in a rating life (L10) of 106 revolutions.
25 The expression „equivalent dynamic load rating (P)‟ used hereinafter in this
specification refers to, a hypothetical load, acting radial on radial bearings and
axially and centrically on thrust bearings, which when applied, will have the same
3
influence on bearing life as the actual loads to which a bearing is subjected.
Actual loads are a combination (vector summation) of the simultaneously acting
radial load Fr and axial load Fa that are constant in magnitude and direction.
P = X Fr + YFa
where X and Y are radial load factor for the bearing and axial load factor for 5 the
bearing respectively.
BACKGROUND
The background information herein below relates to the present disclosure but is
not necessarily prior art.
10 Bearing steel, also known as low alloy chrome steel, of the types SAE 52100 steel
SUJ2, as well as SUJ3, SUJ4 and SUJ5 steel, is the most widely used bearing
material for decades, primarily due to strength, higher hardness and low cost.
Besides, bearing steel is also used for making metallic element such as knives,
slitting rolls, forming rolls, disintegrator rolls spacers, guides, collars, pins,
15 spindles, precision instrument parts, collets, stamping tools, gears, seals, bushings,
sleeves, cylinder liners, dies, gauges, moulds, machine tool components, thrust
collars, engine parts, parts of hydraulic machinery, parts of pumps and clutch
faces. To impart higher hardness and to increase fatigue life, elements made of
bearing steel are subjected to martensitic hardening by performing heat treatment
20 processes. During quenching, all austenite is not converted to martensite, a small
amount of austenite still in the matrix and this austenite is called retained austenite
(for example, 8-15% in SAE 52100). In the hardening cycle, elements made of
bearing steel are heated above 727oC at which ferrite and pearlite phases start
converting to austenite and cementite starts to dissolve into austenite. During the
quenching cycle, at 115o25 C, most of the austenite gets converted to martensite.
Some of the austenite, which does not transfer completely, gets retained in the
matrix of the steel. Depending on the application, retained austenite in the
microstructure of bearing steel can have detrimental effect on the fatigue life of
4
the bearing. Particularly, the existence of retained austenite has a detrimental
effect, where the element is subjected to high load and high temperature. At room
temperatures, austenite is not a stable phase. Due to mechanical stresses acting on
the component during operation, Retained austenite gets converted into lower
bainite or ferrite. This transformation causes volume changes in the bearin5 g
components. In the case of machine components such as roller bearings, such
volume change influences clearances between bearing elements, thus leading to
vibration and noise. Ultimately, the bearing may fail prematurely.
SAE 52100 steel is used in normal and moderate operating temperature ranges,
usually in applications where temperature does not exceed 120o10 C.
Still, above operating temperature of 120oC (upto 200°), retained austenite in SAE
52100 steel is likely to convert to lower bainite, ferrite or cementite and thereby
undergo volume changes. To eliminate this possibility, thermal stabilization (also
called high temperature tempering) treatment is necessary to reduce retained
15 austenite content in elements made of bearing steel. Though thermal stabilization
prevents premature failure, the hardness of the elements is reduced due to thermal
stabilization. This affects the fatigue life (L10) of the elements.
There is, therefore, felt a need of a process for hardening a metallic element that
obviates the above mentioned drawbacks of conventional processes for hardening.
20 OBJECTS
Some of the objects of the present disclosure, which at least one embodiment
herein satisfies, are as follows:
It is an object of the present disclosure to ameliorate one or more problems of the
prior art or to at least provide a useful alternative.
25 An object of the present disclosure is to provide a process for hardening a metallic
element which improves wear resistance of the metallic element.
5
Another object of the present disclosure is to provide a process for hardening a
metallic element which improves hardness of the metallic element.
Still another object of the present disclosure is to provide process for hardening a
metallic element that improves fatigue life of the metallic element.
Other objects and advantages of the present disclosure will be more apparent fr5 om
the following description, which is not intended to limit the scope of the present
disclosure.
SUMMARY
The present disclosure envisages a process for hardening a metallic element.
10 The first step of the process is heating the metallic element above 727°C
preferably in the range of 830oC to 850oC. Preferably, this step is carried out in an
endothermic gas environment to prevent decarburization.
The second step of the process is quenching the heated metallic element until the
heated metallic element attains a second temperature within the range of 105oC to
115o15 C.
The third step of the process is treating the quenched metallic element shallowcryogenically
until the quenched metallic element attains a third temperature
preferably lesser than -145 °C. In an embodiment, this step is carried out in liquid
nitrogen for a time period ranging from 30 minutes to 45 minutes.
20 The fourth step of the process is tempering the shallow-cryogenically-treated
metallic element by heating the shallow-cryogenically-treated metallic element at
a fourth temperature within the range of 165oC to 170oC for a predetermined time
period.
In an embodiment, the metallic element is selected from a group consisting of a
25 component of a bearing, knives, slitting rolls, forming rolls, disintegrator rolls
spacers, guides, collars, pins, spindles, precision instrument parts, collets,
6
stamping tools, gears, seals, bushings, sleeves, cylinder liners, dies, gauges,
moulds, machine tool components, thrust collars, engine parts, parts of hydraulic
machinery, parts of pumps and clutch faces. In another embodiment, the
component of a bearing is selected from a group consisting of an inner race and an
outer race5 .
In accordance with another aspect of the disclosure, the disclosure relates to a
hardened metallic element made by a process described above.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
A process for hardening a metallic element of the present disclosure will now be
10 described with the help of the accompanying drawing, in which:
Figure 1 illustrates a flowchart of a conventional process for hardening a metallic
element which is an SAE 52100 bearing element;
Figure 2 illustrates a flowchart of a process for hardening a metallic element of
the present disclosure;
15 Figure 3 illustrates a flowchart of phase transformation occurring during the
process for hardening a metallic element of the present disclosure;
Figure 4 illustrates a graph depicting a comparative analysis of hardness of a
bearing component obtained by the process of the present disclosure and that
obtained by prior art; and
20 Figure 5 illustrates a bar chart representing life factors of a bearing element when
subjected to the conventional process for hardening and when subjected to the
process for hardening of the present disclosure.
DETAILED DESCRIPTION
Bearing steel, also known as low alloy chrome steel, of the types SAE 52100 steel
25 SUJ2, as well as SUJ3, SUJ4 and SUJ5 steel, is the most widely used bearing
material for decades, primarily due to strength, higher hardness and low cost.
7
Besides, bearing steel is also used for making metallic element such as knives,
slitting rolls, forming rolls, disintegrator rolls spacers, guides, collars, pins,
spindles, precision instrument parts, collets, stamping tools, gears, seals, bushings,
sleeves, cylinder liners, dies, gauges, moulds, machine tool components, thrust
collars, engine parts, parts of hydraulic machinery, parts of pumps and clutc5 h
faces. To impart higher hardness and to increase fatigue life, elements made of
bearing steel are subjected to martensitic hardening by performing heat treatment
processes. During quenching, all austenite is not converted to martensite, a small
amount of austenite still in the matrix and this austenite is called retained austenite
10 (for example, 8-15% in SAE 52100). In the hardening cycle, elements made of
bearing steel are heated above 727oC at which ferrite and pearlite phases start
converting to austenite and cementite starts to dissolve into austenite. During the
quenching cycle, at 115oC, most of the austenite gets converted to martensite.
Some of the austenite, which does not transfer completely, gets retained in the
15 matrix of the steel. Depending on the application, retained austenite in the
microstructure of bearing steel can have detrimental effect on the fatigue life of
the bearing. Particularly, the existence of retained austenite has a detrimental
effect, where the element is subjected to high load and high temperature. At room
temperatures, austenite is not a stable phase. Due to mechanical stresses acting on
20 the component during operation, Retained austenite gets converted into lower
bainite or ferrite. This transformation causes volume changes in the bearing
components. In the case of machine components such as roller bearings, such
volume change influences clearances between bearing elements, thus leading to
vibration and noise. Ultimately, the bearing may fail prematurely.
25 SAE 52100 steel is used in normal and moderate operating temperature ranges,
usually in applications where temperature does not exceed 120oC.
Still, above operating temperature of 120oC (upto 200°), retained austenite in SAE
52100 steel is likely to convert to lower bainite, ferrite or cementite and thereby
undergo volume changes. To eliminate this possibility, thermal stabilization (also
30 called high temperature tempering) treatment is necessary to reduce retained
8
austenite content in elements made of bearing steel. Though thermal stabilization
prevents premature failure, the hardness of the elements is reduced due to thermal
stabilization. This affects the fatigue life (L10) of the elements.
There is, therefore, felt a need of a process for hardening a metallic element that
obviates the above mentioned drawbacks of conventional processes for hardening5 .
A preferred embodiment of the process for hardening a metallic element
according to the present disclosure is described hereforth with reference to Figure
2. The process involves following steps.
The first step of the process involves heating the metallic element until the
metallic element attains a first temperature above 727o10 C, preferably within the
range of 830oC to 850oC. 727oC is the temperature at which plain carbon steel
starts undergoing transformation from ferrite to austenite. In an embodiment, this
step is carried out in an endothermic gas environment, to prevent decarburization
and oxidation on the surface of the element. The endothermic gas may comprise a
15 suitable composition of carbon monoxide, hydrogen, nitrogen and traces of water
and carbon dioxide, depending upon carbon potential of the furnace environment.
The second step of the process is quenching the heated metallic element until the
heated metallic element attains a second temperature within the range of 105oC to
115oC. Preferably, the step of quenching is performed using oil or salt bath.
20 The third step of the process is treating the quenched metallic element shallowcryogenically
until the quenched metallic element attains a third temperature. In
an embodiment, this step is carried out for a time period ranging from 30 minutes
to 45 minutes. The shallow cryogenic treatment can be carried out using a cryogen
such as liquid nitrogen. The temperature to which the metallic element may be
cryo-cooled may be below-120 o25 C.
The fourth step of the process is tempering the shallow-cryogenically-treated
metallic element by heating the shallow-cryogenically-treated metallic element at
9
a fourth temperature within the range of 165oC to 170 oC for a predetermined time
period.
For illustrative purpose, in a first experiment, one batch of inner rings and outer
rings of a deep groove ball bearing made out of SUJ2 steel was subjected to
conventional process for hardening, and in a second experiment, another batch 5 h of
samples was subjected to the process for hardening according to this disclosure.
Experiment 1
In a first experiment, a set of inner rings and outer rings of a 6203 deep groove
ball bearing made out of SUJ2 steel was subjected to a conventional process for
10 hardening, shown in the flowchart in Figure 1, which involves heating for
hardening, quenching and tempering only. In particular, for heating for
martensitic hardening, the rings were heated until they attained a temperature
above 727oC, preferably within the range of 830 oC and 850oC, preferably in an
endothermic gas environment (which is above the critical temperature at which
15 ferrite and pearlite matrix starts converting to austenite, i.e., austenitizing
temperature) for a desired time based on the cross-sectional thickness of the rings
(step 12). A soft microstructural phase of austenite exists at this stage. Later, oil
quenching was performed by cooling the heated rings upto 115oC (step 14).
During quenching, when temperature reaches below the starting point of
20 martensite conversion (Ms), austenite starts converting to martensite. Generally
all austenite are converted to martensite as, at the finishing point of martensite
conversion (Mf), which is generally is lower than the room temperature. Hence,
during quenching some percentage of austenite is thus left in the structure and is
called retained austenite (RA). Generally, SUJ2 steel contains 8-15% RA. After
25 quenching, at 115°C, the hardness remained around 62HRc. Retained austenite
being an unstable microstructure, due to mechanical stresses acting on the
component during operation, austenite would get converted into lower bainite or
ferrite or cementite matrix. This transformation would cause volumetric distortion
in the component, thus causing vibration, which would lead to premature failure.
10
After quenching, the rings were tempered at around 170°C for thermal
stabilization (step 16), which converts some of the retained austenite to
bainite/cementite, thus averting the danger of premature failure as described above
due to transformation of retained austenite. Still, some austenite is retained in the
structure. After tempering at 170°C, the hardness remained around 60HRc, whic5 h
is evidently lower than that obtained before thermal stabilization is performed.
This adversely affects fatigue life of the component.
Experiment 2
In a second experiment, the process for hardening a metallic element in
10 accordance with an embodiment of the present disclosure is described with
reference to figure 2, 3 and 4, wherein another set of rings of a 6203 deep groove
ball bearing made out of SUJ2 steel was subjected to the process for hardening
according to the present disclosure.
In this process, the steps of heating (step 22) and quenching (step 24) were
15 performed at exactly the same conditions as that in the conventional process for
hardening described above. In particular, for heating for martensitic hardening, the
rings were placed in an endothermic gas environment until the rings attained a
temperature within the range of 830oC to 850oC (which is above the critical
temperature of 727oC for plain-carbon steel at which ferrite and pearlite starts
20 converting to austenite, i.e., austenitizing temperature) for a desired time based on
the cross-sectional thickness of the rings (step 22). However, it is to be noted that
the bearing component can be heated to any other suitable high temperature above
the austenitizing temperature of the steel. Later, oil quenching was performed on
the rings upto a temperature within the range of 105oC to 115oC (step 24). After
25 the step of quenching (step 24), shallow cryogenic treatment was performed on
the rings (step 26). Preferably, liquid nitrogen is used for this step. The
temperature upto which the shallow cryogenic treatment cools the rings is less
than -120oC. The rings were kept in liquid nitrogen to convert retained austenite
11
for a period ranging from 30 minutes to 45 minutes. Further, tempering was
performed at around 170oC on the rings (step 28).
As illustrated in the flowchart of phase transformation occurring during a process
for hardening of Figure 3, the step of shallow cryogenic treatment on the bearing
component converts almost all retained austenite to ferrite/bainite (step 36)5 .
Hardness of the rings was around 64 HRc at the end of this step of cryogenic
treatment, which is higher than that obtained by heating at a temperature above
austenitizing temperature followed by quenching (62HRc) or heating at a
temperature above austenitizing temperature followed by quenching followed by
10 thermal stabilization (60HRc). The final microstructure obtained by the process of
the present disclosure does not practically contain any retained austenite (step 38).
Figure 4 illustrates a graph in which hardness values in HRc against distance from
the raceway in micron obtained by various processes are compared. Line 1
indicates hardness obtained by the process for hardening in accordance with an
15 embodiment of the present disclosure. Line 2 indicates hardness obtained by the
conventional process for hardening. Line 3 indicates hardness of a bearing
component obtained by the conventional process for hardening followed by
tempering. It can be inferred from this comparative analysis that the hardness of
the bearing component obtained by the process for hardening of the present
20 disclosure for various distances from the raceway is higher than that obtained by
the conventional processes for hardening.
Therefore, without any retained austenite present in the microstructure as well as
by having a higher hardness, a bearing component in particular, and a metallic
element in general, treated by the process for hardening of the present disclosure
25 has an improved wear resistance and a higher fatigue life at the same time.
Endurance test details and results:
12
An endurance test was performed on both type of rings, i.e., one subjected to
conventional process for hardening and the other subjected to a process of the
present disclosure. The parameters of the test are enlisted in the following table.
Bearing type 6203
Bearing size
(outer and inner diameter in mm)
17 & 40
Basic Dynamic Load Rating (Cr) 9.6 kN
Applied load Radial Load, Fr (35 % of Cr) 3.36 kN
Operating temperature 60°C
Speed 3000 RPM
Lubrication Enklo 68®
Theoretical L10 life 130 hours
Table 1
Life factor of a bearing is defined 5 d as
where,
10 and
13
where,
n: rotational speed, min-1;
C: Basic dynamic load rating, N {kgf}; and
P: Equivalent dynamic load rating, N {kgf}.
As illustrated in Figure 5, the life factor for a ring of bearing type 6203 5 obtained
by the conventional process for hardening was 3 and that by the process of the
present disclosure was 13. Therefore, a marked improvement in fatigue life of the
rolling contact is observed using the process for hardening according to the
present disclosure.
10 The comparison between the results obtained by the two experiments conclusively
confirms that the life of a metallic component is enhanced significantly by treating
it with the process of the present disclosure, as compared with a conventional
process of heat treatment. This result is anticipated, as the process of the present
disclosure ensures complete conversion of the soft and unstable austenite phase to
15 the relatively hard and stable phases of tempered martensite and bainite (ferrite
and cementite), thereby eliminating the possibility of distortion occurring in the
metallic component over a large number of loading and unloading cycles.
TECHNICAL ADVANCES AND ECONOMICAL SIGNIFICANCE
The present disclosure described herein above has several technical advantages
20 including, but not limited to, the realization of a process for hardening a metallic
element, which improves wear resistance and hardness without reducing fatigue
life of the metallic element.
The embodiments herein and the various features and advantageous details thereof
are explained with reference to the non-limiting embodiments in the following
25 description. Descriptions of well-known components and processing techniques
are omitted so as to not unnecessarily obscure the embodiments herein. The
examples used herein are intended merely to facilitate an understanding of ways
14
in which the embodiments herein may be practiced and to further enable those of
skill in the art to practice the embodiments herein. Accordingly, the examples
should not be construed as limiting the scope of the embodiments herein.
The foregoing description of the specific embodiments will so fully reveal the
general nature of the embodiments herein that others can, by applying curre5 nt
knowledge, readily modify and/or adapt for various applications such specific
embodiments without departing from the generic concept, and, therefore, such
adaptations and modifications should and are intended to be comprehended within
the meaning and range of equivalents of the disclosed embodiments. It is to be
10 understood that the phraseology or terminology employed herein is for the
purpose of description and not of limitation. Therefore, while the embodiments
herein have been described in terms of preferred embodiments, those skilled in the
art will recognize that the embodiments herein can be practiced with modification
within the spirit and scope of the embodiments as described herein.
15 Throughout this specification the word “comprise”, or variations such as
“comprises” or “comprising”, will be understood to imply the inclusion of a stated
element, integer or step, or group of elements, integers or steps, but not the
exclusion of any other element, integer or step, or group of elements, integers or
steps.
20 The use of the expression “at least” or “at least one” suggests the use of one or
more elements or ingredients or quantities, as the use may be in the embodiment
of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has
been included in this specification is solely for the purpose of providing a context
25 for the disclosure. It is not to be taken as an admission that any or all of these
matters form a part of the prior art base or were common general knowledge in the
field relevant to the disclosure as it existed anywhere before the priority date of
this application.
15
The numerical values mentioned for the various physical parameters, dimensions
or quantities are only approximations and it is envisaged that the values
higher/lower than the numerical values assigned to the parameters, dimensions or
quantities fall within the scope of the disclosure, unless there is a statement in the
specification specific to the contrary5 .
While considerable emphasis has been placed herein on the components and
component parts of the preferred embodiments, it will be appreciated that many
embodiments can be made and that many changes can be made in the preferred
embodiments without departing from the principles of the disclosure. These and
10 other changes in the preferred embodiment as well as other embodiments of the
disclosure will be apparent to those skilled in the art from the disclosure herein,
whereby it is to be distinctly understood that the foregoing descriptive matter is to
be interpreted merely as illustrative of the disclosure and not as a limitation.

WE CLAIM:
1. A process for hardening a metallic element, said process comprising the
following steps:
a. heating said metallic element until said metallic element attains a first
temperature within the range of 830oC to 8505 oC;
b. quenching the heated metallic element until said heated metallic
element attains a second temperature within the range of 105oC to
115oC;
c. treating the quenched metallic element shallow-cryogenically under
10 liquid nitrogen or any other method; and
d. tempering the shallow-cryogenically-treated metallic element by
heating said shallow-cryogenically-treated metallic element at a fourth
temperature within the range of 165oC to 170oC for a predetermined
time period.
15 2. The process as claimed in claim 1, wherein said step (a) is carried out in an
endothermic gas environment.
3. The process as claimed in claim 1, wherein step (c) is carried out in liquid
nitrogen for a time period ranging from 30 minutes to 45 minutes.
4. The process as claimed in claim 1, wherein said predetermined time period
20 ranges from 105 minutes to 115 minutes.
5. The process as claimed in claim 1, wherein said metallic element is
selected from a group consisting of components of a bearing, knives,
slitting rolls, forming rolls, disintegrator rolls spacers, guides, collars, pins,
spindles, precision instrument parts, collets, stamping tools, gears, seals,
25 bushings, sleeves, cylinder liners, dies, gauges, moulds, machine tool
17
components, thrust collars, engine parts, parts of hydraulic machinery,
parts of pumps, and clutch faces.
6. The process as claimed in claim 5, wherein said components of a bearing
are selected from a group consisting of an inner race, and an outer race.
7. A hardened metallic element made in accordance with the process a5 s
claimed in any of the preceding claims.
8. A metallic element as claimed in claim 7, being an element selected from a
group consisting of components of a bearing, knives, slitting rolls, forming
rolls, disintegrator rolls spacers, guides, collars, pins, spindles, precision
10 instrument parts, collets, stamping tools, gears, seals, bushings, sleeves,
cylinder liners, dies, gauges, moulds, machine tool components, thrust
collars, engine parts, parts of hydraulic machinery, parts of pumps, and
clutch faces.