WO2014019670A1

WO2014019670A1 - Low temperature heat treatment for steel alloy

Info

Publication number: WO2014019670A1
Application number: PCT/EP2013/002238
Authority: WO
Inventors: Mohamed Sherif
Original assignee: Aktiebolaget Skf
Priority date: 2012-07-30
Filing date: 2013-07-29
Publication date: 2014-02-06

Abstract

A method of heat-treating a steel alloy comprising: (i) providing a steel alloy composition comprising: from 0.5 to 1.2 wt.% carbon, from 1 to 2 wt.% silicon, from 0.25 to 2.2 wt.% manganese, from 0.85 to 2 wt.% chromium, from 0.5 to 5 wt.% cobalt, from 0.1 to 0.6 wt.% molybdenum, optionally up to 2 wt.% aluminium, and the balance iron together with unavoidable impurities; (ii) heating the composition to a temperature of from 800 to 950 °C to at least partially austenise the composition; (iii) quenching the composition to a first temperature T1, wherein 0.7M_S≤ T1 ≤1.3M_S, M_s being the martensite start temperature of the composition; and (iv) heating the composition to a second temperature T2 below the bainite start temperature of the composition B_s.

Description

LOW TEMPERATURE HEAT TREATMENT FOR STEEL ALLOY

Technical Field

The present invention relates generally to the field of metallurgy. More specifically, the present invention relates to a method of heat-treating a steel alloy, which may be used in the manufacture of, for example, bearings. Background

Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer raceways and a plurality of rolling elements (for example balls and/or rollers) disposed therebetween. For long-term reliability and performance it is important that the various elements have a high resistance to rolling fatigue, wear and creep.

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 or semifinished components so as to improve, for example, wear resistance and fatigue resistance. A number of surface or case hardening processes are known for improving rolling contact fatigue resistance.

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. For through-hardened bearing 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.

The martensite through-hardening process involves austenitising the steel prior to quenching below the martensite start temperature. The steel may then be iow- temperature tempered to stabilize the microstructure.

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.

Bainitic steel structures are produced by the transformation of austenite to bainitic-ferrite at intermediate temperatures of typically from 190 to 500^°C. The cooling of the austenite leads to a microstructure comprising ferrite, carbides and retained austenite. Bainite itself comprises a structure of supersaturated ferrite containing particles of carbide, the dispersion of the latter depending on the formation temperature. The hardness of bainite is usually somewhere

intermediate between that of pearlite and martensite.

The steel known as SP10 has the following chemical composition: Fe-0.8C-1.5Si- 2Mn-1AI-1 Cr-0.25Mo-1.5Co in wt.%. Austenisation followed by bainite hardening (200 °C, 72 hours) results in a fine microstructure comprising retained austenite and bainitic ferrite. However, the hardness and dimensional stability of this alloy are deemed too low for bearing applications. It is an object of the present invention to address some of the problems associated with the prior art, or at least to provide a commercially useful alternative thereto. Summary of the Invention

According to a first aspect, there is provided a method of heat-treating a steel alloy comprising:

(i) providing a steel alloy composition comprising:

from 0.5 to 1.2 wt.% carbon,

from 1 to 2 wt.% silicon,

from 0.25 to 2.2 wt.% manganese,

from 0.85 to 2 wt.% chromium,

from 0.5 to 5 wt.% cobalt,

from 0.1 to 0.6 wt.% molybdenum,

optionally up to 2 wt.% aluminium,

and the balance iron together with unavoidable impurities; (ii) heating the composition to a temperature of from 800 to 950 °C to at least partially austenise the composition;

(iii) quenching the composition to a first temperature T1 , wherein 0.7M_S≤ T1 < 1.3M_S, Ms being the martensite start temperature of the composition; and

(iv) heating the composition to a second temperature T2 below the bainite start temperature of the composition B_s.

The resulting alloy exhibits high hardness and/or dimensional stability. This means that it can usefully find application in the manufacture of, for example, a bearing component such as, for example, an inner or outer raceway.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects 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.

The term "martensite start temperature" as used herein refers to the temperature at which the transformation from austenite to martensite begins on cooling. The martensite start temperature is typically denoted M_s. The term "bainite start temperature" as used herein refers to the highest temperature at which ferrite can transform by a displacive transformation. The bainite start temperature is typically denoted B_s.

The microstructure of the resulting steel alloy typically comprises nano-structured bainitic ferrite and retained austenite. The microstrucure is typically substantially carbide-free. The microstructure may optionally contain some tempered martensite.

In greater detail, the microstructure of the resulting steel alloy typically comprises at least 60 vol.% bainite, more typically at least 80 vol.% bainite, still more typically at least 90 vol.% bainite. The bainite is preferably lower bainite and preferably has a very fine structure. Steps (iii) and (iv) of the method of the present invention typically result in bainite transformation. This bainite

transformation is typically carried out at a temperature of less than 300 °C, more typically less than 280 °C. One result of the low transformation temperature is that the plates of bainitic-ferrite are very fine. In particular, the material preferably has a microstructure comprising plates of bainitic-ferrite of less than 200 nm, typically from 10 to 100 nm, more typically from 20 to 80 nm. The plates of bainitic-ferrite are typically interspersed with retained austenite. The bainite typically forms at least 60% of the microstructure (by volume), more typically at least 80%, still more typically at least 90%. The microstructure may also contain small carbide, nitride and/or carbo-nitride precipitates, for example nano-scale precipitates, typically 5 -30 nm average size. Any such precipitates typically make up no more than 5 vol%, more typically no more than 3 vol% of the microstructure, for example from 0.5 to 3 vol%. In one embodiment, the structure is free or at least essentially free of carbides, nitrides and/or carbo-nitrides. The structure of the steel alloys may be determined by conventional microstructural characterisation techniques such as, for example, optical microscopy, TEM, SEM, AP-FIM, and X-ray diffraction, including combinations of two or more of these techniques.

Following steps (i) to (iii), the steel alloy composition is heated to a second temperature T2 below the bainite start temperature of the composition B_s. This heating step (iv) results in an acceleration of the bainite transformation kinetics. As a result of this acceleration, for the same transformation time at temperature, the final steel alloy typically contains less retained austensite. This results in increased strength and hardness, and better dimensional stability. The

dimensional stability is critical when the steel alloy is in the form of a bearing component, which operate at warm-to-elevated temperatures, typically 80°C and above. The amount of retained austenite is typically less than 20 vol.%, more typically less than 10 vol.%, even more typically less than 8 vol%. IN one embodiment the amount of retained austenite is about 6 vol%.

In addition, the acceleration of the bainite transformation kinetics may result in a shorter transformation time for a given retained austenite content in the final alloy. For example, in comparison to a conventional heat treatment (for example, austenisation followed by heating at 200 °C for 72 hours), the overall bainite transformation time of the method of the present invention may be reduced by at least 12 hours. This may result in significant cost and time savings.

The steel alloy composition comprises from 0.5 to 1.2 wt% carbon, preferably from 0.7 to 0.9 wt% carbon, more preferably about 0.8 wt% carbon. In

combination with the other alloying elements, this results in the desired fine bainitic structure. Carbon acts to lower the bainite transformation temperature so that a fine structure is achievable. When the carbon content is higher than 1.2 wt% there is a reduction in the maximum volume fraction of the bainitic ferrite portion of the microstructure. When the carbon content is lower than 0.5 wt% the alloys have a higher martensite start temperature.

The steel alloy composition comprises from 1 to 2 wt% silicon, preferably from 1.3 to 1.7 wt% silicon, more preferably about 1.5 wt% silicon. In combination with the other alloying elements, this results in the desired fine bainitic structure with a minimum amount of retained austenite. Silicon helps to suppress the

precipitation of cementite and carbide formation. However, too high a silicon content may result in undesirable surface oxides and a poor surface finish. For this reason, the maximum silicon content is 2 wt%.

The alloy composition comprises from 0.25 to 2.2 wt% manganese, preferably from 1.8 to 2.2 wt% manganese, more preferably about 2 wt% manganese.

Manganese acts to increase the stability of austenite relative to ferrite. However, manganese levels above 2.2 wt% may serve to increase the amount of retained austenite and to decrease the rate of transformation to bainite.

The steel alloy composition comprises from 0.85 to 2 wt% chromium, preferably from 0.9 to 1.1 wt% chromium, more preferably about 1 wt% chromium.

Chromium acts to increase hardenability and reduce the bainite start

temperature. Chromium may also be beneficial in terms of corrosion resistance.

The steel alloy composition comprises from 0.5 to 5 wt% cobalt. In one

embodiment, the steel alloy composition preferably comprises from 1.3 to 1.7 wt% cobalt, more preferably about 1.5 wt% cobalt. In an alternative embodiment, the steel alloy composition preferably comprises from 3.5 to 4.5 wt% cobalt, more preferably about 4 wt% cobalt. Cobalt has been found to improve the corrosion resistance of the steel alloy. This is very important if the steel alloy is used in bearing components for wind turbines or marine pods, for example. Such bearings may become contaminated by sea water, which can drastically reduce the service life of the bearing. The addition of cobalt further increases the rate of superbainite formation. The steel composition comprises from 0.1 to 0.6 wt% molybdenum, preferably from 0.2 to 0.3 wt% molybdenum, more preferably about 0.25 wt% molybdenum. Molybdenum acts to avoid austenite grain boundary embrittlement owing to impurities such as, for example, phosphorus. Molybdenum also acts to increase hardenability and reduce the bainite start temperature. The molybdenum content in the alloy is preferably no more than about 0.6 wt%, otherwise the austenite transformation into bainitic ferrite may cease too early, which can result in significant amounts of austenite being retained in the structure.

The steel alloy composition may optionally comprise up to 2 wt% aluminium, preferably from 0.9 to 1.1 wt% aluminium, more preferably about 1 wt%

aluminium. Aluminium has been found to improve the intrinsic toughness of the bearing component, possibly due to it suppressing carbide formation. Aluminium may also serve as a deoxidiser. However, the use of aluminium requires stringent steel production controls to ensure cleanliness and this increases the processing costs. In one embodiment, the steel alloy composition comprises substantially no aluminium. It will be appreciated that the steel alloy referred to herein may contain

unavoidable impurities, although, in total, these are unlikely to exceed 0.5 wt% of the composition. Preferably, the alloys contain unavoidable impurities in an amount of not more than 0.3 wt% of the composition, more preferably not more than 0.1 wt% of the composition. Phosphorous and sulphur are preferably kept to a minimum.

The steel alloy composition may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements that are mandatory other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence.

In a preferred embodiment the steel alloy composition may comprise: from 0.7 to 0.9 wt.% carbon,

from 1.3 to .7 wt.% silicon,

from 1.8 to 2.2 wt.% manganese,

from 0.9 to 1.1 wt.% chromium,

from 1.3 to 1.7 wt.% cobalt,

from 0.2 to 0.3 wt.% molybdenum,

optionally from 0.9 to 1.1 wt.% aluminium,

and the balance iron together with unavoidable impurities. In step (ii), the composition is heated to a temperature of 800 to 950 °C to at least partially austenise the composition. In a typical embodiment, the composition may be heated to a temperature of from 850 to 925 °C, more typically from 880 to 920 °C. The composition is typically held at such temperatures for at least 15 minutes, typically less than 60 minutes, more typically for about 30 minutes.

In one embodiment, T1 is above the martensite start temperature. This may result in deformation of the residual austenite, i.e. the induction internal stresses. During the subsequent step (iv), the bainite transformation may be substantially accelerated. Accordingly, in comparison to a conventional bainite transformation step of heating at 200 °C for 72 hours, the overall bainite transformation time of the method described herein may be particularly shortened.

In this embodiment, T1 is preferably from 190 to 210 °C, more preferably about 200 °C. Such temperatures are suitable for deforming the retained austenite.

In this embodiment, during step (iii), the composition is preferably held at T1 for at least 5 hours, more preferably from 12 to 36 hours, even more preferably from 12 to 24 hours, still even more preferably from 12 to 16 hours. The time at which the composition is held at T1 is preferably minimised in view of cost. Holding the composition at T1 for at least 5 hours, preferably at least 12 hours, may result in particularly advantageous levels of retained austenite deformation.

In an alternative embodiment, T1 is below the martensite start temperature. This may result in the presence of small amounts of martensite in the final steel alloy, thereby increasing the strength and hardness. In addition, the martensitic transformation may result in an increase in austenite deformation. Since the martensitic transformation is immediate, it is not necessary to hold the alloy composition at T1 for long periods of time. Accordingly, the composition is typically held at T1 for less than 30 minutes, preferably about 15 minutes or less. In this embodiment, the microstructure of the resulting steel alloy preferably comprises from 10 to 50 vol% martensite, more preferably from 15 to 40 vol% martensite, the remainder being bainitic ferrite and retained austenite. T2 at its upper limit may be just below the bainite start temperature. T2 is preferably from 50 to 150 °C below the bainite start temperature, more preferably from 90 to 110 °C below the bainite start temperature. T2 is preferably from 200 to 280 °C, more preferably from 210 to 260 °C, even more preferably about 250 °C. Lower temperatures may result in only a minimal reduction in the retained austenite levels of the resulting steel alloys. Higher temperatures are preferably avoided in view of cost and the somewhat weaker structure obtained.

During step (iv) the composition is typically heated isothermally. The method may further comprise (v) cooling the composition to room

temperature.

Preferably the method further comprises (vi) cooling the composition to a temperature of less than 0°C. This may reduce the austenite content of the resulting steel alloy, thereby increasing its strength, hardness and dimensional ¹ stability.

Preferably the method further comprises (vii) tempering at a temperature of from 100 to 200 °C for at least one hour. Such tempering may serve to reduce the occurrence of cracking in the resulting steel alloy. Preferably, such tempering is carried out after step (vi). In a preferred embodiment, the composition is double or triple tempered with freezing (step (vi)) in between tempering steps. When both steps (vi) and (vii) are carried out, the steel alloy composition is typically allowed to cool to room temperature before subsequent freezing. In addition, the final tempering step is typically followed by air cooling to room temperature.

The method preferably further comprises (viii) subjecting the steel alloy to a surface finishing technique. The hardened bearing steel components may optionally be burnished, especially the raceways, followed by tempering and air- cooling. Afterwards, the bearing steel components are finished by means of hard- turning and/or grinding operations such as lapping and honing.

The burnishing and tempering operations may cause the yield strength of the affected areas to increase dramatically with significant improvement in hardness, compressive residual stress and better resistance to rolling contact fatigue.

The steel alloy composition may be a bearing steel alloy. The steel alloy may be in the form of a bearing component, preferably at least one of a rolling element, an inner ring, and an outer ring.

In a further aspect, the present invention provides a steel alloy produced according to the method described herein. In a further aspect, the present invention provides a method of heat-treating a steel component made from an alloy composition that comprises:

from 0.5 to 1.2 wt.% carbon,

from 1 to 2 wt.% silicon,

from 0.25 to 2.2 wt.% manganese,

from 0.85 to 2 wt.% chromium,

from 0.5 to 5 wt.% cobalt,

from 0.1 to 0.6 wt.% molybdenum,

optionally up to 2 wt.% aluminium,

and the balance iron, together with unavoidable impurities;

the method comprising a step of austentising the steel component,

characterized in that the method comprises further steps of:

(a) quenching the austentised steel component to a temperature around the martensite-start (Ms) temperature of M_s +/- 30%; (b) heating the quenched component to a temperature slightly below the bainite-start (Bs) temperature of the residual austenite and holding it at this temperature until bainitic transformation has ceased; and

(c) cooling the bainitically transformed component to room temperature.

The invention will now be described with reference to the following non-limiting Figures, in which:

Figure 1 is a flowchart of an embodiment of the method of the present invention.

Referring to Figure 1 , the following steps were carried out:

(i) providing a steel alloy composition comprising:

from 0.5 to 1.2 wt.% carbon,

from 1 to 2 wt.% silicon,

from 0.25 to 2.2 wt.% manganese,

from 0.85 to 2 wt.% chromium,

from 0.5 to 5 wt.% cobalt,

from 0.1 to 0.6 wt.% molybdenum,

optionally up to 2 wt.% aluminium,

and the balance iron together with unavoidable impurities;

(ii) heating the composition to a temperature of from 800 to 950 °C to at least partially austenise the composition;

(iii) quenching the composition to a first temperature T1 , wherein 0.7M_S≤ T1 < 1.3M_S, M_s being the martensite start temperature of the composition; and

The invention will now be described with reference to the following non-limiting Examples. Example 1

A steel alloy composition was prepared having the following chemical

composition: Fe-0.8C-1.5Si-2Mn-1AI-1Cr-0.25Mo-1.5Co in wt.%. The

composition was then heated to 920 °C and held for 30 minutes, followed by being quenched to a temperature of 200 °C and held at that temperature for 72 hours. The temperature was then increased to 250 °C and the composition was held at that temperature for 100 hours before being cooled to room temperature. Comparative Example 1

A steel alloy was prepared in a similar manner to Example 1. However, the alloy composition was cooled to room temperature after being held at 200 °C for 72 hours. In other words, the step of increasing the temperature to 250 °C was omitted.

The hardness values and retained austenite levels of the alloys of Example 1 and Comparative Example 1 were calculated, and the results are set out in Table 1.

Table 1 : HV1 (Vicker's) hardness values and retained austenite contents

(calculated by X-ray diffraction) of Example 1 and Comparative Example 2. For comparison, values are also shown for 100CrMnMoSi8-4-6 having undergone the heating regime of Comparative Example 1 (1) and having undergone the heating regime of Example 1 (2).

Example 2 A steel alloy composition in the form of a ring was prepared having the following chemical composition: Fe-0.8C-1.5Si-2Mn-1AI-1 Cr-0.25Mo-1.5Co in wt.%. The composition was then heated to 900 °C and held for 15 minutes, followed by being quenched to a temperature of 200 °C and held at that temperature for 72 hours. The temperature was then increased to 250 °C and the composition was held at that temperature for 24 hours before being cooled to room temperature.

Comparative Example 2 A steel alloy ring was prepared in a similar manner to Example 2. However, the alloy composition was cooled to room temperature after being held at 200 °C for 72 hours. In other words, the step of increasing the temperature to 250 °C was omitted. Example 3

A steel alloy composition in the form of a ring was prepared having the following chemical composition: Fe-0.8C-1.5Si-2Mn-1AI-1Cr-0.25Mo-1.5Co in wt.%. The composition was then heated to 840 °C and held for 30 minutes, followed by being quenched to a temperature of 190 °C and held at that temperature for 72 hours. The temperature was then increased to 235 °C and the composition was held at that temperature for 20 hours. The temperature was then increased to 250 °C and held at that temperature for 4 hours. The temperature was then increased to 300 °C and held at that temperature for 4 hours. The composition was then cooled to room temperature.

Example 4

A steel alloy was prepared in a similar manner to Example 3 with the exception of a prior triple QT (quench and temper) heat treatment. Each QT cycle was carried out as follows: 840 °C/30 minutes + 60°C (oil bath)/10 minutes + 10 °C (water bath)/10 minutes + 390°C/30 minutes. The dimensional stabilities of the rings of Example 2-4 and Comparative Example 2 were then tested. Two rings were tested in each case. The outer diameters (OD) of the rings were measured at room temperature to a precision of less than 1 μηι. The rings were then placed in a furnace at 235 °C for varying lengths of time. After cooling to room temperature, the outer diameters were measured again. The results are shown in Table 2.

Table 2: Dimensional stability test results of Examples 2-4 and Comparative Example 2. Results for 100CrMo7 and 100CrMo7-3 steels are also given for comparison. The hardness values and levels of retained austenite were also calculated for Examples 2-4 and Comparative Example 2, and the results are shown in Table 3.

Table 3: HRC (Rockwell) Hardness and retained austenite contents. ^*Calculated by X-ray diffraction. **Converted from HV 0 measurements (3 measurements) according to D30 Ed. 2, Page 252,3.

The levels of retained austenite in Example 3 and 4 are particularly low. Without being bound by theory, it is considered that the reduction of the retained austensite content in the final hardened structure could be partially attributed to austenising at 840 °C, which may not be adequate to dissolve the carbides entirely and guarantee a fully austenitic structure prior to quenching, which has been the conventional process route with such types of steel.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

Claims

CLAIMS:

1. A method of heat-treating a steel alloy comprising: (i) providing a steel alloy composition comprising:

from 0.5 to 1.2 wt.% carbon,

from 1 to 2 wt.% silicon,

from 0.25 to 2.2 wt.% manganese,

from 0.85 to 2 wt.% chromium,

from 0.5 to 5 wt.% cobalt,

from 0.1 to 0.6 wt.% molybdenum,

optionally up to 2 wt.% aluminium,

2. The method of claim 1 , wherein T1 is above the martensite start temperature.

3. The method of claim 1 or claim 2, wherein T1 is from 190 to 210 °C, preferably about 200 °C. 4. The method of any preceding claim, wherein during step (iii) the composition is held at T1 for at least 5 hours, preferably from 12 to 36 hours, more preferably from 12 to 24 hours, even more preferably from 12 to 16 hours.

5. The method of claim 1 , wherein T1 is below the martensite start temperature Ms.

6. The method of any preceding claim, wherein T2 is from 50 to 150 °C below the bainite start temperature, preferably from 90 to 110 °C below the bainite start temperature.

7. The method of any preceding claim, wherein T2 is from 220 to 280 °C, preferably from 240 to 260 °C, more preferably about 250 °C.

8. The method of any preceding claim, wherein during step (iv) the composition is heated isothermally.

9. The method of any preceding claim, further comprising:

(v) cooling the composition to room temperature.

10. The method of any preceding claim further comprising:

(vi) cooling the composition to a temperature of less than 0°C. 11. The method of any preceding claim further comprising:

(vii) tempering at a temperature of from 100 to 200 °C for at least one hour.

12. The method of any preceding claim further comprising:

(viii) subjecting the steel alloy to a surface finishing technique. 3. The method of any preceding claim, wherein the alloy comprises: from 0.7 to 0.9 wt.% carbon,

from 1.3 to 1.7 wt.% silicon,

from 1.8 to 2.2 wt.% manganese,

from 0.9 to 1.1 wt.% chromium,

from 1.3 to 1.7 wt.% cobalt,

from 0.2 to 0.3 wt.% molybdenum, and

optionally from 0.9 to 1.1 wt.% aluminium.

14. The method of any preceding claim, wherein the steel alloy is in the form of a bearing component, preferably at least one of a rolling element, an inner ring, and an outer ring.

15. A steel alloy produced according to the method of any preceding claim.

16. A method of heat-treating a steel component made from an alloy

composition that comprises:

from 0.5 to 1.2 wt.% carbon,

from 1 to 2 wt.% silicon,

from 0.25 to 2.2 wt.% manganese,

from 0.85 to 2 wt.% chromium,

from 0.5 to 5 wt.% cobalt,

from 0.1 to 0.6 wt.% molybdenum,

optionally up to 2 wt.% aluminium,

and the balance iron, together with unavoidable impurities;

the method comprising a step of austenitising the steel component,

characterized in that the method comprises further steps of:

(a) quenching the austentised steel component to a temperature around the martensite-start (M_s) temperature of M_s +/- 30%;

(b) heating the quenched component to a temperature slightly below the bainite-start (Bs) temperature of the residual austenite and holding it at this temperature until bainitic transformation has ceased; and

(c) cooling the bainitically transformed component to room temperature.