EP3155134A1

EP3155134A1 - Method of heat treatment of bearing steel

Info

Publication number: EP3155134A1
Application number: EP15741719.7A
Authority: EP
Inventors: Daniela HAUSEROVÁ; Zbysek Novy; Jaromir DLOUHY; Uwe Diekmann
Original assignee: Comtes FHT AS
Current assignee: Comtes FHT AS
Priority date: 2014-06-12
Filing date: 2015-06-11
Publication date: 2017-04-19
Also published as: CZ2014405A3; WO2015188796A4; CZ305587B6; WO2015188796A1

Abstract

This heat treatment of bearing steel comprises annealing and hardening. Hardening is carried out after annealing and after the manufacture of structural parts from this steel. The first part of the annealing process is heating of the steel at a rate of more than 1 °C/s to a temperature in the range between 750 °C and 900 °C. It is followed by holding at the temperature achieved for at least 400 seconds. This is followed by cooling at a rate higher than 0.02 °C/s to a temperature, at which austenite begins to transform to ferrite and carbides. In the next step, there is cooling to such temperature and at such rate that complete transformation of austenite to ferrite and carbides takes place at least in the desired location of the steel. The cycle of heating, holding and cooling is carried out at least once. Then the steel is cooled to the ambient temperature. In the preferred embodiment, the quenching temperature during subsequent hardening is lower than the quenching temperature of the conventionally annealed steel in question. The steel may be heated by induction.

Description

Method of Heat Treatment of Bearing Steel

Technical Field

The present invention belongs to a group of heat treatment methods and relates, in particular, to heat treatment of bearing steel

Background Art

Heat treatment of bearing steels is a very complex process which is carried out using various procedures, depending on the part's size and the bearing steel type.

Most often, the heat treatment of bearing steels comprises two fundamental operations incorporated in the entire manufacturing route.

The first operation in the heat treatment of bearing steels is soft annealing of semi-finished products. This annealing is ordinarily carried out after hot forming. The first objective of soft annealing is to obtain a microstructure which is favourable for subsequent machining, e.g. machining of a bearing ring, to dimensions which are close to the final dimensions; i.e. a microstructure which provides good machinability and chip breakability. The second objective of soft annealing is to obtain an optimum microstructure for subsequent quenching. Soft annealing is typically carried out in continuous or box furnaces; it is generally a longtime annealing process, the duration of which depends on the size of the charge and may reach tens of hours. Slow heating to and holding at a temperature in the vicinity of the A_ci is followed by very slow cooling from this temperature, which may be interrupted by holds. The decision to incorporate holds and their durations are dictated by the bearing steel type, as well as by the size of the treated part

The soft-annealed microstructure should contain spheroidised carbides embedded in recrystallized matrix with small relative density of defects; and its hardness should normally be no more than the maximum specified hardness. The size and/or density of the spheroidised carbides and the maximum hardness are usually set out in the in-house standard of the particular bearing manufacturer. These parameters and the size and the related density of carbides have substantial influence on both technological and end-use properties of bearing steels. In terms of technological properties, very fine carbides, if present in a steel semifinished product upon forming and annealing, increase hardness and strength but reduce machinability. In terms of end use properties, very fine carbides improve the combination of the properties of hardening structure upon subsequent hardening, most notably toughness, abrasion resistance and resistance to contact fatigue. At the same time, very fine carbides enable the heat treater to reduce the austenitizing temperature for subsequent quenching, thus mitigating the risk of stress and distortion occurring in the final part.

Another factor that can favourably affect the life of bearing steels is refinement of austenite grain which is the initial state for the hardening structure. The refinement of austenite has a positive impact on the morphology of the hardening structure which is also finer and provides better notch toughness, abrasion resistance and life under contact fatigue conditions. A number of studies describe various annealing methods by which a microstructure with spheroidised carbides of various sizes and densities embedded in a ferritic matrix of various grain sizes can be obtained in bearing steels. All available studies report the duration of these annealing methods, which are used for semi-finished products or specimens, as several hours or tens of hours.

An alternative to conventional soft annealing methods is represented by the method described in CZ302676 patent file and its equivalent patent document AT508101. This treatment method allows the process of carbide spheroidisation in a ferritic matrix to be accelerated considerably.

The second heat treatment operation in the bearing manufacturing process is hardening, i.e. quenching from austenitizing temperature and subsequent tempering. In the manufacturing sequence, this operation is carried out after machining and is followed by grinding and, if required, polishing of the bearing component. Hardening of the material is either carried out as through hardening or as surface hardening; in the latter case, the surface layer is thermochemically treated before surface hardening. The thermochemical treatment may involve, for instance, carburizing or nitrocarburizing. Heating to the austenitizing temperature takes place in an electric air furnace or by induction or in a furnace with a special atmosphere for thermochemical treatment. Besides thermochemical enrichment of the surface of a bearing ring, superfast induction heating represents an important trend in advanced treatment of bearing rings.

For the present patent, a sequence of through hardening and subsequent tempering is important. Depending on the type of steel in question, this operation is implemented in various ways. The main distinction is the difference between martensitic quenching and bainitic hardening. Martensitic quenching is based on very rapid cooling that is achievable in small-size semi-finished products. In some cases, deep freezing is used to advantage, which involves cooling down to cryogenic temperatures, thanks to which the subsequent tempering leads to precipitation of very fine and morphologically favourable temper carbides. In larger semi-finished products, bainitic hardening is effective, during which the cooling rates are slower; in some cases, the cooling is interrupted by holding the product at a certain temperature, at which isothermal bainitic transformation takes place.

In any case, austenitizing temperature and the holding time at this temperature must be chosen correctly for hardening. The austenitizing temperature must not be too low because the amount of carbon that dissolves in austenite should be as large as possible and carbon should become uniformly distributed throughout the volume of the work. On the other hand, the austenitizing temperature must not be too high because cooling from high temperatures increases the level of internal stresses created during quenching and the microstructure may coarsen undesirably. Internal stresses cause elastic-plastic deformation in the material, the amount of which critically depends on the stress level in the material.

The hardening method has a substantial impact on another microstructural parameter: the resulting fraction of retained austenite. For a long time, the presence of retained austenite, a soft microstructure constituent, was considered undesirable. However, recent studies showed that its increased amount in the microstructure, by contrast, enhances the life of the part under contact fatigue conditions. Increased amounts of retained austenite, a metastable phase, can be preserved in the microstructure by, for instance, slowing the rate of cooling or by interrupting the cooling schedule.

Disclosure of Invention

This invention relates to a method of heat treatment of bearing steel which comprises annealing and hardening. In this method, hardening is carried out after annealing and after the manufacture of structural parts from this steel.

The first part of the annealing process is heating of the steel at a rate of more than 1 °C/s to a temperature in the range between 750 °C and 900 °C. It is followed by holding at this temperature for no more than 400 seconds. In practice, however, even very short holding times of several seconds or tens of seconds appear sufficient as well. The heating temperature and the holding time are chosen with respect to the steel's composition and to the properties which are to be obtained by this method.

In the next step, there is cooling at a rate higher than 0.02 °C/s to a temperature, at which the transformation of austenite to ferrite and carbides begins in the steel. The cooling to this temperature is followed by cooling to such temperature and at such rate that complete transformation of austenite to ferrite and carbides occurs at least in the desired location of the steel. In practice, this means that the cooling rate may be either accelerated or retarded. In some cases, the cooling rate may remain unchanged, depending again on the composition of the steel and on the properties which are to be obtained by this method. It is possible to choose either full transformation of austenite to ferrite and carbides throughout the cross- section of the semi-finished product or only in its desired parts, depending on the requirement. The cycle that comprises the heating, holding and cooling steps is carried out at least once. However, it is normally advantageous to repeat it several times. After a sufficient number of cycles, the steel is cooled to the ambient temperature.

In the preferred embodiment, the quenching temperature for subsequent hardening of the previously manufactured structural parts which have been annealed at an accelerated rate is lower than the quenching temperature of the steel in question upon conventional annealing. This is thanks to the above-described method of heating, holding and controlled cooling. The advantages include lower energy consumption and time demands of hardening, more favourable and finer microstructure and lower internal stresses than in the conventional method.

In another preferred embodiment, the steel is heated by induction. The advantages of this heating method include, in particular, the speed and uniformity of heating.

To achieve very favourable final properties in structural parts of a bearing steel, the method described herein which is submitted for patent protection relies on a comprehensive heat treatment that consists of two fundamental processes described in the Prior Art section: soft annealing and hardening. Between these processes, the semi-finished product is machined to dimensions very close to the final part. In the presently proposed method, the soft annealing process is similar but not identical to the route described in the CZ302676 patent. In repeated heating to temperatures above Aci, the new method introduces holds at these temperatures in order to promote carbide spheroidisation. For bearing steels alloyed with carbide-forming elements (Cr, Mo), the incorporation of one or more holds is very desirable for higher stability of carbides and for their slower dissolution.

This new soft-annealing sequence produces fine and uniformly distributed carbides containing iron and other elements. During the subsequent heat treatment operation, hardening, these carbides can be dissolved in austenite solid solution using a lower quenching temperature than that required after conventional soft-annealing, upon which the carbides are coarser and less uniformly dispersed. The difference between such quenching temperatures may be up to 40 °C. The lower quenching temperature imparts additional advantages to the bearing ring material. Quenching from a lower temperature leads to lower internal stresses and distortion. In addition, the hardening structure produced by quenching from a lower temperature is substantially finer (thanks to several factors, including the finer initial dispersion of carbides) and has improved properties. The main properties which receive attention are abrasion resistance and resistance to contact fatigue. Another advantage of the newly-proposed quenching process is the rapid induction heating which is suitable when the very fine microstructure obtained by rapid spheroidisation of carbides is to be retained.

The above-described procedure represents a heat treatment of bearing steel which, at lower time and energy demands, produces improved results and properties in a steel semi-finished product, as evidenced by the following examples.

Brief Description of Drawings

An example embodiment of the proposed solution is described with reference to the drawings, which show the following:

Fig. 1 - Microstructure of steel upon accelerated soft annealing by induction,

Fig. 2 - Microstructure of steel upon conventional several-hour soft annealing in furnace,

Fig. 3 - Microstructure of steel upon accelerated annealing and quenching in oil,

Fig. 4 - Microstructure of steel upon long-time conventional annealing and quenching in oil,

Fig. 5 - Microstructure of steel upon accelerated annealing, quenching in oil and tempering in furnace at 200 °C for 2 hours,

Fig. 6 - Microstructure of steel upon long-time conventional annealing, quenching in oil and tempering in furnace at 200 °C for 2 hours, Fig. 7 - Comparison between wear rates in steel test pieces where A test piece is an accelerated-annealed and hardened specimen and B test piece is a conventionally-annealed and hardened specimen. The y-axis of the graph shows the wear rate in mm³ m,

Fig. 8 - Diagram of the entire accelerated treatment of bearing steel according to the invention, including hardening. Stage 1 comprises the accelerated soft-annealing according to the invention, stage 2 is quenching in in oil and stage 3 comprises tempering.

Modes for Carrying Out the Invention Example 1

This example describes an annealing and hardening sequence. The workpiece used was hot- formed 16 mm-diarneter bar stock of a steel of the following chemical composition: 1.00 weight percent C, 1.11 weight percent Mn, 0.55 weight percent Si, 1.56 weight percent Cr, 0.012 weight percent P, 0.003 weight percent S and a balance of Fe. The notch toughness of the stock was KCVmini - 7 J/cm², the proof stress was Rp_0.2 ⁼ 849 MPa, ultimate strength was R_m = 1355 MPa, elongation was A₅ = 8 % and hardness was 383 HV30.

Using induction, the steel stock was heated to a temperature of 800 °C at an average rate of 15 °C/s and then held 15 seconds at this temperature. This was followed by cooling in still air to a temperature of 660 °C. After that, there was reheating to 800 °C at an average rate of 15 °C/s, a 15-second hold, cooling to 660 °C, reheating to 800 °C at an average rate of 15 °C/s, a 15-second hold and cooling in air to the ambient temperature. In this way, the accelerated annealing was performed. The microstructure consisted of fine globular carbides in a fine-grained matrix (Fig. 1). The notch toughness of the accelerated-annealed specimen was KCVmini = 49 J/cm², the lower yield point was Re_L = 640 MPa, the upper yield point was eH^■" 666 MPa, ultimate strength was Rm = 930 MPa, elongation was A₅ 23 % and hardness was 261 HV30. For comparison, the microstructure upon conventional soft annealing carried out for several hours in a furnace is shown (Fig. 2). This reference specimen had a hardness of 208 HV. In the annealed condition, the bearing is manufactured and then quenched (Figs. 3 and 4) and tempered, i.e. hardened in a conventional manner in a furnace or by induction (Figs. 5 and 6). The specimen was heated to a temperature of 860 °C and held for 25 minutes in a furnace, or for a shorter time when induction heating was used, and then quenched in oil and tempered at 200 °C for 2 hours. Where induction heating was used, the time was shorter as well. The cooling took place in still air. The hardness of the accelerated- annealed and quenched specimen was 838 HV and the hardness of the reference conventionally-annealed and quenched specimen was 826 HV. The microstructure of the accelerated-treated specimen is much finer and contains finer carbides (Fig. 3) than the conventionally-quenched specimen (Fig. 4). The final hardness of the accelerated-annealed and hardened specimen was 745 HV and the hardness of the conventionally-annealed and hardened specimen was 732 HV. Again, the microstructure upon the accelerated process is much finer with finer carbides (Figs. 5 and 6). For this reason, the life of the final bearing is longer. This is evidenced by the pin-on-disc wear test (Fig. 7) of the hardened specimens. The wear rate in the accelerated-annealed and hardened specimen is lower than that upon the conventional method. The diagram of the entire sequence of accelerated treatment of bearing steel is shown in Fig. 8.

Example 2

This example describes an annealing and hardening sequence. Reference steel stock supplied had the following chemical composition: 1.03 weight percent C, 1.08 weight percent Mn, 0.52 weight percent Si, 1.54 weight percent Cr, 0.011 weight percent P, 0.002 weight percent S and a balance of Fe, and had been conventionally annealed in a furnace for several hours and furnace-cooled, and subsequently quenched and tempered (hardening process). The chosen quenching temperature was 840 °C and the tempering at the temperature of 240 °C took 4 hours. The hardness of this reference stock upon quenching from 840 °C reached 710 HV10.

Stock which had been accelerated-annealed according to the schedule described in Example 1 was quenched and tempered (hardening process) in the same manner as the above-described conventionally-annealed reference stock. Upon this treatment, the hardness was 734 HV10. The fact that the finer initial structure produced by accelerated annealing is more advantageous for hardening was verified by quenching from a lower quenching temperature of 800 °C. The hardness of this specimen was 715 HV10. Hence, the outcome is that the quenching of an accelerated-annealed specimen and a conventionally-annealed specimen from the same temperature leads to a higher hardness in the accelerated-annealed specimen. Upon accelerated annealing and quenching from a temperature of 800 °C, the hardness achieved was the same as the hardness after conventional annealing and quenching from 840 °C.

Example 3

This example provides a description of annealing of hot-formed 16 mm-diameter bar stock of a steel of the following chemical composition: 1.00 weight percent C, 1.11 weight percent Mn, 0.55 weight percent Si, 1.56 weight percent Cr, 0.012 weight percent P, 0.003 weight percent S and a balance of Fe. The hardness of the stock was 383 HV10. The stock was heated to 780 °C at an average rate of 15 °C/s, held at this temperature for 300 seconds, and cooled in still air to the ambient temperature. In this way, the accelerated annealing was performed. The microstructure consisted of fine globular carbides, scarce cementite lamellae and a fine ferritic matrix. Upon this heat treatment, the hardness was 279 HV10.

Example 4

This example provides a description of annealing of hot-formed 16 mm-diameter bar stock of a steel of the following chemical composition: 1.00 weight percent C, 1.11 weight percent Mn, 0.55 weight percent Si, 1.56 weight percent Cr, 0.012 weight percent P, 0.003 weight percent S and a balance of Fe. The hardness of the stock was 383 HV10. The stock was induction-heated to 760 °C at an average rate of 5 °C/s, held at this temperature for 15 seconds, and cooled in still air to a temperature of 630 °C. After that, there was reheating to 760 °C at an average rate of 5 °C/s, a 15-second hold, cooling to 630 °C, reheating to 760 °C at an average rate of 5 °C/s, a 15-second hold and cooling in air to the ambient temperature. In this way, the accelerated annealing was performed. The microstructure consisted of fine globular carbides and remnants of undissolved lamellae embedded in a finegrained matrix. Upon this treatment, the hardness of the specimen was 281 HV10. Example 5

This example provides a description of annealing of hot-formed 16 mm-diameter bar stock of a steel of the following chemical composition: 1.00 weight percent C, 1.11 weight percent Mn, 0.55 weight percent Si, 1.56 weight percent Cr, 0.012 weight percent P, 0.003 weight percent S and a balance of Fe. The hardness of the stock was 383 HV10. The stock was induction-heated to 880 °C at an average rate of 90 °C/s, held at this temperature for 5 seconds, and cooled in still air to a temperature of 630 °C. After that, there was reheating to 880 °C at an average rate of 90 °C/s, a 5-second hold, cooling to 630 °C, reheating to 880 °C at an average rate of 90 °C/s, a 5-second hold and cooling in air to the ambient temperature. In this way, the accelerated annealing was performed. The microstructure consisted of finegrained matrix, fine globular carbides and a small amount of newly-formed lamellae resulting from a greater extent of structure dissolution. Upon this treatment, the hardness of the specimen was 297 HV10.

Example 6

This example describes an annealing process of steel stock. It was hot-formed stock of a steel of the following chemical composition: 1.04 weight percent C, 1.10 weight percent Mn, 0.55 weight percent Si, 1.45 weight percent Cr, 0.013 weight percent P, 0.004 weight percent S and a balance of Fe. The hardness of the stock was 351 HV10. It was induction-heated to 780 °C at an average rate of 33 °C/s, held at this temperature for 15 seconds, and cooled in still air to a temperature of 660 °C. After that, there was reheating to 780 °C at an average rate of 33 °C/s, a 15-second hold, cooling to 660 °C, reheating to 780 °C at an average rate of 33 °C/s, and a 15-second hold. It was followed by controlled cooling to a temperature of 630 °C at a rate of 0.04 °C/s. In this way, annealed structure with globular carbides in a finegrained matrix was produced. The hardness of the treated stock reached 260 HV10.

Claims

1. A method of heat treatment of bearing steel comprising annealing and hardening where hardening takes place after annealing and after the manufacture of structural parts of this steel characterised in that in the annealing process, the first step is heating of the steel at a rate of more than 1 °C/s to a temperature in the range between 750 °C and 900 °C,

after which the steel is held at the temperature achieved for no more than 400 s,

and then cooled at a rate higher than 0.02 °C/s to a temperature, at which austenite starts to transform into ferrite and carbides, which is followed by cooling to such temperature and at such rate that, at least in the desired location of the steel, austenite completely transforms to ferrite and carbides,

where the cycle of heating, holding and cooling is carried out at least once

and followed by cooling the steel to the ambient temperature.

2. The method of heat treatment of bearing steel according to Claim 1 characterised in that the quenching temperature in subsequent quenching is lower than the quenching temperature of the steel in question upon conventional annealing.

3. The method of heat treatment of bearing steel according to Claim 1 or Claim 2 characterised in that the steel is heated by electrical induction.