Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
  • C21D1/32—Soft annealing, e.g. spheroidising
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/78—Combined heat-treatments not provided for above
  • C21D1/785—Thermocycling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/40—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rings; for bearing races
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
  • F16C33/00—Parts of bearings; Special methods for making bearings or parts thereof
  • F16C33/30—Parts of ball or roller bearings
  • F16C33/58—Raceways; Race rings
  • F16C33/62—Selection of substances
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
  • F16C2202/00—Solid materials defined by their properties
  • F16C2202/02—Mechanical properties
  • F16C2202/04—Hardness
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
  • F16C2204/00—Metallic materials; Alloys
  • F16C2204/60—Ferrous alloys, e.g. steel alloys
  • F16C2204/66—High carbon steel, i.e. carbon content above 0.8 wt%, e.g. through-hardenable steel
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
  • F16C2223/00—Surface treatments; Hardening; Coating
  • F16C2223/10—Hardening, e.g. carburizing, carbo-nitriding
  • F16C2223/18—Hardening, e.g. carburizing, carbo-nitriding with induction hardening

Definitions

• the present invention belongs to a group of heat treatment methods and relates, in particular, to heat treatment of bearing steel
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Martensitic quenching is based on very rapid cooling that is achievable in small-size semi-finished products.
• 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.
• 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.
• 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.
• 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.
• the presence of retained austenite, a soft microstructure constituent was considered undesirable.
• 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.
• This invention relates to a method of heat treatment of bearing steel which 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 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.
• 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.
• 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.
• 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.
• the steel is heated by induction.
• the advantages of this heating method include, in particular, the speed and uniformity of heating.
• 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.
• the soft annealing process is similar but not identical to the route described in the CZ302676 patent.
• the new method introduces holds at these temperatures in order to promote carbide spheroidisation.
• 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.
• 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.
• 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.
• 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 mm 3 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.
• 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 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).
• Fig. 2 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
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• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
• General Engineering & Computer Science (AREA)
• Heat Treatment Of Articles (AREA)
• Rolling Contact Bearings (AREA)

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• 2014
  • 2014-06-12 CZ CZ2014-405A patent/CZ2014405A3/cs not_active IP Right Cessation
• 2015
  • 2015-06-11 EP EP15741719.7A patent/EP3155134A1/en not_active Withdrawn
  • 2015-06-11 WO PCT/CZ2015/000060 patent/WO2015188796A1/en active Application Filing

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