[0001] The present invention relates to through hardening and case hardening steel components, and more particularly to methods to achieve improved rolling contact fatigue (RCF) resistance characteristics in through hardened and case hardened steel components, and the components that are through hardened and case hardened using such methods.

BACKGROUND

[0002] Through hardening is a heat treatment process that includes heating a steel component to a high temperature as an austenitizing step to change the microstructure of the steel to a pure austenite microstructure. The austenitizing process may include one or more heating cycles or one or more austenitization cycles. The component is then rapidly quenched to increase a hardness throughout a steel component, which increases the strength of the component. Different quench mediums include forced air or gas, still air or gas, quench oil, water and a liquid salt. The component may be quenched to a temperature, such as near or below a martensite start temperature (TMS), which enables further transformation of the microstructure of the component. For example, the microstructure can be transformed from austenite to martensite, bainite, pearlite, or a combination of these microstructures. Additionally, the component may be tempered after the component is quenched by re-heating the component to decrease a brittleness of the component.

[0003] Case hardening is another known heat treatment process in which a hardened, “case region” is created only at the surface of the component and to a depth extending at most to two centimeters below the surface of the component. The case hardening can be done in two different manners. A first is by preferentially enriching the steel surface with carbon and/or nitrogen followed by a quenching step. A second is by preferentially austenitizing the near surface material (case region) by induction heating followed by a quench step.

SUMMARY

[0004] In one aspect, the improved heat treating process provides improved rolling contact fatigue (RCF) resistance, which has been found to improve the life of bearings used in high-debris applications, such as earth-moving and drilling equipment. In those applications, debris (e.g., sand, grit, etc.) gets into the bearing and dents the races and/or rollers. It has been found that bearing steel heat treated with the process described below increases the life expectancy of these bearings that see significant debris denting. To be clear, the process does not necessarily prevent or reduce the denting, but rather improves the rolling contact fatigue (RCF) resistance such that even when heavily dented by debris, the bearings still last longer.

[0005] The present disclosure provides, in one aspect, a method for through hardening steel, and particularly bearing steel components. The method includes austenitizing a steel component, quenching the steel component to a first quench temperature below the martensite start temperature (Ms) and within a range of 0.8Ms to 0.98Ms, holding the steel component at the first quench temperature for a duration of 0.5 hours to 8 hours, and quenching the steel component to a second quench temperature below 100 degrees C. The resulting component has a through hardness of at least 50 HRC, a retained austenite content of at least 10% to a depth of at least 1 mm from a surface of the steel component, residual tensile stresses to a depth of at least 1 mm from the surface of the steel component, and a microstructure of 5-30% bainite, 10-35% retained austenite, less than 7% carbides, and a remainder of martensite throughout a cross-section of the steel component.

[0006] The present disclosure provides, in another aspect, a method for case hardening steel, and particularly bearing steel components. The method includes preferentially austenitizing a near surface material of the steel component by induction heating and quenching the steel component to a first quench temperature to create a case region. Because of preferential austenitizing and quenching, the case region has a higher hardness than the material in the core, and in this context, the case has hardness of at least 50HRC. The case region has a depth from a surface of the steel component ranging from 100 microns to 1 cm, and the first quench temperature is below a martensite start temperature (Ms) of material in the case region and within a range of 0.8Ms to 0.98Ms. The method further includes holding the steel component at the first quench temperature for a duration of 0.5 hours to 8 hours, and quenching the steel component to a second quench temperature below 100 degrees C. The resulting steel component has a hardness of at least 50 HRC in the case region of the steel component, a retained austenite content of at least 10% to a depth of at least 100 microns from the surface of the steel component, residual compressive stresses to a depth of at least 100 microns to 2 cm from the surface of the steel component, and a microstructure of 5-30% bainite, 10-35% retained austenite, less than 7% carbides, and a remainder of martensite in the case region of the steel component. [0007] The present disclosure provides, in another aspect, a method for case hardening steel, and particularly bearing steel components. The method includes diffusing carbon and/or nitrogen into a surface of the steel component while austenitizing the steel component and quenching the steel component to a first quench temperature to create a case region. Because of enriched carbon and/or nitrogen in the near surface material, the case region has higher hardness than the material in the core, and in this context, the case has a hardness of at least 50HRC. The case region has a depth from a surface of the steel component ranging from 100 microns to 1 cm, and the first quench temperature is below a martensite start temperature (Ms) of material in the case region and within a range of 0.8Ms to 0.98Ms. The method further includes holding the steel component at the first quench temperature for a duration of 0.5 hours to 8 hours, and quenching the steel component to a second quench temperature below 100 degrees C. The resulting steel component has a hardness of at least 50 HRC to a depth of at least 100 microns from the surface of the steel component, a retained austenite content of at least 10% to a depth of at least 100 microns from the surface of the steel component, residual compressive stresses to a depth of at least 100 microns to 2 cm from the surface of the steel component, and a microstructure of 5-30% bainite, 10-35% retained austenite, less than 7% carbides, and a remainder of martensite in the case region of the steel component.

[0008] Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 is a time-temperature-transformation chart of the austenite to martensite+bainite+retained austenite conversion in various stages.

[0010] Figs. 2 illustrates the microstructure of an example component that has been heat treated using the method described herein.

[0011] Fig. 3 is an alternative time-temperature-transformation chart of the austenite to martensite+bainite+retained austenite conversion in various stages.

[0012] Fig. 4 illustrates a graph of bearing life for bearings produced by the heat treating processes of Figs. 1 and 3 as compared to other conventional heat treating processes. [0013] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION

[0014] The present disclosure is related to a heat treatment process explained using an example of austempering of bearing steel grades, such as 100CrMo7-3 or!00CrMnSi6-4, or any other steel grade having a carbon composition of at least 0.7 weight% (wt%). However, those of ordinary skill in the art would understand that the inventive heat treatment process could be applied to other steels as well. The typical chemical composition of 100CrMo7-3 steel includes 0.9 wt% carbon, 0.3 wt% silicon, 0.7 wt% manganese, 1.8 wt% chromium, 0.3 wt% molybdenum. The typical chemical composition of 100CrMnSi6-4 steel includes 0.9 wt% carbon, 0.6 wt% silicon, 1.1 wt% manganese, 1.5 wt% chromium. Typical through hardening bearing steels with carbon content of more than 0.7% can be found in the ISO standard ISO 683-17:2014(en).

[0015] The through hardening process described herein is a dual step quenched martensite process for a bearing steel component (e.g., a bearing race or roller, such as a bearing steel component 20 represented in the microstructure view of Fig. 2). As shown in Fig. 1, the steel component may first be austenitized throughout its cross section by heating the component to an initial temperature Ti, which in the illustrated embodiment is a temperature from 700 degrees Celsius to 1000 degrees Celsius. To fully austenitize, the component is held at the temperature Ti for a duration of time 5 minutes to 240 minutes. One or more heating cycles may be used to austenitize the component or one or more austenitizing cycles may be used. After the austenitization is complete and the component has a microstructure of austenite and some carbide particles, the component is then heat treated using the dual step quenched martensite process described herein beginning at an initial time to, which is shown in Fig. 1. [0016] Fig. 1 depicts a time-temperature-transformation (TTT) chart 4 showing a transformation curve 8 of the heat treatment process described herein. Beginning at a time ti, the component is quenched from the initial temperature Ti to a temperature T2 by a time t2. In some instances, the quench rate from Ti to T2 also touches and/or crosses the noses of the C-shaped curves. If that were the case, the microstructure could contain small amounts of pearlite and/or upper bainite. The temperature T2 is below the martensite start temperature TMS of the material, such as from 0.8*TMS to 0.98*TMS, and in some instances from 0.8*TMS to 0.9*TMS. For the example type of bearing steel described herein, TMS is about 200 degrees Celsius, and the temperature T2 ranges from 160 degrees C to 195 degrees C. For other steels having higher a higher TMS temperature, the T2 range would also be correspondingly higher. The initial amount of martensite formed upon reaching the quench temperature T2 is typically about 5% to about 40%. The component is then held at the temperature T2 for a time interval Ati from time t2 to time b. The quench and holding medium includes a liquid salt bath or a quench oil bath. The time interval Ati may be any length of time from 0.5 hours to 8 hours, and alternatively may be from 0.5 hours to 4 hours. During the time interval Ati, the component is held at temperature T2, and during this time, isothermal tempering of the formed martensite occurs. In addition, some formation of bainite (e.g., 5-30%) has been observed during this isothermal tempering.

[0017] At time t3, the component is quenched to a temperature at or below a room temperature, 100 degrees Celsius, or an ambient temperature. The quench medium includes water, oil, still air or gas, forced air or gas. After quenching, the resulting microstructure has been found to contain 5-30% bainite (lower bainite), 10-35% retained austenite, less than 7% carbides, and the remainder being martensite. Fig. 2 illustrates the microstructure, with areas labeled “B” for bainite, “RA” for retained austenite, and “M” for martensite. Carbides are also indicated. The component has a through hardness of at least 50 HRC and retained austenite content of at least 10% to a depth of at least 1 mm from the surface of the steel component. The component also exhibits residual tensile stresses to a depth of at least 1mm from the surface of the steel component. Those residual stresses range from 10 to 150 MPa.

[0018] The component may be tempered after the second quench or may be used without tempering. To temper the steel component, the steel component may be heated to a fourth temperature that is from 100 degrees Celsius to 350 degrees Celsius and quenched to an ambient or room temperature. The through hardness of the steel part remains at least 50 Rockwell C (HRC) throughout the cross-section of the component after the quench tempering heat treatment (i.e., after t4). The quench tempering heat treatment 12 after t4 can be done to reduce the amount of retained austenite to less than 15%. The quench tempering heat treatment 12 begins at time t4 and includes heating the component to a temperature T4 for a short period of time At2, after which the component is again quenched to a temperature at or below a room temperature, 100 degrees Celsius, or an ambient temperature. The quench tempering heat treatment 12 is optional.

[0019] The resulting microstructure increase the rolling contact fatigue life of the component, thus enabling the components to last longer. The increased rolling contact fatigue life is evident from the bearing life testing as shown in Fig. 4. Rolling contact surfaces were dented by debris as per standard procedures and then life tested in viscosity grade 10 mineral oil. Fig. 4 shows the life of bearings made using conventional heat treats that are used in the field as compared to the inventive dual quench martensite heat treatment described above. The improved fatigue resistance properties are attributed to martensite forming at the second quench step and the bainite formed during the isothermal tempering.

[0020] Fig. 3 is an alternative time-temperature-transformation chart 30 of the austenite to martensite+bainite+retained austenite conversion in various stages. The steel used in the illustrated example is bearing steel grades, such as 100CrMo7-3 orl00CrMnSi6-4, or any other steel having a carbon composition of at least 0.7 wt%. The heat treatment process shown in the alternative time-temperature-transformation chart 30 of Fig. 3 includes an additional heating and quenching cycle 34 that may be added to the heat treatment process 4 described above. In the illustrated example, the component is first heated to a high temperature of T-2, which is a carbide dissolution temperature of approximately 900 to 1100 degrees Celsius. This heating stage has a duration Ats of at least 30 minutes. In one example, the duration Ats is from 30 minutes to 300 minutes. The duration Ats is inclusive of the heating time, holding time, and cooling time of the heating stage. The temperature T-2 is above the Ai and the ACM temperatures for the steel component. The Ai transformation temperature is the temperature at which the ferritic phase of the steel starts to transform into austenite. The ACM temperature is the temperature at which the ferritic phase of the steel is completely transformed into austenite. During this heat treatment stage 34 at least some of the carbides in the component are dissolved. After the heating stage 34, the component is quenched to a temperature T-i. The temperature T-i is less than 500 degrees Celsius, but in one embodiment may be less than 300 degrees Celsius.

[0021] Next, the component is then reheated to a temperature above the Ai temperature, and may also be above the ACM temperature. The ACM transformation temperature is the temperature at which the ferrite phase of the steel completely transforms into austenite. In the illustrated example, this temperature is the same as the temperature Ti of the heat treatment cycle described in Fig. 1. Alternatively, the temperature of the second heating stage may be within the range of 750 to 900 degrees Celsius. Following the second heating stage, the heat treatment cycle of Fig. 3 is identical to the heat treatment cycle of Fig. 1. However, the additional heating stage results in components with improved fatigue resistance and refined microstructures. Thus, the heat treatment cycle of Fig. 3 may be better for some applications than the heat treatment cycle of Fig. 1.

[0022] Fig. 4 illustrates the results of bearing life testing conducted for bearings heat- treated using the Dual Quench Martensite processes of Fig. 1, as compared to bearings that underwent different heat treat processes. Rolling contact surfaces of all the bearings were debris dented first as per a standard procedure, and then life tested in a viscosity grade 10 mineral oil. In contrast to smoother bearing surfaces, the dented bearing surfaces decrease the bearing lives drastically. This testing is done typically to evaluate the durability of bearings used in high-debris applications, such as earth-moving and drilling equipment. In Fig. 4, the illustrated Heat Treat 2 results depict results from an austempering process. The illustrated Heat Treat 3 results depict results from a dual step austempering process. The illustrated Heat Treat 4 results depict a case carburized bearing with martensitic microstructure. Also shown for comparison purposes is the estimated bearing life without debris denting of the bearing races.

[0023] The above-described processes described with respect to Fig. 1 can also be performed in conjunction with, or after, a case hardening process is performed on the bearing component. For example, in a case carburizing process, only the near surface material can be hardened by diffusing carbon and/or nitrogen into the steel surface during the austenitization step followed by quenching. With case carburizing, the case region can be created in a separate process prior to the Dual Quench Martensite process 4 described above with respect to Fig. 1, or simultaneously during the austenitizing step of the Dual Quench Martensite processes described above with respect to Fig. 1. When quenching below the Ms temperature, the Ms temperature is based on the material in the case region. It should also be noted that a lower carbon steel (i.e., lower than 0.7 wt. %) may be used as the base material for this case carburized bearing component, but that the additional diffused carbon in the case region will lead to similar results observed in the case region.

[0024] Alternatively to case carburizing, only the near surface material can be hardened to create the case region by selectively heating the near surface material via induction heating followed by quenching. In this case, the induction heating of the near surface region functions as the austenitizing step(s) of the Dual Quench Martensite processes 4 or 30 described above with respect to Figs. 1 and 3. With this induction hardening process, when quenching below the Ms temperature, the Ms temperature is based on the material in the case region.

[0025] In either method of case hardening, the case-hardened region typically has a depth of 100 microns to 1 cm, a hardness of at least 50 HRC in that case-hardened region, and the microstructure has at least 10% retained austenite in that case hardened region. The overall resulting microstructure in the case region has been found to contain 5-30% bainite, 10-35% retained austenite, less than 7% carbides, and the remainder being martensite. In these case- hardened components, residual compressive stresses at the surface region to a depth of at least 100 microns to 2 cm have been observed.

[0026] Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects of the invention as described.