The Ultra Large Bearing (ULB) industry can increase the production performances by using induction heating on a full range of thermal processes. The paper presents the technological, economical, and process optimizations that can be achieved using induction heating technology in both hardening and tempering. Two different solutions are available for (seamless) race hardening: a high-power induction single shot process for small to medium size rings and induction seamless scan hardening for large sized bearings. The ultra-low frequency induction tempering process is described and compared with a traditional furnace. These technologies are presented and compared to show application ranges, specific features, metallurgical results, and efficiencies in processing and cost.

Through hardened steel ball fatigue failure is an atypical mode of failure in a rolling element bearing. A recent full-scale bench test resulted in ball spalling well below calculated bearing life. Subsequent metallurgical analysis of the spalled balls found inferior microstructure and manufacturing methods. Microstructural analysis revealed significant carbide segregation and inclusions in the steel. These can result from substandard spheroidized annealing and steel making practices. In addition, the grain flow of the balls revealed a manufacturing anomaly which produced a stress riser in the material making it more susceptible to crack initiation. The inferior manufactured balls caused at least an 80% reduction in rolling contact fatigue life of the bearing.

AISI 52100 is a high carbon alloy steel typically used in bearings. One hardening heat treatment method for AISI 52100 is austempering, in which the steel is heated to above austenitizing temperature, cooled to just above martensite starting (Ms) temperature in quench media (typically molten salt), held at that temperature until the transformation to bainite is completed and then cooled further to room temperature. Different austempering temperatures and holding times will develop different bainite percentages in the steel and result in different mechanical properties. In the present work, the bainitic transformation kinetics of AISI 52100 were investigated through experiments and simulation. Molten salt austempering trials of AISI 52100 were conducted at selected austempering temperatures and holding times. The austempered samples were characterized and the bainitic transformation kinetics were analyzed by Avrami equations using measured hardness data. The CHTE quench probe was used to measure the cooling curves in the molten salt from austenitizing temperature to the selected austempering temperatures. The heat transfer coefficient (HTC) was calculated with the measured cooling rates and used to calculate the bainitic transformation kinetics via DANTE software. The experimental results were compared with the calculated results and they had good agreement.

Retained austenite may be helpful or detrimental to the life of heat-treated components, but it can be difficult to accurately measure in manufactured steels. Commonly used visual sample investigations are subjective and often incorrect, magnetic measurements require part-specific calibration, and electron backscattering involves expensive equipment, intensive sample preparation, and long measurement times. Recent developments in X-ray diffractometry, however, provide measurements in minutes and can compensate for the influence of carbides in high-carbon steels as well as texture orientations in rolled sheet metals. This paper discusses the use of X-ray diffraction for measuring retained austenite and compares and contrasts it with other methods. It also provides a brief review of the formation of austenite and its effect on carburized gears, TRIP steels, and bearings.

Large slewing bearings are employed in wind turbines and other energy industry applications where they are subjected to harsh working conditions. In order to bear heavy dynamic loads, slewing ring tracks can be surface hardened by induction heating with a seamless process which allows for a uniform heat treatment without soft zones. In comparison with the traditional furnace carburizing, seamless induction hardening is faster, consumes less energy, and has been developed to achieve the same results utilizing medium carbon steel. The presence of a pre-heating coil, with an independent power source, allows for the adjustment of the heat input rate in order to tune the heating process according to the steel characteristics. The pre-heating operation allows for case depths up to 10 mm to be reached without a reduction in scanning speed or productivity. A mechanical tracking system adjusts the coils to compensate for ring deformation and thus assure a uniform heating pattern. Surface hardness tests and metallography have been performed in different process stages to verify the process consistency. A fine grain microstructure in the end zone has been obtained thanks to the pre-heating coil, which avoids surface overheating.

Microstructure refinement strategies for carburized steel were evaluated to assess their effect on the fatigue performance of case carburized components. Commercial 52100 steel samples were subjected to various treatments and analyzed to determine the micro-geometry of plate martensite and the size distribution of retained-austenite regions. Decreasing reheat temperature produced finer austenite grain size, while multiple reheating cycles helped narrow grain size distribution. The refinement of austenite grain size also led to a reduction in martensite plate size and finer distribution of retained austenite.

Induction quenching can be used for hardening the outer surface of large bearing rings, achieving superior mechanical properties, with the advantage that heating the part in its entirety is not required. The desired goal is to obtain a hardened layer on the active surface of the bearing ring, uniform in both section and circumference, having a predetermined thickness. The case study showed that, after the induction hardening process, on a limited number of specimens, the following problems could be observed: uneven depth of the hardened layer in different zones along the ring circumference, uneven depth of the hardened layer reported in the ring section, and cracks in the bearing material. Therefore, the main objective of the optimization process is to pinpoint the specific parameters involved in the bearing rings quenching operation and their influence on the surface hardening depth. It was noticed that the position and number of insulating/concentrator plates in the inductor stack influences the surface hardening depth on the rolling path.

To experimentally investigate the effect of tempering temperature and time on the structure and composition of martensite, AISI 52100 was austenized at 1000°C for 40 minutes and quenched in agitated water at 21°C. The as-quenched steel contained body-centered tetragonal (BCT) martensite with 22% retained austenite. These samples were tempered at 100°C, 200°C, and 300°C with different holding times and then were characterized by x-ray diffraction (XRD) to determine the effect on the structure of the martensite. It was found that the content of retained austenite did not change after tempering at 100°C. Retained austenite decomposed after tempering for 40 minutes at 300°C. The changes in crystal structures and lattice parameters for tempered martensite with different holding times and temperatures were measured. The effect of sample preparation on retained austenite and the structure of martensite and tempered martensite was evaluated. An effective technique for carbide extraction and collection in steel is introduced.

Press quenching is a specialized quenching technique used in heat treating operations to minimize the distortion of complex components such as spiral bevel gears and high quality bearing races. The quenching machine is designed to control the geometrical characteristics of components such as out-of-round, flatness, and (if the tooling is designed to accommodate it) taper. The achievement of final dimensional tolerances is accomplished through a trial and error process where the incoming machined sizes of the components are adjusted based upon measurement data taken from the initial sets of quenched and tempered components that have already been processed through the press quenching operation. Oil flow rates can be altered during the different stages of the quenching cycle, and through the use of specialized tooling the oil flow pathways can be selectively adjusted to meter the oil flow towards specific areas of the part surface while baffling it away from others in order to provide a more uniform overall quench. Complex metallurgical changes take place during austenitizing and quenching, resulting in corresponding mechanical property changes. Accompanying these changes are the generation of thermal and transformation induced stresses, which produce in-process and final residual stresses. During press quenching, dimensional restrictions add additional complexity to the combined effects of thermal and mechanical process sensitivities on these stresses. And if the stresses are severe enough, quench cracking can result. In this investigation the quench cracking of an asymmetrical AISI 52100 bearing ring is evaluated through physical experiments and through corresponding heat treatment process modeling using DANTE. The effects of quench rate, die load pulsing, and several other process variables are examined experimentally and/or analytically to illustrate how they can impact the resulting stresses generated during the press quenching operation.

Eddy current is a non-destructive testing technique proven for use in heat treat and material structure verification. Modern multi-frequency eddy current instruments can test for conditions such as misplaced case, shallow case, short heat, short quench, and delayed quench. Eddy current testing offers many benefits over traditional heat treat validation methods. Unlike sample testing processes using cut, polish, etch, and visual inspection techniques, eddy current testing provides a clean, fast, and repeatable process that can perform in-line inspections of all parts produced. Eddy current inspections have traditionally focused on symmetrical parts such as wheel bearings and gears. However, advances in robotics have paved the way for cost-effective inspection of non-symmetrical, complex components that would have previously required multiple test stations. Robotics also provides a low-cost way to retest, null, and periodically proof the testing process using multiple conditions of masters. This has been difficult and expensive with other types of automation and operator involvement.

With the growing shift toward renewable energy, the production of large gears and bearing rings, such as those used in wind turbines, is becoming increasingly important. Until recently, these large components were typically custom-manufactured to order, but today, they are often produced in small series. This shift has driven the need for cost reduction, quality improvement, and process automation. For case-hardened transmission and roller bearing components, case depths of several millimeters are required to compensate for the significant distortion caused by conventional quenching—often resulting in out-of-roundness of several millimeters. Correcting this distortion requires time-consuming and costly grinding processes. Additionally, deep carburization increases manufacturing costs due to extended furnace holding times. An in-depth analysis of the fixture hardening process for large gears and bearing rings was conducted to address these challenges. The findings from this project enabled targeted measures to counteract shape deviations such as out-of-roundness and concentricity during quenching, significantly reducing distortion and improving overall process efficiency.

In the last centuries carbonitriding was mainly used to enhance the hardenability of unalloyed steels. IWT developed gas-carbonitriding and low-pressure-carbonitriding processes to increase fatigue behavior and quality compared to case hardening. For example, modern gas-carbonitriding processes make it possible to extend materials ́ strength, so that the limit of use of a given alloy can be expanded. The paper shows examples for the treatment of ball bearing and case hardening steels. The treatment results in microstructures, which are unusual, compared with conventional heat treated parts. They are characterized by high amounts of retained austenite and carbonitride precipitations. By a controlled process, which has been developed in cooperation with PROCESS-ELECTRONIC, it is possible to adjust surface carbon- and nitrogen content independently. Low pressure carburized parts have the advantage that no internal oxidation occurs. So they have the potential of leading to a higher strength. Nowadays LP-carburizing is used in a wide range, whereas LP-carbonitriding processes are at a starting point. In this paper possibilities and limitations of this process are shown. So, inline controlling of LP-processes in a classical way is not possible, but simulation guided process control. The paper will give examples for LP-carbonitriding processes and the resulting microstructure.