Torsional fatigue behavior of a USCB gear bearing steel was studied through the combined treatment of carburized and shot peening, in order to optimize the process parameters and explore the torsional fatigue cracking mechanism. The results show that the carburized layer of the USCB gear bearing steel is mainly composed of martensite and a large number of long strip or elliptic carbides. After shot peening, the torsional fatigue strength increases somewhat, and with the increase of shot peening intensity, the torsional fatigue strength increases first and then decreases, and the optimal torsional fatigue strength is obtained when the shot peening intensity is 0.2 mmA. Torsional fatigue cracking is caused by cleavage cracking of large size carbide clusters. The crack initiation plane is 45° from the loading axis, and the crack initiation is dominated by normal stress. Accordingly, when the fast prediction model of torsional fatigue strength of carburized gear bearing steel proposed earlier is applied to the tested USCB gear bearing steel, the prediction error is about 10%, and the accuracy is better than the Murakami model.

Effect of rapid cooling process after rolling(final cooling temperature) on microstructure evolution and mechanical properties of the tested bearing steel during hot rolling and spheroidizing annealing was systematically studied by means of Gleeble-3500 thermal simulator, OM, SEM, EBSD, tensile test and hardness test. The results show that when the final cooling temperature decreases from 650 ℃ to 570 ℃, the pearlite lamellar spacing decreases from 109.87 nm to 68.77 nm, and the yield strength and tensile strength increase from 899 MPa and 1127 MPa to 1333 MPa and 1655 MPa, respectively. After isothermal spheroidizing annealing, the spheroidization rate of cementite is more than 85%, and the hardness of the tested steel is between 230 HV0.5 and 240 HV0.5. As the pearlite lamellar spacing increases, the size of carbide particles and ferrite grains gradually decreases after spheroidizing annealing, and the hardness of the microstructure gradually increases. After comprehensive comparison of the microstructure and mechanical properties after hot rolling and spheroidizing annealing, the pearlite lamellar spacing of the specimen with a final cooling temperature of 570 ℃ is the smallest, the spheroidization rate of the cementite is the highest after spheroidizing annealing, the hardness is relatively low, and the processing property is better.

Novel microalloyed bearing steel GCr15Si1MoV was performed with austempering treatment above Ms and tempering. The effects of different austempering processes on the microstructure and corrosion resistance of the tested steel were studied by means of scanning electron microscope, X-ray diffractometer and electrochemical workstation. The results show that the microstructure of the tested steel after austempering and tempering is mainly composed of bainite, film-like retained austenite and a small amount of tempered martensite, with distribution of spherical undissolved carbides. After austempering at 200 ℃ for 24 h, the steel achieves the best corrosion resistance. With the extension of austempering time, the volume fraction of retained austenite decreases with its increasing stability, resulting in an improvement in the corrosion resistance of the tested steel. When austempered for 24 h, as the austempering temperature increases, the corrosion resistance of the tested steel first increases and then decreases. The increase in content of undissolved carbides reduces the corrosion resistance of the steel.

Effect of cryogenic treatment on microstructure and mechanical properties of a low-carbon Cr-Co-Mo-Ni high alloy bearing steel was investigated through microstructure characterization by SEM, TEM, EBSD and XRD. The results show that microstructure of the steel after conventional quenching and tempering (QT) contains irregularly sized retained austenite, of which the volume fraction is over 35%, mainly in block-like morphologies. Introducing cryogenic treatment between quenching and tempering (QCT) leads to a reduction by about 70% in volume fraction, a refinement in sizes and evener distribution in matrix for retained austenite due to transformation to martensite. This transformation also results in an increase in dislocation density and a decrease in martensite lath size, both retain after tempering. As a results, the strengths are significantly enhanced while the ductility and toughness are slightly reduced. Comparing to processing by QT, the yield strength and tensile strength of the tested steel after processing by QCT increase by about 860 MPa (about 173%) and about 310 MPa (about 20%), respectively. Additionally, due to the presence of a certain amount of evenly distributed retained austenite in the matrix, the steel still possesses a quite good ductility and toughness after QCT.

Formation of abnormal ultra-fine microstructure on the surface of GCr15 bearing steel was studied. The evolution of the ultra-fine microstructure during annealing was simulated through high-temperature metallographic tests. The temperature after breaking out of the water to suppress the formation of ultra-fine microstructure on the surface of bearing steel was obtained through simulated water cooling tests, and the elimination method of the ultra-fine microstructure was determined. The results indicate that during the water cooling process, due to the high water flow rate and pressure, the instantaneous surface temperature may be lower than the Ms point of the bearing steel, resulting in the formation of martensite on the surface. After entering the water, it reheats to above 600 ℃, forming the tempered martensite. The cooling intensity is the main cause of the formation of ultra-fine microstructure, and when the cooling intensity is high, the ultra-fine microstructure is easily to completely cover the decarburization layer. Under the heating condition of 800 ℃, the ultra-fine microstructure is completely eliminated, and most of it constitutes the decarburization layer of the bearing steel. The original acicular tempered martensite transforms into resembling spheroidized pearlite and lamellar pearlite. In the simulated water cooling test, when the measured surface temperature of the bearing steel after breaking out of the water is ≥520 ℃, the microstructure of the specimen surface is composed of pearlite. When the measured surface temperature of the bearing steel after breaking out of the water is ≤330 ℃, the martensite layer on the surface completely covers the decarburization layer. And under the same cooling conditions, the appearance of decarburization layer can easily deepen the depth of martensite layer on the surface.

Preparation of bearing steel entails casting, solidification, and solid-state phase transformation, each crucial in controlling its microstructure evolution, particularly the precipitation behavior of carbides. This precipitation behavior represents a key technical bottleneck in enhancing rolling contact fatigue properties. After decades of research, the mechanisms by which strong magnetic fields influence phase transformations in steel are becoming increasingly understood, suggesting magnetic fields as a valuable tool for structural refinement in steel. Findings show that magnetic fields can enhance melt flow and solid particle migration by altering dendrite growth and morphology, resulting in improved hardness and wear resistance. Furthermore, during spheroidizing annealing, magnetic fields can promote carbide dissolution and uniform refinement, increase dislocation diffusion driving force, and optimize the formation of martensite and carbide precipitation during quenching and tempering processes. However, the response mechanisms of carbide electronic structures in strong magnetic fields remain unclear, with practical challenges such as high costs and limited processing capacity. Thus, further exploration is needed to make magnetic field applications in bearing steel and other high-performance steels more cost-effective and scalable.

By oil quenching and 200 ℃ isothermal salt quenching for GCr15 bearing steel, composite structures (M/B_n) with martensite (M) and different contents of nano bainite (B_n) were obtained. The microstructure, hardness, and impact properties of the GCr15 bearing steel were characterized by means of scanning electron microscope, Rockwell hardness tester and impact testing machine to investigate the effect of nano bainite content in M/B_n composite microstructure on the microstructure and properties of the GCr15 bearing steel. The results show that the microstructure of oil quenched GCr15 bearing steel is mainly composed of spherical particle carbides, tempered martensite, and retained austenite. After isothermal salt quenching, the microstructure of bearing steel is mainly composed of spherical carbides, tempered martensite, bainite, retained austenite, and secondary precipitated fine carbides. The hardness of GCr15 steel gradually decreases with the increase of content of nano bainite. After austenitizing at 860 ℃, salt bath at 200 ℃ for 2 h, and then tempering at 200 ℃, the hardness of the M/B_n composite structure is equivalent to that of the fully martensitic microstructure obtained by austenitizing at 845 ℃, oil quenching and then tempering at 155 ℃. At this time, the content of nano bainite in the steel with M/B_n composite structure is 47%, with a hardness of 61.5 HRC, while that of the steel with fully martensitic microstructure is 61.1 HRC. Compared with the impact absorbed energy (7.67 J) of the steel with fully martensitic microstructure, the impact absorbed energy (9.92 J) of the steel with M/B_n composite structure is increased by 29.34%, indicating that under similar hardness, the M/B_n composite structure achieves better comprehensive mechanical properties.

Isothermal and continuous cooling transformation tests of pearlite in GCr15 bearing steel were carried out by using a dynamic phase transformation instrument. The TTT curve was plotted, the lamellar spacing of pearlite was measured, and the critical cooling rate for the complete transformation of pearlite was calculated. The results show that the nose temperature of the TTT curve for the isothermal pearlite transformation of deformed austenite in the GCr15 steel is 625 ℃, with incubation period of 7 s and complete transformation time of 190 s at this temperature. The lower the isothermal temperature is, the smaller the lamellar spacing of pearlite in the GCr15 steel is. The lamellar spacing of pearlite calculated by using the bulk diffusion mechanism and relevant parameters with the Thermo-calc software is in good agreement with the experimental results. It is calculated that when the isothermal temperature is lower than 680 ℃, the lamellar spacing of pearlite is less than 100 nm. The volume fraction of pearlite transformation in the GCr15 steel decreases with the increase of cooling rate. The volume fraction of pearlite transformation during the continuous cooling process calculated by Umemoto prediction model is in good agreement with the experimental results, and the critical cooling rate for the complete pearlite transformation is deduced to be 1.68 ℃/s.

Effect of addition of 4.6%Mo on high-temperature (500 ℃) long-term mechanical properties and microstructure evolution of a heat-resistant bearing steel was studied by means of universal material testing machine, Rockwell hardness tester and scanning electron microscope. The results show that the addition of 4.6%Mo improves the decreasing trend of tensile strength and hardness of the tested bearing steel without Mo addition under 500 ℃ long-term tempering. After tempering for 100 h, the tensile strength and the hardness of 4.6Mo steel show an upward trend, and the yield strength begins to decrease when the tempering time is 80 h. During the entire tempering process, the percentage reduction of area of 0Mo steel shows an overall trend of first decreasing and then increasing, reaching a minimum value of 43.60% after tempering for 80 h. The percentage reduction of area of 4.6Mo steel does not show significant change within the tempering time range of 0-50 h, and then decreases, reaching a maximum value of 62.16% after tempering for 0 h and a minimum value of 51.84% after tempering for 80 h. With the extension of tempering time, the number and size of Fe₃C phase in the 0Mo tested steel decrease, while the size of M₂₃C₆ phase increases, growing from 25-42 nm after tempering for 10 h to 167-333 nm after tempering for 100 h. As the tempering time increases, there is no significant change in the size of M₂C phase in the 4.6Mo tested steel, but the quantity increases, however, the quantity of M₂₃C₆ phase does not change significantly, but the size increases slightly. The addition of Mo has a significant impact on the nucleation and growth mechanism of carbides, which can improve the tempering stability of the tested steel.

Austenite grain growth behavior at different temperatures (920-1150 ℃) and holding time (0-600 min) of the 100CrMo7-3 bearing steel for wind power gearbox was studied. The austenite grain size was measured by metallographic quantitative method. The austenite grain growth model of the 100CrMo7-3 bearing steel was established by Sellars model and repeated trial method. Based on the austenite grain size at each position of the workpiece under actual production conditions, the austenite grain size at each position at “0” time of the workpiece and the change of austenite grain size with holding time were calculated by DEFORM finite element software. The results show that the austenite grain growth rate is slow when the heating temperature is below 1000 ℃, while the austenite grain growth rate is fast when the temperature is higher than 1100 ℃. At the same heating temperature, the grain growth is rapid during the first 30 min of holding, and gradually slows down after 30 min. The austenite grain growth model of 100CrMo7-3 bearing steel fitted by experimental data is as follows: D^5.02_t-D^5.02₀=7.77×10³⁶texp-764 980RT. Under industrial conditions, due to the influence of specimen size, the austenite grain size of on-site workpieces is larger than that of laboratory specimen. The primary factor affecting the austenite grain size of 100CrMo7-3 bearing steel during the heating and holding process under on-site production conditions is the heating temperature, but the effect of holding time is not significant.

Carburizing, high-temperature tempering and salt bath quenching of a novel 22Cr2Ni3SiMo bearing steel was carried out, and the microstructure and properties under different heat treatment processes were studied. The results show that after carburizing+oil quenching and then tempering at 650 ℃ for 3 h, the carbides volumes fraction on surface layer of the tested steel can be about 7.3%, meeting the technical requirement. After carburizing without high-temperature tempering, the surface structure of the bearing steel directly austenitized and subjected to salt bath quenching consists of martensite+bainite+retained austenite. For the tested steel tempered at high-temperature after carburizing, with the prolongation of salt bath quenching time after austenitizing, the retained austenite content shows a downward trend. The retained austenite content of oil quenched tested steel after carburizing and tempering at high-temperature is relatively lower than that of the salt bath quenched, its surface hardness reaches 63.4 HRC, which is 62.5 and 60.8 HRC when salt bath quenched at 200 ℃ for 4 h and 8 h, respectively, in line with the technical requirements of wind power bearing steel surface hardness of more than 60 HRC.

Effect of a small amount addition of 0.30%Ni on continuous cooling transformation (CCT) behavior and hardenability of a novel Si-Mn-Cr-B type ultrahigh strength spring steel was explored by using thermal dilatometer and end-quenching test, and their CCT curves of undercooled austenite were obtained combined with metallurgical and Vickers hardness tests. The results show that adding 0.30%Ni has considerable effects on the CCT curves of the experimental steel, that is, the phase transformation temperatures decrease at different cooling rates, and the bainite and martensite transformations are promoted. The starting cooling rate for martensite transformation decreases from about 0.78 ℃/s to about 0.48 ℃/s, but the critical cooling rate of obtaining full martensite is barely changed. The hardness test results show that the hardness at different cooling rates of the Ni added steel is higher than that of the Ni-free steel, and their hardness difference exhibits a sharp increase-decrease trend with cooling rate in the range of 0.28-4.40 ℃/s, and adding Ni has almost no effect on the hardness of full martensite. The result of the hardenability tests show that adding Ni element significantly improves the hardenability of the experimental steel, making the declining trend of the hardenability curve gentler, and the maximum dimension of the steel with full martensite structure increases from 30 mm to 40 mm.

In order to solve the problem of intermetallic compounds produced during diffusion annealing of 4343/AZ31B/4343 alloy laminate, the growth kinetics of intermetallic compounds at the interface of 4343/AZ31B/4343 alloy laminate was studied with 1060/AZ31B/1060 alloy laminate as the control group. Two kinds of laminates were diffusion annealed between 200-400 ℃ for 0.5-3.0 h, respectively. The thickness of intermetallic compound layers at different diffusion annealing temperatures and time was obtained through BSE characteristic and EDS line scanning of the Mg-Al interface. The diffusion kinetics equation was derived, and the growth kinetics models of intermetallic compounds for two kinds of laminates were established based on the obtained thickness of the intermetallic compounds. Comparing the model calculated values with the actual results, it can be seen that the model can simulate the growth of intermetallic compounds during the actual diffusion annealing process, and provide a theoretical basis for the heat treatment of laminates. In addition, in the annealing experiment at 250 ℃ for 0.5 h, there is no intermetallic compound in the 4343/AZ31B/4343 alloy laminate, while 1060/AZ31B/1060 alloy laminate shows a layer of intermetallic compounds with uniform thickness, showing that 1060/AZ31B/1060 alloy laminate is more prone to the precipitation of intermetallic compounds at 250 ℃. During the diffusion annealing process at higher temperatures, both laminates produce two types of intermetallic compounds, with Al₃Mg₂ on the Al side and Mg₁₇Al₁₂ on the Mg side.

Continuous cooling transformation behavior and isothermal transformation kinetics of undercooled austenite of a newly developed martensitic stainless steel 4Cr13Cu at different cooling rates were studied. The linear expansion behavior of the 4Cr13Cu steel was measured by DIL805A high-precision differential thermal dilatometer, and the cooling transformation products were analyzed by OM and SEM. According to the JMA formula, the activation energy and Avrami index of pearlite isothermal phase transformation in 4Cr13Cu steel were calculated. The results show that during the continuous cooling process of undercooled austenite of the 4Cr13Cu steel, when the cooling rate is greater than 0.1 ℃/s, the transformation product is full martensite. When the cooling rate is less than 0.1 ℃/s, the transformation product is a mixture of martensite and pearlite, and the content of pearlite increases with the decrease of cooling rate. The isothermal transformation range of pearlite in the 4Cr13Cu steel is 600-680 ℃, and the nose transformation temperature is about 635 ℃. The calculated values of JMA equations under different isothermal conditions are in good agreement with the measured values. The activation energy of pearlite transformation is about 93.1 kJ/mol, and the Avrami index n is in the range of 1.5-2.5, the nucleation rate of pearlite decreases with the prolongation of isothermal time.

Continuous cooling transformation test of 80QL steel at deformation temperature of 950 ℃, deformation rate of 10 s^-1, deformation amount of 40%, and cooling rate ranging from 0.5 ℃/s to 80 ℃/s was carried out by using Gleeble-3500 thermal simulation machine. The transformation point temperature was measured by dilatometry method and the CCT curve was plotted. The microstructure evolution under different cooling rates was investigated by using SEM and EBSD, and Rockwell hardness was measured. The results show that the Ac₁ temperature of the 80QL steel is 729 ℃, and the Ac₃ temperature is 886 ℃. When the cooling rate is in the range of 0.5-5 ℃/s, only pearlite transformation occurs. With the increase of cooling rate, the lamellar spacing of pearlite decreases from 138 nm to 127 nm, and the grain size decreases from 9.15 μm to 6.64 μm. When the cooling rate is within the range of 10-20 ℃/s, both pearlite and bainite phase transformations occur simultaneously, and with the increase of cooling rate, the bainite content increases. When the cooling rate reaches 30 ℃/s, martensitic transformation occurs, and the martensite content gradually increases with the further increase of cooling rate. When the cooling rate is 80 ℃/s, it is all martensite and the hardness reaches 64 HRC. According to the EBSD test results, as the cooling rate increases, the grain size decreases and the dislocation density increases, which is also the reason for the increase in hardness.