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The effects of traditional heat treatment (quenching and then tempering) and deep cryogenic treatment on the microstructure and mechanical properties of a low-carbon high-alloy martensitic bearing steel were studied by Rockwell hardness test, X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. The results show that the deep cryogenic treatment promotes the transformation of the retained austenite to martensite during cooling, which leads to the hardness of the sample after deep cryogenic treatment higher than that at the quenched state. Also, the carbon content in the martensite matrix after different treatments was calculated and the results indicated that deep cryogenic treatment can promote the segregation of carbon atoms in martensite to dislocations. The segregated carbon atoms act as and grow into nuclei for the formation of fine carbide particles during subsequent tempering. And this resulted in the fact that the hardness of the tempered experimental steel after deep cryogenic treatment is higher than that without deep cryogenic treatment.

The fretting wear performance of high-nitrogen stainless bearing steel (40Cr15Mo2VN) under lubrication conditions was researched. Lithium-based grease was prepared by using MoS2 and carbon nanotubes (CNTs) as additives. AISI 52100 steel ball was used in four-ball test to evaluate the extreme pressure property and wear resistance of grease. After four-ball test, the grease adding 0.8 mass% MoS2 and 0.8 mass% CNTs, respectively, was chosen and used for fretting test. AISI 52100 ball and 40Cr15Mo2VN steel disc were used as the upper and lower samples for fretting test. The results showed that wear power consumption increased with the increase in both sliding velocity and contact stress. When initial contact stress was 2.047 GPa, the main wear mechanisms were abrasive wear and plastic deformation as the velocity increased from 0.028 to 0.112 m/s. When the velocity was 0.028 m/s, the main wear mechanisms changed from abrasive wear to adhesion wear and finally to abrasive wear and adhesion wear as the initial contact stress increased from 1.788 to 2.579 GPa. The volume loss grew sharply because of the changes in wear mechanisms. In this condition, the volume loss growth rate can be divided into three regions according to different wear power consumption ranges corresponding to different wear mechanisms.

The effect of heat treatment and deep cryogenic treatment on microstructural evolution of low carbon martensitic bearing steel was investigated. The experimental results showed that the lath martensite was obtained by quenching and a few twins as substructures formed in some martensitic laths. The rudiment of sub-interfaces of martensitic lath was formed in the high-density dislocation regions after deep cryogenic treatment; meanwhile, the number of twins increased, especially in the high-density dislocation regions. This phenomenon is due to the increase in internal stress caused by cryogenic treatment. After tempering, the rudiment of sub-interface further evolved into the martensitic lath boundary, and thus the original martensitic laths were refined. The twins formed by cryogenic treatment did not disappear after tempering. In addition, small quantities of annealing twins formed in tempering process. Martensitic laths morphology and substructures in different stages of the heat and deep cryogenic treatment were observed by transmission electron microscopy.

The carbides precipitation and their evolution at elevated tempering temperature in Cr15Co10Mo5-alloyed heat-resistant bearing steel were investigated by means of the transmission electron microscope. The results show that there is no carbide precipitated from the martensitic matrix when the sample was tempered at 480 °C. However, when the sample was tempered at 540 °C, a large number of stable spherical M6C carbides precipitated in the test steel. Nevertheless, there are three types of carbides precipitated from the matrix including M6C, M2C and M23C6 carbides when the tempering temperature reached 600 °C. The mechanical properties also present a correlation with the evolution of carbides during tempering at different temperatures. This indicates that the strengthening mechanism of the steel is mainly attributed to the precipitation of carbides and their evolution, including the morphologies and types of carbides at different temperatures. In addition, the austenitic layers with a thickness of about 30 nm have been obtained between the martensite laths after tempering at 600 °C. The austenitic layers will produce transformation-induced plasticity effects to improve the toughness of the steel.

It is evident from controversial statements in the literature that the effects of deep cryogenic treatment on the microstructural evolution of low carbon martensitic steels are not fully understood yet. An investigation has been started to clarify this situation by analyzing the microstructural evolution and alloy precipitation after aging. The results show that the hardness is enhanced after deep cryogenic treatment and this is mainly contributed to the transformation of retained austenite to martensite at low temperature. By comparing with the samples subjected to quenching and deep cryogenic treatment, the hardness behaviors produced different variation tendency and characteristics during aging. According to obtained results and comparing with the characterization of carbides during aging, it showed that deep cryogenic treatment is an advantage for the formation and precipitation of alloy carbides during aging. The results indicated that deep cryogenic treatment increases the diffusion driving force of atoms (especially the carbon atoms) which promotes the formation of fine carbides in the process of aging.

The microstructural changes and thermal stability of a cobalt-containing martensitic heat resistance bearing steel have been investigated in this paper. The hardness variation showed a progressive hardness decrease associated with coarsening of fine carbides at elevated temperatures. The precipitation of secondary phases during tempering at 500 °C for 10 h and 100 h has been characterized and identified in detail using transmission electron microscopy. The results revealed that the aging treatment induced very fine secondary M6C precipitates which were responsible for the secondary hardening peak when tempered at 500 °C for 30 h. But the hardness gradually decreased due to the coarsening of M6C carbide and other secondary phases (such as μ phase, σ phase, and χ phase) precipitation when the samples were tempered over 30 h at 500 °C.

Wear behavior and mechanism of plasma nitrided steel oscillating against a heat-treated and an untreated aluminum bronze alloy were investigated using an Optimol SRV tribometer. The influence of heat treatment on the mechanical properties of the alloy was evaluated. Furthermore, the wear debris was also examined to understand the wear mechanisms. The results show that a 220–230 μm nitrided layer, which was harder than the substrate, was obtained on the steel surface. The tensile strength and hardness of the alloy are found to be significantly improved by the heat treatment associated with low impact toughness. The heat treatment of the alloy did not obviously decrease the friction coefficient of the nitrided steel-bronze couple. However, the wear loss of the nitrided steel increased when it mated with the treated bronze by a severe three-body abrasion. The nitrided steel was mainly damaged by fatigue spalling. Under plane contact conditions, the wear debris was mainly generated from the bronze part and can escape from the interface before being oxidized, leading to the phase structure of all the debris being copper rather than copper oxides.

The dynamic recrystallization and carbides precipitation of the Cr-Co-Mo-Ni bearing steel were investigated by hot compression tests performed at temperatures ranging from 850 °C to 1080 °C with strain rate of 1-20 s-1. The activation energy (Q) for the tested steel is calculated to be around 682.99 kJ/mol at a deformation strain of 0.6. Microstructural analysis by SEM shows that the dynamic recrystallization (DRX) behavior is dependent sensitively on the deformation strain, temperature and strain rate, while an exponential relationship between DRX grain size and Z parameter is obtained from the computational formula. Moreover, the M6C-type carbides (

Utilizing Pro-cast software, the whole vacuum arc remelting process of high-alloy bearing steel ingot (the diameter was 160 mm and the high was 600 mm) was simulated. And moving face quality and moving boundary conditions were added to the simulation. Purposes of the simulation were to explore the influence of smelting powers on the temperature field, pool shape and solidification microstructure in vacuum arc remelting process. The depth of molten bath gradually increased and stabilized finally and the pool shape transferred from flat to funnel. When smelting power increased, the depth of molten pool became deeper and the width of mushy zone slightly reduced; the size of primary dendrite and secondary dendrite spacing increased significantly; the percentage and size of columnar crystals also increased. A reasonable power-time cure was given to guide industrial melting after simulation, the size of molten pool morphology and microstructure were controlled in an ideal range under the reasonable cure. The simulated grain morphology agreed well with the experimental pickling result.

Due to the difficulty of manufacturing complex small pieces ofthe new bearing steel0.30C-Cr-W with high-performance in the aviation industry, the electron beam welding process are usually used for preparing the complex parts.Thewelding performance of 0.30C-Cr-W steelwas calculated by theory and confirmedby actual experiments.The base metal was preheated,then welded and heattreated for weldments.The purpose of this work is to study the properties and the microstructure changes of base metal after heat treatment, weldments before heat treatment and weldments after heat treatment.The results show that the base metalafter heat treatment is sorbitic,weldzone of weldments after heat treatmentismartensite and austenite, theprecipitated M23C6 and M6C in latter (the weld zoneof weldmentsafter heat treatment) were more thanthe former (the base metalafter heat treatment), and the strengthof the latter is 23.4%lower than the former, theroom temperature impact absorbing energy and hardness of the latter increased 13.5% and 6.6%. It may be considered that0.30-C-Cr-W steel is not suitablefor welding by the theoretical calculations, but after reasonable preheat and post-weld heat treatment, the mechanical properties of the weld afterheat treatment can satisfy the requirements.

Microstructural transformations and mechanical properties of a low carbon martensite stainless bearing steel treated with different heat treatment parameters and cryogenic treatment (-82°C) were investigated. The function of microstructural transformations on strengthening and toughening process was quantitatively characterized as well. These analyses were performed by the optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and electron back scattering diffraction (EBSD) technique. The obtained results show that with execution of cryogenic treatment and tempering, the tensile strength increase owing to the reduction of retained austenite and fine carbides precipitating respectively. The effect of martensitic microstructure on yield strength increment can be regarded as packet size and block width which conform to Hall-Petch relationship. Meanwhile, the results suggest that the block width is the key structural controlling unit when analyzing the strength-structure relationship of lath martensite in low carbon martensite stainless bearing steel. In addition, packet size can be related to toughness controlling as well because of the same size as cleavage plane.

The low-carbon Cr-Ni-Mo carburizing bearing steel was tested with different heat treatment processes. Quenching-tempering temperature and cryogenic treatment (-73°C) wasstudied respectively onthe mechanical properties and microstructure.Results show thatthe increase of quenching temperature causes the micron-sized Cr-rich carbide re-dissolution and smaller quantity of retained austenite, makingthe strength and hardness of the tested steel increase and the impact toughness decrease. The tempering temperaturerising causesthe reduction of micro-residual stresses and smallerdegree of lattice distortion andlower dislocation density, resulting in the decrease of strength and the increase of impact toughness. Cryogenic treatment contributes to the refinement of martensite lath and precipitation of nanosized carbide and lowest quantity of retained austenite, improving the strength and impact toughness of the steel.The good comprehensive mechanical propertieswith the hardness of HRC41.3, tensile strength of 1413MPa,yield strength of 1168MPa, and impact toughness of 162J/cm2 can be obtainedby optimizing the heat treatment process parameters.

The effects of different heat treatment parameters and cryogenic treatment (−75 °C) on microstructural changes and mechanical properties of a low-carbon martensitic stainless bearing steel were investigated. These analyses were performed via the optical microscope (OM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The obtained results showed that the execution of cryogenic treatment on quenched and tempered bearing steel increases hardness, tensile strength and decreases toughness with the increment of cryogenic treatment and tempering cycles. This paper also showed that the cryogenic cycle׳s treatment incorporating tempering can refine the martensite laths resulting in improvement of tensile strength. In addition, cryogenic treatment further reduces the retained austenite content but it cannot make retained austenite transform into martensite completely even tempering at high temperature.

The carburizing behavior of low-carbon Cr-Ni-Mo, the nitriding behavior of C-Cr-W bearing steel and the carburizing process prior to nitriding of low-carbon Cr-Ni-Mo bearing steel were mainly investigated in this paper through the contrastive analysis of microstructures and properties by employing scanning electron microscope (SEM), transmission electron microscope (TEM) and X-Ray Diffraction XRD, etc. The result indicated that compared to the simple carburizing and nitriding process, the material that experienced the duplex surface finishing process could obtain a good infiltrated layer’s microstructure and excellent mechanical properties. The thickness of infiltrated layer was 1.8mm. The hardness of surface could reach 1100HV, and the infiltrated layer had a reasonable hardness gradient distribution. Meanwhile the test materials through duplex surface finishing process exhibited good friction properties in the aspect of point contact in dry friction.

To reveal the effect of deformation ratio on high nitrogen martensitic stainless steel, the room temperature mechanical properties, microstructures and precipitates morphology were studied by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) as well as mechanical tests. The results indicate that the tensile strength, yield strength and impact energy were raised 241MPa, 180MPa and 10.0J respectively, at a higher deformation ratio. While, the average original austenite grain size was refined from 14.93μm to 9.57μm when the deformation ratio was increased from 0.65 to 0.83 as well as the martensite lath width thinned from 400nm to 250nm. Furthermore, at various deformation ratios, the precipitates morphology was different in quantity, grain size and dispersion. There are more eutectic precipitates and secondary precipitates dispersed along the grain boundary as continuous chains with lower deformation ratio. On the contrary, the precipitates in specimen with higher deformation ratio were uniform and fine without chain or network. It can be inferred that the improvements on mechanical properties, microstructures and precipitates morphology are mainly due to the deformation ratio.

The influence of deformation and quenching temperature on microstructure and properties for high nitrogen stainless bearing steel was studied using the calculation of Thermo-Calc software and relevant Fe-based database and some experiments. The results showed that there was a small precipitation amount of carbide in the sample after a large deformation. The value of impact toughness (AKU) was increased from 2.0 J to 12.0 J with the deformation increasing from 71% to 83%. When the quenching temperature reached 1050, the value of the hardness was up to the tiptop. Additionally, according to the calculation, it was found that there was a linear functional relation between the precipitation of carbide M23C6(M) and quenching temperature (T). And the relational model could be expressed as:MM23C6=55.34 - 0.05T(T1000).

The solidification microstructure and crystallization of Cr-Co-Mo-Ni bearing steel were simulated in solidification process based on CAFE methods in the electro-slag remelting process. The optimization of the intensity of cooling and molten pool temperature was simulated for Cr-Co-Mo-Ni bearing steel. The result shows that raising the cooling intensity to 7.5 m3/h and reducing the superheat temperature to 1600 can increase the grain nucleation number about 13.83%, and reduce the average columnar grain radius about 10.61%, to have a very good effect on refining the grains and get dense uniform solidification, which improve the homogeneity of the solidification structure, and reduce the performance differences of the materials.

In order to inhibit its decomposition and improve its compatibility with the metal matrix during laser cladding, WS2 powder was encapsulated with a layer of micro Ni–P by electroless plating. The microstructure and tribological properties of the NiCr–Cr3C2/30%WS2 and NiCr–Cr3C2/30%WS2(Ni–P) high temperature self-lubricating wear-resistant composite coatings at RT (room temperature), 300 °C and 600 °C were investigated, respectively. It was found that the NiCr–Cr3C2/30%WS2(Ni–P) coating had a microstructure consisting of primary Cr7C3 dendrite, γ-(Fe,Ni)/Cr7C3 eutectic and solid lubricant particles WS2 and CrS, Ni–P electroless plating had decreased the decomposition of WS2 to some extent, the WS2 solid lubricant particles dispersed in the ductile γ-(Fe,Ni)/Cr7C3 matrix. Friction and wear experiments indicated that the tribological properties of the NiCr–Cr3C2/30%WS2(Ni–P) coating was better than that of NiCr–Cr3C2/30%WS2 coating, the NiCr–Cr3C2/30%WS2(Ni–P) coating presents lower friction coefficient at RT and 300 °C and lower wear rate from RT to the elevated temperature of 600 °C.

The high temperature self-lubricating wear-resistant NiCr/Cr3C2–30%WS2 coating and wear-resistant NiCr/Cr3C2 coating were fabricated on 0Cr18Ni9 austenitic stainless steel by laser cladding. Phase constitutions and microstructures were investigated, and the tribological properties were evaluated using a ball-on-disc wear tester under dry sliding condition at room-temperature (17 °C), 300 °C and 600 °C, respectively. Results indicated that the laser clad NiCr/Cr3C2 coating consisted of Cr7C3 primary phase and γ-(Fe,Ni)/Cr7C3 eutectic colony, while the coating added with WS2 was mainly composed of Cr7C3 and (Cr,W)C carbides, with the lubricating WS2 and CrS sulfides as the minor phases. The wear tests showed that the friction coefficients of two coatings both decrease with the increasing temperature, while the both wear rates increase. The friction coefficient of laser clad NiCr/Cr3C2–30%WS2 is lower than the coating without WS2 whatever at room-temperature, 300 °C, 600 °C, but its wear rate is only lower at 300 °C. It is considered that the laser clad NiCr/Cr3C2–30%WS2 composite coating has good combination of anti-wear and friction-reducing capabilities at room-temperature up to 300 °C.

In order to improve the high-temperature wear resistance of austenitic stainless steel, a wear resistant composite coating reinforced with hard (Cr,Fe)7C3 carbide and toughened by ductile γ-(Ni,Fe)/(Cr,Fe)7C3 eutectic matrix was fabricated by a novel central hollow laser cladding technique. The constituent phases and microstructure as well as high-temperature tribological behaviors of the Ni-based coating were investigated, respectively, and the corresponding wear mechanisms were discussed. It has been found that the composite coating exhibits superior wear resistance than substrate either at ambient or high temperatures. The coating shows better sliding wear resistance at 600 °C than 300 °C owing to high-temperature stability of the reinforced carbide and polishing effect as well as formation of continuous lubricious films, which implied it has large potential industrial applications at relatively higher temperatures.