In this work, a strategy is proposed by combining prior cold deformation with chronological control of phase transformation to achieve ultrafine martensite-bainite duplex microstructures in GCr15 bearing steel, which is the specimens with 0 similar to 45 % prior cold deformation are subjected to first martensite transformation at below martensite start (Ms) temperature to form different fraction of first transformed martensite due to the decreasing of Ms temperature caused by prior cold deformation and subsequent transfer to isothermal holding at above Ms temperature for bainite transformation. It is found that prior cold deformation can further accelerates bainite transformation by shortening incubation and growth period, and increasing bainite transformation rate. The specimen with 30 % prior cold deformation after heat-treated by austenitizing at 850 degrees C for 30min, first martensite transformation at 200 degrees C for 5min and subsequent bainite transformation at 240 degrees C for 30min, a marginal improvement of strength and significant improvement of impact toughness are achieved, which is increased by 2.8 % and 20.9 % compared to the specimen with 0 prior cold deformation. Meanwhile, ultrafine bainite platelets of similar to 220 nm are achieved, which is refined by 35 % compared to the specimen with 0 prior cold deformation. In addition, the effect of prior cold deformation on the evolution of martensite-bainite duplex microstructures is also discussed.

In this work, the effect of prior cold deformation on the bainite transformation kinetics of austempered GCr15 bearing steel was investigated by means of dilatometry. It is found that prior cold deformation retards bainite transformation by increasing the incubation and growth time and decreasing the transformation rate, indicating that the effect of carbon content in parent austenite on bainite transformation kinetics is larger than the refinement of prior austenite grain size. Meanwhile, it also found from microstructures analysis that the prior cold deformation is beneficial to refine prior austenite grain size, facilitate dissolution of spherical carbides and improve its aspect ratio. After the heat treatment of austenitizing at 850 °C for 30 min and following austempering at 240 °C for 30 min, a marginal improvement of tensile strength and significant improvement of impact toughness were achieved in specimen with 30% prior cold deformation, which is increased by 6.2% and 29.3%, as compared to that of the specimen without prior cold deformation. In addition, the effect of prior cold deformation on microstructural evolution is also discussed.

目前,采用宏观塑性有限元方法进行环件轧制成形模拟已成为企业提高环件生产效率和降低制造成本的重要方式.同时,通过多尺度数值模拟方法研究环件轧制过程中成形参数与内部组织演变规律的交互关系,建立环件轧制过程中传热-变形-组织演变定量模型成为近年来的研究热点.简述了环件轧制领域宏观和介观尺度数值模拟建模方法的国内外研究现状以及发展趋势,探讨了目前环件轧制多尺度数值模拟技术的局限性,指出未来建立准确的内变量模型以及多数值模拟方法耦合模型是实现环件轧制传热-变形-组织演变动态同步数值模拟的发展方向.

In this work, effect of martensite pre-quenching on the subsequent bainite transformation kinetics of GCr15 bearing steel has been investigated by means of dilatometry. The results show that the pre-quenched martensite is significantly accelerate nucleation of subsequent bainite transformation in comparison with direct bainite austempering. Meanwhile, specimen of martensite pre-quenching at 200 °C has the shortest incubation period of bainite transformation at 240 °C. Moreover, the transformation rate of bainite transformation after martensite pre-quenching is faster than direct bainite austempering in the initial growth stage, and decrease with the decreasing of martensite pre-quenching temperature. Compared with the conventional quenched and tempered (QT) specimen, the tensile strength is slightly decrease from 2170 MPa to 2143 MPa, but the impact toughness is obviously increase from 43 J to 71 J in specimen heat-treated by martensite pre-quenching at 200 °C and subsequent bainite transformation at 240 °C. In addition, the mechanical stability of retained austenite in all martensite pre-quenching specimens are higher than that of QT specimen.

In this work, the ultrafine spheroidized carbides were obtained by combining warm deformation with the divorced eutectoid transformation (DET) in GCr15 bearing steel. The results show that the temperature range of DET is approximately 730 °C to 700 °C at a cooling rate of 0.05 °C s−1. It is also found that excellent spheroidized microstructure can be obtained and less deformation resistance can be generated when the deformation is carried out at 720 °C with a strain rate of 0.5 s−1. This result could be ascribed to the fact that deformation provides the driving force for transformation, dynamic recrystallization (DRX) and reduced dislocation density through the coordination of warm deformation and DET. This suggests that the deformation at the initial temperature is more favorable to the spheroidization than that at the intermediate temperature of DET. In addition, it is also proved that the warm rolling forming in the actual production line can shorten the spheroidization time from at least 10 h to 2 h and obtain ultrafine spheroidized carbides with the roundness close to 1. This research may provide a novel pathway to obtain good spheroidization effect with less energy conservation.

The microstructure and mechanical properties of M50 steel subjected to combining cold rolling (CR) with austempering are investigated. The microstructure is characterized using X-ray diffraction and scanning and transmission electron microscopy. The mechanical properties are measured using the uniaxial tensile and Charpy impact tests. It is observed that an excellent combination of ultimate tensile strength (2536 MPa) and impact toughness (128 J) was achieved by combining austempering with CR; of which the ultimate tensile strength increased by 10% compared with traditional martensite quenching-tempering (Q-T) specimen and the impact absorbed energy exhibited 2.3 times of that in Q-T specimen. The observation of the microstructure indicates that CR obviously refines the thickness of bainite sheaves, which is favorable for the formation of ultrafine equiaxed ferrite during tempering.

This work reasonably utilizes the decreasing of martensite start (Ms) temperature caused by prior cold deformation, combining martensite pre-quenching and bainite transformation to develop ultrafine martensite-bainite duplex microstructures for improves toughness of GCr15 bearing steel. Comparison with the conventional quenched and tempered (QT) specimen, the remarkable improvement of impact toughness (similar to 87 J) and fracture toughness (similar to 39 MPa.m(1/2)) were achieved in 30% prior cold deformed specimen, while with a slight decrease in the hardness. The enhanced toughness was attributed to the nanometric martensite-bainite duplex microstructures and the formation of film-like retained austenite. (C) 2018 Elsevier B.V. All rights reserved.

In this work, the effects on dry wear behavior of cold ring rolling (CRR) of GCr15 bearing steel, after quenching and tempering (QT) heat treatment are investigated. The effects on steel microstructures and wear mechanisms of CRR with different austenitizing times are also discussed. The results show that, with a short austenitizing time of 10 min, CRR can increase the retained austenite content, decrease the undissolved carbide content and improve the hardness of the specimen, thus reducing ploughing and fatigue flaking, and decreasing the wear loss of the CRR specimen. With the longer austenitizing time of 20 min, the retained austenite content increases, the undissolved carbide content decreases, and the hardness increases significantly, both in specimens with and without CRR, so that ploughing, fatigue flaking, and wear loss can all be decreased. However, with an austenitizing time of less than 20 min, the effects of CRR on retained austenite content, undissolved carbide content, and hardness are not significant. Thus, CRR of less than 20 min cannot further improve wear morphology or decrease wear loss.

In this work, the carbide dissolution and precipitation of M50 bearing steel were investigated as a function of cold rolling reduction. It is observed that the prior cold rolling not only enhances the dissolution of primary carbides, but also contributes to the carbide precipitation. Additionally, the influence of prior cold rolling on the dry wear behaviors of M50 bearing steel was studied, finding that the friction coefficient as well as wear rate decreases with the increasing cold rolling reduction. This result could be attributed to the formation of stable self-lubricating film within the subsurface layer for the specimens with cold rolling.

This study investigates the correlation between the residual stress and distortion behavior of a cold-rolled ring from the annealing to quenching-tempering (QT) process. Due to the cold-rolled process, the external periphery of the bearing ring experiences a compressive residual stress. To relieve the residual stress, cold-rolled rings are annealed at 700 °C which is higher than the starting temperature of recrystallization. When cold-rolled rings are annealed at 700 °C for 15 min, the compressive residual stress is reduced to zero and the outer diameter of the annealed ring becomes larger than that of a non-annealed sample, which is unrelated to annealing time. Simultaneously, the roundness and taper deviation do not obviously change compared with those of non-annealed sample. The stress relaxation during the annealing process was attributed to the recovery and recrystallization of ferrite. Annealing has a genetic influence on the following QT heat treatment, wherein the lowest residual stress is in the non-annealed cold-rolled ring. From the annealing to QT process, the deviation of the outer diameter, roundness, and taper increased with annealing time, a large extend than that of non-annealed samples.

In this work, the effect of prior cold deformation on the stability of retained austenite in GCr15 bearing steel was investigated after quenching and tempering treatment. The thermal stability was evaluated by calculating thermal activation energy for decomposition of retained austenite using differential scanning calorimeter. The mechanical stability was investigated according to the strain-induced martensitic transformation behavior of retained austenite under the standard compression testing. It is found that the prior cold deformation not only accelerates the carbide dissolution during the austenitization process but also contributes to the carbon partitioning in the tempering stage due to the higher density of phase boundaries, which results in the improvement of the thermal stability of retained austenite. Due to the enhanced carbide dissolution, the higher carbon content in the prior austenite will intensify the isotropic strain of martensitic transformation. As a consequence, the film-like retained austenite is likely to form under a higher hydrostatic pressure and thus shows a higher mechanical stability. Additionally, it is noteworthy that the benefits of the prior cold deformation to the stability of retained austenite would be saturated when the cold deformation degree is larger than 40%.

The susceptibility of hydrogen embrittlement (HE) in GCr15 bearing steel under two types of heat treatments: quenching and tempering (QT) and pre-quenching and austempering (PQA) were investigated. Results showed that PQA-treated specimen have higher mechanical stability of the retained austenite (RA) compared with QT-treated specimen. The experiment of indentation test and hydrogen bubbles suggested that PQA-treated specimen was less susceptible to hydrogen than the QT-treated sample. Subsequently, hydrogen permeation tests revealed significant differences in diffusion coefficient and the number of hydrogen trapping sites between QT and PQA specimens. It was demonstrated that PQA is an appropriate heat treatment to tailor the stability of the RA and enhances the resistance to HE of GCr15 bearing steel. This paper is part of a thematic issue on Hydrogen in Metallic Alloys

This paper concerns the function of retained austenite to resist local plastic indentation deformation in bearing steel. Three austenitizing temperatures were used to adjust the morphology, volume fraction and carbon content of retained austenite in samples. The volume fraction of retained austenite was measured by magnetic method. The higher austenitizing temperature result in higher fraction of retained austenite and higher carbon content in it, and changes the morphology of retained austenite from film-like to blocky. Resistance to plastic indentation of retained austenite was measured by nanoindentation on samples with different heat treatments. Strain induced martensite transformation was observed by pop-in phenomena on indentation curves. Results showed that indentation resistance depends mostly on the local stability of retained austenite rather than the fraction and morphology. Furthermore, the local stability of retained austenite under indentation deformation is mainly attributed to the carbon content.

This study presents mechanical spectroscopy of bearing steel subjected to different heat treatments. A non-thermally activated maximum, P1, was found at 130°C, in quenched martensitic samples, which were austenitized at 1050°C and 860°C, and presented twin martensite microstructures. It is suggested that the mechanism of the P1 maximum, observed on the low-temperature side of Snoek-Köster peak, is related to the change of defect configurations in twinned martensite assisted with high mobility of the solute carbon atoms under an external harmonic stress field applied during mechanical loss measurements.

In this work, the influence of sub-zero Celsius treatment and tempering on the mechanical and thermal stability of retained austenite in bearing steel were assessed by tensile test and DSC. Compared with traditional quenched and tempered treatment, sub-zero Celsius treatment obviously decreases the volume fraction of retained austenite. Moreover,the mechanical stability of retained austenite was enhanced due to the accumulation of compressive stresses in retained austenite after sub-zero Celsius treatment and tempering. Meanwhile, the morphology of retained austenite changed from film-like to blocky with austenitization temperature increasing, and the mechanical stability of film-like retained austenite is higher than that of blocky one. The DSC results showed that the activation energy of retained austenite decomposition slightly increased through sub-zero Celsius treatment and tempering. This result may probably be ascribed to partitioning of carbon during tempering. However, the temperature at which retained austenite starts to decompose is unchanged.

利用双层辉光等离子渗金属技术,在20号钢表面进行钨-钼-钇和钨-钼共渗,并获得合金渗层.利用扫描电镜(SEM)和能谱仪(EDS)进行微观组织分析和成分分布检测,用X射线衍射仪(XRD)进行结构分析,用Fick第二扩散定律分别计算上述二元共渗和三元共渗渗层的钨、钼扩散系数以及扩散激活能.研究表明:钨-钼-钇和钨-钼共渗渗层组织均为柱状晶,渗层与基体之间有一条分界线.稀土钇的加入细化了渗层的组织;钨-钼-钇共渗层物相主要为:Fe(W,Mo,Y),Fe3Mo,Fe17Y2,W,Y等;钨-钼共渗渗层物相主要为:Fe(W,Mo);钇的加入使渗层中钼在距表面12～15,24 ～ 25,35～ 36 μm处的扩散系数分别提高了1.72,1.85,2.10倍,平均增大1.89倍.而钨原子的扩散系数在加入稀土元素后减小了0.92,0.99,0.76,平均降低了0.89倍;稀土钇的加入降低了钼原子的扩散激活能,说明了稀土钇对钼具有催渗作用,但对钨没有催渗作用.

The BCB (BaCu(B2O5)) powders were synthesized by solid state reaction method and its effects on the microstructures, the phase formation and the microwave dielectric properties of MCT (Mg0.95Ca0.05TiO3) ceramics were investigated. BaO and B2O3 first forms BaB2O4, and then BaB2O4 reacts with CuO forming BCB. The sintering temperature of MCT ceramics with as-synthesized BCB addition can be effectively lowered from 1450°C to 1100°C due to the liquid phase effect. The formation of second phase (MgTi2O5) was restrained. The 3wt% BCB-doped MCT ceramics sintered at 1100°C for 3h have optimum microwave dielectric properties of Kr=21.5, Q×f=28000GHz, and TCF=-3.3ppm/°C. Obviously, BCB could be a suitable sintering aid that improves densification and microwave dielectric properties of the MCT ceramics.

The effects of BaCu(B2O5) (BCB) and ZnO co-doping on the sintering properties and dielectric properties of 0.95MgTiO3-0.05CaTiO3(95MCT) microwave dielectric ceramics were investigated. The crystal phase structure and micromorphology of the 95MCT ceramics were studied by XRD and SEM. The results show that the sintering temperature of the 95MCT ceramic can be reduced from 1 400 ℃ to 1 050 ℃ and it can be co-fired with Cu. Formation of second phase MgTi2O5 can be effectively restrained through the addition of ZnO. After sintering at 1 050 ℃ for 3 h, the 95MCT ceramic co-doped with 3.00% BaCu (B2O5) and 1.00% ZnO (mass fraction) shows good dielectric properties of er= 20.5,Q·f = 21 133 and τf = – 10.1×10–6/ ℃ at 7 GHz.

The Effects of BaCu(B2O5 )on the dielectric properties of 0.95MgTiO3-0.05CaTiO3 microwave dielectric ceramics were investigated.The crystalline structure and micromorphology were studied by XRD and SEM,respectively.The sintering temperature of BaCu(B2O5 )-doped 0.95MgTiO3 -0.05CaTiO3 ceramics can be lowed to 1100℃and effectively inhibited the formation of second phase MgTi2O5 .The microwave dielectric Properties of 0.95MgTiO3-0.05CaTiO3Ceramics with 3wt%BaCu(B2O5)additions sintered at 1100 for 3h showed ε r of 22.9,Q×f of 25,000(7 GHz),τ f of-3.3 ppm/℃.

The effects of BaCu(B2O5) (BCB) addition on the microstructure and microwave dielectric properties of 2.5ZnO-2.5Nb2O5-5TiO2 (ZNT) ceramics were investigated. The bulk density of the specimens is higher than that of pure ZNT specimens when the sintering temperature is above 900°C. The dielectric constant εr increases initially and then decreases slightly with the increase of BCB content. The product of quality factor and resonance frequency of Qf value degrades with the addition of BCB because of the presence of the liquid phase in the sintering. The temperature coefficient of the resonance frequency τf is effectively lowered compared with the pure ZNT specimens. The 3% in mass BCB-added ZNT ceramics sintered at 900°C for 3 h have good dielectric properties of εr=48, Qf = 15 258 GHz, and τf = 41 × 10-6/°C at 5 GHz.