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Microstructure and mechanical properties in core of a carburizing 20CrNi2MoV bearing steel subjected to cryogenic treatment were investigated. Conventional treatment sample was quenched and tempered at 180 °C for 2 h. Cryogenic treatment samples were quenched, cryogenically treated at − 80 and − 196 °C for 4 h, slowly returned to room temperature and thereafter tempered at 180 °C for 2 h, and finally tempered at 180 °C for 2 h. The scanning electron microscope, electron backscattering diffraction, X-ray diffraction and transmission electron microscope were adopted for microstructure characterization. The results show that cryogenic treatment increases the fraction of high-angle grain boundaries and the precipitation of finely dispersed carbides in the matrix, decreases the volume fraction of inter-lath retained austenite, and hence improves the strength and hardness. Compared with the conventional treatment, the hardness, yield strength and ultimate tensile strength of the steel after cryogenic treatment are increased by 11.7%, 12.6% and 18.3%, respectively, while the impact energy is decreased by 9.8%.

In the present study, double-sided gas shield welding (GSW), in a single pass, was applied to Fe-21Cr-15Ni-6Mn-Nb nonmagnetic stainless steel plate. The microstructure and performance of the welded joint were investigated via microstructural and phase analysis, mechanical testing and magnetic analysis. The results showed a satisfactory micrographic appearance with full penetration, and no solidification defects observed in the welded joint, which showed typical epitaxial growth characteristics in the welding solidification. Slight grain growth, with an average size of ~ 18.9 μm, occurred in the heat-affected zone (HAZ). The primary dendrite arms, on both sides of welding joint, grew along the direction of the temperature gradient and met at the centerline of the weld with no parting microstructure in evidence. The secondary dendrite arm spacing was stable in all parts of the weld zone (WZ). The hardness of the weld zone was higher than that of the HAZ and the base metal (BM), with the lowest hardness occurring in the HAZ. The results of tensile testing showed a higher strength of the welded metal (WM) compared to the BM, and a good strength-ductility balance of the welded joint. The impact energy of the HAZ was higher than that of the WZ, corresponding to 60.5 J at room temperature and 52.7 J at − 196 °C. Microvoid coalescence, concurrent with crack propagation along the direction of the long axis of the dendritic sub-grains, was observed during plastic deformation in the WZ. Additionally, the excellent paramagnetic characteristics of the welded specimens were revealed.

The effect of martensite–austenite (M–A) constituents formed in thermo-mechanical controlled process on impact toughness of 20CrNi2MoV steel was investigated. The variation in fraction, size and morphology of M–A constituent and its effect on toughness under different cooling rates were carried out. The result shows that there was no significant change in the fraction of M–A constituent under different cooling rates, but the distribution and size of M–A constituent were greatly influenced by cooling rate, which consequently influenced toughness. The amount of large blocky M–A constituents decreased from 4.7 to 1.7%, while that of elongated M–A constituents increased from 3.8 to 8.6% with the cooling rate increasing from 7 to 26 °C/s, and the corresponding impact energy decreased from 132 to 84 J. The deterioration of impact toughness could be related to the increase in the elongated M–A constituents. The elongated M–A constituents existing along the prior austenite grain boundaries in samples of 26 °C/s could easily lead to the formation of cleavage crack, which then results in the lower crack initiation energy than that of low cooling rate samples.

A systematic investigation on grain growth behavior, grain boundary character distribution and mechanical properties of Fe-21Cr-15Ni-6Mn-Nb non-magnetic stainless steel was performed after solution treatment in temperature range of 1100–1200 °C with various durations. The results showed the hot-rolled plate still maintained fine microstructure with the grain size range of 11.5–19.6 µm after solution treatment. A Nb(C, N) drag mechanism concurrent with a restraining effect of Σ3n boundaries during solution treatment resulted in a slow kinetics with grain growth exponent n of ~ 4. The proliferation mechanism of Σ3n boundaries mainly varied from Σ3 regeneration to new twinning accompanied by grain growth. Twin boundary has an effective strengthening role like a grain boundary. The YS, UTS and elongation of solution-treated plate was above 634.0 MPa, 838.9 MPa, 45.3％, respectively, which were superior to the conventional austenitic stainless steel. The Hall-Petch relationship was determined well between the grain size under the condition of either including or excluding twin boundaries and tensile results. An inherent relationship between k ′ and k ( k ′ and k correspond to the circumstance of including and excluding twin boundaries, respectively), was expressed by k ′ = 1.52037 + 1.67168 k .

The microstructural evolution and mechanical properties of a low-carbon high-strength bainitic steel under different final cooling temperatures were studied. The microstructures of the experimental steel at different final cooling temperatures were composed of acicular ferrite and granular bainite. A decrease in final cooling temperature resulted in an increase in volume fraction of granular bainite and a decrease in volume fraction of acicular ferrite. The specimen with the lowest final cooling temperature (459 °C) exhibited the highest yield strength, tensile strength, and yield ratio, and a lower total elongation than the specimens with the highest final cooling temperatures (501 and 535 °C) because of the higher volume fraction of granular bainite and finer bainitic ferrite lath. The specimen with the lowest final cooling temperature had a lower absorbed energy than that of the highest final cooling temperature, because the strip-like martensite–austenite (M-A) constituents that existed between the bainitic ferrite or in prior austenite grain boundaries reduced its toughness. At an intermediate final cooling temperature of 501 °C, the experimental steel exhibited excellent mechanical properties with a yield strength, tensile strength, and absorbed energy of 825 MPa, 1232 MPa, and 102 J, respectively.

Hot deformation behaviour of Fe–21Cr–15Ni–Nb–V stainless steel was investigated by isothermal compression in the temperature range of 950–1150°C with a strain rate of 0.01–10 s−1. The results showed that complete recrystallisation occurred beyond 1050°C, resulting from the pinning effect of (Nb, V)(C, N). The nucleation of dynamic recrystallisation (DRX) was performed by the bulging, sub-grain swallowing and twinning mechanism. With increasing strain rate, new twinning was transformed into the Σ3 regeneration mechanism in the partial DRX region, while an opposite transformation was observed in the complete DRX region. In the partial recrystallisation region, grain rotation resulted in the formation of 110 orientation. In the complete recrystallisation region, the texture tended to distribute randomly at a high strain rate, and the grain growth was accompanied by the emergence of stable 100 orientation.

Using thermo-mechanical control process, the online spheroidization annealing process of 1.0C-1.5Cr bearing steel was designed. Apart from intercritical online spheroidization (IS), a novel subcritical online spheroidization (SS) process was proposed, which is characterized by water-cooling to around 773 K (500 °C) after the final rolling pass, and then directly reheating to 973 K (700 °C) for isothermal holding. Compared with the results from the traditional offline spheroidization (TS) process, the size of spheroidized carbides is similar in both the TS and IS processes, whereas it is much smaller in the SS process. After spheroidization annealing, microstructure evolution during austenitization and quenching treatment was examined. It is shown that the refining of spheroidized carbides accelerates the dissolution of carbides during the austenitizing process, and decreases the size of undissolved carbides. In addition, the SS process can obtain finer prior austenite grain after quenching, which contributes to the enhancement of final hardness.

The microstructural transition and mechanical properties of Fe-Cr-2Ni-Mo-V steel were investigated in a carburizing process with carburizing at 935 °C, oil quenched, re-quenched at 880 °C,and low-temperature tempered at 180 °C. X-ray diffraction was employed for phase analysis and Ir Carbon-sulfur Spectrometer was used for carbon content detection. The morphologies of martensite were observed by scanning electron microscopy. The Vickers hardness was measured and the impact toughness was measured by means of Charpy’s V-notch tests. The results show that the carbon concentration and hardness gradients are established after carburizing process as a function of distance from the case to core. The carbon content gradient results in a gradient of martensitic microstructures, ranging from plate martensitic morphologies with large amounts of retained austenite in case to lath martensitic microstructures with essentially no retained austenite in core. The mechanical properties are significantly improved with adding re-quenching and low-temperature tempering. The case hardness, core hardness, tensile strength and impact energy are 851 HV, 402 HV, 1310 MPa and 92 J, respectively.

The spheroidized 1.0C–1.5Cr bearing steel with 30% cold deformation in thickness was subjected to austenitizing treatment. Interrupted quenching was employed to study the effect of prior cold deformation on the microstructure evolution during austenitizing. The results indicate that, due to the formation of many low angle boundaries, the prior cold deformation facilitates the initial dissolution of cementite during the austenitizing and decreases the final prior austenite grain size. With the dissolution of cementite, mean diameter of cementite particles decreases first and then increases. When the volume fraction of undissolved cementite was within 3.7–4.9%, the mean diameter of cementite particles would have the smallest value of approximately 0.21 µm. After tempering at 170 °C for 2 h, the impact toughness and hardness of the experimental steels with the austenitizing temperature of 850 °C were measured. The prior cold deformation increases the impact absorbed energy by approximately 7%. The fracture surface exhibits quasi-cleavage fracture. At the austenitizing time of 30 min and 60 min, nearly no fibrous region can be observed, and fracture occurs by the nucleation of microcracks at the interfaces of martensite-cementite.

The effect of final rolling temperature and cooling process on the microstructure of 1.0C-1.5Cr bearing steel was studied, and the relationship between the microstructure parameters and subsequent spheroidization annealing was analyzed. The results indicate that the increase of water-cooling rate after hot rolling and the decrease of final cooling temperature are beneficial to reducing both the pearlite interlamellar spacing and pearlite colony size. Prior austenite grain size can be reduced by decreasing the final rolling temperature and increasing the water-cooling rate. When the final rolling temperature was controlled around 1103 K (830 °C), the subsequent cooling rate was set to 10 K/s and final cooling temperature was 953 K (680 °C), the precipitation of grain boundary cementite was suppressed effectively and lots of rod-like cementite particles were observed in the microstructure. Interrupted quenching was employed to study the dissolution behavior of cementite during the austenitizing at 1073 K (800 °C). The decrease of both pearlite interlamellar spacing and pearlite colony size could facilitate the initial dissolution and fragmentation of cementite lamellae, which could shorten the spheroidization time. The fragmentation of grain boundary cementite tends to form large-size undissolved cementite particles. With the increase of austenitizing time from 20 to 300 minutes, mean diameter of undissolved cementite particles increases, indicating the cementite particle coarsening and cementite dissolution occuring simultaneously. Mean diameter of cementite particles in the final spheroidized microstructure is proportional to the mean diameter of undissolved cementite particles formed during partial austenitizing.

Effects of cooling rate (Vcr) and final cooling temperature (Tft), after hot deformation, on microstructure and hardness for 1·0C–1·5Cr bearing steel were investigated. The results show that if Vcr increases from 2 to 25°C s−1 and Tft remains at 650°C, pearlite colony size and grain size both decrease, hardness increases. When Vcr exceeds 8°C s−1, carbide network can be restrained effectively. TEM micrographs indicate that there exist branches in the local region of lamellar cementite and ferrite, and a ferrite thin film is also found around the proeutectoid carbide. Under the cooling rate of 10°C s−1, with the increase in Tft, the microstructure changes from martensite into pearlite, carbide network becomes more serious and hardness decreases.