A novel online subcritical spheroidization annealing technology is proposed. To verify the validity and advantage, microstructure, and properties of 1.0C-1.5Cr bearing steel in three process of hot rolling, spheroidization, quenching, and tempering are investigated. It is demonstrated that the refining of hot rolled microstructure is beneficial to obtaining finer spheroidized microstructure, which accelerates cementite dissolution during the austenitization process of subsequent quenching treatment. When the ferrite grain size of spheroidized microstructure decreases from 8.3 to 1.7 mm, and mean diameter of spheroidized cementite decreases from 0.38 to 0.22 mm, the prior austenite grain size of the microstructure quenched from 850 C decreases by about 27%, and mean diameter of undissolved cementite decreases by at least 20%. The fatigue limit of the tested steel after quenching and tempering is measured. It is increased from 789 to 986 MPa due to the comprehensive effect of the refining of both undissolved cementite and prior austenite grain, and the increase of retained austenite content in the martensite matrix.
Herein, the effect of austempering treatment on martensite/bainite multiphase microstructure evolution and mechanical properties of 3Cr2MnNiMo mold steel are investigated, and the mechanical properties are compared with the conventional quenching and tempering (QT) treatment. It indicated that the morphology of martensite/bainite can be effectively tailored by changing the austempering temperature. With the increase of temperature from 280 to 360 °C, the microstructure changes from tempered martensite + bainite + blocky retained austenite (RA) to martensite + coarse bainite plate + blocky RA and irregular martensite/austenite (M/A) island. When the temperature reaches 420 °C, the martensite content reaches the maximum. The bainite morphology is surprisingly transformed from coarse bainite plate to ultrafine acicular shape owing to the reduction of the bainitic driving force and the sharp increase of carbon diffusion coefficient. Moreover, blocky RA and M/A islands disappear and are replaced by nanoscale filmy RA. The alternating distribution of variants has a refinement effect on the microstructure and the effective grain size reaches a minimum value. Compared with the conventional QT treatment, the tensile strength of austempering at 420 °C increased by 702 MPa, but the toughness and ductility levels are almost the same.
Thermal simulated specimens with the heat inputs of 20, 50 and 80 kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe–2Cr–Mo–0.12C pressure-vessel steel. The results indicated that the microstructures in the coarse-grained heat affected zone of tested steels with various heat inputs were mainly consisted of lath martensite and bainite ferrite. As the heat input increased, the fraction of martensite decreased and the bainite ferrite fraction increased. The toughness (tested at −40°C) and hardness for the heat input of 50 kJ/cm were 102 J and 346 HV, respectively, which was attributed to the high-volume fraction (60%) of the high-angle grain-boundary and the fine bainite lath. This paper is part of a thematic issue on Nuclear Materials.
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