Microstructure and mechanical properties of a low-carbon Co-Cr-Mo-Ni steel processed by thermo-mechanical treatment
In order to improve the strength of low-carbon high-alloy steels,the influence of deformation of thermo-mechanical treatment on microstructure evolution and mechanical properties was studied.The results show that the microstructures of the steel after processing by thermo-mechanical treatment with deformation in the range of 20%-60%,cryogenic treatment and tempering contain the complex phases with tempered martensite,retained austenite and a few carbides.Compared with the conventional quenching and tempering sample,the strength of steel decreases with the elongation increase due to the thicker tempered martensite and a higher fraction of retained austenite after the thermo-mechanical treatment with the deformation of 20%.With deformation increasing from 20%to 60%,the dislocation density increases,the width of tempered martensite lath and the fraction of retained austenite decreases simultaneously,which leads to an improvement in the strength and a decrease in ductility.When the deformation reaches to 60%,the yield strength is higher by about 13%than that of the un-deformed quenching-tempering specimen,and the yield strength,ultimate tensile strength and elongation of the steel are 1 547 MPa,1 810 MPa and 17%,respectively,achieving a good strength-ductility matching.
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