Influence of temper technology on mechanical properties and microstructure of 1200 MPa bainitic rail steel

The influence of difference temper technologies on microstructure and mechanical behaviors of the bainitic steel was studied. The results show that the yield strength, ultimate tensile strength, elongation, reduction of area of the alloy are above 1000 MPa, 1200 MPa, 15% and 45%, respectively, when the bainitic steel is tempered at 350°C for 4 h or longer. However, the steel appears obviously temper brittleness when the temper temperature is 450-550°C. TEM and optical observations show that the microstructure of the bainitic steel is composed of mainly granular bainite, and that the retained austenite is distributed as M-A island structure in lathes. The retained austenite in bainitic steel tempered at different temperatures and deformed by 3% stretching strain was tested. The results show that the mechanical stability of the retained austenite is the best when the steel is tempered at 350°C. The strength and toughness of the bainitic steel tempered with different technologies have a close relation to mechanical stability of the retained austenite.