Heat Treatment Process Of The Organization And Performance Of New Bainitic Steel

15Cr2Ni3MoW steel which was studied in this paper is a new bainitic steel, and it is mainly used to manufacture heat-resisting die, such as hot-rolled piercing plug. Its microstructure is granular bainite when austenitized in 1050℃ and isothermally quenched in 300℃, but this microstructure has a low ductility and yield ratio, which results in the failure of piercing plug made in 15Cr2Ni3MoW steel when working. In order to resolve this problem, the heat-treated experimental steel was tempered in different temperatures and at different time, hoping that the aim of improving ductility and yield strength of the experimental steel could be achieved by proper tempering process. In order to supply theoretical warrant to make heat-treating process of the experimental steel, CCT curve of the experimental steel was firstly plotted by expansion method assisted by metallographic method and hardness method in the Gleeble-1500 thermal analog machine. Moreover, the experimental steel was treated in different temperatures, at different time and by different cooling velocities. Microstructure and mechanical property of the heat-treated, tempered and treated in different heat-treating processes experimental steel were performed by optical microscopy (OM) scanning electric microscopy (SEM) electric universal material testing machine microhardness tester. Microstructure changes of Granular bainite and chemical constitution of carbide precipitated when tempered were performed by transmission electron microscope (SEM) and energy spectrometer. The mechanism of remarkable strengthening-toughening and temper brittleness of the experimental steel when tempered were discussed according to experimental results. Through the study, the following conclusions were obtained:CCT curve of the experimental steel was firstly plotted. Phase transformation didn't happened in the high temperature area of CCT curve, as well as proeutectoid ferrite or pearlite wasn't formed in the high temperature area, which reasons were that "C" curve of phase transformation in the high temperature area was greatly moved to theright because many alloy elements were added to the experimental steel. Full bainite was obtained when cooling velocity was lower than 70°C/min;martensite and bainite were obtained when cooling velocity was between 70°C/min and 160°C/min;full martensite was obtained when cooling velocity was more than 160°C/min. Microstructure hardness was not remarkable discrepant when cooled in the range of cooling velocity in which full bainite was obtained.Microstructure obtained when the experimental steel was treated by different heat-treating processes was mainly bar-like granular bainite, but grain structure that granular M/A island distributing in block bainitic ferrite could be obtained when cooling velocity was very slow (furnace cooling) and austenitized temperature was high (^1100°C). Gain structure had lower strength, plasticity and very bad impact toughness, so it was necessary to distinguish gain structure from granular bainite. At different austenitized temperature strength and ductility were the best by air cooling, it was in the middle by cooling in sand, and it was the worst by furnace cooling. The strength and ductility of the experimental steel sharply decreased especially by furnace cooling when austenitized at 1200°C. The reasons were possibly that sulfide and phosphide in the steel precipitating along austenite grain boundary avianized it when the steel was austenitized in higher temperature, and austenite grain fully grew and became very large because the experimental steel stayed for a long time in the high temperature.Microstructure of the experimental steel was strip-like granular bainite when austenitized in 1050°C and isothermal quenched in 300°C, but this microstructure had a low ductility and yield ratio. However the optimum combination property of the experimental steel was obtained when tempered for three hours at 350 °C .And the aim of improving ductility and yield strength of the experimental steel has been achieved, and the plant requirement to the steel property was met too. At this situation tensile strength, yield strength, reduction of cross-sectional area, elongation percentage, impact toughness and yield ratio of experimental steel were respectively 1167MPa, 988MPa, 36.1, 15.8, 57.7J/cm2 and 0.85. The remarkable strengthening-toughening of the experimental steelcould be obtained when tempered in about 350°C .Its mechanism was as followed: First, disperse, fine and needle-like carbide precipitated from bainitic ferrite could effectively anchor dislocation and strongly block its movement, so improving yield strength of the experimental steel. Second, Transformation of residual austenite (AR)always began from the residual austenite of low carbon concentration when tempered, which made carbon concentration of other residual austenite which didn't transit increase and drove to stability. And there was a lot of stable M/A island in the steel by TEM. And stable residual austenite could block crack propagation, so improving the ductility of the experimental steel. So the remarkable strengthening-toughening of the experimental steel was obtained. Temper brittleness occurred when the experimental steel was tempered between 500°C and 550°C.The reasons inducing temper brittleness were as followed: Microstructure similar to brittle upper bainite was formed because carbide precipitating from M/A island distributed along the boundary of bainitic ferrite bar. And carbide precipitating from austenite grain continuously distributed along the austenite grain boundary. The carbide located in the boundary of bainitic ferrite bar and austenite grain not only was crack nucleation site but also was easily cracked paths, so greatly decreasing the impact toughness of the experimental steel. So temper brittleness occurred when the experimental steel was tempered.