Transformation behavior and acceleration of low-temperature bainite in high carbon chrominum steel

For high carbon chrominum steel, better comprehensive mechanical properties were obtained after low temperature bainitic treatment compared to martensitic, but much longer duration time was taken simultaneously. Therefore, the transformation kinetic of low-temperature bainite was investigated in this paper. The database of TCFE7 of Thermo-Calc software was used to calculate the T₀ line. DIL805A dilatometer was used to simulate the heat treatment. The microstructure was observed by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and x-ray diffraction (XRD). The mechanical properties were tested by multifunctional tensile testing machine. The results indicate that the holding times at intercritical austenitization temperature influence the subsequent bainitic transformation. Under-cooled austenite (UA) has different stability after different bainitic holding times, and transforms into martensite while cooling to room temperature if the holding time is less than 1200 s. Pre-strain at different temperatures or different pre-strains at the same temperature shorten the incubation time and accelerate the initial bainitic transformation, but have negligible effect on the full transformation process. The transformation from austenite into bainite could be accelerated by a novel two-step heat treatment in which the isothermal temperature is increased to a higher temperature at the second step. The rational cooperation of the two-step process sharply reduces the lower bainitic transformation time to about 27% percent of the conventional full transformation process. (paper)