Special Features of Structural and Phase Transformations in Bainitic Steel

Special features of structural and phase transformations during the formation of bainitic steel structure were studied. The possibilities of identifying plate (PM) and lath (LM) martensite after quenching, and of tempered martensite (TM) along with upper (UB) and lower (LB) bainite after tempering of quenched steel are described. The morphology and specifics of the formation of UB, LB, and TM in the steel structure were studied. The x-ray phase analysis method was used to show the possibility of identifying phases (PM, LM, residual austenite ( A res ), TM, UP and LB) and calculating their mass fractions in steel by decomposing the reflections into components. Quenching at 950°C was shown to produce PM and LM in steel with different morphologies and dislocation densities, contents of twins and carbon concentrations in the matrix. Tempering of the quenched steel at 500°C was shown to cause the following phase transition: PM + LM + A res → UB + LB + TM + PM+ LM+ cementite. The resistance of bainitic steel to sulfide corrosion cracking underloading and to hydrogen embrittlement was assessed.