Characterization of microstructure obtained by quenching and partitioning process in low alloy martensitic steel

Microstructure of low alloy martensitic steel treated by quenching and partitioning (Q&P) process was characterized by means of SEM, TEM, EBSD and XRD. Mechanical properties of steel processed by Q&P and the same steel processed by quenching and tempering (Q&T) were measured by uniaxial tensile test. The study suggests that microstructure is mainly composed of three phases, i.e. initial martensite, fresh martensite and retained austenite. The initial martensite formed at the first quenching step is easily etched; the fresh martensite is formed at the final quenching step and looks like 'blocky' type phase with size about 0.2-3 μm, and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary. The measured volume fractions and carbon contents of these phases were slightly different from those predicted by the constrained paraequilibrium (CPE) model proposed by Speer, which was interpreted by the effects of the different grain sizes of the untransformed austenite after first quenching. Mechanical properties of steels processed by Q&P assume much higher strength and ductility than those processed by Q&T. It is concluded that Q&P process is a promising approach to control the multiphase structure with hard matrix and a ductile retained austenite, which gives an excellent combination of strength and ductility.