Strengthening mechanisms in low carbon martensitic steels

There are several operative strengthening mechanisms in low carbon martensitic steels that depend on the alloy and prior processing conditions. It is very challenging to deconvolute these strengthening mechanisms because lath martensite is a complicated, multiscale microstructure. In an effort to identify prominent strengthening mechanisms in low carbon lath martensite, an Nb-microalloyed low carbon steel was subjected to different austenite conditioning and postquench tempering treatments. Austenite conditioning was performed to generate different prior austenite grain sizes and to thermomechanically process austenite below its recrystallization temperature. After processing, the strength of the as-quenched or as-quenched and tempered martensite was assessed through tensile or hardness experiments at room temperature. The results were correlated to measurements of dislocation density in the martensite, martensite microstructure including martensite block and packet size measured using electron backscatter diffraction imaging, and precipitation strengthening. For the processing conditions that were selected, strength was predominantly controlled by martensite dislocation density, which can be altered by reducing the prior austenite grain size and thermomechanical processing. The tempering response of the alloy was also sensitive to prior austenite grain size.