Precipitation behavior of nanoscale cementite in hypoeutectoid steels during ultra fast cooling (UFC) and their strengthening effects

We describe here the unique effects of ultra fast cooling (UFC) on strengthening induced by nanoscale cementite precipitation in hypoeutectoid steels containing 0.17 and 0.33 wt% of carbon. In the absence of nanoscale cementite, fine ferrite grain size, refinement of pearlite, and reduced lamellar spacing contributed to strengthening of 0.04 wt% C and 0.5 wt% C steels, respectively. The contribution of nanoscale cementite precipitates of size ∼20-30 nm to yield strength was ∼100 MPa in 0.17C and 0.33C steels. Carbon content and the degree of undercooling were the primary factors that govern nanoscale cementite precipitation. The transformation driving force for the undercooled austenite was calculated using the Kaufman-Radcliffe-Cohen (KRC) model to thermodynamically analyze the possibility of precipitation of nanoscale cementite. It was also observed that dislocations could be “frozen” in austenite because of UFC, which provide channels for diffusion of carbon atoms and consequent nucleation of cementite.