The Effect of Transformation Latent Heat on the Progress of Phase Transformation during Continuous Cooling of Steel

A study was undertaken to make clear the effect of latent heat evolution on the cooling transformation behaviour of steel. A method to estimate the influence of transformation latent heat on a cooling curve was developed on the basis of the cooling transformation kinetics. The present method was justified by comparing the cooling curve measured in steel SKS5 (an eutectoid steel containing Ni and Cr) with that estimated. The effect of latent heat evolution on the critical cooling rate was investigated. It was found that the effect of latent heat evolution was much larger on the lower critical cooling rate than on the upper critical cooling rate. Two types of continuous cooling transformation (CCT) diagrams were calculated; one with the cooling curves influenced by the latent heat evolution, and other with those controlled not to be influenced (i.e, controlled to be equal to those of transformation free). It was found that the C curves of the former CCT diagrams were located at higher temperature than the corresponding C curves of the latter, and this trend was stronger for the C curve of larger fraction transformed.