Heat transfer during the transformation of steels.

The extent to which heat conduction within large steel samples (billets) influences and is influenced by solid state transformation reactions has been studied under conditions of one dimensional heat flow with heat removed by convection with a constant heat transfer coefficient. Values of the heat transfer coefficient in the range 130-220 (W/m [20]C) has been used. These values were independently measured using steady state heat transfer and mass transfer analogy techniques which gave results in fair agreement. The progress of transformation front was followed using thermal analysis technique.The effects of different values of the heat transfer coefficient during transformation were investigated. The principal purpose of this investigation was to try to discover whether the assumptions made in establishing the theoretical method provided a basis for predicting the rates at which steel transformation reactions took place. The theoretical method assumed that the rate of the transformation was controlled by the rate of cooling and hence by the rates at which heat was transferred from the cooling steel component.The rate of transformation could thus be predicted by solving the heat transfer equation. The theory developed ignored any interaction between the kinetics of the transformation and the heat transfer process. The reaction was assumed to take place over so narrow a range of temperature that the latent heat of the reaction could be assumed to be liberated at a single unique temperature. The method used to solve the heat transfer equation, and hence to predict the transformation rates was the integral profile method. This is an approximate method that uses assumed cuadratic temperature profiles within the transformed and untransformed steel in order to obtain average solutions to the heat transfer equation in the two regions.The experimental results obtained are relevant to cooling rates significantly lower than those involved in most previous investigations and extend our current knowledge concerning heat transfer during transformation in large steel components.