QUANTITATIVE-ANALYSIS OF THE DILATATION BY DECOMPOSITION OF FE-C AUSTENITES - CALCULATION OF VOLUME CHANGE UPON TRANSFORMATION

A detailed analysis has been made of the length changes due to decompo sition of iron-carbon austenites into ferrite and carbon enriched aust enite above the A(1) temperature, and into ferrite and cementite below the A(1) temperature, using lattice-parameter data for the phases inv olved. The calculations have been performed as a function of carbon co ntent, temperature and degree of transformation. A general definition for the total degree of transformation has been proposed for the case that more than one product phase develops. This definition also holds when the product phases develop consecutively, allowing an analysis of the kinetics of the austenite decomposition. Above the A(1) temperatu re, the relation between the degree of transformation and the dilatati on is linear within good approximation. However, below the A(1) temper ature a strong deviation from such a linear relationship occurs, as th e result of subsequent decomposition of austenite into ferrite and car bon enriched austenite, and decomposition of the carbon enriched auste nite into ferrite and cementite.