1EFFECT OF PRIOR MICROSTRUCTURE ON AUSTENITE DECOMPOSITION AND ASSOCIATED DISTORTION

Steel components are heat treated in order to obtain desired microstructure and properties. However, such heat treatments often also impart unwanted and often unpredictable dimensional changes [1]. In the present work, Jominy bars are used to characterize the pattern of the dimension change versus distance from the quenched end and versus initial starting microstructure. The behavior of nine different kinds of steels is investigated. Size distortion is characterized as part growth or shrinkage. It is generally attributed to the volume differences between the various phases developed in the microstructure of a steel component during heat treatment. The martensitic microstructure of a particular steel, for example, has a slightly higher specific volume (lower density) than the same steel with a ferrite-pearlite microstructure. The specific volume differences between the various phases generally increase with the alloy carbon content. Shape distortion is associated with mechanisms that produce microstructure variations within the component. These microstructure differences may be inherited from casting and hot working or may result from an uneven thermal cycle. Experiments with carburized and oil-quenched annular discs produced from different steels have shown that the dimension changes could be related to steel hardenability. The dimensiona