Materials and Structure

The quantity of martensite formed in a carbon steel depends upon its carbon content and the lowest temperature to which the material is taken. The micrographs below show the effect of lowest temperature on the formation of martensite in an Fe -1.8 wt % C alloy. The material is initially heated above 727 C and converted to a homogeneous austenite. The sample is then cooled through the martensite start temperature and the process is stopped at three different low temperatures. For (a) the lowest temperature is 24 C and very little martensite (dark region) is formed. In (b) the sample was taken to - 60 C and more martensite has been formed. For (c) the minimum temperature was - 100 C and the transformation has progressed even further. Because the transformation process is non-diffusive, the transformation rate is not reduced by reducing the temperature. The micrographs were taken at room temperature at which the martensite is meta-stable. The morphology of the material can be changed by heating to higher temperatures where diffusion of carbon in the alloy will convert the material to the stable form at the diffusion temperature. Reheating will also reduce the lattice strain and make the material more ductile.

From: Schafer, Saxena, Antolovich, Sanders and Warner, "The Science and Design of Engineering Materials," McGraw Hill (1999)