In situ observations of continuous cooling transformations in low alloy steels

A typical heat treatment for a low alloy steel will often involve a quenching heat treatment step, in which the steel is cooled from high temperatures to trigger austenite decomposition. The particular cooling rate during the quenching step can have a marked influence on the phase transformations taking place, and the resulting steel microstructure and mechanical properties. Although methods such as dilatometry have been available for many decades to characterise continuous-cooling transformation (CCT) behaviour in steels, the use of in situ synchrotron X-ray diffraction (SXRD) to elucidate CCT behaviour in a systematic way has not been reported. In this work, we measure the CCT behaviours of two pressure vessel steels in situ using simultaneous dilatometry and SXRD. Both steels are subject to austenitisation followed by quenching at a range of cooling rates. On comparing results from SXRD and dilatometry, it is found that recorded starts of transformations appear to be in good agreement. However, calculations of phase fractions derived from dilatometry data significantly overestimate the fraction of ferrite that forms in comparison to SXRD when the formation involves the partitioning of carbon. This happens for two reasons: first, because the method to extract ferrite volume fractions from dilatometry data generally ignores the presence of any retained austenite at low temperatures, and second, because analyses of dilatometry data do not account for the expansion of the austenite during transformation due to enrichment in carbon. This enrichment leads to an increase in strain, and the standard analysis method falsely attributes this increase to ferrite formation, thereby overestimating it. The results highlight that caution must be exercised when interpreting the results of dilatometry, since levels of ferrite (especially diffusively-formed) and retained austenite are important quantities for the prediction of mechanical behaviour, and they are not readily quantified by the analysis of dilatometry data alone.