Effect of reverted austenite on tensile and impact strength in a martensitic stainless steel−An in-situ X-ray diffraction study

The transformation kinetics from martensite to reverted austenite and partially back to martensite during tempering of a martensitic stainless steel is investigated by in-situ high temperature X-ray diffraction (HT-XRD). Phase fractions and microstructural parameters such as the dislocation density are deduced by means of the Rietveld method in combination with the double-Voigt peak broadening model. The dislocation densities of the phases investigated are related to the square of the microstrain by a pre-factor that depends on Poisson's ratio and lattice constant only. For low tempering temperatures the dislocation density in martensite remains high and the mass fraction of austenite at room temperature remains low. The highest value of the mass fraction of austenite - stable at room temperature - occurs at intermediate tempering temperatures and this condition corresponds to a maximum in impact strength and a minimum in tensile strength. At higher temperatures austenite formed during tempering partially retransforms to martensite during cooling. The average dislocation density in martensite increases with increasing fraction of newly transformed martensite resulting in lower values of impact strength. The in-situ XRD experiments can be used to analyze structural changes in detail and offer thereby a powerful tool to design appropriate heat treatments for engineering steels to obtain tailored mechanical properties.