We describe the analysis of in-situ HT-XRD data of a dual phase stainless steel exposed to a complex thermal cycle of heating, holding and cooling. For the conditions used only low quality diffraction data could be collected. Peak positions, peak areas and peak broadening are modeled by the Rietveld method. The low signal-to noise ratio and the presence of artificial peaks due to tube tails complicate the data evaluation. In a first attempt the parameters are refined by a local optimization procedure (e.g. Levenberg-Marquardt). However, this procedure fails by being caught in one of several local minima. Next, a Bayesian approach with a Markov Chain Monte Carlo (MCMC) algorithm is used as a global optimization procedure to refine the simulated Rietveld diffractograms. Accurate estimates of the evolution of the phase fractions and dislocation densities in martensite and austenite during all stages of the thermal cycle are obtained by this MCMC algorithm. While an approach based on multivariate second order Taylor series completely underestimates the error, the uncertainties in the model parameters could be estimated appropriately from histograms obtained by the MCMC method.@en

Direct microscopic observation of the isothermal bainite evolution in terms of nucleation events, the location of the nuclei, as well as their growth is very valuable for the refinement of models predicting the kinetics of bainite transformation. To this aim, the microstructural evolution in a Fe-0.2C-1.5Mn-2.0Cr alloy during isothermal bainite formation at temperatures between 723 K and 923 K is monitored in situ using high temperature laser scanning confocal microscopy (LSCM). Both the nucleation and the growth kinetics of the bainitic plates are analyzed quantitatively. Bainitic plates are observed to nucleate on three different types of locations in the grain: at austenitic grain boundaries, on newly-formed bainite plates and at unspecific sites within the austenite grains. Grain boundary nucleation is observed to be the dominant nucleation mode at all transformation temperatures. The rate of nucleation is found to vary markedly between different austenite grains. The temperature dependence of the average bainite nucleation rate is in qualitative agreement with the classical nucleation theory. Analysis of plate growth reveals that also the lengthening rates of bainite plates differ strongly between different grains. However, the lengthening rates do not seem to be related to the type of nucleation site. Analysis of the temperature dependence of the growth rate shows that the lengthening rates at high temperatures are in line with a diffusional model when a growth barrier of 400 J mol–1 is considered.@en

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.@en