Quantitative SEM analysis of the evolution of microstructure in tempered martensite ferritic steels during creep and high temperature fracture

The present work focuses on the evolution of microstructure during high-temperature creep and fracture in tempered martensite ferritic steel. Data have been collected on miniature C(T) specimens of a 9-12% chromium steel, designated X20CrMoV12-1, exposed to high-temperature cracking in the range of 575 to 650°C. Coarse-grained as well as fine-grained prior austenite grain sizes were produced via two different heat treatment routes to study the dependence of mechanical behaviour and microstructural evolution on the initial microstructural state. Microstructures having finer grains (higher boundary densities) exhibit higher strength, as stated by the Hall-Petch relationship. However, one of the downsides of grain boundaries is that they are preferential sites for crack nucleation and growth and fast diffusion paths during creep. Several phenomena were observed at the crack tip area. As the distance to the crack tip decreased, the microstructure was exposed to higher creep strains. The most obvious finding was the significant microstructural evolution ahead of the crack tip which was also temperature-dependent. The findings suggest that, at all temperatures, coarsening took place at the (prior) sub-block, block and packet hierarchical levels. The fracture mechanism was observed to be mainly transgranular at temperatures higher than 620°C. Evolution of SBB, BB, PB, NM and VLA boundary densities against the distance to the crack tip as well as at areas around the pores and on the ligaments connecting the pores (bridges) were studied in detail. A positive correlation seems to exist between hardness and the „martensitic“ boundary populations. On the contrary, a negative correlation was found between hardness and the density of non-martensitic boundaries.