Microstructural characterization of ultra-high strength martensitic steels

In the development of steels with increasing strength and toughness, ultra-high strength steel grades with a martensitic microstructure are gaining more importance. From the martensitic transformation product, however, strength-specific quantities derived from the former austenite grain cannot be accessed conveniently. A profound analysis of the prior austenite microstructure, its size distribution and aspect ratio is essential in order to allow conclusions on the mechanical properties. This paper presents an etchant to reveal the prior austenite grains of thermomechanical processed and press hardened martensitic steels. Furthermore, alternative detecting methods such as electron backscatter diffraction (EBSD) and high-temperature laser scanning confocal microscopy were evaluated. It was found that a picric-acid etchant in combination with a prior tempering treatment of the steel enables the visualization of prior austenite grains and their elongation in a micrometer-scale.