PH 13-8 Mo alloys belong to the group of stainless maraging steels that are mainly strengthened by the precipitation of the ordered β-NiAl intermetallic phase during an ageing treatment. Due to the excellent combination of high strength and good toughness, this alloy is commercially used for applications where reliability under extreme loads is required, such as for instance in the aerospace industry.
This work focuses on the characterization of toughness-relevant microstructural features such as martensitic blocks, prior austenite grains, reverted austenite and delta ferrite. The martensitic transformation was examined by using high temperature confocal laser scanning microscopy and the martensitic block size was determined by means of electron backscattering diffraction. The obtained data was subsequently used to calculate the prior austenite grains. Parallel to this, electrolytically etching of the prior austenite grain boundaries by using nitric acid was performed. The proposed method enables a more rapid and cost-efficient way to reveal the prior austenite grain size and study the influence of heat treatment and hot forming on the grain size. Refining the prior austenite grain size and subsequently the martensitic structure increases both, strength and toughness simultaneously according to the Hall-Petch relation. As reported, the toughness of PH 13-8 Mo maraging steel is more affected by the amount of reverted austenite than by the prior austenite grain size. By means of X-ray diffraction, the phase fraction of austenite was determined after different heat treatments. Electron backscattering diffraction and transmission electron microscopy were used to reveal the morphology and nucleation sites. It was found that reverted austenite has an elongated morphology and tends to nucleate at martensitic laths. Residual delta ferrite originates from non-equilibrium solidification and has a stringer-like morphology after hot forming. Delta ferrite was highlighted by etching in accordance to Lichtenegger and Bloech. This etchant colors martensite, while leaving delta ferrite uncolored. Delta ferrite was also highlighted by calculating the kernel average misorientation from the data obtained by electron backscattering diffraction.