Effect of intercritical annealing on the microstructure and mechanical properties of a PH 13-8 Mo maraging steel

One method of achieving exceptional ductility and toughness of PH 13-8 Mo maraging steels is to perform aging at high temperatures or for prolonged dwell times, which is referred to as overaging. The increase in ductility and toughness is primarily related to the formation of high amounts of reverted austenite during aging. An alternative approach to elevate the reverted austenite content is to perform intercritical annealing, i.e., annealing in the dual phase field of martensite and austenite, prior to aging. Due to partitioning of substitutional elements during intercritical annealing, the freshly formed martensite is enriched in Ni after cooling. As a result, the formation of reverted austenite is facilitated, and high phase fractions can be achieved even at moderate aging temperatures. This study aims to shed light on the full potential of implementing intercritical annealing in the heat treatment route of PH 13-8 Mo maraging steels by thoroughly investigating the effect of this heat treatment adaption on the microstructure, mechanical properties and austenite stability. Overall, it is demonstrated that the addition of intercritical annealing enables to achieve a well-balanced microstructure showing a promising combination of strength, ductility and toughness. By performing intercritical annealing for shorter dwell times, high reverted austenite contents comparable to those after overaging can be reached. Resulting from a moderate aging temperature, fine beta-NiAl precipitates, which were detected by atom probe tomography, are formed within martensite, leading to considerably higher strength compared to after overaging. However, the high matrix strength restricts the mechanically induced transformation of reverted austenite to martensite, as found by in-situ high-energy X-ray diffraction tensile tests.