Influence of delta ferrite on the impact toughness of a PH 13-8 Mo maraging steel

Abstract

Due to the fact that maraging steels are widely utilized as structural parts for the aerospace industry, high and consistent impact toughness is crucial to ensure reliability under extreme mechanical loads. The toughness of maraging steels is heavily influenced by the martensitic structure and reverted austenite. Another microstructural constituent is residual delta ferrite that originates from non-equilibrium solidification. This work focuses on the effect of delta ferrite on the impact toughness of a PH 13-8 Mo maraging steel, while keeping other toughness-influencing factors as constant as possible. Three-step heat treatments were applied to samples for adjusting different phase fractions of delta ferrite. Charpy impact testing revealed that the impact toughness decreases with an increasing phase fraction of delta ferrite. However, no significant influence on the lower energy shelf, i.e. the impact energies below the ductile-to-brittle transition temperature range, was found. In addition, no decrease in hardness at room temperature was measured when delta ferrite is present in the microstructure. Particle analysis by APT measurements revealed that delta ferrite contains Ni- and Al-rich precipitates. It is assumed that those precipitates in combination with effective solid solution hardening by Cr, Mo and Al significantly contribute to the hardness of delta ferrite, which is in the range of martensite. Furthermore, EDS analysis showed a depletion in Ni in delta ferrite, presumably resulting in a lower cleavage fracture resistance compared to martensite, and, therefore, causing embrittlement. Moreover, the interface between delta ferrite and martensite possibly plays an additional role for crack initiation due to amplified local stresses.