EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Compared to other types of steels, maraging steels show an excellent combination of high strength and good toughness. This is achieved by precipitation of intermetallic compounds during ageing of a martensitic matrix. However, delta ferrite might be formed during processing due to non-equilibrium solidification. The present work investigates the influence of delta ferrite on the impact toughness of PH 13-8 Mo.

Computational modelling of the development of the phase fraction and chemical composition of delta ferrite during solidification was performed by applying the moving boundary model within the diffusion module (DICTRA) in Thermo-Calc for PH 13-8 Mo with varying chemical compositions. Furthermore, the chemical composition of delta ferrite was estimated by equilibrium calculations and the Scheil-Gulliver model. Results were validated by analysing the microstructure of PH 13-8 Mo in as-forged condition. Charpy V-notch impact testing was performed on specimens with different phase fractions of delta ferrite in order to evaluate its influence on the impact toughness. Since carbide precipitation was largely suppressed due to the low carbon content, it is assumed that solely the effect of delta ferrite on the impact properties was investigated.
The results show that a significant effect of the chemical composition on the phase fraction of delta ferrite was predicted by computational modelling and subsequently validated by means of metallographic examination and energy dispersive X-ray analysis.
Charpy V-notch testing revealed a clear trend, where the presence of delta ferrite led to lower impact toughness, which correlates with results from literature obtained for a low carbon 13Cr-4Ni martensitic stainless steel.

The present work outlines that even small variations within the chemical composition of PH 13-8 Mo might have significant influence on the phase fraction of delta ferrite and subsequently on the impact toughness.