In-situ neutron diffraction study of wrought and selective laser melted maraging stainless steels

Highlights: • SLM-ed as-built 17-4PH steel is essentially ferritic. • Martensite is recovered after conventional re-austenitizing treatment. • 17-4PH steel has a δ-ferritic domain close to melting temperature (1370 °C). • Dislocation density in martensite is 5 10¹⁵ m⁻². • Dislocation density in ferrite is 4 10¹⁴ m⁻². Bulk phase transformations and dislocation density were monitored in a maraging stainless steel on a bulk level using in-situ neutron diffraction up to 1340 °C, i.e. 30 °C below the melting temperature. Three materials with different initial microstructure and/or different composition were studied: wrought, as-built SLM-ed (selective laser melted) and re-austenitized SLM-ed. In contrast to the wrought martensitic steel, the as-built SLM-ed steel was essentially ferritic. However, re-austenitized SLM-ed steel recovered the usual martensitic microstructure. A δ-ferritic domain above 1200 °C was confirmed for all the materials studied. However, the amount of δ-ferrite at the melting point strongly depends upon steel composition. Additionally, it was shown that the initial microstructure of the steel (ferritic or martensitic) has little to no influence on re-austenization. Dislocation densities were estimated from diffraction peak broadening. The ferritic as-built SLM material contains a high dislocation density (~4 × 10¹⁴ m⁻²), which is however far less than in the martensitic materials (~5 × 10¹⁵ m⁻²). Dislocations start to annihilate from 550 °C / 600 °C in all the materials studied, but a measurable dislocation density of ~10¹³ m⁻² is still observed at 950 °C / 1000 °C.