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

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 degrees C, i.e. 30 degrees 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 delta-ferritic domain above 1200 degrees C was confirmed for all the materials studied. However, the amount of delta-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 (similar to 4 x 10(14) m(-2)), which is however far less than in the martensitic materials (similar to 5 x 10(15) m(-2)). Dislocations start to annihilate from 550 degrees C / 600 degrees C in all the materials studied, but a measurable dislocation density of similar to 10(13) m(-2) is still observed at 950 degrees C / 1000 degrees C.

Automated summary

Bulk phase transformations and dislocation density were examined in maraging stainless steel at high temperatures using in-situ neutron diffraction. The study revealed the impact of steel composition on the amount of delta-ferrite at the melting point, while the initial microstructure had little influence on re-austenization process. Dislocation densities were found to vary among different materials, with dislocations starting to annihilate at temperatures above 550 degrees C.