Martensitic transformation during nitrogen-gas atomization of 17-4 PH martensitic steel

This work was undertaken to investigate the unusual character of martensitic transformations occurring during the inert-gas atomization of martensitic steels. It was believed that the extreme characteristics of the atomization technology — including the extremely high cooling rate, a small size of atomized particles, and a nitrogen atmosphere — may exert an influence on the martensitic transformation. For the purpose of the present study, 17-4 PH steel was used as a program material, and electron-backscatter diffraction (EBSD) was employed for microstructural characterizations. The atomized particles exhibited a significant variability in size, ranging from 1 to ≈30 μm. Moreover, the powdered material also contained a significant fraction of retained austenite (≈14 %), presumably due to the high cooling rate and the influence of the nitrogen atmosphere. It was found that the martensitic transformation preferentially developed within only comparatively coarse (>10 μm) particles. In the finer particles, any solid-state phase transformations were suppressed by the particularly high cooling rate, and therefore those typically had a δ-ferrite structure (sometimes even a single-crystal one). The limited dimension of the atomized particles naturally restricted the austenite grain size, which was typically below 15 μm. Moreover, extensive twinning was observed in the austenitic phase. Both these effects resulted in a pronounced variant selection during subsequent martensitic transformation with a clear preference for the twin-related martensite variants.

1. Government assignment, Ministry of Science and Higher Education of the Russian Federation - FZWG-2026-0005 "Development of scientific foundations for optimization of selective laser melting and subsequent heat treatment of precipitation-hardening martensitic steel 17-4 PH"