Phase transformation and microstructure study of nano-structured austenitic and ferritic stainless steel powders prepared by planetary milling

In the present work, nano-structured austenitic and ferritic stainless steel powders were prepared in bulk by milling elemental powders in a specially designed dual-drive planetary mill (DDPM) and Fritsch pulverisette planetary mill (P5 mill) for 10 and 40 h respectively. The progress of milling and phase transition of stainless steel have been studied by means of X-ray diffraction. The crystallite size and the lattice strain of the austenitic stainless steel after 10 h of milling are 9 nm and 5.59 × 10− 3 respectively. Similarly, the crystallite size and the lattice strain of the ferritic stainless steel after 10 h of milling are 8 nm and 9.05 × 10− 3 respectively. Annealing of milled powder at 750 °C promotes ferritic to austenitic transformation in both argon and nitrogen atmospheres as limited transformation takes place after milling. However, nitrogen favors the transformation to a greater extent than argon. Lattice parameters calculated from both high resolution transmission electron micrographs (HRTEM) and Nelson–Riley method match with austenitic and ferritic stainless steels. It has been found that initially particles are flattened and finally become almost spherical of size around 10–15 μm in both cases.