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

摘要

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.