Fabrication of stainless steels with composite structure by Nitrogen Gas Atmosphere Sintering

Mixtures of ferritic stainless steel powders with 12mass%Cr and 23mass%Cr were compacted and subjected to Nitrogen Gas Atmosphere Sintering (NGAS) treatment at 1473 K in N₂ gas atmosphere of 0.1 MPa in order to produce high nitrogen stainless steels with a (martensite (α′)+austenite (γ)) composite structure. Microstructure of the NGAS-treated materials was investigated by means of optical microscopy and X-ray diffractometry in relation to the mixture ratio of both powders. The mechanical property of the materials was evaluated by three-points bending test and tensile test. During the NGAS treatment, the 12mass%Cr and 23mass%Cr powders absorb 0.3mass%N and 1.0mass%N respectively, and this leads to the transformation from ferrite (α) to γ. On the following air-cooling from 1473 K, the γ phase transforms to α′ in the 12mass%Cr material, while it retains stably at room temperature in the 23mass%Cr material. The structure ratio (α′/γ) of the NGAS-treated materials can be controlled by changing the mixture ratio of both powders. Results of mechanical testings for the NGAS-treated materials indicated that both of strength and ductility were reduced with increasing the volume fraction of the 12mass%Cr powder, because the powder produces α′ with brittle nature and the α′ give a route of crack propagation.