Effect of different nitrogen content on microstructure and properties of powder metallurgy 316L stainless steel

【Objective】 The 316L stainless steel is a crucial engineering material, widely used in the manufacturing of wear-resistant and corrosion-resistant components. In order to further enhance the performance of 316L powder metallurgy steel, nitrogen, showing strong strengthening effect on powder metallurgy components, was added to the powder metallurgy stainless steel to improve its microstructure and material properties.
【Method】 In this paper, the 316L nitrogen-containing powder metallurgy stainless steel was prepared by PM techniques with Cr₂N additive as a nitrogen resource, then, the microstructure and performance of PM 316L nitrogen-containing stainless steel were studied.
【Result】 Results show that the grain size of 316L nitrogen-containing stainless steel reduces gradually, and the particle shape becomes round with continued increase of N content regardless of as-sintered state or as-heat treated state, large size of blocky and banding particles are observed with further increase of N content. Results of mechanical properties indicate that the hardness of 316L nitrogen-containing stainless steel increases monotonously with the increase of N content, and the hardness of 316L nitrogen-containing stainless steel as-sintered state and as-heat treated state is 75.4HRB and 90.2HRB respectively at the designed N content 1.5%. The tensile strength of 316L nitrogen-containing stainless steel increases firstly and then decreases with the increase of N content, in addition, the tensile strength of PM 316L nitrogen-containing as-heat treated state is superior to that of the samples as-sintered state, while the tensile strength of PM 316L nitrogen-containing as-sintered state and as-heat treated state reaches maximum value 518.3 MPa and 597.1 MPa respectively at N content 0.9%. The impact toughness of 316L nitrogen-containing stainless steel as-sintered state changes little when the designed N content is no more than 0.9%. However, the value of impact toughness decreases remarkably with the continuously increased N content. The impact toughness of the 316L nitrogen-containing stainless steel as-heat treated state is inferior to that of the as-sintered stainless steel. The impact toughness of the 316L nitrogen-containing stainless steel as-heat treatment state increases slightly at 0.3% N designed content, subsequently, it decreases monotonously with the increase of N content and desreases abruptly when designed N content exceeding 0.9%.
【Conclusion】 The nitrogen content in PM stainless steel was retained only 25%~30% for high volatility of nitrogen element, however, even a small quantity of nitrogen has an excellent strengthening effect on properties of PM 316L steel, so it is a feasible method to manufacture high-performance PM 316L steel with nitrogen element additive.