Change in mechanical properties and microstructure of 201 stainless steel with increased nitrogen alloying

It is well known that mechanical properties of commercial stainless steel are improved by alloying with nitrogen. In this study a series of nitrogenated commercial 201 stainless steel alloys with nitrogen levels as high as 2.6 wt. % were obtained by melting in a hot-isostatic-pressure furnace using nitrogen as the pressurizing gas. Nitrogen concentrations in excess of 1.25 wt. % formed a series of different chromium nitride precipitates and morphologies depending upon the nitrogen concentration. Five different nitrogen levels were fabricated using the same processing conditions recommended for 201 stainless steel including hot-and cold-working, and heat-treating at two different temperatures. Tensile strength of the nitrogenated materials at each processing step was related to the interstitial nitrogen concentration and the presence or absence of precipitates. The presence of chromium precipitates did reduce the fracture ductility and changed the fracture features. This U.S. Bureau of Mines study shows that increasing the nitrogen concentration in commercial steels above their current level has positive effects on mechanical properties as long as the nitrogen solubility level is not exceeded and chromium nitride precipitates begin to form.