CHANGE IN MECHANICAL-PROPERTIES AND MICROSTRUCTURE OF 201 STAINLESS-STEEL WITH INCREASED NITROGEN ALLOYING

It is well known that mechanical properties of commercial stainless st eel are improved by alloying with nitrogen. In this study a series of nitrogenated commercial 201 stainless steel alloys with nitrogen level s as high as 2.6 wt.% were obtained by melting in a hot-isostatic-pres sure furnace using nitrogen as the pressurizing gas. Nitrogen concentr ations in excess of 1.25 wt.% formed a series of different chromium ni tride precipitates and morphologies depending upon the nitrogen concen tration. Five different nitrogen levels were fabricated using the same processing conditions recommended for 201 stainless steel including h ot-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 presen ce or absence of precipitates. The presence of chromium precipitates d id reduce the fracture ductility and changed the fracture features. Th is U.S. Bureau of Mines study shows that increasing the nitrogen conce ntration in commercial steels above their current level has positive e ffects on mechanical properties as long as the nitrogen solubility lev el is not exceeded and chromium nitride precipitates begin to form.