Change in mechanical properties and microstructure of 201 stainless steel with increased nitrogen alloying
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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.
I. INTRODUCTION
Although studies have shown that nitrogen addition to stainless steel improves many material properties, these studies have been limited in the extent of nitrogen addition. There have been few studies to determine the effects of ultrahigh nitrogen additions (greater than 1.0 wt. %) on stainless steels. This study examined the effects of varying the nitrogen concentrations from 0.07 to 2.54 wt. % on commercial 201 SS and for different fabrication processing conditions. Nitrogen is added to stainless steels to enhance mechanical and corrosion properties.1 Reducing the nickel and increasing the manganese concentration in stainless steels can increase nitrogen concentration from approximately 0.2 wt. % for Fe-18Cr-8Ni to as high as 0.5 wt. % for Fe-18Cr-18Mn.2 Recently, it has been shown that the nitrogen concentration in stainless steels can be further increased by melting under elevated nitrogen pressure.3'4 Nitrogen concentration in liquid steels has been shown to closely follow Sieverts' law and, thus, is proportional to the square root of the nitrogen pressure.5'6 Nitrogen concentrations approaching 1.0 wt. % have been achieved by commercial electroslag remelting under 5.0 MPa.3 Nitrogen concentrations exceeding 5.0 wt. % have been achieved in laboratory studies using hot-isostatic-pressure melting (200 MPa) with nitrogen as the pressurizing gas.4 Depending upon the alloy composition, the nitrogen concentration in stainless steels can reach approximately 1.5 wt. % before 3160 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 12, Dec 1994 Downloaded: 14 Mar 2015
the nitrogen begins to react with chromium to form chromium nitrides (Cr2N and CrN).4'7 Tensile properties of high-nitrogen alloys increase with increasing nitrogen concentration as long as the
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