Characterization of stainless steels melted under high nitrogen pressure
- PDF / 2,040,575 Bytes
- 8 Pages / 597 x 774 pts Page_size
- 44 Downloads / 227 Views
I.
INTRODUCTION
MODERNengineering designs are placing ever greater demands on materials. Some of today's applications for stainless steels demand not only good corrosion properties, but also enhanced tensile strength and improved wear. Recent gas alloying research has shown that nitrogen addition to stainless steel has positive effects on both mechanical and corrosion properties, tt'2j Currently, several commercial 300 series stainless steels are being alloyed with nitrogen to increase tensile strength and improve wear. I3.41 Nitrogen alloying of stainless steels is important because, atom for atom, nitrogen has twice the strengthening effect of carbon I~j and the solubility of nitrogen is significantly greater than that of carbon, fSj Nitrogen solubility in commercial 300 series ironchromium-nickel stainless steels made at atmospheric pressure is less than 0.4 wt pct. 161 Nitrogen concentration in iron alloys has been shown to follow Sievert's l a w [2'7-91 (i.e., nitrogen concentration is proportional to the square root of the nitrogen pressure). The nitrogen concentration in commercial stainless steel alloys has been increased to 0.6 to 0.8 wt pct by melting under 4.2 MPa. Ij~ All the nitrogen in these alloys was interstitial; no second-phase nitrides were observed. However, recent nitrogen high-pressure melting (N-HPM) at 200 MPa experiments have shown that massive nitrogen additions to iron-chromium-nickel alloys result in the formation of several different metal nitrides in addition to the interstitial nitrogen, t~2,~31 In this study, elevated nitrogen concentration in commercial type 300 stainless steels was evaluated. As the total nitrogen concentration increased above 1 wt pct, the nitrogen solubility limit was exceeded and several different chromium nitrides formed. The composition and microstructures of these nitrides were determined. Tensile properties of the N-HPM alloys were found to be proportional to the interstitial nitrogen concentration and J.C. RAWERS, Materials Scientist, J.S. DUNNING, Research Supervisor, and G. ASAI, Materials Scientist, are with the Albany Research Center, United State Bureau of Mines, Albany, OR 97321. R.P. REED, formerly Research Scientist, National Institute of Science and Technology, Boulder, CO, is retired. Manuscript submitted April 8, 1991. METALLURGICAL TRANSACTIONS A
were not affected by the presence of nitrides. Other material properties, such as hardness, wear resistance, and elastic modules, were found to be related to total nitrogen concentration and not to the interstitial nitrogen concentration which decreased with increasing total nitrogen concentration.
II.
EXPERIMENTATION
Commercial type 304 and 316 stainless steels were placed in a hot-isostatic-pressure (HIP) furnace heated to 1650 ~ under nitrogen pressure of 200 MPa. The resulting castings were analyzed for nitrogen concentration, phase composition and structure, and microstructure. Mechanical and physical properties of nitrogenated alloys were also determined. Samples for nitrogen processing were
Data Loading...
