Plasma-Melted Nitrogen-Bearing Cast Stainless Steels Microstructure and Tensile Properties O.P. SINHA, A.K. SINGH, C. RAMACHANDRA, and R.C. GUPTA Alloys of Fe-Cr-Ni and Fe-Cr-Mn were plasma arc-melted and chill cast in the form of ingots. Exposure of liquid melts to a nitrogen plasma for the purpose of adding nitrogen to the above alloys was used to dissolve varying amounts (up to 0.32 wt pct) of nitrogen. Carbon and sulfur were varied up to 0.5 and 0.056 wt pct, respectively. The alloys were evaluated for their mono- tonic behavior. It was observed that while strength and ductility parameters increased consid- erably with increasing nitrogen and carbon contents, both these parameters deteriorated with sulfur content. The analysis of the present results, along with the data from the literature, sug- gests that the strength parameters are predominantly chemistry dependent, particularly controlled by the nitrogen and carbon contents. The results also show that the alloys produced by this relatively new technique, plasma arc-melting, are comparable to those produced by other stan- dard techniques. I. INTRODUCTION EXTENSIVE efforts are being made to utilize nitro- gen as an alloying element in stainless steels on com- mercial scale. Nitrogen offers several advantages. It acts as a strong austenite stabilizer, offers very effective solid- solution strengthening together with toughness, im- proves pitting corrosion resistance and above all, renders an inexpensive substitute to expensive and scarce nickel) u Use of nitrogen-rich master alloys, t2,31the bubbling of nitrogen gas through a melt, t4] nitrogen gas exposure to liquid melt at suitable pressure, I5-81 electroslag remelt- ing, 18'9]and nitrogen plasma exposure to a liquid melt [9.~°.~ u are various methods used to add nitrogen to alloys. Each method has its own merits and demerits which have been discussed elsewhere, t~2] Among the various techniques listed, the exposure of liquid melt to nitrogen plasma for adding controlled quantity of nitrogen is relatively new and is associated with several advantages, t~3.~41 The ni- trogen gas mass transfer in a liquid melt is very rapid under plasma arc melting, and hence, it possesses good potential for large-scale commercial applications. In the present investigation, an effort has been made to assess the nitrogen-bearing Fe-Cr-Ni and Fe-Cr-Mn alloys pre- pared using plasma arc-melting technique by studying their tensile properties at room temperature. A compar- ison of the same is made with those alloys 1~5-2°1prepared by other techniques. II. EXPERIMENTAL The Fe-Cr-Ni and Fe-Cr-Mn alloys used in the present investigation were plasma arc-melted and chill cast (cooling rate -40 ks -~) in the form of ingots of 125 x O.P. SINHA, Research Associate, C. RAMACHANDRA, Reader, and R.C. GUPTA, Professor, are with the Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India. A.K. SINGH, formerly Postdoctoral Fellow, Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, is Scientist, with the Defence Metallurgical Research Laboratory, Hyderabad, India. Manuscript submitted November 18, 1991. 75 x 25 mm size. The detailed melting procedures using a furnace designed and fabricated, have been described elsewhere.[~-~4~ The nitrogen contents in the alloys made were varied. Variations in the carbon and sulfur contents were also affected during the process. The interstitial elements were analyzed using high-temperature vacuum fusion techniques, namely, LECO* model TC-236 for *LECO is a trademark of LECO Corporation, St. Joseph, MI. nitrogen and oxygen and LECO model CS-244 for car- bon and sulfur. The final melt analysis for all other ele- ments was carried out using a X-ray flourescence analyzer (model MSS 400). The chemical compositions of the al- loys are presented in Table I. Small sections from the cast ingots were prepared for optical metallography by conventional mechanical pol- ishing techniques and etched with FeC13 solution. The polished and etched specimens were then examined using Labor Lux Optical Metallography. The microstructural features were quantified using an image analyzer (Model VIDS III, Analytical Measuring System, United Kingdom). Tensile specimens with gage diameter 4.5 mm and gage length 15 mm were machined from the cast ingots. Ten- sile tests were performed at a nominal strain rate of =2.22 x 10 4 s-~ at room temperature. In each case, the reported value is an average of three tests with a variation of ---5 pct. Fractured surfaces of tensile specimens were exam- ined using a JEOL scanning electron microscope JSM-840A for fracture topography. III. RESULTS A. Microstructure The microstructural features as quantified for all the alloys of the present investigation are described in Table I. The optical microstructures in Figures l(a) through (c) illustrate the three typical microstructures, namely, austenite (rolled and annealed), austenite and METALLURGICAL TRANSACTIONS A VOLUME 23A, DECEMBER 1992--3317