Studies on Production and Thermo-Mechanical Treatment of 0.32% Nitrogen Alloyed Duplex Stainless Steel P. Chandramohan, S.S. Mohamed Nazirudeen, and S.S. Ramakrishnan (Submitted April 19, 2006; in revised form May 5, 2007) Duplex stainless steel with nitrogen content 0.32% was produced using a conventional induction furnace under normal ambient atmosphere. The samples were subsequently hot forged to various size reductions (23-57%). Both the as-cast and the hot-forged samples were examined for changes in the microstructure and mechanical properties. The results reveal that the optimal-mechanical properties were noticed for a forging deformation of 48%. Texture analysis was carried out in these samples using Inverse Pole Figures (IPF) and Orientation Distribution Function (ODF). However, IPF and ODF results revealed that bulk texture was weak after hot forging. Keywords austenite, duplex stainless steel, ferrite, forging, texture 1. Introduction Duplex stainless steel (DSS) belongs to the family of stainless steels which contain two phases, i.e., ferrite and austenite in almost equal proportions. The ‘‘Duplex’’ micro- structure is obtained by controlled alloying with nitrogen, nickel (austenite stabilizer), and chromium (ferrite stabilizer). Small percentages of Mo, Mn, Si, and other alloying elements are also present in these alloys (Ref 1). One of the most common failure mechanisms in stainless steel is chloride-induced Stress Corrosion Cracking (SCC). The conventional austenitic grades are particularly susceptible to this failure mode. However, Duplex stainless steels offer significant resistance to this form of cracking (Ref 2-4). In addition to, they have better corrosion resistance in severe chloride atmosphere besides better mechanical properties. In the early stages, the commercial products in duplex stainless steels were developed using nickel as the austenite stabilizer. Subsequently, nickel was found to be expensive and allergy causing and was partially replaced by nitrogen. Nitrogen bearing duplex stainless steels were manufactured by various controlled atmospheric techniques (Ref 5-8). The production of nitrogen alloyed DSS in a conventional furnace under normal atmospheric condition was also reported in the literature as this meets the competitive requirement of low-cost manufacture without sophisticated equipment and controls (Ref 9). Thermo-mechanical treatment is expected to improve the mechanical properties of DSS. In these alloys, hot deformation was done in the temperature range of 1000-1300 °C followed by solution annealing and quenching (Ref 10). After hot- forging DSS, solutionizing is followed by water quenching to avoid precipitation (Ref 11). A study on 22 Cr DSS reveals the fact that it should not be hot worked at temperature above 1200 °C, in order to avoid the transformation of austenite to ferrite (Ref 12). The higher the deformation temperature, the greater is the probability for the formation of ferrite from austenite especially in specimens containing less than 2% nitrogen (Ref 13, 14). It is a proven fact that, texturing influences the properties of the mechanically worked material to a remarkable extent. The extent of texture development caused by treatments like rolling and extrusion in various steels, aluminum alloys, and Zr-Nb alloys has been studied by several researchers (Ref 15, 16). The general inference is that cold rolling is insignificant in two-phase alloys, but very noticeable in single-phase alloys. The authors felt that the study of texture development caused by hot-forging DSS was rarely reported in the literature, and hence this work was attempted using Inverse Pole Figure (IPF) and Orientation Distribution Function (ODF). Duplex stainless steel alloys with 0.15 and 0.23% N had been produced in a conventional induction furnace under atmospheric pressure and subjected to various solution treatment temperatures ranging from 1010 to 1140 °C. But, better mechanical properties were observed at the solution treatment temperature of 1060 °C by Chandramohan et al. (Ref 9). The higher the nitrogen content (0.23% compared to 0.15%), the better were the mechanical properties. It was therefore expected that, by increasing the nitrogen content to 0.32% would very likely increase the mechanical properties further. The scope of the present work is therefore (i) To produce DSS with higher nitrogen content (0.32% N) followed by heat treatment and hot forging. (ii) To study the effects of hot forging on microstructure, mechanical properties, and bulk texture. P. Chandramohan, Sri Krishna College of Engg and Tech, Coimbatore 641 008, India; and S.S. Mohamed Nazirudeen, PSG College of Technology, Coimbatore 641004, India; and S.S. Ramakrishnan, TamilNadu College of Engineering, Coimbatore 641004, India. Contact e-mail: pcmohu@yahoo.co.in. JMEPEG (2008) 17:271–279 ÓASM International DOI: 10.1007/s11665-007-9140-1 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 17(2) April 2008—271