Embrittlement in CN3MN Grade Superaustenitic Stainless Steels
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INTRODUCTION
AUSTENITIC stainless steels are a subclass of stainless steels containing austenite as a primary phase at room temperature due to the high amount of nickel constituent. Like other types of stainless steels, most of them are susceptible to pitting corrosion in chlorine environments. A common index known as pitting resistance equivalent number (PREN30) is commonly used in stainless steels to give a quantitative measure of pitting corrosion resistance. The empirical formula representing PREN30 is given as[1]: PREN30 ¼ pctCr þ 3:3 ðpctMoÞ þ 30 ðpctNÞ: ½1 The steels which have higher PREN30 than 45 are generally known as superaustenitic stainless steels (SSS) and they combine high strength and toughness with excellent corrosion performance. As compared to conventional austenitic stainless steels, they are widely used in extreme environments due to their superior resistance to pitting and crevice corrosion at high temperatures and in seawater. They also have high resistance to chloride-induced corrosion and stress corrosion cracking. Off-shore oil wells, seawater handling, chemical and
MERTCAN BAS¸ KAN, Research Assistant and YUNUS EREN KALAY, Assistant Professor, are with the Department of Metallurgical and Materials Engineering, Middle East Technical University 06800 Ankara, Turkey. Contact e-mail: [email protected] SCOTT L. CHUMBLEY, Professor, is with the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50010. Manuscript submitted September 19, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
food processing equipment in addition to heat exchangers and piping in purification systems are the application areas of SSS.[2] Equation [1] clearly shows that pitting resistance can be enhanced by increasing the amount of chromium and molybdenum. However, when superaustenitic steels are exposed to high temperatures, such as 823 K to 1223 K (550 °C to 950 °C), the formation of intermetallic phases is almost inevitable due to the high alloying content.[3,4] Most of the intermetallics are detrimental for both corrosion resistance and mechanical properties; therefore, they have been investigated and classified in various studies. The most common intermetallics observed in SSSs are sigma (r), chi (v), laves phase, nitrides (Cr2N, CrN, and p), and some carbides like M23C6 and M6C depending on the alloying elements and composition.[5–8] Proper heat treatment is very crucial in SSSs because inadequate heat treatment is almost always followed by intermetallic formation, degrading the mechanical properties. Several studies have shown that the sigma phase is responsible for the embrittlement observed in the SSSs.[9,10] Sigma precipitates at grain boundaries, therefore, small grained-structures provide a greater opportunity for grain boundary nucleation.[11] They can also form intragranularly, but intergranular formed sigma particles have faster growth kinetics and a higher coarsening rate as opposed to intragranular ones.[12] Similar to sigma phase, chi also prefers grain boundaries and t
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