Simulation of the precipitation of sigma phase in duplex stainless steels
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I.
INTRODUCTION
DUPLEX stainless steel, the microstructure of which contains between 25 and 70 pct of the austenite phase (g), with the balance being ferrite (a), offers certain technological advantages over other types of stainless steel. These include good resistance to chloride-induced stress-corrosion cracking, good resistance to pitting corrosion and other localized forms of corrosive attack, a relatively high strength, and a Ni content lower than that of most austenitic grades.[1,2] The alloy UNS S31803 (commonly denoted as ‘2205’) is typical of a modern duplex stainless steel and is normally used with a microstructure of about 55 pct g and 45 pct a. It and similar alloys have been used in various chemical process industries, including the oil and gas industry, the paper and pulp industry, and the beverage industry.[3] Application of the duplex grades is usually straightforward, since they can be formed and welded with standard equipment and techniques. However, consideration of the relevant phase equilibria shows that their duplex microstructure is most often a metastable state, preserved by fast cooling after a solution heat treatment at a temperature above 1030 7C. The equilibrium amount of austenite varies as a function of temperature, and, in the case of 2205 for example, would peak at about 800 7C if only austenite and ferrite were involved.[4] Unfortunately, the situation at temperatures in the range of 400 7C to 950 7C is complicated by the precipitation of the intermetallic compounds sigma (s), chi (x), and a' within the highly alloyed a. The formation of these compounds has a detrimental effect on workability, strength, corrosion resistance, and impact strength.[2,5] Below 550 7C, the s phase would in theory decompose to a mixture of a and a' phases by a eutectoid transformation, but in practice such a transformation is negligible in extent.[6] Information regarding the precipitation of intermetallic compounds in these alloys may be widely found in the literature (for example, References 7 through 12). However, M.B. CORTIE, Director, and E.M.L.E.M. JACKSON, Senior Scientist, are with the Physical Metallurgy Division, Mintek, Randburg 2125, South Africa. Manuscript submitted July 2, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
some interesting aspects of the problem will be mentioned here. For example, the precipitation of the s phase is not necessarily a rapid process in all susceptible alloys, and it takes several tens of hours of annealing to form s in binary Fe-Cr alloys of reasonable purity.[13] However, the process is rapid in most wrought duplex stainless steels, primarily due to the significant levels of sigma-enhancing alloying elements in those alloys. A few alloying elements are, however, known to retard the formation of s.[14] The s and x phases that form in duplex stainless steels have a complicated stoichiometry: (Fe, Ni)x(Cr, Mo)y in the case of s and Fe36Cr12Mo10 in the case of x.[14,15] The problem in duplex stainless steels is exacerbated by the natural partitioning of alloyi
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