The effect of temperature and nitrogen content on the partitioning of alloy elements in duplex stainless steels
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I.
INTRODUCTION
D LEX stainless steels consist of approximately equal proportions of the phases austenite (3') and ferrite (or). The two phases, a and 3', possess a varying affinity for the alloying elements which are common in duplex stainless steels. In general, the so-called austenite stabilizers, such as nickel and nitrogen, become somewhat concentrated in the 3' phase, and the ferrite stabilizers, such as chromium and molybdenum, become somewhat concentrated in the a phase. This phenomenon is known as partitioning, and the degree of partitioning that occurs is measured by various types of partition coefficients. The degree of partitioning in a binary system can be determined directly from a binary phase diagram by use of the "lever rule." It cannot be determined directly from pseudobinary-phase diagrams for ternary and higher systems, because the tie lines of the latter do not generally lie on the plane of the paper on which the diagrams are drawn. Partitioning has several important practical implications. In particular, the partitioning of chromium, molybdenum, and nitrogen is known to influence the general corrosion resistance, tl,2] the resistance to pitting and crevice attack, f3,4J the susceptibility to common kinds of stress corrosion cracking, t5~ the mechanical properties of both the 3' and the ct phases, tll and the susceptibility of the a phase to either precipitate sigma phase or undergo spinoidal decomposition, tlj The degree of partitioning that occurs can be calculated by use of the principles of thermodynamics t61 for the Cr-Ni-Fe ternary, as well as for selected duplex stainless steels, but the calculations are relatively complex and are not easily carried out by a nonspecialist. Accordingly, this study has endeavored to present an alternative, simpler means of estimating the degree o f partitioning in alloys. In particular, since commercial grades of duplex stainless steels usually contain significant quantities of nitrogen, which has been found to confer numerous advantages, the study has examined the effect M.B. CORTIE, Assistant Director, Stainless Steel Metallurgy, and J.H. POTGIETER, Senior Scientist, are with Mintek, Randburg 2125, Republic of South Africa. Manuscript submitted October 1, 1990. METALLURGICAL TRANSACTIONS A
of nitrogen content on partitioning. Nitrogen additions are believed to be beneficial in duplex stainless steels because they promote the stabilization of austenite, t2,7-91 increase strength, t~,71 improve the resistance to localized corrosive attack, t~-5,7-91 and decrease the susceptibility of the alloy to form deleterious intermetaUic precipitates, tg,l~ There is some confusion in the literature regarding the effect of the nitrogen content on the amount of partitioning. Some workers claim that there is little or no effect of nitrogen on the partitioning of the alloying elements, t2,aj whereas others claim that an increased nitrogen content results in a reduction in the degree of partitioning in either the chromium content or of all of the alloying elements, t3,7
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