Structural stability of super duplex stainless weld metals and its dependence on tungsten and copper
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I.
INTRODUCTION
DUPLEX stainless steels (DSS) constitute a group of very attractive steels possessing a favorable combination of mechanical strength and corrosion resistance in chloride ion containing environments. Superduplex stainless steels (SDSS) are characterized by an unusually high resistance to pitting corrosion and are commonly defined as the subset of DSS that shows a pitting resistance equivalent (PRE N = wt pet Cr + 3.3 • wt pet Mo + 16 • wt pet N) above 40. The resistance to pitting corrosion in the base material can be optimized by selecting a diffusion annealing temperature which is such that the concentration of alloying elements corresponds to equal pitting resistance in ferrite and austenite. However, the situation in the weld metal is more complex than in the base metal, since the advantages of diffusion annealing cannot be used. The balance of alloying elements can be disturbed, either by the formation of intermetallic precipitates and Cr2N or by excessive precipitation of so-called secondary austenite (%). Redistribution of alloying elements, as a result of such transformations, may lead to local reductions in pitting resistance and associated corrosion attack, either in regions that are depleted with respect to crucial alloying elements or in the precipitates themselves. In theory, pitting corrosion resistance in DSS could be
J.-O. NILSSON, Manager, Physical Metallurgy, and A. WILSON, Research Metallurgist, Tube Laboratory, are with the R&D Center, AB Sandvik Steel, S-811 81, Sandviken, Sweden. T. HUHTALA, Research Metallurgist, and L. KARLSSON, Senior Research Metallurgist, are with Central Laboratories, Esab AB, S-402 77, Grteborg, Sweden. P. JONSSON, Graduate Student, Department of Engineering Materials, is with the Lule~ University of Technology, S-951 63, Lule~, Sweden. Manuscript submitted April 11, 1995. 2196~VOLUME 27A, AUGUST 1996
further improved by additions of chromium, molybdenum, and nitrogen. However, since these elements also promote the formation of intermetallic precipitates such as o- phase, X phase, and R phase and nitrides,m there are practical limits regarding the amount of alloying additions. It has recently been suggested that tungsten in superduplex stainless steel base metal improves the resistance to pitting corrosion but, unlike molybdenum, hinders the formation of intermetallic phase, t2] This contradicts findings by Charles, t3j who reports faster precipitation kinetics and a higher dissolution temperature of intermetallic phase as a result of tungsten additions and thereby points out similarities rather than differences between tungsten and molybdenum. The kinetics of austenite formation is also of importance for the following reason. The DSS are almost entirely ferritic at temperatures just below solidus. However, on cooling, transformation of ferrite to austenite will occur.t~.4] The austenite formed in this manner at relatively high temperatures is usually termed primary austenite. Additional transformation of ferrite to Y2 may occur after the duplex struc
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