Interstitial precipitation in Fe-Cr-Al alloys
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I.
INTRODUCTION
FERRITIC Fe-Cr stainless steels are susceptible to three distinct forms of precipitation that severely reduce the room-temperature ductility and toughness; these sources of embrittlement have been traced to interstitial C and N, sigma-phase precipitation, and the precipitation of the a' phase, which is also identified as the 475 ~ embrittlement reaction. The relative influence of each of these embrittling mechanisms depends on the alloy chemistry and thermal history. Sigma-phase precipitation and the 475 ~ reaction become increasingly important in compositions exceeding 15 pct Cr, whereas interstitial embrittlement is usually associated with residual concentrations of C and N. One of the important keys to improving toughness and expanding the application of ferritic stainless steels has been the reduction of interstitial C and N impurities by using argon-oxygen decarburization (AOD), vacuum induction melting (VIM), or Ti and Nb additions to combine with C. m Ferritic stainless steels containing intermediate to high interstitial contents (greater than - 2 0 0 ppm total C + N) exhibit severe embrittlement and susceptibility to intergranular corrosion after heating to 950 ~ and cooling rapidly to room temperature; this phenomenon is commonly referred to as "high-temperature" embrittlement. The embrittlement effects become more severe with increasing Cr and/or interstitial content.t2] Demot3] showed the underlying cause in type 446 stainless steel is the rapid precipitation of Cr carbides and nitrides at the grain boundaries and dislocations sites during cooling from temperatures above 950 ~ where the interstitial atoms are in solution. The high interstitial content (C + N > 1000 ppm) of common ferritic steels, combined with the high diffusivity of C and N, make it impossible to completely avoid precipitation, even by water quenching from the solution temperature. The ductility, toughness, W.S. SPEAR, Principal Engineer, is with Boeing Company, Seattle, WA 98124. D.H. POLONIS, Professor, is with the Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195. Manuscript submitted November 19, 1993. METALLURGICALAND MATERIALSTRANSACTIONSA
and corrosion resistance of an embrittled steel can be partially restored by annealing for a short time at 800 ~ to 850 ~ to allow some Cr diffusion to the boundaries and to form large stable carbides and nitrides within the matrix and at the grain boundaries at the expense of interstitials previously precipitated at the dislocations. The complex nature of carbonitride precipitation in Fe-Cr alloys has been the subject of several previous studies of the 475 ~ embrittlement reaction, t4-7j The formation of CrzN was reported within the first few hours of aging, t4,51the structure of this precipitate is similar to that of e-carbide, thereby leading to the argument that Fe and C may substitute partially for Cr and N in the MzX phase.[Sl Such a substitution offsets the requirement for long-range Cr diffusion since the formation of the MzX
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