An analytical electron microscope investigation of the phase transformations in a simulated heat-affected zone in alloy
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I.
INTRODUCTION
ALLOY800
is a fully austenitic Fe-Ni-Cr stainless steel which provides a combination of strength and corrosion resistance at temperatures up to 650 ~ Alloy 800 was originally developed as a heater element sheath material in electrical appliances, but more recently, it has been utilized as a structural material in high temperature components. For example, Alloy 800 tubing has been used extensively in heat exchangers for both conventional and nuclear power plants. Since welding is an important fabrication step in the implementation of any structural alloy, considerable attention has been focused on the weld cracking susceptibility of Alloy 800. A. Heat-Affected Zone Cracking Heat-affected zone (HAZ) cracking susceptibility is a problem which plagues many of the highly alloyed austenitic stainless steels and nickel-based superalloys. Fusion zone cracking is also encountered in these materials but is usually avoided by the selection of appropriate weld filler materials. HAZ cracking is more insidious, since the phenomenon is often related to the composition of the material and its microstructure, both of which have been optimized to achieve desirable base metal properties. Several investigators have studied HAZ cracking in Alloy 800 weldments. 1-4 In general, the HAZ cracking was restricted to grain boundaries adjacent to the fusion line and appeared to result from localized melting along these boundaries at temperatures slightly below the bulk solidus temperature.l'4 Electron probe microanalysis (EPMA) of these liquated boundaries revealed a nearly 50-fold increase in Ti content4 relative to the average alloy composition (in wt pct: 46Fe-32.5Ni-21Cr-0.38Cu-0.38A1-0.38Ti-0.05C-0.008S).5 Several mechanisms have been proposed to explain the HAZ hot cracking phenomenon. 4'6-12 One mechanism involves the segregation of minor elements or impurities to the grain boundary, leading to a localized composition with a A. D. ROMIG, Jr., Physical Metallurgy, and M. J. CIESLAK, Process Metallurgy, are with Sandia National Laboratories, Albuquerque, NM 87185. J.C. LIPPOLD is with Edison Welding Institute, Columbus, OH. Manuscript submitted May 9, 1986. METALLURGICAL TRANSACTIONS A
low melting point. 13If the impurity is present in the matrix as a supersaturated solution and given sufficient time at temperature, the impurity will segregate to the grain boundary, the segregation being driven by a chemical potential gradient toward the grain boundary. If there is insufficient time for the elements to diffuse to the boundary, a moving boundary may sweep up the impurity through a "grain boundary sweeper" mechanism. In the case of Alloy 800, if the grain boundaries become enriched in Ti, either through volume diffusion or a grain boundary sweeper, one might postulate melting at the grain boundaries via the Fe-Ti and/or Ni-Ti eutectic reactions. Alternatively, a "constitutional liquation" mechanism has also been used to explain HAZ cracking in several nickelbase alloys, 7'8'9 high alloy stainless steels, 11 and maraging
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