Effect of boron segregation at grain boundaries on heat-affected zone cracking in wrought INCONEL 718
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HEAT-AFFECTED zone (HAZ) liquation cracking in INCONEL 718* has been extensively studied and has been *INCONEL is a trademark of INCO Alloys, Huntington, WV.
attributed to different factors. The original theory of liquation cracking was related to the formation of grain-boundary liquid through the constitutional liquation of grain-boundary precipitates and the inability of the liquid films to support the tensile stress developed during cooling after welding.[1,2,3] The phases that may induce constitutional liquation in the alloy have been identified to be primarily NbC carbides[1,2,4–6] in wrought material and carbides and Laves phase[5,7,8] in the cast alloy. The application of the constitutional liquation mechanism to HAZ cracking is generally satisfactory, yet is challenged by some observations. For example, the alloy’s resistance or susceptibility to HAZ liquation cracking is not proportional to the volume fraction of grain-boundary precipitates that are prone to constitutional liquation, but is significantly influenced by heat treatment, which, in fact, may not cause a significant change in the grain-boundary precipitation phases likely to undergo constitutional liquation.[5] When material was found not to contain grain-boundary precipitates, a susceptibility to HAZ liquation cracking still existed. Borland,[9] for example, in 1960, suggested that segregation of elements with high relative potency factor values (leading to wide freezing ranges), combined with suitable wetting of grain surfaces, could result in weld cracking. Other possibilities, according to Savage and Krantz,[10] include sweeping up of solute by migrating boundaries and the influx of solute down the grain boundaries due to epitaxial effects from the weld
W. CHEN, Assistant Professor, is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. M.C. CHATURVEDI, Senior NSERC Industrial Research Professor, and N.L. RICHARDS, Associate Professor, are with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6. Manuscript submitted June 5, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
pool. Thus, several possibilities exist to enable species to segregate on grain boundaries and reduce the melting temperature of the grain boundaries relative to the surrounding matrix. The segregation of impurities to grain boundaries can take place during the heat treatment before welding. In general, it can occur by two mechanisms, viz., equilibrium segregation[11] and nonequilibrium segregation.[12–19] In equilibrium segregation, solute atoms that diffuse to grain boundaries are actually bound to grain-boundary sites. The extent of this segregation increases with decreasing temperature and increasing solute concentration in the matrix. Nonequilibrium segregation occurs during cooling from high temperature[13,15,16] and during annealing following plastic deformation.[20,21] This type of segregation requires the formation of solute-vacancy compl
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