Summary: Effects of minor elements on weld cracking in superalloys

There are several distinct types of cracking which can occur during the fusion welding of superalloys, and they can all occur at high temperature. The cracking may occur in the weld metal or the heat-affected zone (HAZ), either during the making of the weld or during subsequent post-weld heat treatment. The latter applies only to precipitationhardened alloys, and has been described as strain-age cracking, but heat-treatment cracking is the preferred term. Solidification cracking occurs in the weld metal during the freezing of the weld pool and is usually referred to as being super-solidus. Liquation cracking occurs either in the HAZ or in previously deposited weld metal, reheated by an adjacent subsequent pass; it is associated with microsegregation. Ductility-dip cracking occurs in the HAZ, in weld metal, or in weld metal reheated by subsequent passes, and the same is true of heat-treatment cracking. Superalloys can be based on Fe, Ni, or Co, but most of the reported information relates to Ni-based alloys and very little to Co-based alloys. Nearly all of it deals with contents of minor elements above those that would normally be regarded as trace levels. The information available in the literature contains numerous apparent contradictions concerning the effects of elements on both individual crack mechanisms and different types of cracking. The influence of the various elements on weld cracking is discussed by grouping together the reported effects of each element on the various alloys and mechanisms which have been investigated. The behaviour of C provides an example of the confusing results that have been reported. Although one leading authority states that C has no effect on the weldability of Ni-Cr alloys, others have found that it promotes HAZ liquation, that it should be increased to stop HAZ liquation, and at low levels that it either aggravates or ameliorates post-weld heat-treatment cracking. There is general agreement on the detrimental effects of S, P, Pb, Sn, and Zr on high-temperature cracking resistance, and that high levels of Ti +Al promote postweld heat-treatment cracking. However, the effects of C, Si, Mg, and La are variable, and elements such as B have been shown to act in opposite senses for different crack mechanisms. Nb and Mn are generally accepted as having beneficial influences on weld cracking, although both have been demonstrated by microanalysis techniques to show an association with liquated (but not necessarily cracked) grain boundaries. In part, these contradictions can perhaps be explained by the existence of critical ranges within which a given element is harmful. This behaviour is perhaps best known and documented for AI alloys, where the effect of a given element on solidification cracking passes through a maximum at some given concentration. There is also the possibility of interaction between elements, minor and/or major, which may well influence the effect of any given element; this is particularly likely to be true of superalloys in which alloying additions vary widely. The difficulty in studying the effect of minor elements on weld cracking is partly related to the inherent inconsistency of welding and weld-simulation techniques, but the use of a wide variety of commercial alloys also undoubtedly tends to obscure the important interactive effects of various combinations of intentionally added and impurity elements.