Enhanced resistance to gas tungsten arc weld heat-affected zone cracking in a newly developed Co-based superalloy

The occurrence of heat-affected zone (HAZ) cracking and the effect of pre-weld microstructural modifications on HAZ cracking susceptibility of newly developed cobalt-based superalloy welded by gas tungsten arc welding (GTAW) process is investigated. Microstructural examination shows that a primary cause of HAZ cracking is the sub-solidus liquation of MC-type carbide and gamma' precipitates, which are identified by transmission electron microscopy (TEM) as present in the material before the welding process. Total crack length (TCL) analysis on pre-weld heat-treated samples reveals that pre-weld hardness does not control the HAZ cracking susceptibility during welding. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) coupled with HAZ TCL analysis confirms that grain boundary (GB) elemental segregation of Boron (B) controls the cracking susceptibility during welding. In addition, it is observed that reduction in grain size aids the alloy's resistance to HAZ cracking. Accordingly, a new pre-weld heat treatment that couples reduction in GB segregation of B with small grain size, is found to effectively suppress the HAZ cracking not only after welding, but also after post-weld heat treatment (PWHT). Furthermore, the new pre-weld heat treatment produces tensile properties, after PWHT, that are comparable to those of the alloy without the welding operation, which demonstrates that the pre-weld heat treatment prevents the deleterious effects of welding on tensile properties of the new superalloy.