Effect of Filler Metal La Additions on Micro-Cracking in Multi-Pass Laser Overlay Weld Metal of Alloy 690

摘要

The effect of La addition on microcracking susceptibility in the multipass laser overlay welds of alloy 690 (Inconel 690) was investigated. Multipass laser overlay welding of alloy 690 was conducted on alloy 132 (Inconel 132) in air (in-air welding) and under water (underwater welding) by using a YAG laser with the power of 2 kW. Argon gas shielding was applied in both cases with the gas flow rate of 20-60 L/min. Five kinds of filler metals, which were varied in La content from 0 to 0.09 mass%, were used. Microcracks in both weld metals produced by in-air and underwater welding were categorised as ductility-dip cracks. Ductility-dip cracks occurring in the weld metal could be completely prevented by using 0.01-0.03 mass% La containing filler metals in both in-air and underwater welding. However, excessive La addition more than 0.04 mass% to the filler metal adversely led to the occurrence of weld metal solidification/liquation cracks also in both in-air and underwater welding. The improved ductility-dip cracking susceptibility was attributed to the desegregation of impurity elements of P and S to grain boundaries due to the scavenging effect of La. Excessive La addition to the weld metal caused the formation of a Ni-La intermetallic compound with low melting point, resulting in solidification/liquation cracking. Thermo elasto-plastic analysis indicated that the plastic strain-temperature curve slightly intersected the ductility-dip temperature range (DTR) of the La-free weld metal in the cooling stage of overlay welding, however, the plastic strain-temperature curves did not cross the DTRs of the La-added weld metals. The analysed results were fairly consistent with the experimental results that a few ductility-dip cracks occurred in the La-free weld metal, while no ductility-dip cracks occurred in the La-added weld metal of multipass laser overlay welds.