Effect of Preheating on Hot Cracking Susceptibility in Pulsed Laser Welding of Invar Alloy

Abstract

Hot cracking is a serious problem in welding of Invar alloy. The weld hot cracking susceptibility of Invar was evaluated using pulsed laser welding on fish-bone sheet experiment. The pulse wave consisted of preheating pulse and welding pulse. Hot cracks that formed along the grain boundary propagated from the weld upper surface to the inside. The experiments show that adding a preheating pulse can effectively reduce the hot cracking susceptibility of Invar alloy. Finite Element Modeling (FEM) calculations and experimental measurement results show that the welding temperature gradient and cooling rate decrease with increasing preheating pulse duration. However, as the preheating pulse duration increases, the hot cracking susceptibility of the Invar alloy does not decrease all the time, but decreases first and then increases. This is because the increase of heat input leads to the increase of shrinkage plastic strain when the preheating pulse duration increases. The maximum tensile strength of the butt welded joint of the Invar alloy was 467.3 MPa, which is 92.3% of the base metal when the preheating pulse duration is 3 ms. Introduction The Invar alloy is widely applied in precision measurement devices and low temperature resistant structures for its low thermal expansion coefficient, which is less than 1.6 X 10-6 k-1, about 1/10 of low-carbon steel, and it changes very little within large temperature range (Park et al. 2011; Oh et al. 2014; Qiu et al. 2016). With the increase demand for natural gas, liquefied natural gas (LNG) carriers are being built to support long-distance transport. Invar is used for primary and secondary barriers on the containment insulation system of membrane-type LNG carrier. The Invar membranes have thickness of .7 mm are directly in contact the -163°C LNG (American Bureau of Shipping 2006; Bureau Veritas 2011; Zhao et al. 2013; Zhao et al. 2015; Zhao et al. 2019). The total weld length of Invar alloy can reach up to 100 km according to the statistics on a 13 million cubic meter LNG carrier.