Microstructure and Mechanical Properties of GTAW Welded Joint of Invar Alloy

Abstract

Experiment of automatic gas tungsten arc welding of liquefied natural gas carrier Invar alloy with a thickness of .7 mm was completed, and the welding parameters were optimized, as well as microstructure and mechanical properties of the welded joint were measured and analyzed. The grain size of the area near the weld centerline was small, mainly cellular dendrites, and the grain size on both sides of the weld centerline increased gradually, mainly dendrites, whereas the grain size near fusion line was larger, and there were more columnar crystals. The heat-affected zone was composed of coarse austenite grains. Transgranular cracks were the main cracks in the welding seam. When welding current was 40 A, frequency was 120 Hz, and welding speed was 350 mm/min, tensile strength of the welded joint was 446.9 MPa, which 88.1% of the base metal’s tensile strength and 10.3% of the fracture elongation. The fracture surface of tensile specimens showed typical plastic fracture characteristics, with no obvious crack characteristics, and no eutectic liquid films were observed. Invar alloy is a Fe–36%Ni alloy. Its thermal expansion coefficient at room temperature is less than 1.6X10-6/k, which is about one-tenth of that of low carbon steel, and changes little in a larger temperature range. Therefore, Invar alloy is widely used in making precision measuring instruments and low temperature-resistant materials (Corbacho et al. 1998; Park et al. 2011; Qiu et al. 2016; Yakout et al. 2018). In recent years, with the increasing demand for clean energy, liquefied natural gas (LNG) carriers are developing rapidly (Zhao et al. 2015; Oh et al. 2018). LNG liquefies at temperatures below -161.5°C. The volume of LNG is about 1/ 625 of the volume of gaseous natural gas of the same mass; thus, it can transport large quantities of natural gas over long distances.