Improvement of hot cracking susceptibility and productivity using Super-TIG welding for 9% Nickel-steel

Abstract

9% nickel steel has been widely used for storage of liquefied gases, oxygen, and nitrogen in competition with aluminum and austenitic alloys due to its low cost. The development of 9% nickel steel with low ductile-brittle transition temperatures reformed the LNG industry by extensive storage and transportation. In recent years, the demand for 9% nickel steel fuel tanks has increased due to the International Maritime Organization regulations on the reduction of sulfur oxides from 3.5% to 0.5%. Hence, the LNG fuel tanks made with 9% nickel steel became a major part of the international energy industry. However, the weldability, fabrication cost, and productivity of these steels are some of the major concerns for manufacturers. The flux-cored arc welding (FCAW) process is the most commonly used joining method for 9% nickel steel with Hastelloy fillers. In FCAW, the loss of deposited metal due to interpass cleaning by grinding action caused a reduction in total productivity. Demands for higher production, low cost, better mechanical properties, and reliability are the main driving forces for new developments in this area. To achieve these, the most recently developed “Super-TIG Welding” with C-type filler was successfully applied to improve the 9% nickel steel weldability, productivity, and mechanical and metallurgical properties of the butt welds. In Super TIG welding, alloy 625 filler was used, which is cheaper and has better weldability than the Hastelloy fillers. The weldability evaluation tests resulted in improved resistance to hot cracking susceptibility using Super-TIG welding. The mechanical properties of the Super-TIG welded joints performed better than the FCAW joints. Most importantly, the productivity of the welding dramatically improved with Super-TIG welding. Super-TIG welding is capable of producing higher feeding rates with clean beads. Therefore, the alloy 625 C-Filler and avoidance of interpass cleaning in Super-TIG welding result in increased productivity. The newly developed Super-TIG welding achieved many things to obtain satisfactory products in the recent welding industry.