The characteristics of GTA (gas tungsten arc) under high pressure has been studied in this paper to reveal the special arc phenomenon from the ambient pressure. A totally new hyperbaric GTAW experiment system has been built, including a hyperbaric environment simulator (0.1-3.5 MPa), an automatic GTAWmachine with cooling, the moving equipment, a camera with filter and a central control unit. The arc morphology, the electric static characteristic, the arc voltage and arc length are studied with the increment of ambient pressure from 0.1 to 3.5 MPa, respectively. The relationship between the ambient pressure and the arcvoltage is deduced according to the balance of heat-input and heat-output. The deduced function also verified the results from the welding experiment.

Ferrite number (FN) is a crucial parameter for austenite steel welding product, since it has significant influences on the crack sensitivity and ductility at cryogenic temperature. Normally, the FN is measured using a Feritscope or predicted according to WRC-1992 and the results generally match. This investigation detected the FN of 304L stainless steel multi-pass GTAW joint protected by mixed shielding gas with different N₂ additions; the measured FN did not match with the FN predicted through WRC-1992 as the N₂ content wasmore than 1%. The microstructure showed that a fully austenitized zone formed at the inter-bead of the joint shielded with Ar–1.0%N₂ and Ar–1.5%N₂, which resulted in the inaccuracy of the FN predicted through WRC-1992. The formation mechanism of the fully austenitized zone was proposed based on the nitrogen solubility and equilibrium phase transformation map. It is suggested that traditional prediction diagrams cannot give an accurate prediction of the FN during the multi-pass process shielded with N₂ addition.