METALLURGICAL AND MECHANICAL INVESTIGATION OF TIG ARC WELDMENTS FOR API 32″ GAS PIPE
Keywords:steel pipes, Weldability, Tungsten inert gas (TIG), Acicular ferrite, Impact transition temperature (ITT)
API steel gas pipe of 32" diameter and 15.9 mm thickness was investigated to characterize the steel grade and assess weldability for fourteen-consecutive passes of TIG arc welding technology. The parent Metal (PM) contains 0.02% C in addition to 0.02% Nb. A steel electrode for TIG process was used containing 5.6% Cr and 0.56% Mo. Standard V-groove was mechanically prepared to suite butt welding. The microstructure of the PM was containing mainly fine acicular ferrite satisfying the essential requirements of API specifications for grade X60. The weld metal (WM) was containing very fine acicular ferrite and showing the maximum hardiness values. Heat affected zone (HAZ) structure still contains acicular ferrite but it became coarse, possessing lower hardness than that of WM. HAZ does not exceed 2-2.5 mm adjacent to the welded V-groove. The fractured tensile welded specimens were necked and failed at the HAZ. The impact transition temperature (ITT) was detected as -35 oC. However, the mechanical properties of the WM did not negatively affected by the welding process and still fair satisfying the requirements of API X60.