Failure Analysis of a High Pressure Butt-Weld Flange

This paper constitutes the failure analysis of a 24-inch diameter ‘Flange’ that had been welded to a high-pressure gas transmission pipeline. The flange had ruptured catastrophically during the hydrostatic test that was conducted to testify the integrity of the flange as well as that of the weld joint. The rupture had occurred along the weld through the heat-affected-zone (HAZ) on the flange side. The flange, which was a steel forging (of Class 600 as per ANSI Standards) made from ~ 0.25 % carbon steel, was expected to exhibit good weldability. However, the metallographic examination revealed that the steel had a coarse grain-size and an in-homogenous microstructure wherein pearlite-rich regions, ~2-3 mm across in size, were sporadically distributed in the microstructure. The coarse grain-size coupled with the presence of pearlite-rich patches in the microstructure had locally reduced the weldability of the steel, i.e., increased its tendency towards HAZ cracking during welding.