Failure of Piping System Cross by Intergranular Cracking Traceable to Improper Heat Treatment
Linear indications on the outer surface of a cross in a piping system were revealed by dye-penetrant examination. The cross was specified to be SA403 type WP 304 stainless steel. The cross had been subjected to induction-heating stress improvement. The linear indications on the cross were located in wide bands running circumferentially below the cross-to-cap weld and above the cap-to-discharge-pipe weld. The material was found to conform to the requirements both in terms of hardness and strength. Intergranular cracks filled with oxide were observed on metallographic analysis of a sectioned and oxalic acid etched sample. The grain size was found to exceed the ASTM standard. No indications of sensitization were observed during testing with practice A of ASTM A 262. Definitive evidence of contaminants to support SCC as the failure mechanism was not disclosed during analysis. It was concluded that overheating or burning of the forging, which classically results in large grain size, intergranular fractures, and fine oxide particles dispersed throughout the grains was the possible reason for the failure.