Failure of weld joints between carbon steel pipe and 304 stainless steel elbows

A number of weld joints between carbon steel (CS) pipe and type 304 stainless steel (SS) elbows constituting a gas piping system of a petrochemical unit developed cracks after a relatively short period of usage, resulting in leakage. The gas flowing through the pipe, was hydrogen rich at a temperature of 45 °C and a pressure of 16 kg/cm2. Light optical metallography and scanning electron microscopy, combined with energy dispersive X-ray spectroscopy, inductively coupled plasma and microhardness testing were used to determine the most probable cause of failure. Analysis showed that the cracks originated at the interface between the CS pipe and the SS root weld. A narrow band between the CS pipe and SS weld exhibited a hardness of Rockwell C 60 suggesting the formation of martensite due to C segregation at welding temperature and subsequent quenching during cooling. The ferritic region of CS adjacent to the weld was decarburized and was devoid of pearlite; corroborating C diffusion. The weld region was diluted comprising mainly of Fe with small amounts of Cr and Ni. Cracking is thought to have initiated at the hardened region. However, the failure might have been aided by hydrogen rich medium and soft C-depleted ferrite region.