Metallurgical analysis of failed jaws used in joint of a water supply pipe line

In this paper, the cause of failure of the 420 stainless steel jaws in a joint was investigated. It was found that the martensitic stainless steel ASTM type 420 jaws used in connection joints of a water supply pipeline were cracked after 6 months in service. The failed parts were investigated by means of stereoscopic microscope, optical microscope, scanning electron microscope equipped with energy dispersive X-ray spectroscopy micro-analysis, inductively coupled plasma atomic emission spectroscopy, carbon–sulfur analyzer and Rockwell hardness tester, in order to identify the causes of failure and suggest preventive solutions. The study shows that failure was mainly due to intergranular corrosion cracking caused by the precipitation of chromium-rich carbides (or oxides) formed during heat treatment along grain boundaries.

► Fracture surfaces revealed typical intergranular fracture features.
► Cr-rich carbides are generated during heat treatment along the grain boundary.
► Failure was mainly due to intergranular stress corrosion cracking (IGSCC).
► IGSCC caused by the precipitation of Cr-rich carbides along grain boundaries.