Broken Full Lift Disk of a Safety Valve

A full lift disk, made of die cast brass, which served as a lifting aid in a safety valve, had cracked in service at a number of locations in the vicinity of the threaded hole. During microscopic examination, agglomeration of oxide inclusions were noted in the region of the cracks. Because the die cast brass was alloyed with aluminum, these inclusions consisted predominantly of aluminum oxide. The tolerable limit in pores and oxide inclusions was greatly exceeded in the lift disk under examination. Above all, the numerous oxide skins disrupted the cohesion of the microstructure and were primarily responsible for the failure of the lift disk.