Failure of Feed Cock Studs
The cover of a feed cock fitted to an economic boiler suddenly blew off and the plug lifted sufficiently to permit the escape of water. It was found that all four of the studs securing the cover had fractured. In each case fracture had occurred at the end of one of the screwed portions adjacent to the shank. The fractures were of a short nature, with no evidence of progressive cracking by fatigue, nor was there any sign of stretching prior to failure. The fractured faces and the shanks of the studs were of rusted appearance. Microscopic examination of the material showed it to be an austenitic nickel-chromium stainless steel, stabilized by titanium and of the free machining type. Multiple transgranular cracking characteristic of failure from stress-corrosion cracking, was present to an extensive degree. It was considered probable that there had been slight leakage of water from the valve over a period and evaporation resulted in a solution which favored failure from stress-corrosion cracking. If corrosion resistant studs were desired, those of bronze or Monel metal are to be preferred.