Failure analysis of a 34CrMo4 seamless steel gas cylinder filled with a mixture of fluorine and nitrogen gases

Abstract

A fifty litre seamless gas cylinder manufactured from a 34CrMo4 low carbon alloy steel which was in service for five years failed in an unexpected manner, causing severe damage. The cylinder was filled with a mixture of fluorine and nitrogen. The fluorine was first filled into the cylinder at a pressure of 4 MPa and then topped up with nitrogen at 14 MPa. The filled cylinder was then allowed a 90 min settling period before coupling. Unfortunately, 20 min into the allowed 90 min of settling period, the fracture occurred leading to an explosion and damage. The fractured cylinder was subjected to chemical analysis, microstructural analysis, mechanical testing and stress calculation analysis. The mechanical and material properties of the steel were found to be of good quality indicating good manufacturing and heat treatment practices. The stress analysis also indicated that the applied pressure was within the applicable limit of the material. It was however found that, the failure occurred due to fluorine attack (burning) of the steel.