Failure analysis of a stainless steel socket-welding flange due to improper manufacturing process and chemical composition

This paper presents the failure of a socket-welding flange that cracked during the tightening of the bolts. The component failed, despite of the correct tightening procedure. The flange, supposedly produced by forging, should meet the specification ASTM A182 Grade F316L. The visual inspection revealed the presence of several cracks located at the flange back face (surface in contact with the nuts). The investigation involved visual inspection, chemical analysis via optical emission spectroscopy (OES), metallography, fractography, hardness test, bend test and finite element stress analysis, aiming to determine the mechanisms and the root cause of the failure. The analysis revealed that casting was the actual manufacturing process of the flange, instead of forging. Besides, the carbon, chromium, nickel and molybdenum contents were not in accordance with chemical requirements of AISI 316L. Therefore, the flange did not meet ASTM A182 Grade F316L specification. The improper manufacturing process and chemical composition promoted embrittlement of the flange material, favoring the premature failure during the tightening of the bolts.