Metallurgical Investigation into the Incidence of Delayed Catastrophic Cracking in Low Nickel Austenitic Stainless Steel Coils

Abstract

An incidence of delayed catastrophic cracking in commercially processed low nickel austenitic stainless steel coils during unwinding of the material at a customer’s end has been investigated. The fracture surfaces of failed steel coil samples revealed a brittle intercrystalline mode of fracture with typical manifestations of clear grain facets. Optical and scanning electron microscopy of polished, unetched steel specimens exposed a preponderance of branched as well as discrete stepwise microcracks, commonly observed in hydrogen-induced delayed failures. The specimens, electrolytically etched in 10% oxalic acid, were found to exhibit pronounced intergranular cracking, stepwise microcracking, grain dropping, and intergranular ditching in the annealed austenite microstructure of low nickel stainless steel. Investigations also revealed unusual levels of residual hydrogen to the extent of 11 ppm in steel specimens and a marked loss of tensile ductility was also noticeable in the defective steel with elongations barely approaching 8% in comparison to an overall elongation of 50% at the time of its dispatch to the customer. Based on the delayed nature of cracking, which transpired at customer’s premises after several days/months of dispatch, and the fact that the failure of the coils occurred at the time of uncoiling, when the operating stresses are well below the yield point of the material, it is surmised that the failure is attributable to hydrogen-induced damage. This article also takes a critical look at the possible sources of hydrogen pickup during steel making and elucidates remedial measures for control of hydrogen content in steels.