Sulphur segregation and intergranular microcracking in ferritic steel weld metal

Abstract

Abstract Metallographic and fractographic studies were undertaken to investigate weld metal microcracking in a 2·25Cr–1 M o steel fabrication. Numerous small microcracks were found in single-pass TIG welds in the as-welded condition. An unusual form of brittle fracture on prior austenite grain boundaries was observed, associated with a dense array of intergranular submicrometre sulphide inclusions. The observations indicated that sulphide precipitation had occurred by a solidstate mechanism as the weld metal cooled through the austenite range, while elemental sulphur segregation promoted grain-boundary faceting and embrittlement. The microcracks appeared to have formed on the previously faceted and embrittled grain boundaries by a low-temperature quench cracking mechanism. The factors responsible for the severe sulphur segregation are reviewed, and the implications are discussed in the context of recent interest in the role of grain-boundary sulphides in weld stress relief cracking.