This paper addresses and comprehensively answers topical questions about the role of niobium during austenite transformation in the coarse-grained heat affected zone (CGHAZ) of modern HSLA linepipe steels during welding. It has been categorically demonstrated that there is no justification for the application of any carbon equivalent formula, containing an additive factor for niobium, to be used in the assessment of CGHAZ transformation for the rapid cooling regime in which resistance to cold cracking susceptibility may require consideration. Indeed, there is every reason to expect niobium treated steels, which are resistant to austenite grain growth during welding, to exhibit significantly superior performance in this respect.
At higher welding heat inputs, as encountered in submerged arc welding pipe manufacture with multi arc procedures or in ‘double jointing’ girth welding, the finer grain sized HAZ’s are still encouraged to transform in an appropriate lower temperature range to achieve optimum microstructure and toughness through the role of niobium in solution in the transforming austenite. The way in which niobium achieves these unique benefits is thoroughly explained.