Role of Ti and N in line pipe steel welds
High strength line pipe steels exhibit a combination of excellent toughness and high strength achieved through microalloy additions and thermomechanical controlled processing. During welding, severe thermal cycles experienced by the heat affected zone (HAZ) result in precipitate coarsening/dissolution and subsequent grain growth. This significantly reduces toughness in this region. It is well known that small Ti additions are utilised to control grain growth in the HAZ through grain boundary pinning action of TiN precipitates. Because of a lack of systematic and controlled study, it has been difficult to quantify the effect of TiN in the variety of steels. Hence, the optimum levels proposed in the literature are inconsistent and even contradict each other when compared. This paper mainly reviews the effect of different levels of Ti, N and Ti/N ratios on steels and pipes manufactured using different processes, with particular focus on the HAZ toughness.
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