Local Brittle Zones and the Role of Niobium

Abstract. There is a confused and contradictory literature on the role of small concentrations of niobium on the development of the so-called local brittle zones in steels. These zones consist of a mixture of martensite and austenite and hence, their formation should be predictable using modern microstructure calculation methods. Following an assessment of the most relevant literature, a mathematical model is presented which enables three quantities to be calculated, the fraction of martensite, the carbon concentration of the martensite, and its ability to influence toughness. Examples are presented for particular linepipe steels, and then the generic effect of alloying elements other than niobium, on the development of local zones. The Local Brittle Zone as an Entity Local brittle zones (LBZs) are small regions of hard, brittle phase that form in the heat affected zones (HAZs) of multipass welds [1]. They usually contain untempered martensite that can lead to scatter in toughness data when the test specimen samples a brittle zone [2]. Such scatter reduces confidence in design because of the existence of a few poor values which fall out of the desired range. Experimental data have been interpreted to conclude that the lower bound toughness values deteriorate as the volume fraction of martensite in the form of LBZs increases [3]. However, th