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.
Materials Science Forum 783-786 (2014) 2129-2135.
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