Local Brittle Zones and the Role of Niobium

There is a confused and contradictory literature on the role of small concentrationsof niobium on the development of the so-called local brittle zones in steels. These zones consistof a mixture of martensite and austenite and hence, their formation should be predictable usingmodern microstructure calculation methods. Following an assessment of the most relevant literature,a mathematical model is presented which enables three quantities to be calculated, the fractionof 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 elementsother than niobium, on the development of local zones.