Influence of nickel on toughness and ductile-brittle transition in low-carbon martensite steels

Abstract

The ductile-cleavage transition has been studied with respect to grain size, yield strength, and nickel content by impact testing of low-carbon < 0.1%C) lath martensite steels with 5%Mn, 5%Ni, and 9%Ni, respectively. The impact transition temperature (ITT) is found to be controlled by the martensite packet size and the yield strength analogously with the general case reported for ferritic steels. Regardless of packet size and yield strength, nickel additions imply powerful intrinsic toughness improvements and may decrease the ITT by as much as 20°C per percent nickel. At the same time, nickel moderated the effect of packet size and yield strength on ITT. These important effects of nickel additions are mainly attributed to a corresponding increase in cleavage fracture resistance and form the basis for the use of Ni steels in cryogenic applications. The effects of grain size, yield strength, and nickel additions on ITT are adequately described by a simple model, based on the assumption that the ductile-brittle transition occurs at the intersection between temperature dependent yield stress and cleavage fracture stress curves. The cleavage fracture stress is strongly dependent on packet size, but is also found to increase with increasing temperature in the manner expected for a cross-slip-influenced fracture process.