Brittle intergranular fracture at elevated temperatures in low–alloy steel

Abstract

Recent studies of stress-relief cracking in low-alloy steels have focused attention on a novel mode of brittle intergranular fracture which occurs at elevated temperatures (300–650°C) in hard, coarse–grained heat–affected–zone microstructures. Fracture initiates at stress concentrators such as sharp cracks or inclusions, and can propagate under static loading at rates of 10⁻¹¹−10⁻⁵ ms⁻¹ to produce intergranular facets with very little associated plastic deformation. The stress-intensity parameter K has been used to characterize crack growth, and three regimes of behaviour have been observed: (i) a threshold region at growth rates of 10⁻¹¹−10⁻¹⁰ m S⁻¹, (ii)a plateau region, in which growth rates are independent of K between 10⁻¹⁰ and 10⁻⁸ m S⁻¹, and (iii) a region of highly K-sensitive crack growth between 10⁻⁸ and ⁻⁵ m S⁻¹. Independent Auger electron spectroscopy analyses have demonstrated that sulphur segregates locally to the high-temperature crack tip, giving rise to the embrittlement of a limited area of grain boundary. Together with other presegregated solutes, this enables brittle fracture to occur at high temperature, and the transfer of sulphur to the crack tip controls the rate of crack growth. Two models describing crack-tip sulphur segregation are currently proposed. In the first model, a quantitative analysis demonstrates that the crack-tip stress field will drive undersize solute atoms such as sulphur to the physical crack tip. In the second, the intergranular crack is modelled as a sharp cavity. Grain-boundary sulphides which are exposed by cavity formation become unstable and dissolve, saturating the cavity surface with sulphur, which is then drawn into the tip as part of the cavity growth process.

MSTj77