Fractures of as-cast low-carbon steels in the plasticity degradation area at high testing temperatures

Abstract

The work studies the fractures of low-carbon steels after tensile tests in a temperature interval from ca 1200°C, where a plasticity degradation takes place, up to the liquid phase formation temperature. Statistical analyses of the temperature dependence of the reduction of area were made with the aim to determine integral effects of the chemical composition on the plasticity course.

In the plasticity degradation temperature area, intercrystalline fractures occur, which have several morphologies, even within the same steel grade. Besides typical intercrystalline fractures, a fine-grained, interdendritic fracture often occurs. A specific case is the failure of steels in the γ + δ two-phase area, where fractures are formed at the phase interface. According to the failure type, fractures have been classified under 4 basic groups.

The fracture process and subsequently the plasticity degradation temperature and the plasticity course are determined by the intensity of decohesion of polygonal boundaries of austenite grains, or δ-ferrite grains, and of boundaries of interdendritic space. With increasing content of phosphorus, which segregates on the polygonal grain boundaries, the plasticity degradation start temperature decreases. At the highest testing temperatures, phosphorus degrades the cohesive strength of grain boundaries by formation of a liquid phase.

The slope of the temperature dependence of plasticity degradation and the temperature of plasticity degradation to a nearly non-measurable level, th5, are mainly determined by the Al/N ratio. An increase of the Al/N ratio, which was in the interval from 2.2 to 16 for the tested steels, shifts the th5 temperature to the right.