Plasticity drop at temperatures above 1200 °C in the cast state of low carbon steels and its analysis

Abstract

Fractures of low carbon steels after tensile strength tests performed in a temperature interval from 1200 °C up to the liquid phase occurrence temperature are analysed in the contribution. Statistical analyses of the reduction of area values were also made with the aim to establish an integral influence of chemical composition. Intercrystalline fractures with various morphologies are formed, also within individual steels, in the temperature region of plasticity drop. Beside typical intercrystalline fractures, also fine grained interdendrite fractures are often present. Fracturing of steel in the two phase γ + δ region, in which fractures originate on the interphase boundary, is a specific case. The portion of the fine grained interdendrite fracture in the steel increases with increasing the C content and decreasing the portion of Al/N. The fracture process and thereafter the plasticity drop temperature and its course are determined on the interdendrite space boundaries by decohesion intensity of the polyhedrical austenite and/or δ ferrite grain boundaries. Increasing the phosphorus content, which segregates on the polyhedric grain boundaries, the cohesion strength decreases. At the highest testing temperatures, the phosphorus causes degradation of the cohesion strength via liquid phase formation. The portion of fine grained interdendrite fracture is increasing with increasing the C content and decreasing the Al/N ratio. The slope of the temperature course of plasticity loss and the temperature of plasticity drop to a nearly non-measurable level, th5, are preferable determined by the Al/N ratio mostly. The increasing of Al/N ratio which, in case of investigated steels was in an interval from 2.2 to 16, shifts the th5 temperature to the right.