Deformation and fracture of Cr – Mn – C – N steel in as-cast condition

Abstract

The paper studies the influence of boundary conditions and the loading rate on the strain behavior and fracture of Cr – Mn – C – N austenitic steel in the cast state without additional heat treatment. Regularities of steel strain and fracture were analyzed on the basis of three-point bending test data of square-section samples with and without a notch, placed with a rib on supports. In addition to the initial stage of the steel elastic strain, this unconventional arrangement of the sample on supports enabled the detection of two more stages of strain development under the effect of an external applied force: the stage of nonlinear strain and the stage of discontinuous strain preceding the moment of sample failure. As the loading rate increases, it was demonstrated that the fracture resistance and the extent of the nonlin-ear strain stage of the sample with a notch increases, and the extent of the discontinuous strain stage decreases. The sample without a notch has a prolonged nonlinear strain stage and exhibits maximum strength in the absence of the discontinuous stage. The end of the nonlinear strain stage corresponds to the moment of sample failure. A characteristic property of cast steel under the given loading conditions is that the fracture of the sample is brittle, despite the prolonged stage of non-linear strain. Structural metallographic and diffractometric studies have shown that in all tests the steel fracture is brittle with-out traces of plastic yield. The nonlinear strain stage of steel is determined not by dislocation plastic yield, but by the mechanism of γ → αʹ transformation in austenitic interlayers between nitride and car-bide particles under the effect of an external applied force. The discontinuous strain stage of steel is associated with the process of stable crack propagation across the sample.