The effect of cold work on low temperature (0.35 Tm) creep crack growth in CMn steels

Rapid low displacement creep crack growth has been observed in cold-worked CMn steels tested in air at 320–380°C. Cracking was predominantly but not exclusively intergranular with respect to ferrite grains and pearlite colonies. Crack velocities were dependent on the degree of prestrain, hardness, prior thermal history, deoxidation practice and chemical composition. For 15% prestrained HFS35 steel with a Vickers hardness of 250 HV the displacements accompanying crack growth were predominantly elastic and the non-dimensional crack velocity may have a value greater than 100. The effect of prestrain was achieved both by an increase in creep resistance and through the formation of residual stress fields or microcracks by dislocation pile-ups at carbides on ferrite-ferrite or ferrite-pearlite boundaries. Subsequent crack propagation was by a continuum growth mechanism.