Influence of dislocation hardening, precipitation hardening, grain elongation and sulphides on fissure formation in HSLA steels having a ferrite/pearlite microstructure

Abstract

A number of examinations have been carried out on HSLA control rolled ferrite/pearlite steels to establish the influence of grain shape, precipitation hardening, dislocation hardening and inclusions on fissure formation. Grain elongation has been found to be mainly responsible. Decohesion of the grain surfaces is much more common than generally thought and occurs even in normalised steels. Warm rolling introduces dislocations into the structure and the increased strength of the matrix relative to the grain boundary strength helps to separate the grain surfaces but the main cause of fissuring is the grain elongation that occurs on warm rolling and this is enhanced the smaller the step height between the grains and the smoother the grain surfaces. Elongated austenite grains can also produce elongated ferrite grains which can also lead to fissuring.Precipitation hardening was found to cause only a small increase in the degree of fissuring as it does not influence the grain elongation and also its strengthening effect in the steels examined was generally relatively small, not greater than 100 MPa. The sulphur content of the steel was also found to have only a small influence on fissure formation. Reducing the S content did not remove the fissures but at similar grain aspect ratios fewer fissures were produced. To produce steels that have few fissures, it is important to reduce the elongation of the ferrite or austenite grains. Since dislocation hardening is normally associated with grain elongation the favoured form of increasing the strength to meet the strength requirement in these high strength steels is to produce a uniformly fine grain size with as a low an aspect ratio as possible and to use solid solution hardening and precipitation hardening for further strengthening.