Developing higher strength microalloyed medium carbon steel capable of being fracture split after laser transformation notching

Abstract

High proof strength (>600 MPa) microalloyed medium carbon steels are being developed to replace high carbon steels in the production of internal combustion engine components which are manufactured by processes involving fracture splitting. The target steels are required to achieve a balance between the ability to fracture when initiated by a novel laser transformation notch (LTN) and the properties required by the application. In this study, the effect of phosphorus (P) and sulphur (S) and the depths of LTN on the fracture process of these forged steels were investigated by using an instrumented Charpy impact tester. The impact specimens were notched by a fibre laser with no removal of material. The depth of the notch showed a significant influence on the force–displacement curves, the Charpy impact energy and the associated fracture surfaces. The steel with combined high S and P contents showed the lowest Charpy impact energy of 2·9 J when the depth of LTN was >0·4 mm compared to 5·5 J with the steel containing only a higher S content and 11·2 J for the reference base steel with normal S and P contents. It was observed that MnS inclusions in the originally forged materials were redistributed to the resolidified grain boundaries in the melted region of the LTN. The distribution of P could not be identified in the LTN but presumably it also segregated to the interdendritic regions and the columnar grain boundaries during freezing. The steel containing enhanced contents of S and P was shown to be potentially suitable for fracture splitting and higher load applications.