AbstractIn this paper, the changes in fracture mode that occur when a phosphorus–doped steel is tempered at temperatures between 150 and 650°C are reported. The primary technique used in the study is high–resolution scanning electron microscopy. It was shown by the results that when the steels were tempered at temperatures<250°C, the fracture was usually of a ductile intergranular type. Large shallow microvoids formed along the grain boundaries and were nucleated on second–phase particles. At temperatures between 250 and 400°C, the fracture mode changed to very brittle intergranular fracture. The fracture proceeded along the grain boundaries with very little evidence of plastic tearing. Since tempering at these temperatures does not cause additional segregation of phosphorus, it can be assumed that the onset of brittle fracture occurs because of the precipitation of M3C carbides along the grain boundaries. It is suggested that the carbides contribute to both nucleation and propagation of the intergranular cracks. It is also shown by the results that the ease with which a grain boundary fractures does not simply depend on the amount of impurity segregation, but also on the structure and orientation of the grain boundary.MST/215