EFFECT OF THE DELTA-FERRITE PHASE ON THE IMPACT PROPERTIES OF MARTENSITIC CHROMIUM STEELS

It is evident from controversial statements in the literature that the effects of the delta-ferrite phase on the impact properties of martensitic Cr-steels are not fully understood, especially with respect to the development of martensitic low-activation steels. An investigation has been started to clarify this situation using instrumented impact testing and SEM investigations. By analyzing the fracture behavior of alloys exhibiting between 0 and 25% delta-ferrite it could be shown that small amounts of this phase (about 1%) improve the toughness. On the other hand, higher amounts of the delta-phase significantly favour cleavage fracture. As SEM investigations revealed, these opposing effects are caused by the formation of a brittle M23C6 layer in the interface between larger delta-ferrite grains and the martensitic matrix. If this interface layer is avoided, delta-ferrite containing alloys show a somewhat better behaviour in the transition region than fully martensitic heats. The possible fracture mechanisms are discussed.