Cr Segregation and Impact Fracture in a Martensitic Stainless Steel

The fracture surfaces of a 10.5 wt.% Cr martensitic stainless steel broken in Charpy testshave been investigated through X-ray photoelectron spectroscopy (XPS). The specimens have beenexamined in two different conditions: as-quenched and heat treated for 10 h at 700 °C. The trends ofFe/Cr ratio vs. test temperature are similar to the sigmoidal curves of absorbed energy and, after bothductile and quasi-cleavage brittle fractures, such ratio is always significantly lower than the nominalvalue of the steel chemical composition. Cr segregation does not occur on a macroscopic scale buttakes place in microscopic zones which represent weaker spots in the steel matrix and a preferredpath for moving cracks. Small area (diameter 300 m) XPS measurements evidenced a higher densityof such microscopic zones in the inner part of probes; this is explained by the different diffusionlength of Cr atoms in the external and inner parts during quenching from austenitic field whichhas been calculated through FEM simulations. No significant differences of Cr concentration wereobserved in fracture surfaces of probes with and without heat treatment. The results highlight how Crsegregation plays a role not only in the intergranular mode of fracture but also in the quasi-cleavageand ductile ones.