Mechanism for suppression of surface hot-shortness in Cu containing ferritic stainless steel

This paper investigates why Cu containing ferritic stainless steel does not exhibit surface hot-shortness even when it contains much more than enough Cu to show the hot-shortness in mild steel. A 2.4% Cu containing 16% Cr stainless steel and a 0.3% Cu containing mild steel were employed and they were heated up to 1250degreesC in atmospheres of 20%H2O-1%O-2-bal.N-2 and 10%H2O-1%O-2 bal.N-2 for the stainless and the mild steels, respectively, in order to produce the same amount of scale. Their surface hot-shortness was assessed by measuring the number of cracks occurring in hot-deformed specimens and the microstructure around the scale/steel interface was closely observed by optical microscopy and SEM. No cracks were observed in the 2.4%Cu-16%Cr stainless steel while severe cracks were found in the 0.3% Cu mild steel although the amount of scale for the former was nearly the same or a little larger than that for the latter. For the 2.4%Cu-16%Cr stainless steel, Cu enriched liquid alloys were observed in the inner scale zone and no concentration of Cu was found at the scale/steel interface. In addition, a complicated structural zone called metal/oxide mixed zone covered the steel surface. The suppression of surface hot-shortness in Cu containing ferritic stainless steel was explained in terms of the structural characteristics of the scale/steel interface and the different diffusion rate of Cu atoms between alpha-phase matrix and gamma-phase matrix at 1250degreesC.