Caburisation of heat-resistant steels

The effects of alloying additions of silicon; molybdenum, reactive elements (Ti, Y, Ta, Hf, Nb, Zr), and aluminium, in cast, heat-resistant steels on their carburisation resistance have been investigated at temperatures of 900 to 1150 degrees C. Under strongly reducing conditions, where neither silicon nor chromium oxidise, it is found that the alloying additions slow carburisation rates significantly. In the case of molybdenum and reactive elements, the beneficial effect is attributed to diffusional blocking of carbon flux due to the formation of carbide precipitates of these elements. Silicon operates by altering the solubility and diffusivity of carbon in the matrix. Aluminium appeared to exclude carbon almost completely from the alloy, probably by forming a tenacious external oxide scale. Comparison between a wrought and a cast alloy showed that the cast alloy was more resistant to carburisation. Under conditions oxidising to silicon and chromium, but not to iron and nickel, carburisation rates are slower. The most important factor is then alloy silicon content, which controls the formation of an external SiO2 layer.