CABURISATION OF HEAT-RESISTANT STEELS

The effects of alloying additions of silicon; molybdenum, reactive ele ments (Ti, Y, Ta, Hf, Nb, Zr), and aluminium, in cast, heat-resistant steels on their carburisation resistance have been investigated at tem peratures of 900 to 1150 degrees C. Under strongly reducing conditions , where neither silicon nor chromium oxidise, it is found that the all oying 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 carbid e precipitates of these elements. Silicon operates by altering the sol ubility 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 c ast alloy showed that the cast alloy was more resistant to carburisati on. Under conditions oxidising to silicon and chromium, but not to iro n and nickel, carburisation rates are slower. The most important facto r is then alloy silicon content, which controls the formation of an ex ternal SiO2 layer.