HOT DUCTILITY OF AN AUSTENITIC AND A FERRITIC STAINLESS-STEEL

The influence of strain rate and grain size on the hot ductility of an austenitic and a ferritic stainless steel has been examined. Samples were cooled from the austenitising temperature to temperatures in the range 1000 to 700 degrees C and tensile tested at strain rates in the range 10(-1) to 10(-4) s(-1). The austenitising temperature was varied to give two grain sizes, coarse, 600 mu m and fine, similar to 30 mu m. For both steels, ductility was excellent at fine grain size through out the temperature range and strain rates examined. The ferritic and austenitic stainless steels both gave ductility troughs at the coarse grain size, but the trough was favoured by higher strain rates in the ferritic steel and lower strain rates in the austenitic steel. The poo r ductility was related to the presence of precipitation, mainly at th e grain boundaries; this being FeTi phosphides in the case of the ferr itic stainless steel and coarse chromium carbides in the austenitic st eel. Grain boundary sliding was the major mode of intergranular failur e in the austenitic steel while normal microvoid coalescent failure co ntrolled ductility in the ferritic stainless steel. (C) 1997 The insti tute of Materials.