Influence of strain rate and temperature on serrated flow in 9Cr–1Mo ferritic steel

Tensile tests were performed on specimens in normalised and tempered condition at temperatures ranging from 300 to 873 K and at four strain rates in the range 6.33×10−5 to 6.33×10−3 s−1 to examine serrated flow behaviour in 9Cr–1Mo ferritic steel. At all strain rates, the steel exhibited different types of serrations namely type A, B and C serrations at intermediate temperatures, and the nature and type of serrations were strongly dependent on temperature and applied strain rate. Serrations were observed only after a specimen was deformed beyond a critical plastic strain. Critical strain for type A and A+B serrations decreases with increase in temperature and decrease in strain rate. Inverse temperature dependence of critical strain for type C serrations was observed. The activation energy of 86 kJ mol−1 obtained for serrated flow suggested that diffusion of an interstitial solute such as carbon is responsible for dynamic strain ageing in 9Cr–1Mo steel.