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.