Effect of Heat Treatment on Ductile-Brittle Transition Behaviour of 9Cr-1Mo Steel

The 9Cr-1Mo steel used in fast breeder reactor is exposed to irradiation during service which severely affects the dynamic fracture resistance by increasing the ductile to brittle transition temperature (DBTT). Thus, even at room temperature, the steel can become brittle and prone to cracking. The microstructural features especially prior austenite grain size (PAGS), martensitic lath and packet size originating from prior processing routes has an effect on the toughness properties of 9Cr-1Mo steel, which is yet to be understood fully. In the present investigation, three different austenitizing temperatures (950 °C, 1025 °C and 1100 °C for 1 h) have been selected to vary the PAGS. Impact energy, lateral expansion, dynamic yield stress as the function of temperature has been estimated from Charpy impact tests carried out on normalized and tempered specimens. The results show lower DBTT for the samples reheated at 1025 °C followed by tempering.