Change of microstructures and mechanical properties in Mn-Mo-Ni low alloy steel with respect to intercritical heat treatment conditions

The change of microstructures and mechanical properties in a Mn-MoNi low alloy reactor pressure vessel steel with respect to intercritical heat treatment (IHT) conditions has been examined. The introduction of IHT after quenching step of commercial heat treatment process of the steel significantly improved the Charpy impact toughness and J-R fracture resistance. The IHT conditions showing improved toughness were the temperature range of 710-740 deg C and the holding time of 4-8 hours. At the early stage of IHT, fine distribution of newly formed martensite (austenite in the intercritical temperature region) in tempered bainite matrix produced the fine microstructure with small rounded carbides after tempering, which resulted in the improved toughness. With the higher IHT temperature or longer time, however, the positive effect of IHT disappeared due to the coarsening of austenite. High toughness was obtained when the austenite volume fractions formed during heating to the intercritical temperature region were about 20-60%. (author)