AbstractTo investigate the effect of microstructural evolution during heat treatment in newly developed Cr–2W ferritic steels containing from 2 to 15%Cr, Charpy impact testing was carried out after quenching, tempering, and subsequent prolonged aging at 873 K for 3600 ks (1000 h). The ductile to brittle transition temperature (DBTT) and the upper shelf energy were determined as functions of heat treatment and Cr concentration. The fracture sequence was examined on fractographs obtained using scanning electron microscopy. The steels consisted of bainite, martensite, andδferrite phases and carbide precipitates, depending on the Cr concentration. The DBTT decreased considerably after tempering compared with after quenching, but increased again somewhat after subsequent prolonged aging. The upper shelf energy exhibited converse behaviour. Concerning the effect of alloy phases, the DBTT was increased significantly by the formation ofδferrite and was slightly higher for the martensite than for the bainite. The effects of the alloy phases and heat treatments on the DBTT are discussed by considering the microstructural evolution, such as dislocation recovery and carbide precipitation, during heat treatment.MST/1573