Abstract
Slow bending and Charpy impact bending tests have been performed with V-notched and fatigue-precracked specimens to study notched toughness of medium carbon Ni-Cr-Mo steels having mixed structure of martensite and residual ferrite.
It has been found that residual ferrite, when it remained in lower tempered martensite, had detrimental effects on the slow bending and Charpy impact bending toughness even though characteristic relationship between the toughness and volume fraction of residual ferrite was found by the variation in the notched radius and loading speed. When the residual ferrite appeared in higher tempered martensite, harmless effect was found on the Charpy impact bending toughness in a state of upper shelf beyond room temperature, but the toughness became to be significantly deteriorated with a decrease in test temperature. From the analyses of microfractographs, the detrimental effect on the toughness primarily results from the fact that residual ferrite under V-notch and fatigue-precrack fractures in a brittle manner.
