Abstract
A study has been made of the strength and ductility of a 0.42% C-Ni-Cr-Mo steel having mixed structure of martensite and bainite.
It was found that lower bainite, which precipitated acicularly so as to partition prior austenite grains, had a desirable effect on the 0.2% proof stress regardless of tempering conditions, but had not necessarily on the elongation. Upper bainite, which precipitated massively so as to fill prior austenite grains, was found to have detrimental effects both on the 0.2% proof stress and elongation irrespective of tempering conditions.
The beneficial effect of lower bainite on the 0.2% proof stress is attributable to the fact that the 0.2% proof stress deviates better from the general law of mixture, it resulting from the improvement of the strength of martensite owing to the refinement of its substructure, besides an increase in the strength of the bainite being due to higher plastic restraining of the bainite by martensite. The detrimental effect of upper bainite on it is attributable to the fact that it deviates worse from the law of mixture being due to ununiform strain of two-phase.
