Abstract
Two vanadium steels B and D containing 0·12%C and 0·45%V with varying %Mn were continuously cooled from austenite over a range of cooling rates and a hardness peak found to occur at 1 to 2 K s−1. Microstructural features known to contribute to strength, such as grain size, dislocation density, and precipitate size and volume fraction were measured. By means of the Hall–Petch equation it was shown that the peak hardening was primarily due to an increase in the precipitation strengthening component σp. When a volume rather than a surface distribution of particles was considered, comparison between σp obtained by difference and by direct measurement showed a reasonable agreement at the cooling rate corresponding to peak hardening, but was less satisfactory at slower cooling rates. The particle size distributions for a variety of cooling rates were obtained from carbon extraction replicas, and the problems and limitations of this method are discussed. Finally, steel D which contained 1·5%Mn showed a lower strength at the peak (620 MN m−2 compared with 750 MN m−2) than steel B (0·02% Mn), although the grain size strengthening contributions were similar for both steels. It was concluded that a greater precipitation strengthening contribution was responsible for the increased strength of steel B.
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