A study was carried out to obtain a detailed knowledge about the change in microstructures in connection with the secondary hardening on tempering of a series of vacuum-melted 0.2% carbon steels containing vanadium up to about 0.5%.The main results are as follows: (1) High resistance for tempering in vanadium steels can be explained in terms of the suppression of dislocation climb and of the reduction of the growth rate of ferrite grains by vanadium atoms in solution particles are coherent with the ferrite matrix, and give rise to remarkable strengthening.(3) Vanadium carbide V4C3 gradually at subboundaries. (4) The orientation relationship between V4C3 and the ferrite matrix is similar to that sugrelationship. To summarize, the high strength of vanadium steels on tempering is attributed to the finely dispersed precipitates of coherent V4C3, a comparatively high density of retained dislocations, and the smallness of grain size. The fine dispersion of V4C3 particles can be attributed largely to the existence of high density of dislocations acting as the preferential nucleation sites.

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