An investigation of secondary hardening of a 1% vanadium - 0.2% carbon steel / E. Smith, Physical Metallurgy Division. : M38-8/61E-PDF

"Steel samples were quenched into oil, tempered and examined by transmission electron microscopy. The structure of the quenched steel was very similar to that of plain carbon steels of similar carbon content. Most of the ferrite grains were needles or large untwinned plates. Within the grains was a very high density of dislocations and also a Widmanstätten precipitate of iron carbide, formed during the quench. This structure was resistant to tempering up to about 20 hr at 500°C when the secondary hardening reaction appeared to begin. Peak hardness coincided with the formation of coherent platelets of vanadium carbide only a few unit cells in thickness and about 100 A in diameter. The dislocation network was stable under these conditions and appeared to be hold by the precipitates. The fall from peak hardness was accompanied by increase in size and reduction in number of the vanadium carbide particles. At the same time the dislocation network cleared from the grains accompanied by polygonization and recrystallization of the ferrite. Attempts were made to correlate measurements of strength and particle density with various theories for precipitation hardening. Moderato agreement was found with theories dependent on particle strength and spacing, but not for hardening based on coherency strains"--p. 583.

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