On the Secondary Hardening on Tempering in Vanadium Steels

Abstract

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%. Specimens were austenitized for 2 hr at 1200°C, quenched into 10% iced-brine, and tempered for 1∼100 hr at various temperatures ranging from 150° to 700°C, and examined by a transmission electron microscope.
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 as well as finely dispersed V₄C₃. (2) Considerable secondary hardening occurs above 550°C, and the hardness reaches a peak by tempering for 1 hr at 625°C in steels containing more than 0.1% vanadium. At the initial stage of secondary hardening, extremely fine V₄C₃ particles less than 20 Å in diameter are preferentially formed on dislocations. These particles are coherent with the ferrite matrix, and give rise to remarkable strengthening. (3) Vanadium carbide V₄C₃ grows into platelets parallel to {100}_α−Fe planes by tempering for 1∼10 hr at 700°C, and then the platelets spheroidize gradually at subboundaries. (4) The orientation relationship between V₄C₃ and the ferrite matrix is similar to that suggested by Baker and Nutting. Excellent lattice coherency is expected along {100}_α−Fe planes from the orientation relationship.
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.