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 2hr at 1200•Ž, quenched into 10% iced-brine, and tempered for 1•`100hr at various temperatures ranging from 150•Kto 700•Ž, 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 we11 as finely dispersed V4C3.(2)Considerable secondary hardening occurs above 550•Ž, and the hardness reaches a peak by tempering for 1hr at 625•Ž in steels containing more than 0.1% vanadium. At the initial stage of secondary hardening, extremely fine V4C3 particles less than 20A 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}ƒ¿-Fe planes by tempering for 1•`10hr at 700•Ž, and then the platelets spheroidize gradually at subboundaries. (4) The orientation relationship between V4C3 and the ferrite matrix is similar to that sug-geste by Baker and.Ntting. Excellent lattice coherency is expected along{100}ƒ¿-Fe planes fromŽRe 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 nuclea-tion sites.