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%.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.

Processes, microstructure and properties of vanadium microalloyed steels

“…This observation was confirmed by Raynor et al 44 and Tanino and Nishida. 45 The latter proposed that the secondary hardness found in vanadium steels was due to the suppression of dislocation climb and the reduction of the growth rate of ferrite grains, by vanadium in solution and finely dispersed carbides. Both sets of authors 44,45 used SAED to confirm the B-N orientation relationship and to 'identify' the platelet precipitates as vanadium carbides.…”

“…37,39 Smith and Nutting 37 considered the possibility that the precipitation might be preceded by the formation of vanadium enriched zones, similar to GP zones observed in Al-4%Cu alloys. 115 This was later considered to be unlikely due the high affinity of V for C. 116 Others claimed that GP zones form in bcc iron alloys [117][118][119] and it was suggested 113,120 that spherical zones rich in vanadium 121 occur before the precipitation of V 4 C 3 . However, it is highly unlikely that coherent precipitation will be the only type of precipitates in tempered steels due to high density of quenched-in dislocations providing ample sites for heterogeneous nucleation.…”

Precipitation of Fine Niobium Carbide Particles in Low Carbon Steels

“…In quenched a nd tempered steels, on the other hand , the secondary hardening can only be observed in case of the austenitization at higher temperatures approximately I 250°C and this is applied to the direct quen ching.I 5 , 16) According to Irani et aU 6 ) the impact tra nsition temperature of niobium conta ining steels d e teriorated significantly on tempering to peak hardness . After tempering to the p eak hardness condition there was no clear microstructural change under bright field conditions of transmission electron microscopy.…”

An electron-microscopical study of the tempering of tungsten high-speed steels

“…The phenomenon of secondary hardening in vanadium (1)(2)(3)(4) and molybdenum steels(5-7) has been thoroughly studied by previous workers, and it has been established that the secondary hardening in such steels is caused by the precipitation of V4C 3 and/or Mo 2 C. It has also been shown that the precipitation of W 2 C leads to secondary hardening in Fe-W-C alloys (8) and also in Fe-W-Cr-C alloys (9)…”