Strengthening in pearlite formed from thermomechanically processed austenite in vanadium steels and implications for toughness

Abstract

Strengthening in the pearlite of 0·75C–1·2Mn (wt-%) steels resulting from microalloying with 0·09–0·16% V was examined. The steels were transformed by controlled cooling in a dilatometer at an average transformation temperature of 600°C, which produced a pearlite interlamellar spacing of 0·11–0·12 μm. Using electron microscopy no hardening precipitates in the as transformed specimens were revealed, but after additional aging V(CN) precipitates were observed. In the investigation of the hardening caused by V(CN) precipitation, a limiting value was found, which could not be increased by a further addition of V. The limit of the precipitation hardening is suggested to be a consequence of the limited amount of C and N dissolved in the ferrite, since no interphase precipitation occurred. The maximum hardness obtained was 400 HV20 and an increment of 75 HV20 (or 260 MN m−2 in strength) was found in comparison with the comparable pearlite structure of a steel containing no V. From examination of the effect of the thermomechanically processed austenite structure on the pearlite morphology, it was found that transformation from fine grained unrecrystrallised austenite produced an ultrafine colony pearlite from which it can be stated that this transformation would result in improved toughness in pearlitic steels.MST/847