Phase transformations in zones of plastic deformation beneath the surface of impact fractures in steel N32T3

Abstract

Impact tests are performed on steel N32T3 in the hardened and aged conditions. The mechanism of failure and martensitic transformation in zones of plastic deformation between the surface of fractures obtained over a wide temperature range is studied. It is shown that the failure mechanism for both hardened and aged steel in the stage of crack propagation depends weakly on test temperature. An increase in KCU and KCT impact strength for the steel with an increase in temperature is due to an increase in the work for crack generation and formation at the failure site of zone L whose microrelief is predominantly folded with elongated pits. Two zones of plastic deformation are detected by means of x-ray structural analysis beneath the surface of fractures in the hardened condition and one in the aged condition. It is established that within the limits of a highly deformed microzone for hardened steel the amount of martensite is constant in spite of presence of a deformation gradient, but in aged steel it decreases constantly. It is noted that for correct evaluation of the effect of phase transformations occurring in zones of plastic deformation on impact strength and failure mechanism for the steel it is necessary to consider local heating and the actual phase composition of the steel at the tip of a propagating crack.