Effect of thermal ageing on tensile-plastic flow and work hardening parameters of Indian reduced activated Ferritic Martensitic steel

Abstract

Effect of thermal ageing (873 K/5000 h) on microstructure, tensile-plastic flow and work hardening parameters of normalized and tempered (N&T) 1.4W-0.06Ta Indian Reduced Activated Ferritic Martensitic (INRAFM) steel was investigated. Hollomon, Ludwigson and Voce equations were used to explain the tensile-plastic flow response of the INRAFM steel over a wide temperature range of 300-873 K. Variations of strain hardening exponent was found to increase on ageing due to increase in work hardening capability of aged steel while strain hardening coefficient decreased on ageing. TEM specimens extracted near the tensile tested specimen of different temperatures were used to correlate the formation and movement of dislocation debris structure of both N&T and thermal aged specimens. Tensile plastic flow behavior at different temperatures was appropriately represented by Hollomon. Ludwigson and Voce equations and best fitting was adjudged by the goodness of fit (i.e. lowest ?2 values). Both yield strength (YS) and ultimate tensile strength (UTS) were fitted by the initial stress and saturation stress respectively as per the Voce constitutive equation and variations of initial stress and saturation stress with temperature were observed for N&T and aged steel. The absolute value of Voce strain component (nv) decreases due to ageing exhibiting two-stage behaviour with increase in temperature. The acceleration of recovery process was found lower at high temperature in case of steel subjected to thermal ageing as compared to N&T steel. Voce relation has been successfully used to predict the yield stress (YS) and ultimate tensile strength (UTS) of both thermal aged and N&T IN-RAFM steel at different temperatures.