Effect of martensite volume fraction on abnormal work hardening behavior of a low carbon low alloy ferrite-martensite dual-phase steel

This paper investigates the work hardening behavior of a low carbon low alloy steel under various ferrite-martensite dual-phase (DP) microstructures. For this purpose, several DP microstructures containing 23-87% martensite volume fraction (MVF) were developed by application of inter-critical annealing heat treatment processes. The Hollomon analysis was employed in conjunction with mechanical testing and electron microscopy observation to describe the work hardening behavior of DP samples. The experimental results indicated that the DP specimens containing lower than 36% MVF were associated with a 'single stage' of work hardening behavior while those of higher than 36 % of martensitic microconstituents were related to a 'double stage' of work hardening pattern. With increasing the MVF from 23 to 87 %, the strain hardening exponent for the first stage of work hardening showed a peak value of 0.111 at MVF = 73 %, while those for the second stage of strain hardening exponents were continuously increased from 0.162 to 0.197. This abnormal trend of strain hardening exponents with MVF was correlated with the variable hardening responses of ferrite and martensite microconstituents.