Effect of martensite volume fraction on work hardening behavior of triple phase (TP) steels

The strain hardening behavior of AISI 4340 steel with dual phase (DP) and triple phase (TP) microstructures with constant ferrite volume fractions of 34% was investigated. The microstructures of specimens were analyzed using an optical microscope. The tensile flow behavior of all the samples was explained using Holloman equation. Results showed that in both DP and TP steels, work hardening takes place in two stages. The onset strain of second stage of work hardening and Holloman equation parameters (work hardening exponent and strength coefficient) in both stages were increased with the increase of martensite volume fraction in TP steels. At first stage of work hardening, ferrite phase deforms plastically and at the second stage, co-deformation of all phases happens. Variations of work hardening behavior with martensite volume fraction were related to internal martensitic transformational stresses and their effects on adjacent ferrite grains. The difference between UTS and YS, and strength ratio as a criterion for work hardening were confirmed Holloman equation parameters’ changes with martensite volume fraction.

Research highlights
► Work hardening of TP steels increased with increasing martensite content.
► The TP steels showed two-stage work hardening in term of Holloman equation.
► The first stage of work hardening related to the ferrite plastic flow.
► At the second stage, co-deformation of both ferrite and hard phases happened.
► Among the hard phases, bainite yielded at lower strains in comparison to martensite.