Effects of tempering on the mechanical and corrosion properties of dual phase steel

Abstract

The mechanical and corrosion behaviors of low carbon dual phase (DP) steel were studied based on different tempering conditions. By increasing the time and temperature of tempering, the ultimate tensile strength (UTS), the hardness, and the corrosion current density (icorr) in the polarization curves decrease while the total elongation and the diameter of the semicircular arc in the Nyquist plots increase. Tempering treatment resulted in the appearance of the yield point phenomenon and an initial rise in the yield stress (YS). While the UTS values were related to hardness through a single relationship for both tempered and DP steels, two separate trends were observed for the relation of the YS and hardness. The latter was related to the high work-hardening rate of DP steels that increased the hardness during indentation. The tempering kinetics were also studied and the average value of the Avrami exponent (n) was determined as 0.65, which was interpreted as precipitation on dislocations (theoretical n-value of 0.667). The tempering activation energy was determined as ∼ 40 kJ/mol, which was indicative of the rapid diffusion of carbon atoms along the dislocation core (pipe diffusion), and hence, it was concluded that the kinetics of tempering is essentially controlled by carbon atom diffusion.