The Effect of Different Heat Treatment Procedures on Duplex Stainless Steel Microstructures and Electrochemical Properties

In this paper, the effect of solutioning and various aging heat treatment processes on the microstructure, the hardness and electrochemical properties of a duplex stainless steel (DSS) were studied. The evaluation of the microstructure and phase compositions were carried out by the optical microscopy (OM) and the X-ray diffraction (XRD). Electrochemical behavior of specimens were evaluated by both potentiodynamic polarization and electrochemical impedance spectra (EIS) tests at 25 and 60 ºC. The obtained results showed that the solutioning heat treatment increased corrosion rates with respect to the blank specimen. The aging process at 490 ºC for 20 hrs increased the volume percent of the carbide phase to the highest value (25.1%) which resulted in an increase in the hardness value up to 170 VHN. The specimen which was aged at 540 ºC for 10 h with the Cr7C3 size of 22.8 μm, exhibited the higher corrosion resistance at both temperatures of 25 and 60 ºC with respect to other aged specimens. In addition, the temperature of 60 ºC promoted the anodic reactions in 3.5 wt.% NaCl solution which decreased impedance modulus values significantly. Consequently, the carbide size was more effective parameter than the carbide content in predicting electrochemical behavior of such alloys.