Microstructure evolution and pitting corrosion resistance of the Gleeble-simulated heat-affected zone of a newly developed lean duplex stainless steel 2002

Abstract

The effect of cooling rate on microstructure evolution and pitting corrosion resistance of the Gleeble-simulated heat-affected zone of a newly developed duplex stainless steel 2002 was investigated. The results showed that the content of austenite phase and the grain size of the austenite increased as the cooling rate decreased from 100 to 10 °C/s in the temperature range of 1350–800 °C. Meanwhile, the critical pitting temperature (CPT) value increased from 31.2 to 43.6 °C and the pitting potential (Epit) increased from 0.322 to 0.477 V, indicating that the pitting corrosion resistance increased with the cooling rate decreasing. With the cooling rate of 100 °C/s, the localized pitting attack of the specimens occurred along the ferrite/austenite boundaries but preferentially inside the ferrite domains, which might be attributed to the fact that the precipitation of Cr2N led to the Cr-depleted region in the ferrite matrix. When the specimens were subjected to cooling rates of 50, 30 and 20 °C/s, the stable pits were all selectively located at the interior of the ferrite phase, suggesting that the ferrite phase was a weaker phase as compared to austenite phase. At 10 °C/s, the pits of specimens preferentially took place across ferrite/austenite phase boundaries, meaning that the pitting resistance of two phases almost achieved equivalent. Moreover, with the decrease of cooling rate, the pitting resistance equivalent number (PREN) of ferrite phase increased, while that of the austenite phase decreased. In addition, it was found that ferrite showed a lower Volta potential than austenite phase and the Volta potential differences between two phases grew smaller with the cooling rate decreasing. The relationship between PREN and Volta potential of specimens with respect to pitting corrosion resistance of steels was discussed.