Effect of nitrogen atmosphere during post-weld heat treatment on microstructure evolution and corrosion behavior of 2205 duplex stainless steel welded joint

Nitrogen gas is used as a protective atmosphere during Post-Weld Heat Treatment (PWHT) of 2205 duplex stainless steel welded joints (DSS). For comparison, similar welded joints were characterized after welding in the same conditions without PWHT. Detailed microstructure examination (optical and SEM) demonstrates the presence of two types of secondary austenite precipitates in weld zone (WZ) for welded samples without heat treatment (W-sample). Nitride precipitation appeared around austenite grains in WZ after PWHT in a nitrogen atmosphere. Microstructural investigation reveals an increase in austenite volume fraction after PWHT across all parts of welded samples (base metal (BM), heat-affected zone (HAZ), and WZ). An enhancement of pitting resistance equivalent number of BM and HAZ for PWHTed samples. Tafel and EIS tests of BM and HAZ showed good corrosion resistance and a low corrosion rate in sample that underwent subsequent PWHT. Corrosion rate of BM was 2.56E-01 and 1.78E-03 mm/y for W and N-sample, respectively, while that of HAZ was 7.34E-04 and 5.01E-04 mm/y for W and N-sample, respectively. It was also noted that precipitation of secondary austenite gives more stability than precipitation of nitride, as a slower corrosion rate was observed for W-sample (1.70E-02 mm/y) that was not heat-treated compared to N-sample (2.52E-02 mm/y). Furthermore, despite high presence of nitride in WZ and minimal nitride in BM for N-sample, compared to W-sample, which contained secondary austenite in WZ and a high precipitation of nitride in BM. W-sample showed a very rapid corrosion rate in three zones combined (BM, HAZ, and WZ) compared to N-sample.