The effect of substituting W for Mo in duplex stainless steel (DSS) was investigated with respect to microstructure and stress corrosion cracking (SCC) behavior. The homogenizing treatment was performed at 1,100°C for 10 min while the aging treatment was performed at 900°C at different holding times. In the homogenized condition, regardless of W substitution, all the specimens had about an equal ratio of ferrite to austenite. In the early stage of aging, the beneficial effect of W modification on the suppression of secondary phase precipitation was very notable. The total amount of secondary phase precipitates was greatly reduced in the W-modified DSS. However, its effect decreased rapidly as the aging time increased.
SCC was examined in boiling 42% magnesium chloride (MgCl2) solution by slow strain rate test (SSRT) and constant load test (CLT) methods. In the homogenized condition, the beneficial effect of W was clearly observed at the low applied stress levels, where an electrochemical reaction plays a dominant role. In the commercial-grade DSS without W modification, the crack propagated in a transphase mode, whereas in the W-modified DSS, the crack propagated in a mixed mode of transphase and interphase because of the barrier effect of the austenite phase against crack growth. In the aged condition, W-modified specimens showed a significant improvement in SCC resistance, which resulted from an increase in toughness attributable to a relatively small amount of brittle secondary phase precipitates. However, the cracks propagated in a transphase mode in the DSS specimens regardless of W modification.
