Duplex austenitic/ferritic stainless steels(DSS) are known to undergo 475℃ Embrittlement' similar to ferritic stainless steels, which affects the mechanical properties of ferritic phase, when exposed to temperature range of 300~550℃ This embrittlement is caused by the formation of Cr-rich α phase. This leads to a subsequent loss of impact toughness because of the restriction of dislocation movement and also a degradation of corrosion resistance owing to Cr-depleted regions around it. In this study, effects of thermal aging at 475℃ and W addition on the mechanical properties and corrosion characteristics of duplex stainless steels were investigated by destructive and nondestructive, electrochemical method.
To evaluate the effects of Mo and W, Fe-25Cr-7Ni-xMo-yW-0.25N (3Mo, 2Mo-2W, 1.5Mo-3W, 6W) were designed to have same PREw value by varying the content of Mo and W. After solution annealing at 1050℃, DSS were aged at 475℃ for 1hr, 10hr, 100hr, and 300hr. A mechanical degradation of the designed alloys with aging were investigated by tensile and Charpy-impact tests. And electrochemical anodic polarization tests were done in NaCl and in HCl solution to evaluate localized corrosion resistance and passivity characteristics of DSS with aging. Also a modified DL-EPR method were used to detect Cr-depleted regions of the aged sample. Finally, gavanic corrosion behaviors between austenitic and ferritic phases in acidic solution were investigated by AFM (atomic force microscope).
A mechanical degradation with aging was measured for the designed alloys. However, W-containing alloys still remained high impact energy after aging. In the anodic polarization responses, it showed that the pitting potentials decreased in NaCl solution and the critical current density and the passivation current density increased in HCl solution with aging. For the sample aged for 300 hr showed second anodic current peak in the passivation range during polarization tests. The degradation of corrosion resistance was retarded as W/Mo (wt%/wt%) ratio increased. Modified EPR test results showed that a recativation current was detected for the aged alloys. Finally, AFM analysis after immersion tests for the aged samples in HCl sol. showed that galvanic corrosion occurred in the countless local cells consisted of the Cr-rich and the Cr-depleted regions within the ferrite matrix as well as in the ferrite-austenite phase boundaries. The dissolution depth of the ferritic phase of the 1.5Mo-3W alloy at the vicinity of phase boundaries was lager than that of 3Mo alloy. It means that the 1.5Mo-3W alloy have more resistance to the precipitation of α' phase than the 3Mo alloy.
A detrimental effect of the precipitation of Cr-rich α' phase on the corrosion characteristics as well as on the mechanical properties of DSS were made clear in this study. And it is made manifest that a substitution of W for Mo in DSS can retard the precipitation of α' phase.