ELECTROCHEMICAL AND ATOM-PROBE STUDIES OF CAST AND AGED DUPLEX STAINLESS-STEEL

Cast duplex stainless steel of CF3M was aged at 450 degrees C for up to 10000 h. The ferritic phase was hardened and embrittled by the ageing. Cyclic polarization curves were measured in 0.5 kmol/m(3) H2SO4+0.01 kmol/m(3) KSCN solution to investigate the electrochemical behaviour during phase decomposition of the ferritic phase. In addition, the 3-D microchemical change was examined by a recently built position sensitive atom probe (POSAP). The reactivation current peak appears in the cyclic polarization measurement after 400 h ageing. This results from the dissolution of the chromium-depleted iron-rich region formed during decomposition of the ferritic phase. The POSAP analysis of the 10000 h aged steel shows a well-developed phase decomposition into the chromium-rich and iron-rich regions in several nanometer scale. Flat plate-like chromium-rich regions (Cr greater than or equal to 50 at%) of a few nanometer thickness are observed at a spacing of a few nanometer, while the iron-rich regions (Cr<12 at%) form the percolated structure surrounding the flat plate-like chromium-rich regions.