Abstract
Low-temperature phase separations (T < 500 °C), resulting in changes in mechanical and corrosion properties, of super duplex stainless steel (SDSS) base and weld metals were investigated for short heat treatment times (0.5 to 600 minutes). A novel heat treatment technique, where a stationary arc produces a steady state temperature gradient for selected times, was employed to fabricate functionally graded materials. Three different initial material conditions including 2507 SDSS, remelted 2507 SDSS, and 2509 SDSS weld metal were investigated. Selective etching of ferrite significantly decreased in regions heat treated at 435 °C to 480 °C already after 3 minutes due to rapid phase separations. Atom probe tomography results revealed spinodal decomposition of ferrite and precipitation of Cu particles. Microhardness mapping showed that as-welded microstructure and/or higher Ni content accelerated decomposition. The arc heat treatment technique combined with microhardness mapping and electrolytical etching was found to be a successful approach to evaluate kinetics of low-temperature phase separations in SDSS, particularly at its earlier stages. A time-temperature transformation diagram was proposed showing the kinetics of 475 °C-embrittlement in 2507 SDSS.
Highlights
DUPLEX stainless steels deliver an excellent combination of mechanical and corrosion properties, making them an attractive alternative to austenitic and super austenitic stainless steels.[1]
An industry datasheet for 2507 super duplex stainless steel (SDSS),[5] in contrast, introduced a time-temperature transformation (TTT) diagram indicating that only 3 minutes aging at 475 °C will result in a 50 pct toughness drop in 2507 SDSS
Low-temperature phase separations (T < 500 °C) and embrittlement of 2507 SDSS were investigated in functionally graded base and weld metals arc heat treated between 0.5 and 600 minutes
Summary
DUPLEX stainless steels deliver an excellent combination of mechanical and corrosion properties, making them an attractive alternative to austenitic and super austenitic stainless steels.[1]. A considerable variation of data reported in the literature and industrial datasheets points to a knowledge gap about the kinetics of 475 °C-embrittlement in SDSSs. Hilders et al found that 9 hours heat treatment of 2507 SDSS at 475 °C did not decrease the impact toughness while it was significantly reduced after 72 hours.[6] Gutierrez-Vargas et al observed only minor changes in microhardness after 1-h heat treatment of 2507 SDSS at 475 °C.[7] An industry datasheet for 2507 SDSS,[5] in contrast, introduced a time-temperature transformation (TTT) diagram indicating that only 3 minutes aging at 475 °C will result in a 50 pct toughness drop in 2507 SDSS
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