Spinodal Decomposition in Functionally Graded Super Duplex Stainless Steel and Weld Metal

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.

Effect of Sigma Phase Morphology on the Degradation of Properties in a Super Duplex Stainless Steel

“…The microstructure and phase transformation in arc heat-treated SDSS and weld metal were investigated using automatic hardness testing and electron backscattered diffraction (EBSD) in previous studies [14,15]. In addition, the kinetics of spinodal decomposition in the same material was the subject of another study [16]. …”

Nanoscale phase separations in as-fabricated thick super duplex stainless steels

“…11. In the previous studies [18][19][20][21], it was illustrated that at an isotherm line of 475°C in the arc heat treated samples, a low etching contrast is seen due to the progress of spinodal decomposition, supported by hardness testing and APT analysis. In the current study, CuHIP160s did not show any low contrast etching around 475°C isotherms after 5 min arc heat treatment, while this was seen after 10 min.…”

“…Phase separation in the ferrite may, however, occur during intermediate temperature applications by the formation of alpha (a)-alpha prime (a') domains, Cu-rich particles (CRPs), G-phase particles, etc., causing a toughness loss, known as ''475°C-embrittlement''. The separation occurs on a nanoscale in the temperature range of about 300-500°C [4][5][6][7][8]. The a-a' separation during quenching and different solutionizing treatments has been the subject of different studies.…”

“…Lemoine et al [13] also mentioned the progression of spinodal decomposition during quenching of a cast DSS alloy. Some recent studies also showed that noticeable phase separation can occur already after 3 min aging in SDSSs, with material datasheets indicating up to 50% toughness loss [4,14]. It, therefore, raises the concern of what the level of this phase separation is in as-fabricated thick SDSS sections and how it impacts the properties.…”

Effect of Cooling Rate after Solution Treatment on Subsequent Phase Separation Evolution in Super Duplex Stainless Steel 25Cr-7Ni (wt.%)

“…The above-mentioned phase decomposition, or phase separation (PS), occurs on the nanoscale in a temperature range of about 275-500 • C for SDSS [4,5]. It is known that the ferrite in the duplex stainless steel (DSS) can already experience PS after solution treatment [6][7][8], depending on the solution treatment temperature [9][10][11] and the cooling rate after solution treatment [12][13][14][15][16].…”