Abstract
The super duplex stainless steel SAF 2507, with a typical composition of ∼25%Cr, 3·8%Mo, ∼7%Ni, and 0·27%N, has been isothermally heat treated in the temperature range 700–1030°C for times between 1 min and 72 h. The influence of heat treatment on impact toughness and pitting corrosion resistance has been investigated and related to the microstructure as observed using light optical microscopy, scanning electron microscopy, and transmission electron microscopy. It was found that the essential features of toughness and pitting corrosion could be directly related to the presence of α phase in much of the time–temperature space. However, in the temperature interval 800–850°C, low critical pitting temperatures were found for relatively short aging times, although virtually no σ phase could be observed. These effects could be directly attributed to the presence of secondary austenite at the ferrite/austenite phase boundaries, which was found, via energy dispersive X-ray (EDX) analysis of thin foils, to be poor in chromium. In addition, thermodynamic calculations showed that the secondary austenite is expected to be poor also in nitrogen. The paucity of chromium and nitrogen in combination with the observation of pitting attack in association with secondary austenite, support the conclusion that secondary austenite can be as detrimental to pitting corrosion as α phase. Both Cr2N and χ phase appear to play an indirect role in pitting by accommodating the crucial alloying elements chromium and nitrogen (Cr2N) and chromium and molybdenum (χ phase).
MST/1755
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