INFLUENCE OF ISOTHERMAL PHASE-TRANSFORMATIONS ON TOUGHNESS AND PITTING CORROSION OF SUPER DUPLEX STAINLESS-STEEL SAF-2507

The super duplex stainless steel SAF 2507, with a typical composition of similar to 25%Cr, 3.8%Mo, similar to 7%Ni, and 0.27%N, has been iso thermally heat treated in the temperature range 700-1030 degrees C for times between 1 min and 72 h. The influence of heat treatment on impa ct toughness and pitting corrosion resistance has been investigated an d related to the microstructure as observed using light optical micros copy, scanning electron microscopy, and transmission electron microsco py. It was found that the essential features of toughness and pitting corrosion could be directly related to the presence of sigma phase in much of the time-temperature space. However, in the temperature interv al 800-850 degrees C, low critical pitting temperatures were found for relatively short aging times, although virtually no sigma phase could be observed. These effects could be directly attributed to the presen ce of secondary austenite at the ferrite/austenite phase boundaries, w hich was found, via energy dispersive X-ray (EDX) analysis of thin foi ls, to be poor in chromium. In addition, thermodynamic calculations sh owed that the secondary austenite is expected to be poor also in nitro gen. The paucity of chromium and nitrogen in combination with the obse rvation of pitting attack in association with secondary austenite, sup port the conclusion that secondary austenite can be as detrimental ta pitting corrosion as a phase. Both Cr2N and chi phase appear to play a n indirect role in pitting by accommodating the crucial alloying eleme nts chromium and nitrogen (Cr2N) and chromium and molybdenum (chi phas e).