EFFECTS OF AGING TEMPERATURE ON PRECIPITATION PHASE OF A CAST SUPER DUPLEX STAINLESS STEEL

Super duplex stainless steels have been extensively used in many applications owing to their excellent mechanical properties and corrosion resistance. But subjected to "aging" treatments at temperatures of 600 to 1000 degrees C, duplex stainless steels will precipitate a certain amount of intermetallic phases such as sigma phase, chi phase and chromium nitride, etc., which decrease mechanical properties and corrosion resistance of these steels. In this paper, the effects of aging treatments on the type, size and quantity of precipitation particles of a cast super duplex stainless steel after solution treatment were studied by means of OM, SEM, XRD and TEM. The results indicate that the soloution annealing cast super duplex stainless steel is aged in temperatures ranging from 650 to 950 degrees C, no phase transformation occurs in the original austenite but the intermetallic phases are precipitated in ferrite and at ferrite/austenite interfaces. The precipitated intermetallic phases are mainly chi phases during aging at 650 degrees C. They are sigma and chi phases at aging temperature of 750 degrees C. When temperature increased to 850 degrees C the intermetallic phase is only sigma phase. At temperature of 950 degrees C, there is a little sigma phase precipitated at ferrite/austenite interfaces. The formation of intermetallic phases is analyzed by thermodynamics, which shows that metastable chi phase can invert to sigma phase at aging temperatures from 650 to 750 degrees C. When temperature ranging from 850 to 9.50 degrees C, sigma phase can be directly precipitated.