[ Super Duplex ]
[ Super Duplex ]
Fig 1 . Optical micrograph of solution annealed sample at 1100 ° C for 2h , electrolytically etched with 20 % NaOH ( 2.5V , 10 seconds ), bright area ( austenite γ ), dark area ( Ferrite δ )
the solubilized etched sample was quantified using ImageJ software . The average area fraction of ferritic ( phase δ ) was quantified at 54 % and the remaining 46 % was austenitic ( phase γ ) without any secondary phase in the cast SDSS ; wrought SDSS has 58 % δ phase and 42 % for the austenitic phase . Ferrite is enriched with chromium and molybdenum , which are known to promote intermetallic phase formation . These microstructural changes can occur during isothermal ageing , welding , or cooling after solution treatment . They are mainly concerned with the a-ferrite as the alloying diffusion rates are about a hundredfold higher in this phase than in the austenite .
Isothermal heat treatment Both σ-phase and χ-phase start their nucleation process soon after the initial ageing treatment and after the precipitation of nitrides ( in between 3 minutes ). When the ageing duration increases , these phases are randomly distributed on the grain boundaries and within the grains ( from 15 minutes onwards ). In the cast sample at 850 ° C , at 3 min chi was the only phase to appear . After increasing the ageing time to 15 min sigma phase starts to nucleate from the chi phase at the ferrite and austenite
phase boundary . At 60- 90 min , both phases nucleated at the grain boundary or within the ferritic grains up to 7.92 %. For wrought SDSS , at 3 min and 5 min no phase was detected . However , at 10 min < 1 % and for 15 min 6.2 % and a maximum of 15 %, both phases were detected at 25 min . In the cast sample at 900 ° C , from 3 to 10 min of ageing > 1 % of both phases appear at the grain boundary . As ageing time increased to 60-90 min , both phases nucleated at the grain boundary and within the grain up to 12.67 %. The rate of precipitation of sigma increased after 15 minutes while the rate of chi phase was lower . For wrought SDSS , χ phase appeared after 5 min at the grain boundaries with some isolated ϭ phase particles . A significant increase in χ amount was detected after 10 min ; χ particles are coarser and more numerous , while the ϭ content is quite constant , with a moderate size coarsening . After 40 min , χ , ϭ and nitrides are present . χ phase is always at the boundary ’ s ferrite / austenite and ferrite / ferrite . The ϭ phase penetrates in the ferrite or grows along the austenite / ferrite boundaries . The ferrite decomposition is not completed . The rate of ϭ precipitation increases after 20 min , while the χ rate is lower . At 950 ° C in the cast sample at 3 min , chi phase appears but when the ageing time is increased to 15 min , sigma phase starts nucleation from chi phase at the grain boundary . After 15 min sigma phase is not only on the grain boundary but also within the
Fig 2 : % Volume Fraction at 850C for cast and wrought SDSS
Fig 3 : % Volume Fraction at 900 ° C for cast and wrought SDSS
26 Stainless Steel World December 2024 www . stainless-steel-world . net