[ Super Duplex ]
[ Super Duplex ]
Comparison of the precipitation of secondary phases in cast & wrought SDSS
The production of sigma and chi phases in cast and wrought super duplex stainless steel at temperatures ranging from 700 to 1000 ° C has been widely studied . However , to date , research has not examined the impact of particle size or interphase distance on the percentage of precipitations . A team at the University of Padua set out to close this research gap .
By Saira Sehar , Mirko Pigato , Claudio Gennari & Irene Calliari , University of Padua , Italy
Stainless steels were developed during the first decades of the twentieth century in the United Kingdom and Germany . There are four primary types of stainless steel : duplex ( high corrosion resistance and strength ), martensite ( high strength ), ferrite ( high temperature strength ), and austenite ( high corrosion resistance ). Because of their superior mechanical qualities and resistance to corrosion , stainless steels are alloys that find extensive applications in several industrial sectors . Duplex stainless steels ( DSSs ) are a class of steels with austenite and ferrite as the two main phases . Combining different phase-stabilizing components of both phases yields the austenite and ferrite phase assemblage . In order to offer enhanced resilience against pitting and crevice corrosion , DSS is combined with several elements , including Cr , Mo , N , W , and Cu . ( 1 ) . The PREN categorizes DSS into four grades : lean , standard , super , and hyper . In an environment containing chloride , PREN is a numerical measure of resistance to pitting corrosion ; the higher the number , the higher the resistance to pitting corrosion . A PREN of 32 or higher indicates that the material is resistant to pitting corrosion in seawater ; super duplex stainless steel ( SDSS ) has a PREN of 40 or higher . SDSS is appropriate for applications involving seawater desalination and offshore plants because of its corrosion resistant qualities . ( 2 )
Table 1 : Nominal Composition of Cast SDSS
These DSS grades are susceptible to the production of harmful complex phases during thermal ageing , such as chi ( χ ), sigma ( σ ), secondary austenite , and / or chromium nitrides , due to their high alloy content . At temperatures of 600 – 1000 ° C , the ferritic phase is extremely unstable and can break down into secondary phases . Phase transformations are vital to the functioning of SDSS , including the creation of χ , σ , and other secondary phases ( 3 , 4 ) . Previous research ( 5 ) has demonstrated that intermetallic secondary phases are harmful to the mechanical and corrosion-resistant qualities of steels . Because sigma can create a substantial amount ( up to roughly 30 vol pct ) in DSS , it is the most harmful . For this reason , understanding the kinetics of sigmaphase development is crucial for managing mechanical qualities and corrosion . Given the variations in the nominal compositions and microstructures of the cast and wrought alloys , it is expected that the phase transition kinetics and even phase equilibria will change . The production of sigma and chi phases in cast and wrought super duplex stainless steel was widely studied at temperatures ranging from 700 to 1000 ° C . Higher sigma phase was observed at 900 ° C for a long duration . Grain size or interphase distance significantly impacts the percentage of secondary phases that precipitate . However , prior research has not examined the impact of particle size or interphase distance on the percentage of precipitations . ( 6 ) Thus , this research gap will be filled by our current investigation . Our present study will contribute to demonstrating the comparison of Phase transformation in Super duplex stainless steel in cast and wrought conditions at different temperatures , soaking times and interphase distances . ( 3 )
Material & method This investigation focused on cast material ( UNS S32760 , ASTM A890 / 6A ); its chemical composition is shown in Table 1 . Compared to other duplex grades , it contains copper and tungsten , which enable them to endure more harsh conditions . The starting material ( designated as received , or AR ) consisted of ingots of cylindrical specimens . Wrought SDSS UNS S32760 SDSS plate , commercially known as Zeron 100 , has a slightly different composition than cast SDSS . The samples are already in a solubilized condition ( at 1100 ° C for 2h ) to have an almost equal duplex microstructure to avoid precipitation of secondary phases and achieve a nearly equal ratio of austenitic and ferritic phases .
Experiment and results The isothermal ageing treatments were carried out at 850 , 900 , and 950 ° C for times ranging from 3 to 90 minutes , followed by water quenching . The volume fraction of ferritic phases in
Elements Cr C Fe Mo Si W Cu Ni N Mo P S
% Comp ( wt ) 24-26 0.03 Bal 3-4 1.0 0.50-1.0 0.50-1.0 6.5-8.5 0.20-0.30 1.0 0.030 0.001
www . stainless-steel-world . net Stainless Steel World December 2024 25