With its austenitic-ferritic ( 50 / 50 ) microstructure , super-duplex has twice the strength of conventional austenitic steels , while providing excellent crevice and pitting resistance . To ensure these properties were met , UNS , ASTM , API , NORSOK , and other regulatory bodies set stringent tests for corrosion , porosity , hardness , and more . However , the hot-work reduction ratio was not specified , nor could it be measured . In recent years , customers have increasingly specified a hot-forged reduction ratio of around 4:1 for wrought super-duplex . This appears to be a carryover practice from producing forged austenitic and higher nickel alloy grades , where a higher reduction ratio minimises the risk of porosity and degradation . However , unlike austenitic and higher nickel alloys , duplexes already have low porosity due to ferritic phase solidification and tight core . Recent research by Alleima shows that super-duplex maintains consistent quality and meets all acceptance criteria in a hot-worked condition down to 2.5:1 . In other words , the 4:1 ratio is not only an overcompensation – but is also irrelevant .

Full-scale production study Carried out in Sandviken , Sweden , Alleima explored the influence of forge reduction ratios on mechanical properties and corrosion resistance between 1.5:1 and 6.0:1 through a thermomechanical process to a final billet dimension and quench annealing . The aim was to determine if it was necessary to manufacture 25Cr duplex with a minimum total hot-work reduction ratio of

Figure 1 . Longitudinal / transverse section of SAF 2507 ^® for the half radius taken from the 1.5:1 and 5.0:1 forge reduction in quench annealed condition . Austenite is in bright contrast . LOM microstructure etched in the 40 % NaOH solution shown at 400 x magnification . Scale bar is 20 microns .

at least 4.0:1 . Proving that this was not the case would generate several advantages for customers . These include the flexibility to order wider diameters with 2.5:1 reduction , faster delivery due to the wider availability of materials , and more sustainable and energy-efficient manufacturing through shorter production times .

Production route and reductions The production route was a basic thermomechanical processing cycle with solution annealing . The heating or reheating temperature was the same for all billets . Each workpiece was subjected to hot forging with a total reduction ratio to obtain a square billet , followed by water quenching . The total hot-forging reduction ratios ( RR ) were 1.5 , 2.0 , 2.5 , 3.0 , 3.5 , 4.0 , 5.0 , and 6.0 , which are equivalent to relative deformations of 34 %, 50 %, 60 %, 67 %, 71 %, 75 %, 80 %, and 84 % respectively .

Eight billets , one batch , same heat Eight billets measuring 365 × 265 mm and produced in a continuous cast from the same heat were analysed . The chemical composition was based on UNS S32750 in accordance with ASTM A479-14 . The mechanical properties were evaluated at several alternative forging reduction ratios ranging from 1.5:1 to 6.0:1 . Samples were cut from a close to mid-radial position ( 1 / 4 thickness ) and assessed at least three times . Structural investigations were conducted using Light Optical Microscopy ( LOM ).

Testing protocols A fully-instrumented 750 J Charpy impact tester was used on standard Charpy V-notch specimens in both longitudinal and transverse directions , managed at −46 ° C . Longitudinal and traverse tensile tests for fracture , rigidity , and other mechanical properties were undertaken at room temperature ( RT ). HRC hardness with transverse direction tests were run at RT . Samples were cut out transversely to the direction of sample forging . Pitting corrosion testing was conducted according to ASTM G48 Method A . Microstructural observations were performed using scanning electron microscopy equipped with an electron backscatter diffraction ( EBSD ) analyser incorporating an orientation imaging microscopy ( OIM ) system . The ferrite fraction was determined by electron diffraction backscattered analysis . The mean austenite / ferrite grain size was evaluated by the line intercept method counting at least 2000-3000 grains on at least three typical EBSD scans . The microstructures were examined using LOM magnified to 400x . The samples were electrolytically etched in a 10 % oxalic acid solution to reveal Cr2N and in 40 % NaOH at

32 Stainless Steel World April 2023 www . stainless-steel-world . net