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Top1. Introduction
Duplex stainless steel is a type of stainless steel which contains both ferritic and austenitic phases in approximately equal proportions. DSS 2205 contain 22% chromium, 3% molybdenum and 5% nickel. Due to its high toughness and strength, it can replace thick sections of parts made up of austenitic stainless steel (ASS). DSS and SDSS materials possess high corrosion resistance qualities, especially against pitting corrosion and stress corrosion. SDSS 2507 contains 25% chromium, 6.5% Nickel and 3.5% Molybdenum. SDSS 2507 possess higher corrosion resistance and strength compared to DSS 2205. Both the materials are considered to be cost effective when compared to ASS. It is appropriate to use these materials within 50 to 300° C. After 300° C, the property of the materials changes and become brittle. SDSS 2507 have high proof strength compared to DSS and ASS materials. Physical and mechanical properties of DSS 2205 and SDSS 2507 are excellent.
SDSS 2507 can be used in petroleum, hydropower, pressure vessels, pulp & paper making industries, structural components and chemical tankers. DSS 2205 has variety of applications in offshore oil and gas industries for pipe work systems and in petrochemical industries in the form of pipelines, pressure vessels, chemical processing and storage equipment. Even though duplex materials have enormous applications especially due to its exceptional non-corrosive characteristics, studies on duplex materials especially super duplex material is less compared to other materials. Previous studies report mainly turning of duplex steels. But reports on CNC milling of SDSS 2507 and DSS 2205 materials are very less mainly due to the difficulty in machining such materials.
Barbara (2014) compared the corrosion behaviour and economical aspects of different stainless steels namely Ferritic grade AISI 430, Austenitic grades AISI 304, AISI 316 along with DSS 2205 duplex grade. Results revealed that, duplex steel can be used without coating due to its high corrosion resistance, which reduces the maintenance cost. Rajguru & Arunachalam (2021) conducted experiments on SDSS material for finding out the stress corrosion cracking. It was found that, SDSS material is a good choice for marine applications due to the exceptional corrosion resistance. They also found the development of machining stresses, due to biaxial tensile residual stresses induced in the work material. This is mainly due to the selection of improper cutting parameters. To avoid this defect, proper selection of cutting parameters is inevitable.
Rajaguru & Arunachalam (2020) investigated stress corrosion cracking resistance (SCC) and machinability of SDSS under distinct cutting fluid application methods. Effect of flood cooling, dry machining and minimum quantity lubrication technique on cutting performance was analysed. Flank wear, surface finish, cutting force, chip morphology and residual stress were considered as performance indicators. It was found that low surface crack density could be achieved with MQL technique.
Gowthaman et al. (2020) reviewed the machining behaviour of DSS. The review results indicated that, machining behaviour of DSS is tough because of its higher rate of strain hardening, improved strength, Built-up edge formation, lack of thermal conductivity and fracture toughness. The work also recommends the need for coated inserts with higher toughness, sharp cutting edges with positive chip breaker and exclusion of excessive cutting speed to avoid Built-up edge formation. Nomani et al. (2013) conducted machinability tests on SDSS 2507 and DSS 2205 materials during drilling process by keeping 316L austenite steel as a reference. Flank wear, surface finish and cutting force were analysed and compared under similar cutting conditions. Both SDSS 2507 and DSS 2205 materials revealed poor machinability responses and flute damage were observed on the drill bit with built-up edge formation. Drilling of SDSS 2507 revealed poorer surface finish and higher cutting force compared to the other two steels (DSS 2205 and 316L ASS).
Rajaguru & Arunachalam (2017) investigated the behaviour of four different PVD and CVD coated tools during dry turning of SDSS material. Cutting tool performance was analysed by considering surface integrity, tool wear, cutting force and cutting temperature. Results revealed that [MT-TiCN]-Al2O3 coated tool can provide better wear resistance along with better surface finish compared to other coated tools.