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Posted by Tocco on February 24th, 2021

What is Duplex?

Duplex and Super Duplex Grades of Stainless Steel offer superior corrosion resistance when 316 grade just isn't enough.

Duplex Stainless-steel Fasteners (SAF 2205 or UNS S31803) or Super Duplex Stainless Steel Fasteners (SAF 2507 or UNS S32750) are grades of Stainless Steel typically needed in the battle versus deterioration when 316 grade just isn't enough, such as being constantly immersed in salt-water. An important feature of Duplex stainless-steel is that its pitting and crevice corrosion resistance is considerably superior to that of 304 or 316 grade Stainless Steel.

This combined with high mechanical strength and ductility makes it an exceptional material for so many applications. Often 316 Grade Stainless-steel is referred to as 'Marine grade' - and yes while it is typically used and ideal for marine environments such as fastenings on a luxury yacht's deck, it is by no suggests the most robust alloy for high chloride environments (E.g. salt-water). Where fastener strength and stability is crucial, such as for bolts in a Luxury yacht's Keel, a Duplex or Super Duplex Fastener is required. Likewise where a Stainless Fastener is submerged in salt-water, Duplex would be the minimum requirement. Duplex and Super Duplex Fasteners are not normally 'off the rack' and equipped by stainless fastener suppliers. They are made to buy and making sure the source is specialized in Duplex fastener manufacture with a high degree of Quality Control is important.

Duplex Manufacturing https://diigo.com/0jsy1o technologies

Duplex fastenersare preferably made with created heads and rolled threads, with the heating in the forging procedure thoroughly temperature controlled.

Duplex fasteners can be CNC machined from round or hex bar - however ideally they would be made with forged heads and rolled threads. Threads ought to be rolled instead of machined to guarantee smoother, stronger threads are acquired as compared to machined/CNC threads - this increases the thread's yield strength, surface area finish, solidity, and use resistance. When Hot Forging the fastener head, temperature level control is critical. When heating Duplex you need to prevent extended time in the critical temperature zone as this encourages undesirable Nitrides and Carbides to form. It is necessary that digital control devices is used to quickly warm through this temperature level zone and immediately eliminate from the heat source when the temperature level is optimal for forging. Any Duplex product must then undergo laser marking to guarantee traceability of the fastener through the supply chain. Test Certificates should likewise always be obtained.

Duplex Applications - where would you use Super Duplex over Duplex?

Applications which can take advantage of using these duplex steels involve piping systems, pumps (where the good disintegration and abrasion resistance is utilized), valves, heat exchangers, Keel Bolts, used with products submerged in salt-water and other equipment.

Duplex has exceptional basic deterioration resistance; exceptional to grade 316 in many environments, consisting of resistance to inter-granular, pitting and crevice corrosion. Super Duplex has even greater pitting resistance. In summary - 316 is great, Duplex is excellent and Super Duplex is well - 'SUPER'! One of the most typical techniques to define resistance to pitting deterioration is the PRE value, Pitting Resistance Equivalent. The PRE can be computed from the composition as: PRE = %Cr + 3.3 %Mo + 16 %N Plainly grades high in the alloying components Chromium and specifically Molybdenum and nitrogen are more resistant.