Last Updated: 17/3/21 at 6:41am
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 often needed in the fight against corrosion when 316 grade just isn’t enough, such as being constantly submerged in salt-water . An important feature of Duplex stainless steel is that its pitting and crevice corrosion resistance is greatly superior to that of 304 or 316 grade Stainless Steel. This combined with high mechanical strength and ductility makes it an excellent material for so many applications.
Often 316 Grade Stainless Steel is referred to as ‘Marine grade’ – and yes while it is often used and ideal for marine environments such as fastenings on a yacht’s deck, it is by no means the most robust alloy for high chloride environments (E.g. salt-water). Where fastener strength and integrity is critical, such as for bolts in a Yacht’s Keel, a Duplex or Super Duplex Fastener is required. Also where a Stainless Fastener is submerged in salt-water, Duplex would be the minimum requirement.
Duplex and Super Duplex Fasteners are not generally ‘off the shelf’ and stocked by stainless fastener distributors. They are made to order and making sure the source is specialized in Duplex fastener manufacture with a high degree of Quality Control is critical.
Duplex fasteners can be CNC machined from round or hex bar – but ideally they would be made with forged heads and rolled threads. Threads should be rolled rather than machined to ensure smoother, stronger threads are obtained as compared with machined/CNC threads – this increases the thread’s yield strength, surface finish, hardness, and wear resistance.
When Hot Forging the fastener head, temperature control is critical. When heating Duplex you must avoid prolonged time in the critical temperature zone as this encourages unwanted Nitrides and Carbides to form. It is important that digital control equipment is used to quickly heat through this temperature zone and automatically remove from the heat source when the temperature is optimum for forging. Any Duplex item should then undergo laser marking to ensure trace ability of the fastener through the supply chain. Test Certificates should also always be obtained.
Duplex Applications – where would you use Super Duplex over Duplex?
Duplex has excellent general corrosion resistance; superior to grade 316 in most environments, including resistance to inter-granular, pitting and crevice corrosion. Super Duplex has even greater pitting resistance. In summary – 316 is good, Duplex is great and Super Duplex is well – ‘SUPER’!
One of the most common methods to define resistance to pitting corrosion is the PRE value, Pitting Resistance Equivalent. The PRE can be calculated from the composition as:
PRE = %Cr + 3.3 %Mo + 16 %N
Clearly grades high in the alloying elements Chromium and especially Molybdenum and nitrogen are more resistant.
Here are the approximate PRE pitting corrosion values for 304 and 316 Stainless Steel plus Duplex and Super Duplex grades:
304 = 17-20
316 = 25-30
Duplex = 30-38
Super Duplex = >40
Duplex Stainless Fastener Availability
Anzor’s Duplex fasteners are a high quality option, made with excellent quality control and traceability. They are made to order at our leading European factory with a 6 week lead time into Australia or New Zealand. All items are produced with laser engraving and Test Certificates.
Our range of Super Duplex Fasteners can be found below: