Super duplex stainless steel fasteners

Duplex stainless steels combine the most favourable properties of austenitic and ferritic stainless steels: good strength and toughness; relative ease of fabrication; and high resistance to corrosion and stress corrosion cracking.

Strength and corrosion resistance

A 2205 duplex stainless steel (UNS S31803 / UNS S32205, F51 / F60, 1.4462) is based on a 22% chromium content, giving a pitting resistance equivalent number (PREN) of 34. A super duplex grade is generally one with a PREN greater than 40, typically based on 25% chromium or higher, which ensures high corrosion resistance across a wide range of environments. Super duplex grades are the default specification for long-term performance in seawater, and for many industrial and chemical processes where selected acids are present.

The strength of duplex stainless steel can be roughly twice that of a standard 3xx-series austenitic grade such as Alloy 316L. A 2205 duplex has a minimum yield strength of 65ksi (450MPa) against just 25ksi (170MPa) for Alloy 316L (UNS S31603, 1.4404). Super duplex grades UNS S32750 (F53, 1.4410) and UNS S32760 (F55, 1.4501) are higher still at 80ksi (550MPa), and Ferralium^® 255-SD50 (F61, UNS S32550, 1.4507) provides the highest strength of any super duplex stainless steel, with a minimum yield strength of 85ksi (586MPa).

Increasing strength further

Strength can be increased by further processing. Duplex stainless steels can be work-hardened, raising the yield stress and ultimate tensile strength at the cost of some elongation. A grade such as UNS S32750 (F53, 1.4410) can double in strength with a reduction in area of around 25%, achieved by cold working such as drawing or cold forging - suitable for smaller diameter bars only. A little cold work is also imparted during machining. Where a component works in tension with minimal shock loading, this combination of high strength and reduced toughness is acceptable.

Another approach for critical components is ageing. Ferralium^® 255-3AF is an aged version of the solution-annealed product. Subjecting the forged bar, or even machined components, to a further heat treatment raises the yield strength marginally and lifts the tensile strength by at least 5-10%.

Fastener supply

We are a specialist supplier of duplex and super duplex stainless steels, stocking the most complete range of S32205, S32750, S32760 and Ferralium^® 255-SD50 from ½" (12.7mm) to 16" (406.4mm) diameter solid bars, plus selected plates, hollow bars, pipes, flanges and fittings.

Fastener companies frequently buy random lengths, up to 6m each, to make multiple components. With our particular focus on duplex and super duplex grades, we hold several hundred tonnes of each grade ex-stock. We are also the preferred distribution partner for Alleima's range of high-performance duplex and super duplex stainless steels, which gives us regular replenishment and the capacity to support the largest projects.