Duplex stainless steels combine the most favourable properties of austenitic and ferritic stainless steels, namely good strength and toughness; relative ease of fabrication; plus they are highly resistant to corrosion and stress corrosion cracking.
A 2205 duplex stainless steel (UNS S31803 / UNS S32205, F51 / F60, 1.4462) is based around a 22% chromium content, which achieves a pitting resistance equivalent number (PREN) of 34. A super duplex stainless steel is generally classified as one having a PREN value greater than 40 and is typically based upon an alloy with 25% chromium or higher. This ensures a high level of corrosion resistance in a wide range of environments. Super duplex grades are the default specification for applications requiring long-term performance in seawater, as well as numerous industrial and chemical process industries where there is exposure to selected acids.
The strength of duplex stainless steel can typically be twice that of a standard 3xx-series austenitic stainless steel such as Alloy 316L. A 2205 duplex stainless steel has a minimum yield strength of 65ksi (450MPa) compared with a minimum of just 25ksi (170MPa) for Alloy 316L (UNS S31603, 1.4404). The yield strength for a super duplex grade, either UNS S32750 (F53, 1.4410) or UNS S32760 (F55, 1.4501) is higher still at 80ksi (550MPa). Ferralium 255-SD50 (F61, UNS S32550, 1.4507) is able to provide the highest strength of any super duplex stainless steel, with a minimum yield strength of 85ksi (586MPa).
Besides the obvious advantages of these alloys previously stated, their strength can be improved by further processing. Duplex stainless steels can be work-hardened, where the yield stress and ultimate tensile strength can be increased, albeit with an associated drop in the elongation value. A standard super duplex stainless steel such as UNS S32750 (F53, 1.4410) can double in strength if subjected to a reduction in the area of around 25%. This can be achieved by cold working, such as drawing or cold forging, suitable for smaller diameter bars only. A small amount of cold working is also imparted during machining. If the component is operating in tension, with minimal shock loading, then this combination of high strength with reduced toughness is acceptable.
Another approach for critical components is to age them. Ferralium 255-3AF is an aged version of the product in the solution annealed condition. By subjecting the forged bar, or even machined components, to a further heat treatment it is possible to increase the yield strength marginally, but uplift the tensile strength by at least 5-10%.
Fastener companies will frequently purchase so-called random lengths, of up to 6m length each, to manufacture multiple components. As we have a particular focus on duplex and super duplex grades, we hold several hundred tonnes of each grade ex-stock. Langley Alloys is also the preferred distribution partner for Sandvik’s range of high-performance duplex and super duplex stainless steels, ensuring we have regular replenishment and the capability to support the largest of projects.
Ferralium is a registered trademark of Langley Alloys Ltd.
Image of socket head fasteners provided by Spring, Bolt and Nut Manufacturing, USA.
Charts sourced from The International Stainless Steel Forum guide to duplex stainless steels.