What are duplex grades F51, F53, F55, F60 and F61?

The American Society for Testing and Materials (ASTM) is one of the world's largest standards organisations. Standards beginning with the letter 'A' cover metals.

ASTM A182 ('Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service') is now in its 19th edition (2019). Across these editions, new alloys have been added and given a new 'grade' number. The 'F' prefix designates relevance to forged products. The number is partly grouped by alloy type, for example austenitic or martensitic, but is not entirely prescriptive. Ferritic-austenitic duplex steels are numbered between F50 and F71, with ascending numbers roughly tracking more recently added grades.

The different grades

ASTM A182 F51 equates to UNS S31803, the original designation for a 22% Cr duplex stainless steel. Manufacturers optimised the composition towards the top of the limits to improve pitting resistance, and this tighter specification is designated F60, equating to UNS S32205. As a result, S32205 can be dual-certified as S31803 but not the other way round. It accounts for around 80% of duplex stainless steel production. We stock Sanmac 2205, Alleima's proprietary product with enhanced machinability as standard, from ½" up to 450mm diameter solid bar, plus hollow bar and plate.

ASTM A182 F53 equates to UNS S32750, the 25% Cr super duplex stainless steel most widely promoted by Alleima as SAF2507. Its increased chromium over F51 offers improved pitting resistance, and its higher yield strength lets designers reduce section size for load-bearing applications. We stock SAF2507 solid bar from Alleima, in sizes from ½" to 16" diameter.

ASTM A182 F55 equates to UNS S32760. This grade traces back to the development of Zeron 100 by Platt & Mather, Manchester, UK. It is a 25% Cr super duplex stainless steel with an addition of tungsten. We stock S32760 solid bar from Alleima, in sizes from ½" to 16" diameter.

ASTM A182 F61 equates to UNS S32550, an approximation of Ferralium^® 255, the original super duplex stainless steel invented by Langley Alloys. Launched in 1969, it has now given more than 50 years of successful service in demanding applications. Compared with F53 and F55 it delivers increased strength and corrosion performance. Its minimum yield strength exceeds 85ksi, where other grades are limited to 80ksi, and it contains up to 2.0% copper, which aids pitting resistance. We stock Ferralium^® 255-SD50 in sizes from 5/8" to 14" diameter solid bar, plus plate up to 3" thickness.

A number of other grades are included in the specification, such as lean duplex (F68, UNS S32304) and hyper duplex (F71, UNS S32808), but these are not available ex-stock from the supply chain.

We offer a comprehensive range of duplex and super duplex stainless steels, operating from locations in the UK and USA.