What Are Duplex Grades F51, F53, F55, F60 and F61?

F51, F53, F55, F60 and F61 are duplex and super duplex stainless steel designations taken from ASTM A182. This standard is one of the most widely referenced standards for the supply of stainless steels.

The American Society of Testing and Materials (ASTM) is one of the world’s largest standards organisations, reviewing, collating and publishing technical standards for an increasingly wide range of materials. Published standards beginning with the letter ‘A’ cover metals.

Standard 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 19^th edition (2019). Over the course of these editions, new alloys have been added and allocated a new ‘grade’ number. The ‘F’ prefix designates the relevance of this standard to forged products. The number suffix is partly grouped by alloy type i.e. austenitic, martensitic, but is not totally prescriptive. So-called ‘Ferritic-Austenitic’ duplex steels are numbered between F50 and F71, with ascending numbers partly approximating to more recently added grades.

The Different Grades of Duplex Stainless Steels

ASTM A182 F51

ASTM A182 F51 equates to UNS S31803. This was the original caption for a 22% Cr duplex stainless steel. However, as explained in an earlier article, manufacturers optimised the composition towards the top end of the limits to improve pitting corrosion resistance. This grade, with a tighter specification, is captioned as F60, equating to UNS S32205. Consequently, S32205 can be dual-certified as S31803, but not vice-versa. It accounts for around 80% of overall duplex stainless steel production. Langley Alloys stocks Alloy 2205+, which is Alleima’s proprietary product that provides ‘enhanced machinability as standard’. Our stock range goes from ½” up to 450mm diameter solid bars, plus hollow bars and plates too.

ASTM A182 F53

ASTM A182 F53 equates to UNS S32750. This is the 25% Cr super duplex stainless steel most widely promoted by Alleima as SAF2507. With an increased chromium content compared with F51 it offers improved pitting corrosion resistance. The yield strength is also higher, allowing component designers to reduce section size for load-bearing applications. Langley Alloys stocks SAF2507 solid bars from Alleima, in sizes from ½” to 16” diameter.

ASTM A182 F55

ASTM A182 F55 equates to UNS S32760. The origins of this grade can be traced back to the development of Zeron 100 by Platt & Mather, Manchester UK. It is another super duplex stainless steel based upon a 25% Cr composition, but with an addition of tungsten. Langley Alloys stocks SAF32760 solid bars from Alleima, in sizes from ½” to 16” diameter.

ASTM A182 F61

ASTM A182 F61 equates to UNS S32550. This, in turn, is an approximation of Ferralium 255, the original super duplex stainless steel invented by Langley Alloys. Launched in 1969, it has now provided more than 50 years of successful service in a wide range of demanding applications. Compared with F53 and F55, it delivers increased strength and corrosion performance. Its minimum yield strength exceeds 85 ksi, whereas other grades are limited to 80 ksi. In addition, it contains up to 2.0% of copper, which aids in pitting corrosion resistance. Langley Alloys stocks Ferralium 255 in sizes from 5/8” to 14” diameter solid bar, plus plates up to 3” thickness.

Other Grades

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

For More Information, Contact Us

Langley Alloys offers the most comprehensive range of duplex and super duplex stainless steels, operating from locations in the UK and USA. For enquiries and quotations, please contact us today. For more information about ASTM A182 F51 and the other grades taken from ASTM A182, our dedicated team will also be happy to help you.