What is austenitic stainless steel?

9th June 2020

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The most well-known grades of stainless steel are the austenitic stainless steel grades 304 and 316. An austenitic stainless steel is so-called because of the crystal structure the atoms take when the composition and thermal processing conditions allow. The austenite crystal structure is also described as face centred cubic (FCC) and defines many of the mechanical and physical properties of the stainless steel.

In order to increase the resistance to pitting of stainless steels, chromium and molybdenum are added in increasing quantities. In order to retain the desirable austenitic microstructure, nickel is usually added. However, highly alloyed super austenitic stainless steels containing high levels of molybdenum (typically greater than 6%) are relatively expensive due to their higher nickel content too.

Austenitic stainless steels are non-magnetic, which is useful for selected applications. They also retain excellent impact strength toughness down to very low cryogenic temperatures. They are straightforward to fabricate, machine and weld, although lower carbon grades such as 304L and 316L (UNS S31603, 1.4404) are less likely to suffer from carbide sensitisation. Corrosion performance is positive, but strength is somewhat limited.

Alloy 304 consists of 18-20% chromium, balanced by 8-10.5% nickel. Alloy 316 has a similar base composition, but also includes 2-3% molybdenum specifically to improve resistance to pitting corrosion. Whilst these standard grades can include up to 0.08% carbon, the lower carbon versions – 304L and 316L – contain less than 0.03%. This reduces the likelihood of forming chromium carbides during welding, which result in a loss of corrosion resistance and toughness.

Although 304 and 316 are the most common grades, Langley Alloys focuses on metals suited to the most demanding of applications, such as long-term marine exposure, Oil and Gas, energy production, mining, water treatment and the chemical processing industries. Therefore, we stock the more highly alloyed grades with increased strength, improved corrosion resistance and enhanced physical properties.

 

Nitronic® 50 is a higher-performance austenitic stainless steel that provides approximately twice the yield strength of regular 3xx series stainless steels and improved corrosion performance. It is widely used in a variety of marine and industrial applications. This alloy is available in several conditions, Solution Annealed, Hot Rolled or Strain Hardened. Langley Alloys carries a comprehensive stock of Fermonic 50 (XM-19, UNS S20910, Nitronic 50, 1.3964) in sizes from 3/4″ (18.9mm) to 10″ (254mm) diameter.

Nitronic® 60 (Fermonic 60, UNS21800) is an austenitic stainless steel with incredibly useful physical properties, specifically wear and galling resistance. Nitronic 60 is used selectively for valve seats, trim and stems, fasteners, couplings and break-away connectors, pins and bushes.

Alloy 254 (254SMO, 6Mo, UNS S31254, F44) is a super austenitic stainless, using raised additions of molybdenum to increase the Pitting Resistance Equivalent Number (PREN) to 43. This compares with a PREN of 25 for Alloy 316, and 34 for a 2205 duplex stainless steel or Nitronic 50. It has been used in applications such as marine exhaust scrubbers, due to the wider temperature operating range possible for an austenitic stainless steel compared with a more cost-effective super duplex stainless steel.

Nitronic is a registered trademark of AK Steel Corporation. 254SMO is a registered trademark of Outokumpu.