Alloy 255

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Definition - What does Alloy 255 mean?

Alloy 255 is a solid solution strengthened, highly alloyed material that offers an outstanding strength, superior resistance to corrosion and high wear resistance. The ferric-austenitic stainless steel combines high corrosion resistance and strength-to-weight ratio suitable for a wide variety of applications and environments. It is widely used in the equipment handling phosphoric acid, pollution control, petrochemical, fertilizer, pulp and paper industries, marine corrosive environments and more.

The super duplex alloy is also known as Ferralium Alloy 255.

Corrosionpedia explains Alloy 255

The alloy 255 consists of about 27% chromium, 6.5 Nickel, 3.9 Molybdenum, 2.5 copper, 0.25 Nitrogen, 1 Silicon 1.5 manganese, 0.04 carbon, 0.04 phosphorus and 0.03 sulphur, while the iron takes the remaining balance of about 57%.

Its microstructure is a balanced mixture of ferrite and austenite, a combination of which results in a high strength and chloride stress-corrosion cracking resistance. The copper content improves it resistance to corrosion in seawater applications as compared to other super duplexes.

Even though the alloy is harder compared to most other stainless alloys, it is still machined using the common methods; however, slow, heavy and constant speeds must be observed so as to overcome the material's tendency to harden. It is readily formed using the common hot and cold working methods, and only hardens by cold work; however, it should be used in its annealed condition so as to maintain its resistance to corrosion.

Alloy 255 characteristics are:

  • Superior resistance to general corrosion, chloride stress-corrosion cracking, pitting and crevice corrosion
  • High strength-to-weight ratio
  • Good fabricability
  • High mechanical strength
  • High resistance to abrasion and cavitation erosion

The combination of high strength, high wear resistance and superior corrosion resistance makes the alloy 255 ideal for demanding applications in a variety of industries. Its resistance to corrosion makes it ideal for chemical processes equipment where phosphoric and chlorides are present, while the erosion resistance makes it suitable for use in pumps and valves that handle abrasive and corrosive hot slurries.

The 255 alloy is used in the manufacture of fasteners, agitators, vessels, pumps, blowers and other components for the pulp and paper, marine, oil and gas, nuclear, chemical processing, and other similar industries. However, it is not suitable for high temperatures and its use is limited to a maximum continuous temperature of about 500°F, otherwise secondary phases may form at the higher temperatures.

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