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Metal Profile: Austenitic Stainless

What is Austenitic Stainless Steel?

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Austenitic steels are non-magnetic stainless steels that contain high levels of chromium and nickel, and low levels of carbon.  Known for their formability and resistance to corrosion, austenitics are the most widely used grade of stainless steel.

While ferritic steels have a body-centered cubic (BCC) grain structure, the austenitic range of stainless steels are defined by their face-centered cubic (FCC) crystal structure, which has one atom at each corner of the cube and one in the middle of each face. This grain structure forms when a sufficient quantity of nickel is added to the alloy (eight to ten percent in a standard 18 percent chromium alloy).

As well as being non-magnetic, austenitic stainless steels are not heat treatable. However, they can be cold worked to improve hardness, strength and stress resistance. A solution anneal (heating to 1045°C followed by quenching or rapid cooling) will restore the alloy's original condition, including removing alloy segregation and re-establishing ductility after cold working.

Nickel-based austenitic steels are classified as 300 series. The most common of these is grade 304, which typically contains 18 percent chromium and eight percent nickel.

Eight percent is the minimum amount of nickel that can be added into a stainless steel containing 18 percent chromium in order to completely convert all the ferrite to austenite.

Molybdenum can also be added to a level of about two percent (grade 316) so as to improve corrosion resistance.

Although nickel is the alloying element most commonly used to produce austenitic steels, nitrogen can also be used to produce austenitic steels. Stainless steels with a low nickel and high nitrogen content are classified as 200 series.

However, because it is a gas, only limited amounts of nitrogen can be added before deleterious affects arise, including the formation of nitrides and gas porosity that weaken the alloy.

The addition of manganese, also an austenite former, combined with the inclusion of nitrogen, allows for greater amounts of the gas to be added. As a result, these two elements, along with copper (which also has austenite forming properties), are often used to replace nickel in 200 series stainless steels.

The 200 series - also referred to as chromium-manganese (CrMn) stainless steels - were developed in the 1940s and 1950s when nickel was in short supply and prices were high. It is now considered a cost effective substitute for 300 series stainless steels that can provide an additional benefit of improved yield strength.

Straight grades of austenitic stainless steels have a maximum carbon content of 0.08 percent. Low carbon grades, or "L" grades, meanwhile, contain a maximum carbon content of 0.03 percent in order to avoid carbide precipitation.

Characteristics:
Austenitic steels are non-magnetic in the annealed condition, although can become slightly magnetic when cold worked.

They have good formability and weldability, as well as excellent toughness, particularly at low, or cryogenic, temperatures. Austenitic grades also have a low yield stress and relatively high tensile strength.

While austenitic steels are more expensive than ferritic stainless steels, they are generally more durable and corrosion resistant.

Applications:
Austenitic stainless steels are used in a wide range of applications, including:

  • Automotive trim
  • Cookware
  • Food and beverage equipment
  • Industrial equipment

Applications by Grade:

304 & 304L (standard grade)

  • Tanks
  • Storage vessels and pipes for corrosive liquids
  • Mining, chemical, cryogenic, food and beverage, and pharmaceutical equipment
  • Cutlery
  • Architecture
  • Sinks

309 & 310 (high chrome and nickel grades)

  • Furnace, kiln and catalytic converter components

318 & 316L (high moly content grades)

  • Chemical storages tanks, pressure vessels and piping

321 & 316Ti ('stabilised' grades)

  • Afterburners
  • Superheaters
  • Compensators
  • Expansion bellows

200 Series (low nickel grades)

  • Dishwashers and washing machines
  • Cutlery and cookware
  • In-house water tanks
  • Indoor and non-structural architecture
  • Food and beverage equipment
  • Automobile parts

Sources:
South Africa Stainless Steel Development Association. Types.
URL: http://www.sassda.co.za/info/types.htm
International Stainless Steel Forum (ISSF)
URL: http://www.worldstainless.org
International Molybdenum Association. Stainless Grades and Properties
URL: http://www.imoa.info/moly_uses/moly_grade_stainless_steels/steel_grades.php

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