Selection of stainless steels for cryogenic applications

stainless steelsIntroduction

  Ferritic, martensitic and duplex stainless steels tend to become brittle as the temperature is reduced, in a similar way to other ferritic / martensitic steels.

  The austenitics stainless steels such as 304 (1.4301) and 316 (1.4401) are however ‘tough’ at cryogenic temperatures and can be classed a cryogenic steels.

They can be considered suitable for sub-zero ‘ambient’ temperatures sometimes mentioned in service specifications sub-arctic and arctic applications and locations (typically down to -40°C).

This is the result of the ‘fcc’ (face centred cube) atomic structure of the austenite, which is the result of the nickel addition to these steels.

  The austenitics do not exhibit an impact ductile / brittle transition, but a progressive reduction in Charpy impact values as the temperature is lowered.

  There is a useful summary of low temperature data for austenitic stainless steels on the Nickel Institute website.

Impact toughness and impact strength measurement

  Impact tests e.g. Charpy, are done to assess the toughness of materials. To assess their suitability for cryogenic applications, the test is done after cooling the test piece.

  The Charpy impact test measures the energy absorbed in Joules when a standard 10mm square test piece (usually with a 2mm deep ‘v’ notch) is fractured by striking it in a pendulum type testing machine.

The more energy absorbed, the tougher the material, and less likely it is to fail ‘catastrophically’ if subject to mechanical shocks or impacts.

The impact toughness of steels varies with temperature.

Ferritic and martensitic steels exhibit what is known as a ‘ductile / brittle transition’ where, over a certain temperature range, there is a pronounced reduction in the impact toughness for a small decrease in test temperature.

  When plotted on a graph, the energy absorbed against temperature produces an ‘S’ curve.

The mid-point on the ‘S’ is known as the ‘transition temperature‘. Here the fracture failure mode changes as the temperature is lowered, from ‘ductile’, where the steel can absorb quite a lot of energy in breaking, to brittle, where only a small of amount of energy is absorbed.

For this reason it is dangerous to use steels in this brittle state in structural applications, as even small shock loads can result in sudden, possiblecatastrophic failures.

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