I have to specify materials for use at cryogenic temperatures. I have heard about the ‘ductile/brittle’ transformation in steels. Are all steels affected, are there any other materials that I could use as corrosion might also be an issue? The materials will need to be weldable.

Many common engineering steels do undergo a ductile to brittle transformation at, or just below room temperature. ‘Cryogenic temperatures’ usually means substantially below room temperature. However, let us look at the options in some different temperature ranges.

BS EN 10025 and BS EN 10028 (parts 4, 5 and 6) list a number of carbon and low alloy steels that have acceptable impact properties, combined with useful mechanical strengths down to –20 to –50^oC. Impact properties, as measured by Charpy or Izod tests are a measure of the energy absorbed in fracturing the sample and are therefore a direct indicator of whether fracture will be ductile or brittle. These specification, BS EN 10025 and BS EN 10028, replace the old BS 1502 and BS 4360. Generally these steels are weldable and offer sensible options for structures operating in this temperature regime. As a rule of thumb remember that the lower the operating temperature the higher the alloy, particularly nickel, content. You should always consult the tables of properties in the Standards before making a final decision on a material.

If your operating temperatures are lower than –50^oC then you must consider some alternatives. If you prefer to use steels then your choices really are limited to the austenitic grades of stainless steel. These steels combine good levels of mechanical properties at temperatures as low as –180^oC with a freedom from any ductile to brittle transformation.

Many nickel based alloys have excellent cryogenic properties combining mechanical strength with formability, weldability and corrosion resistance. Monel 400 and Inconel 600 both offer excellent combinations of mechanical and impact properties, as low as –180 to 190^oC.

Another alternative, that you might not have considered although they may offer a price advantage, is aluminium and aluminium alloys. These alloys do not suffer from any low temperature brittleness problems and again offer a useful combination of formability, weldability, mechanical properties and corrosion resistance for most grades except the 2000 series alloys where corrosion may be an issue. There is a very useful publication from the Aluminium Federation called ‘The Properties of Aluminium and its Alloys’ that provides mechanical and other property data on these alloys.

Further information

Iron & Steel Society

Light Metals Division