Ferritic, martensitic and duplex stainless steel tend to become brittle as the temperature is reduced, in a similar way to other ferritic / martensitic steel. The austenitic stainless steel 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’ temperature sometime in service specification sub-arctic and arctic application and location (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 steel. The austenitic 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 steel on the Nickel Institute.
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 steel varies with temperature. Ferritic and martensitic steels exhibit what is known as a ‘ductile / brittle transition’ where, over a certain temperature range, 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, possible catastrophic failures.
Affect of steel structure on toughness
The toughness of the austenitic relies on their fcc atomic structure. The presence of either ferrite or martensite can limit the cryogenic usefulness of the austenitic stainless steel. The small levels of ferrite usually present in wrought austenitics is not usually detrimental.
Cold working of austenitic stainless steels can also affect their cryogenic toughness. This is due to the progressive formation of martensite from the ‘meta-stable’ austenite. In effect this is similar to the presence of ferrite and can be controlled in the same way through compositional changes that stabilise the austenite. In addition the effects of cold work can be removed by heat treatment. Solution annealing (softening) by heating to around 1050 / 1100 °C and cooling in air, depending on section size, will completely stress relieve the structure and transform the structure back the naturally tough austenitic one.
Welded areas may be at risk of brittle failure at very low temperatures, as ferrite levels in welds are higher than the surrounding wrought steel (to avoid hot cracking on solidification). Special low ferrite level welding consumables are available for cryogenic applications and should be considered for very low, safety critical, temperature applications.
Casting compositions for austenitic stainless steel also have ferrite levels higher than the corresponding wrought grades BS3100 – Steel Castings for General Engineering Purposes, requires special impact tests at -196°C for the cryogenic application grades such as 304C12LT196. Although there are no major restrictions on composition, this grade is required to meet an additional Charpy impact test requirement of 41 Joules minimum at -196°C
Impact toughness of austenitic stainless steel
When austenitic stainless steel are Charpy tested at -196°C the test piece is usually ductile enough not to fracture (which actually invalidates the test).
Data available however quotes impact energies of over 130J for the 304 (1.4301) type. This is well within the 60-Joule minimum required in EN 10028-7 pressure vessel standard for 304 (1.4301) at -196°C. Any of the austenitic stainless steels should be suitable for applications at these temperatures. The best choices of grades for very low temperatures are those with austenite stabilising additions such as nitrogen e.g. asi n grade 304LN (1.4311). (Higher alloy grades such as 310 (1.4845) or 904L (1.4539) which derive their austenite stability from higher nickel levels could also be considered)
Wrought grades with ferrite stabilising additions such as 321 (1.4541) or 347 (1.4550) may not be suitable at very low temperatures e.g. at the liquid helium boiling point of -269°C.
Impact toughness of other stainless steel
The ferritic, martensitic and duplex stainless steels cannot be considered as cryogenic steels. Their impact characteristics change at sub-zero temperatures in a similar way to low alloy steels. The transition temperatures will depend on composition and heat treatment.
Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)
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