suitability of steels to low temperature use

[chrisgibson]

Can anybody help me. I am trying to determine the suitability of particular steels to low-temperature use (-40°c being low). As far as I know the normal way, to be on the safe side, is to look at the impact toughness of the material at the given temperature. If this lies above 27 Joule using the standard Charpy impact test. One can be sure that there is enough rest ductility in the part, that the part will survive at this reduce temperature.
Unfortunately this normally leads to additional cost and one starts to consider whether this is truly necessary. Bearing in mind that there is almost no chance of the part seeing a real impact and that any forces which the component meant see are introduced into the part at a relatively slow rate, is there not another way of defining this problem? The ASTM E812-91 describes a slow-bend test and shows that at slow loading rates the characteristics of steels are totally different to those seen in an impact test. Unfortunately the standard loading rate values used for this test are extremely small and do not compare with the loading rates I would expect to see on my component. I know that I will ultimately have to carry out some tests at reduced temperature on the component to verify this point. I just wanted to get some idea of the chance of success and what would be the best way of performing these tests.

 

[Metalguy]

Is your main concern the cost of performing the Charpy tests or that steel with decent toughness (not ductility) at low temp. costs more?

How critical are the parts involved? What happens if they crack? If you're working with carbon steel, you can *infer* something about how it will behave at low temps. from the chemistry and processing history, but for critical parts you really must have the actual impact test data.

 

[Carburize]

The most realistic assessment would be a fracture toughness test - ASTM E399 or similar - carried out at the operating temperature and strain rate. This data will permit calculation of a critical defect size which will then indicate the inspection requirements and flaw tolerance of the component.

 

[stanweld]

Exposure to low temperature, by itself, will not lead to failure. Sufficient applied stress coupled with residual stresses from welding, forming, machining, etc. and the presence of flaws or other stress intensifiers are needed. If the combined stresses are much less than the materials yield strength and flaws or other stress intensifiers are either not present or minimal, "brittle" steels will not fail in service.

You must first determine the design stress conditions and fabrication residual stress/strain relative to the steel's minimum specified tensile properties and the potential for or presence of stress intensifiers.

If you have determined that loads are such that toughness is a prerequisite to safe operation, Charpy testing is one of the cheapest methods to use. COD tests, etc., while offering superior information, are also considerably more expensive.

 

[dirpes]

Suitability of some steels for low temperature service is done by some allying componens .Low allied steels (less than 0,2%Ni & between 0,8 to 1,2Mn ) have a good stability till -50 if you had treated them before. If the parts you have are designed to be weld you can equivalate your steels to the American ASME or the German Werkstoff.Medium allied steels (allied with Cr,MO,Ni,V,etc.)with the proper heat treatment can give you satisfaction .Stailess steels are stable for much lower temperatures (the austenitic ones )but with their particular mechanical properties .So accordin to the stresses the parts are loaded you can choose the right material .If you ask for more I can answer here more .