low temperature suitability of steels 2

[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.

 

[mloew]

chrisgibson,

You should post this question in the Metal and Metallurgy engineering forum (forum330).


Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Maine]

You might try looking up the specifications on Ice Boat design. The blades are usually made from steel and although the riding edge will have the friction heat, upper surfaces not only are subject to actual air temps, but wind factors and constant jarring and stresses depending on speed and terrain. The races I've seen haven't been on uniform surfaces so the temp variation across the blade itself should have a pretty good range for you.

The Mainer...

 

[SMF1964]

The suitability of the steel for low temperature service is not just an issue with impact loading conditions. It is a question of toughness at temperature, which is another way of saying a material is 'flaw tolerant'. The initial work on the Charpy curve was to address why ships were failing catastrophically during WWII. This is not an impact related loading, because a number of these fractures occurred while the vessel was being loaded in port.

Under identical loading conditions, a material that has poor toughness will suffer catastrophic fracture with a smaller flaw than a material that has good touchness. This being a perfect world, we do not have to worry about fatigue crack propagation while in service, undetected fabrication related weld defects or crevice corrosion/pitting that can instigate these flaws until they reach critical size. The critical flaw size is a function of applied load, crack geometry and (you guessed it) toughness. Designing a widget that is going to be exposed to temperatures below its ductile-brittle transition temperature (or nil ductility transition temperature, or 30 ft-lbs limit, or whatever spec you are using) is risky.

 

[mloew]

Why is this being posted in the Military engineering other topics Forum? Is there a military application or specification?

Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

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[IRstuff]

While the temperature range is military, the question is a metals question. You might try red-flagging your post and ask the administrator to move it to the metals forum

TTFN