At what temperature range can brittle fracture be expected in low carbon steels?

[chetanladha]

Hi.
At what temperature range can brittle fracture be expected in low carbon steels? Are there other parameters which govern its occurrence?

Thanks.

 

[thearny2]

It is around -20 degC I think. Type of steel and manufacturing route I guess, not something I know about. Just use stainless...

 

[chetanladha]

Its certainly higher for low carbon steels.
Liberty ships sank because of brittle fracture, and temperature was about 0-5 C.

 

[Enthalpy]

There is no clear limit. Elongation at break, fracture toughness, impact strength and other indicative and non-reproducible figures vary steadily with temperature.

Steel producers use to give figures or curves down to -50°C and these don't look damning. Steel won't break like glass. The need for toughness depends on you part's use: at a gas tank you want deformation without rupture, at a ball bearing any plastic deformation equals a failure.

For comparison, most hard aluminium alloys have around 6% elongation at break, and brass about 2%. If you were ready to use brass at room temperature, you could use low-carbon steel for the same part at extreme cold weather.

Stainless, yes. That is, the common austenitic family. Martensitic stainless (the one used for knives for instance) behaves much like alloyed tempered steel, and ledeburitic (stainless ball bearings, Fiskar scissors...) like quenched steel.

 

[chetanladha]

There is no clear limit. Elongation at break, fracture toughness, impact strength and other indicative and non-reproducible figures vary steadily with temperature.

Steel producers use to give figures or curves down to -50°C and these don't look damning. Steel won't break like glass. The need for toughness depends on you part's use: at a gas tank you want deformation without rupture, at a ball bearing any plastic deformation equals a failure.

Hmm.. Makes real sense. Let me re-frame my question.
I want to study the effects of low temperature (0-5 C) on a ship structure, which is made using low carbon steel. (Steel Grades commonly used AH36, DH36 & EH36)

Can you please help?
Thanks..!

 

[Enthalpy]

Very uneasy.

Typically at a ship hull, a plastic deformation at impact is acceptable but a crack isn't.

Toughness is defined by dozens of incompatible figures and methods because none works properly, nor permits any numerical prediction - they aren't even repeatable.

If the ship you designed sinks you must provide an answer that judges or their "experts" understand. Not with MPa*sqrt(m) but like "I applied this standard".

So the method I would apply, hence recommend to you, is to look after what the profession does and stick to it. Unless the boat is only for you and needs no certification.