cold embrittlement of steel

[JOM]

Hi All.

As a non-metallurgist I have a question.

If steel that is not rated for low temperature service experiences a low temperature excursion it can become brittle and thereby weakened. Correct, so far?

What I'd like to know is if it subsequently returns to the higher temperatures of its normal service, has it been permanently altered and in what ways? Has its strength been altered?


J.

 

[Salvatio]

perhaps it's like putting a rubber ball into the frezer, let it be hard rock(brittle), then take it out, it'll be in it normal condition again.

What you are referring is the ductile-brittle-transition temperature.

 

[metengr]

JOM;
I can discuss the behavior of ferrous materials used in the boiler and pressure vessel industry. The answer to your first question is no. Ferrous materials that are not designed for low temperature service will exhibit reduced fracture toughness only when exposed to service temperatures at or below their ductile-to-brittle transition temperature (DBTT). The DBTT is a defined temperature where the material will change from ductile to brittle fracture behavior. Once service temperatures have increased or recovered above the DBTT, the material toughness will increase, and will behave as though it never saw the lower temperature service.

If a ferrous material is inadvertently exposed to low temperature service (below the DBTT), this can be ok, as long as the material is not exposed to dynamic service stresses that could result in the introduction of cracks or brittle fracture. There should be no significant change in mechanical properties from inadvertent exposure to low temperature service.

 

[Maui]

JOM, in order to give you a better understanding of this phenomenon, I'll refer you to my reply in thread330-110691, where a detailed explanation is provided.

Maui

 

[unclesyd]

Adding a little to above posts by metengr and Salvito.
Some metals that have very high (DBTT)are actually given a controlled cold treatment (cryogenic temperature processing). When returned to temperatures above their (DBTT) they may have certain improved properties.

The Titanic is prime example of a metal being used below it's (DBTT) with disastrous results. It was quite a strong ship until the metal temperature of the ship got below the (DBTT) in the cold water of the North Atlantic. The biggest problem was that little was known about this brittle transition at the the time. The metal in the Titanic was the best available at the time but very brittle below about 40°F

 

[SMF1964]

For a clearer understanding of the metallurgy of the Titanic, I suggest the following website:

http://www.metallurgy.nist.gov/webpages/TFoecke/titanic/titanic.html

 

[JOM]

Thanks everyone.

The answer seems to be that the steel recovers.


J.

 

[Salvatio]

one question that may be out of topic. Other than failure due to DBTT, didnt the relatively high sulphur content, hence the sulphur precipitates, helped to cause the failure too?

 

[unclesyd]

Yes, there were several things that contributed to the failure and certainly high S in the rivets didn't help matters at all.
You must remember that the materials were the best commercially available at the time.

A lot of ships were built during the same time period from the same or lessor materials but never underwent the loading imposed by the collision.

Several buildings were also built in this time frame using the same rivet material, beams and plate.

 

[rorschach]

Here is an article on the metallugical studies done on the hull plating and rivets.

http://www.tms.org/pubs/journals/JOM/9801/Felkins-9801.html