Temperature and A36 Structural Steel

I must admit that steel and material science are not my fortay, but did I read the question correctly?

"At what enviromental temperature would a person change from A36 structural steel to some other material."

Are you working on a T-1000 robot?

If there is some technical jargon in there that I don't understand, I'm sorry for the joke.

Try this web site www.matweb.com to find charpy impact data for the steel in question, or go to the ASTM web site.

The real question is what are you constructing? Likely you would not want to exceed the yeild point, which is not as temperature dependent. Above the yeild point stress you will start to get plastic deformation which is great for deep drawing an oil pan but not so great for building a bridge.

What you are looking for is the DBT (ductile brittle transition) temperature. This is calculated by doing an izod or charpy impact test of like samples at different temperatures, often down to -196 Celsius. The results are plotted and DBT is determined as ((upper shelf energy - lower shelf energy)/2)+lower shelf energy.

Generally speaking, lower% carbon = lower DBT temperature, manganese content also plays a significant role in determining DBT, higher% Mn = lower DBT.

I do remember some photos in a college text regarding the WW2 "Liberty" ships breaking up near the Alaska region due to low temperature embrittlement around -40 (C or F). In a dorm room discussion, another student recalled a tractor hitch breaking at -50 F, while hauling manure (I went to an Ag school in North Dakota).

I likewise am not a steel expert, but I don't think -20 F is anywhere steel's embrittlement zone.

Piping code ASME B31.3 resticts the use of pipes made from A36 plates to a minimum temperature of (-) 20 deg. F. Refer Table A1 and Table 323.2.2 B2 of this code. you may apply this restriction to plates also -- to be on the safer side.

-20 F is certainly low enough to promote brittle fracture if carbon content is high, as was the case with the Liberty ships which saw service in the north Atlantic.

Other factors are grain size, phosphorous and sulphur content, work hardening and any heat treating or normalizing done.

Speaking of the Liberty ships, the main problem was not so much that the steel was of a grade susectible to low temperature brittle fracture, but the fact that they were welded. This was a new method of fabricating ships at that time. It was done mainly as a time saving method that allowed many ships to be brought on-line in a short time. The problem with welding is that no rivets are used. Rivet holes in a ships hull act as crack arrestors. A crack, even a microscopic one, will drastically affect the amount of stress required to fracture metal as per Griffith's Crack Theory which can be stated; S_c=2S(√(c/r ))

S=stress, c=crack width, r=crack tip radius

Because of this, the actual stress acting on the crack tip or front can be a thousand times greater than the actual applied stress. This is the theory behind stress risers and the reason for rounded corners and fillets in dies and castings.

One can imagine a small crack developing, perhaps due to fatigue, in a cold hull of poor quality, high carbon steel. With nothing to impede its travel and a force multiplier as per Griffith's, this crack would quickly run the entire way around the hull and the ship would suddenly be in two peices. Frightening!

Later Liberty ships were welded in the same manner as before, however, dummy rivets were installed as crack arrestors.

Thanks for taking time to illuminate my ignorance!