Cooling joints rapidly?

On another forum, a friend describes a welder working where the heat needs to be very focused i.e. there is wood and fabric very close by.
"The welder worked very slowly, meaning that he quickly welded about 1/4 inch at a time, then backed off and I put wet rags on the joint to cool it immediately."
I was under the impression one left the weld to cool on its own, otherwise you affected the metal somehow.
Apparently it's time for me to learn how metal is affected by various types of cooling, and what stresses are introduced, so I'll start reading up; however can someone address the rapid cooling question? - Mike
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Rapid cooling is a wide open question, very application dependant.
Structurally, it will affect distortion, and the temperature differential across the weld are may lead to cracking of the weld or the base metal around it, even for a good weld. This can be avoided with careful planning, and is more relevant with some metals and alloys (like a high carbon steel) than with others (like many low carbon steels).
Metalurgically, rapid cooling can have a big effect. This is, after all, a technique for hardening steel. Which can make the steel brittle. Copper, on the other hand, is actually made more ductile with a rapid quench. The rate of quench and the temperature from which the quench begins are both key factors.
With regard to the quote, welding 1/4" and quenching, I would bet that there wasn't enough heat in the joint by the time he quenched for there to be a major effect (heat flow from the joint to the surrounding metal does quite a good job quenching a small bead, and can lead to brittle weld and HAZ in some circumstances WITHOUT further quenching), though I would hope that the welder was good enough to get heat for proper fusion in such a short bead on cold, wet metal.
General rule for small welds on low carbon steel is that once the red is totally gone, the metal is below the hardening temperature, and quenching will not have much effect unless the weld or HAZ is already brittle, in which case the rapid cooling may increase the risk of cracking.
Michael Horowitz wrote:

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and quenching, I would bet that there wasn't enough heat in the joint by the time he quenched for there to be a major effect (clip) ^^^^^^^^^^^^^^ Sounds like the welder knew what he was doing. He evidently had no choice, due to the proximity of heat sensitive and combustible materials. He may have cause some hardening/embrittlement in the steel when he cooled it, but as soon as he came back and did the next bit of welding, he passed it through another heating/cooling cycle, and then another and another. Probably wound up with an annealed weld. We don't know what the composition of the steel was, but if it was low carbon, it probably wouldn't harden anyway.
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Low carbon steel is not affected except for internal stresses. Almost always not a problem. Medium or high carbon steel and alloy steels are affected. Rapid cooling makes them hard, strong, and with high internal stresses which need to be relieved.
Use a grinder to produce sparks from the steel and compare to known steels to determine if it is low carbon, high carbon, or alloy steel.
Dan
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They pretty well said it all. Problems relating to "heat affected zone", cracking, and all that are directly proportional to the percentage of carbon in the steel.
This is why it's dicey to weld rebar. A lot of it is very high carbon. But it's not manufactured to a standard. Therefore, you don't know what you've got.
I believe it was Ted Edwards who pointed out that if you want to know whether rebar is "weldable" here's what you do.
Heat it red hot with a torch. Quench it. If a file won't nick it, or does so, only with difficulty, it's carbon steel. If it's dead soft after a quench it's mild steel.
Mild steel, by definition, contains virtually no carbon. Therefore, it cannot be heat treated. After you quench it it's still soft.
As a point of clarification all steels are "weldable". But as carbon content goes north so does the difficulty. As I understand it the main preparation steps are pre-heat, post-heat, and perhaps, proper filler metal selection. This is where 7018 shines.
Hope this helps.
VT
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