Experience with quenching a red hot steel rod

Do you want a answer? But I might be wrong!

If you put the hot bar into water, the water helps to transport the heat to the other end. Iron isn't such a good thermal conductor. Everyone welding knows how long you can hold a piece of steel in your hands just welded.

Now this, is, ... well, I don't know! Just guessing: Also, it might be interesting to have a curve that shows thermal conductivity vs. temperature of steel. Does it degrease with temperature? There might be a "hidden secret", together with the water's thermal conductivity.

Nick

"s.morra" asked a question about the concept of "thermal inductance," in an effort to explain a heating/cooling phenomenon. Since we know there is thermal resistance (the reciprocal of thermal conductivity,) and thermal capacitance (product of mass and specific heat,) it seems as though, by analogy, one should look for thermal inductance as well.

The first thing to note, here, is that heat flow is described by the diffusion equation, while electrical phenomena obey the potential equation. These are two separate differential equations, so it does not surprise me nor trouble me that inductance is absent from all heat flow phenomena.

I doubt that the fast heating of a quenched rod really happens. If it were a tube, I can attest that it does happen, because steam travels up the inside of the tube. Possibly, something like that can happen to a lesser degree on a solid bar, but I suspect that it is mostly psychological. It would not be hard to do a test. I have a torch, and I have a Fluke meter that can be hooked to a thermocouple. In fact, this would make an excellent science fair project, but I'm a little old for that. However, if there is enough interest, I would be willing to carry it out and report the results.

(snip)

It's an illusion.

If you heat two bars identically, then plunge one in a quenching bath while holding both, the rate of temperature increase at the ends you're holding will initally be about the same.

Heat propagates thru a material at a rate determined by its thermal conductivity, specific heat and temperature gradient. . This process can be (has been) treated as a boundary value problem with partial differential equations.

The illusion is caused by heat that is "on the way" from the heated end to the held end. Quenching the hot end still leaves a hot region in the middle, which will continue to propagate (now in both directions) thru the bar. Quenching the end does not instantly quench the middle, just as heating the end didn't instantly heat the middle.

I know exactly what you mean and you are right! The best explination I have heard is that the heat doesn't like the cold water and runs to the other end to get away.

I have noticed the exact same thing, to the point that I posted my own set of questions about it several years back. I think it was here and in one of the physics groups.

As in your posting, a few people replied saying they had noticed the phenomenon too, several felt that steam played a part, most said it wasn't really happening. No one had any kind of explanation why it might be happening just inside a bar of solid metal.

I had planned to try to instrument some bars with thermocouples or thermistors tied to a logging device and do some experiments trying to compare with and without the fast cooling of the hot end, and also have some kind of shield to block steam. After a few quick tries with stuff at hand, I got distracted and never got a setup that I could use and trust for this. Lots of variables are hard to control like rapidly heating one end with a measurable amount of heat to start.

So I fully believe this is a real phenomenon and remain surprised that nobody seems to have an explanation. Most people with knowledge about such thermodynamics seem to feel it isn't really happening. I believe it is happening, but never found an accurate way to prove it, and certainly have no good explanation of why it would happen.

So if you placed the solid hot bar on two plastic 5 gallon buckets full of water, then the ensuing stampedes of excited and scared molecules would run into each other and generate even more heat than the observed phenomenon? I would think that they would go willingly to the dark medium.

Yes, and the bar will melt in the middle! With this trick you can melt the thickest rods just with two cigarette lighters and two buckets of ice water.

Nick

The heat moving to the other end is a technique used to heat treat carbon tool steel tooling such as lathe tool bits. You heat the working end to cherry red, then plunge the first inch into water, then remove the piece. The heat from the opposite end that was not quenched travels back into the quenched end to temper it. On larger pieces these two steps must be done individually, but on small pieces the remaining heat is enough to reach the 400 degrees needed for tempering.