heat transfer in a steel rod ??

If you leave the end of a rod in a fire long enough, it will get hotter until it reaches equilibrium where the heat lost to the atmosphere outside the forge is equal to the heat being added inside the forge. It takes a while for this to happen, but it will get pretty damned hot on the cool end. I'm assuming that you usually remove it from the forge long before this equilibrium occurs.

If a rod is being heated at one end, heat transfer, in the form of conduction, will travel through the rod (and by radiation and convection to the surroundings). When the rod is removed there is a temperature gradient along it from hot to cool. The end you hold is the coolest, the hot end is red hot, and the middle is very hot, but not red hot. Depending on how long you quench the hot end it will end up at some cooler temperature--possibly cooler than the middle since it is cooling at a much slower rate (in air as opposed to water). Now heat conduction continues from the hot center outward...in other words, the cool end is still getting hotter. Now you put it back into the forge and wait some more and the cool end keeps getting hotter.

When you quench one end, it slows the conduction down a little, but does not stop it. If you quench the whole rod, this reduces the entire gradient and buys you more time.

Don Kansas City

Don Your explanation doesn't address the case of why the rod that is being forged stays cooler than the plunged rod.

Steam rising from the water being felt by your hand holding the bar?

---------- Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)

Nope, bar was held at better than 45 degrees to prevent gettin g scalded. My palm felt the heat so it wasn't steam and why did the rod being forged remain cooler?

Dear Henry Kolesnik:

You work the rod while it lays on the anvil. The anvil "siphons off" the heat.

David A. Smith

This sounds very like the Frog's Dilemma. A frog in a pan on the stove stays and cooks , a frog dropped in hot water jumps out.

In this case, it is possible you are much more aware of a fast hot to cool transition traveling the bar, than a slow, slow hot to hotter change.

Brian Whatcott Altus Ok

As far as I know an anvil can't take heat away quicker than water. Hank "N:dlzc D:aol T:com (dlzc)" wrote in message news:ALmve.3340$Qo.1365@fed1read01...

Hank-

Is it possible to re-do your "experiment" now with two rods?

How much of the 20" bar got plunged into the water? Did it get plunged & removed to left? If removed, how quickly?

Your description of the situation doesn't make sense to me.

The colder plunged end is certainly not going to "chase" the heat to the "held" end.

cheers Bob

As a industrial heat treat furnace designer I can say without question that what you experienced had to be all in your mind.

Once plunged into a quench medium the material looses temperature. I've tons of surveys to prove it.

Steam may affect your perception. A loud noise (which I once experienced quenching a load by hand in a drum of oil which practically made me mess myself) can make you think something other than what is actually happening.

Dear Henry Kolesnik:

When you are working the bar, it is in contact with the anvil. It can therefore transfer heat to the anvil more easily than it can transfer it up the rod. When you go to quench, you move through the air, over some period of time, and the rod is no longer in contact with the "cold sink". The water quench (if it isn't steam) is only a time delay.

David A. Smith

"Henry Kolesnik" wrote in news:aZjve.2191$ snipped-for-privacy@newssvr11.news.prodigy.com:

It's got nothing to do with the quenching. The reason the rod gets hotter faster is due to the rods orientation. Held in a horizontal direction on the anvil the heat doesn't travel as fast through the rod as it does when the rod is held in a more vertical direction. Heat rises. Also the cross section of the rod is thinner when held horizontally so heat will dissipate more readily into the air around it. Held in a vertical direction the rods cross section becomes its length. That is to say the air doesn't conduct heat as well as the metal rod. But when the rod is held in a horizontal position it has a long thin cross section which heats the air around it. That warmed air will quickly rise (convection) and be replaced by cool air, drawn from below the bar. Now picture the heated bar end down toward the ground and the rod held vertical. The heat will conduct to the cooler section of the bar faster than convection can dissipate that heat. That's why radiators in a hot water heating system are laid out horizontally along the floor of a room, instead of vertically in a corner of the room.

When you quench the end of the bar you are likely sticking the end of it in a pail on the ground. That will take away the heat in the end of the rod but will do nothing to the heat that exists in the bar above the water in the pail. The heat that is left will travel rather quickly up the rod (thermal conductivity) if you leave the bar oriented in a vertical position.

Same holds true in a CNC machine tool. Vertical axes have more thermal displacement than the horizontal axes have.

I wish I could redo the expereience but I have no forge. I'm going to look for a blacksmith site and see if someone there can confirm this. tnx

Dear Henry Kolesnik:

It would be really good if you could put numbers to it. A graph of temperature vs. time...

David A. Smith

Dear D Murphy:

Heat rises in a convective medium, when the change in density of the medium allows the hotter, less dense, medium to rise. How does this work for a bound solid... such as steel? Because in steel, heat simply conducts through it, convects from/to its surface, and radiates from/to its surface. Warmer less dense steel is still trapped in the same physical location.

This is at least plausible. But he said he felt the heat in his palm, where it touched the rod.

That, and you get more even distribution of the heat...

"The rod will be wrapped in a hot air blanket, when held vertical." This is good too.

Some good thoughts...

David A. Smith

Could it be that on heating a " pulse " of heat starts down the rod.

You would normally forge the rod before the pulse arrived at the cold end.

When you quenched the steel, the " pulse " would continue ( weakened ) and have arrived at the cold end when you returned to the bar.

Another thought, more ergonomic. When you're hammering much of the weight of the bar is taken up by the anvil, you are only balancing the end with you hand. When quenching you are gripping the bar might tighter as you are providing it's main support. hence the contact resistance to your hand is much less when quenching?

---------- Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)

But the steam rising along the rod may well chase the heat up the rod.

Assuming only the very end is heated cherry red and the rod is plunged a ways under water its possible that the rising steam and surrounding hot water could be hotter than the rod is at the waterline and therefore heating the middle of the rod. In fact it would it is a certainty that if the rod does really get hotter at a faster rate at the point where its held while quenching then at some point between the hand and the hot end the surrounding temperature must be hotter when quenched than when forged. Steam (and/or hot water) would be the only thing that would likely transfer heat to the middle of the rod fast enough that it would be greater than all the heat being lost at the ends.

-jim

Heat can only spontaneously conduct from an area of higher temperature to an area of lower temperature. You cannot have a "pulse" of heat that travels in one direction. There will be a flow of heat from the hot end to the cold end.

The only way for the cold end to get hotter quicker is if the heat transfer is hastened. For a rod of a given length and a given material, the only way to speed up the heat transfer in this case is to *increase* the temperature difference. There is no other way.

Don Kansas City