Heat treating at home (salt pot or temperature controlled kiln)

Shaw Blades

Shaw Blades

Joined
May 18, 2009
Messages
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Pretty soon I'm either going to build my own heat treating oven off of the plans on the web or maybe buy a salt pot heat treat set up. Is there any advantage over either one?
 
Unless you have a very specific need, a HT oven with a controller ( electric or gas) is far more useful than a salt pot.....and much safer.
When you have every major tool and device you can think of, and can use them all proficiently, then a salt pot may be in line.
 
Why not build your own HT oven and give it salt pot capability. This is mine;

DSCF0040.jpg


The oven will work horizontally and this one will take an 18" long workpiece. It will stand on end for use as a salt-pot. The control box is separate, so it doesn't matter what orientation the oven is in, and can be used for other things as well; I'm thinking of a tempering oven and a heating jacket for diffusion bonding Mokume in a workshop press. I've already done a small electric crucible furnace.

P1210161.jpg
 
The green cable on the left?

That's the thermocouple connection.

It should actually be pink to match the socket, for a type N thermocouple.

I use plugs and sockets for all the connections; mains in (as usual), power to the elements, the thermocouple and the door-switch circuit.

The photo was taken when using the control box to test-run a 42" sword oven I'd built. I'd had to add a second (slave) circuit to the control box to run the second element; It's a 6kW oven and UK mains outlets are 230V, 13A, so I needed to use 2. The second circuit is in the yellow cable, just what I had at the time. I used a green type K extension cable on the thermocouple because it was what I had and it let me put the box on the bench, with the oven on the floor, for comfort. The cable on the thermocouple itself is only 1M long.

P1220165.jpg


The control box will still run the smaller oven.

Having everything just plug in makes it easy to play with. If using a Ramp/Soak controller, the controls can make up around half the cost of building an 18" HT oven, so the plug arrangement effectively halves the cost of a second one and makes separate ovens for hardening and tempering affordable.

The controller will actually talk to a PC, and I used the logging on some early testing, but I think it's through an RS485/422 interface and I don't bother any more. It would be worth using if running performance tests on HT programs though.
 
Only problem I see with using one single channel controller with multiple devices is that it will need to be "re-taught" before each initial use. If not you run the risk of it either oscillating or over shooting temps.

If it's a multi-channel unit you can use a discrete channel for each device.
 
Although it's a single-channel controller, I don't find it much of a problem.

For the "odd" things, it's a case of having to manually enter the PID settings before use. It's not a problem as long as they are written down after the first autotune run, preferably on the oven/furnace/whatever.

For using heat and temper ovens, they can be made identical, which overcomes the problem, but I take care of it in the controller.

The AutomationDirect Solo controller that I use has 4 different sets of PID values and automatically uses the set that were tuned at the temperature closest to the setpoint. If I tune the controller on the tempering oven at, say, 150 degC and 350 degC, then change to the Austenitizing oven and tune at 500 degC and 900 degC, storing the PID terms in the controller, I can Austenitize my blade at 790 degC with a correctly-set-up controller, then plug in the tempering oven and temper to 230 degC, also with a correctly-set-up controller.

http://www.automationdirect.com/adc...ers/1-z-4_DIN_Size_(SL9696_Series)/SL9696-VRE

http://www.automationdirect.com/adc...ers/1-z-16_DIN_Size_(SL4848_Series)/SL4848-VR
 
You can buy a vertical, stacked kiln through Evenheat. Tim Zowada is the dealer. Buy one, put a really big iron pipe inside it to add thermal mass. Mine stays within 5degF of the set temp after it warms up and settles down. It is a wonderful too. You could put salts in the pipe later, or have two pipes. One with, one without. I go without, because this level of control is plenty for what I need. The blade comes up to temp with the oven, after the oven settles in, let the blade sit another 5 minutes and you are golden.

Or, put the blade in hot, I allow 10 for a 5/16" thick sword blade to come to temp after the oven settles back. The oven does actually tend to cycle at about 2F cooler than the set point, but that is easy to fix. You can heat treat anything up to 34" blade. Just let the tang stick out the top and wrap everything and close it down with kaowool. Yes, I have done this. Yes, it works fine.

I love this thing. Not very expensive compared to the competition either.

IMG_0612.JPG


The frying pan on top is a joke, I cover with kaowool. It was just for that certain touch of class.
kc
 
Tim,
Great points. You are correct that documenting and using the specific PID parameters will prevent the potential problems I mentioned, and will work for any PID controller. That is the proper way of dealing with it.
Also, the controller you have is excellent for the way you are utilizing it and elimates this need for at least certain tasks. Some of the simple controllers the folks here have been using don't have the multiple routine capacity, that's why I mentioned it the issue - I should have given the solution as well.
However I do have some insite on "identical" pieces of equipment. They don't always behave identical. You can build 50 identical pieces of precision heating equipment and about 45 of them will behave the same. The others will require additional tuning. Been there, done that more than once.
 
I'm with you all the way on "identical" not actually being so.

The Solo was the cheapest controller I could find that seemed to be able to do the job well. It's not perfect, but I feel it's adequate.

For tempering, I use a ramp/soak program. This allows me to slow the rate-of-temperature-rise to a degree at which precise tuning isn't absolutely essential.

There are lots of little things that may not be obvious to most people, but make a big difference to how well a HT setup works. I think I'm starting to get to grips with some of them. Most folk get a glazed expression when I talk about it though.
 
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