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Gerhard

Kiln - to buy or not to buy

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I honestly didn't think I'd get here soon, but recent developments has me considering getting a proper knife makers kiln....

I have severance pay coming my way, theoretically there should be enough left to fit in the kiln.

My reasoning is as follows, I had some bad experience with blades forged from O1 roundbar which air hardened and I couldn't get holes drilled in the tang. Gave up and gave the bar away....

I believe the accuracy of a kiln would allow me to anneal the O1 as well as doing more accurate and optimized heat treats.....as well as opening up the dreaded and despised world of stainless steels :P 

Also, a kiln can be used where a forge of any description simply wouldn't work......

My main concern is electricity usage, how heavy are these things on the juice?

Any pointers, advice or considerations welcome..... 

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I don't understand.  What would you do in a kiln that you could not do in a properly built gas forge?  When you say kiln do you mean a heat treat oven?  With a properly constructed forge and a Pyrometer you can do most anything you need to do with metal.

Let me know if I can help you.

Wayne

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@Wayne Coe I sincerely doubt that, maybe good enough for simple steels, but definitely not the SS types that require specific temps and hold times....?

Also remember I'm at the arse-end of the 3rd world, as well as the wrong side of an exchange rate.

Knife makers over is generally called kilns over here.....same thing I believe......the jobbies with the programmable temp controller hanging off the side.....

 

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I agree, Gerhard.  Even the best gas forge can't be programmed to ramp and hold steady.

There's a couple of good pinned threads on kiln builds here.  If you feel electronically skilled enough it's definitely the cheaper way to go, particularly if you're doing larger items.  I don't know enough about them to build one myself, but I feel sure that can be fixed.  

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WRT the electrical costs:

A heat treat oven will consume it's full rated power until heated up.  A small oven will be at least 1500W, probably up to 3000W for a big one.  (I didn't bother to look up the rating of a large size oven, but it will be listed with the catalog info)

Once heated up, the oven will cycle on and off as it needs heat.  (This is true even for PID control, it just turns on and off more rapidly)  How much time the element spends on is a function of how well insulated it is.  I do not have a proper heat treat oven in my shop, but I have used a lot of similar equipment, and I would say they run between 10% and 20% of the time.

Let's say it takes your oven 1/2 hour to heat up and is a 2000W model.  That is 1/2 * 2000 or 1000 Watt-hours (1KW*Hr)

If you run it for 2 more hours at 20% duty cycle you get another 2Hr*2000W*0.20 or 800 Watt-Hours (0.8KW*hr)

So, that 2 hour run, with extra time to warm, up will consume about 1.8KW*hr of electricity.  I'm not sure about Namibia, but here in the states, you by power by the KW*Hr, and the rate is listed on the monthly bill.

 

Edited by Brian Dougherty
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A Kiln/HT oven will certainly allow you to do things that no forge can realistically manage.

Power input is typically around 3 kW. I have built HT ovens up to 27" long that will reach 1300 degC (in about an hour and 45 minutes) and will run from a standard UK domestic outlet: 13A at 230V nominal rating, so 3 kW. Bigger ones might use more power, smaller ones might use less. 

Lower temperatures are reached much faster. When I first tested the 27" oven from cold (maybe 10 degC, 50 degF), 800 degC, 1472 degF was reached in 22 ½ minutes, 1100 degC, 2012 degF took 54 ½ minutes, the temperature at an hour was 1125 degC, 2057 degF and 1177 degC (2150 degF) took 71 minutes.

As Brian says, once the desired temperature is reached, the PID controller will cycle the power on and off as needed to maintain temperature, so the average power drawn during the "hold" will be less than the heat-up, which will usually be at full power unless you are "ramping". 

A fairly typical HT cycle for O1 might be to heat the oven to 800 degC and stabilize it (call it 30 minutes at 3 kW, for 1.5 kWhr), triple normalize; Work in, allow the temperature to recover, soak for 10 min, pull out and cool to black. Repeat. Repeat. (Call it an hour at 33% power, so 1 hr at 1 kW for 1 kWhr) Then Austenitize: work in, allow temperature to recover, soak for 20 min (call it 30 min at 33% power, 30 min at 1 kW for 0.5 kWhr). Quench workpiece(s) and switch off oven. 

Total power consumed 3 kWhr. 

You might expect to maybe double that for a Stainless HT, but to put things into perspective, unless you only do single blades each time, 6 kWhr is still unlikely to significantly exceed the cost of HT foil. 

The numbers I've given are probably high-side figures. I don't have hard data, but I'm pretty sure the same homebuilt 27" oven was actually showing a steady-state output cycle of only 18% during the 1100 degC, 2012 degF hold section of a Stainless Heat Treat: when I delivered it, the new owner gave it a test run while we drank tea. I use a 2-second output cycle and always have an LED that is switched with the elements. I was surprised the "on" part of the cycle seemed so short and pulled up the output cycle display on the controller. It read 18%.

The heating load of a HT oven is entirely resistive: it does not have the starting surge of a motor load or the horrible Power Factor of some modern electronics. So long as you do not exceed the outlet rating, I would not expect any new and interesting problems with mains power supplies, even in the middle of nowhere. The short-cycling 3 kW output would be harsh on a small generator, were you to run it from one, but I'd not anticipate a problem once the generator gets above maybe 10 kW or so.

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Thanks for the input guys, sounds like a yes.....

I don't know how much the "unit" is that our electricity is sold at, but I do have a meter in my flat so i'll have to test.  Main thing is refund the landlord for any silly usage.....

Maybe fate, saw ovens advertised on a FB group last night half the price of the ones I've been considering, and from a trustworthy source as well......

Only hick-up is I heard today our severance packages will be delayed because the bloody government are taking there sweet time grinding out the necessary paperwork......

@timgunn Fortunately I'm not that middle-of-nowhere, but my mentor is!  No mains power, they use solar, and they only run the generator when it's cloudy because the closest fuel is 50km away.  He uses mostly Elmax, Damasteel and N690, and he can only heat treat on cloudy days when they're running the generator.

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