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Thread: Heat Treatment Kiln
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11th Feb 2020, 08:01 PM #16New Member
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12th Feb 2020, 08:57 AM #17Most Valued Member
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I was interested, so I took to gumtree to have a search. There are lots of pottery kilns available. Cheapest I found was $250 and it was working.
Most interesting one I found was this one. Is the chair for the mother in law?
https://www.gumtree.com.au/s-ad/nort...lpg/1211923597
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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12th Feb 2020, 10:07 AM #18Golden Member
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My kiln is a benchtop model and is quite small - I believe it was originally designed to do glazes on jewelry, interior is about the size of a shoe box. My main engineering interest is model engines so this size is perfect for me, it is plenty big enough to heat treat the camshaft of my current project a miniatureV8 engine - 25cc.
I think the chair must be for cold winter nights.
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12th Feb 2020, 11:36 AM #19New Member
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Highest temp.
Cheers. I’m looking at a small ward one for heat treating knives. Do you know how hot yours gets? I need to get max around 920C for normalising before heat treating (lower than that). Hoping it’ll reach that temp coz it’s a heap cheaper than a new heat treating oven. Cheers
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12th Feb 2020, 12:35 PM #20Golden Member
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I believe my kiln will get to 1200C - made a V spring for an old family owned double shotgun, kiln had no trouble getting it to bright cherry red with simmerstat set to around 5 - 10 is max. it took me a bit by surprise, I had already heated and quenched it with a blow torch and just wanted to temper it back, it wasn't in there long and when I checked it was a bright cherry red - had to go back through the heat and quench process again. To heat treat wound compression springs setting is around 1.5 on the simmerstat.
I plan to set it up with a temp controller in the near future, I feel this is a must for heat treating metals as you need reasonably accurate temp control, the older pottery kilns used a slump cone that slumped when a certain temp was reached, the cone sat inside the kiln and an arm protruded through from the external controller - the cone slumped, the arm dropped and shut off the controller, not really good enough especially if you need to soak at a certain temperature for 15 or 20 min. I see these are quite cheap now, a "kit" of parts - controller, solid state relay, and thermocouple can be had for under $100 then it is just a matter of a wiring, a suitable enclosure and a higher rated thermocouple if needed, this would solve the trial and error on the simmerstat setting.
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12th Feb 2020, 01:03 PM #21New Member
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12th Feb 2020, 02:32 PM #22Member: Blue and white apron brigade
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I can get my mains gas powered furnace up to about 1280ºC.
Here it is annealing a 100 mm diam billet at 1080ºC
1080.jpg
With both torches running it theoretically 22kW,so it heats up real and recovers very quickly when the door is open.
I reckon most of this energy is just lost as the furnace walls are just not tick enough to retain much heat.
Even with one torch on I can't run inside for more than an hour as (despite lots of ventilation) the shed gets too hot.
I measure the temps to about +/- 5º using an optical pyrometer.
It's a pretty scary thing to operate - not recommended for the faint of heart.
If anyone is interested I have several posts on this and can dig these out.
Am unlikely to need so much power in the future so will probably retire this and look at using a 4kW electrical furnace I have sitting on my back veranda or maybe even a smaller shoe box size furnace.
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15th Feb 2020, 06:37 PM #23Most Valued Member
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Hi Bob,
That's neat. I always wondered how you control the temps accurately enough if using gas when heat treating?
I would love to one day make a heat treating oven. While gas would be easier to make, I always wondered how you accurately control the temps.
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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15th Feb 2020, 07:31 PM #24
Hi Simon, Guys,
In exactly the same way as you would for an electric kiln. You would use a motorised gas valve ! A bit like a gas oven control but with an electric drive mechanism.Best Regards:
Baron J.
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15th Feb 2020, 07:33 PM #25Member: Blue and white apron brigade
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In theory if you can measure it, it should be able to be controlled. In practice it's difficult for a bunch of reasons on my mains gas setup mainly because to measure the temp I have to open the door and that drops the temp. What I need is small port hole in the door to be able to measure the temp without affecting the temp. The other problem is that every time the gas input is adjusted the forced air balance has to be adjusted otherwise the flame can become too oxidising which is not desirable. If it gets too hot its hard to dump excess heat without opening the door and then the bits nearer the door can get too cool etc.
My understanding is it's easier with propane in conjunction with TCs.
The truth is mine was built as more of a forge than a furnace as it just gets bloody hot quickly.
I've used it mainly to anneal stuff but for tempering I have been using the electric oven in the kitchen which goes up to a true 250ºC
I must drag out the old electric furnace (it has proper control etc) that has been sitting on my back veranda for 5 years and see where its at. I believe it needs a 20A SP GPO so I will need a sparky to run a dedicated mains line to it.
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15th Feb 2020, 07:44 PM #26Most Valued Member
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OK. So a forge is used purely to get things hot in a "reasonable" amount of time so that you can then hand forge with either a hammer etc. on an anvil or similar. The temp does not need to be precise, similar to a blacksmiths forge, just get it bloody hot so you can form it.
Whereas a heat treatment furnace needs to be reasonably precise. Question as short notice, if you need to heat a piece of steel to (say) 950 deg C in order to quench it and harden it, what happens if you take it too high and heat to 1050 deg C and quench? Lets assume it's for knife making. I understand if you don't get it high enough, you will not have the required hardness or the hardness may be non-uniform but what if you heat too high?
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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15th Feb 2020, 09:04 PM #27Senior Member
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Up to a point, nothing.
Past that point it will start to burn.
If there is oxygen in the atmosphere it will also oxidise. Actually I guess if there is no oxygen it can't burn but that is hard to arrange. So a gas (or coke) forge has a slight advantage in that it reduces the oxygen available.
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15th Feb 2020, 09:09 PM #28Philomath in training
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Not a lot and subsequently quite a bit.
Over heating the steel should not hurt it, but you are wasting time and energy doing so.
However, one of the important things with heat treatment of steels is the rate of cooling. When you quench you are 'chilling' the steel when certain metallurgical phases are present to lock those phases in and so determine the (room temperature) properties. There are time/ temperature diagrams for steels (different ones for different types of steel), like this one (taken from a knife making site)
TTT-quench2.gif
When doing H/T most of the time you are trying to avoid going through that bump. So, if you have a part hotter than it needs to be and then quench it, it may take longer to get past the bulge because of the extra heat held in the steel. Some steels it does not matter but with some there are literally seconds to get the temperature down. If not, you go through regions you don't want to go to and get properties that you were not planning on.
Sorry this sounds so vague - been a while since I had to think about this stuff.
More thinking - the diagram actually shows things reasonably well. For a start, that arrow represents the rate of cooling. Quench in warm water, or oil, or don't agitate sufficiently, and the rate of cooling won't be as great = may hit the gamma + bainite region. Similarly, imagine instead of starting at 950 degrees, you start at 1050 but can only apply the same rate of cooling. The arrow will be the same slope but being further up the temperature axis, will get the cooling closer to that gamma plus bainite region. If you think about annealling (that is, a really slow cool), you go through that area, so your 'planned' treatment will turn into an anneal like experience.
MichaelLast edited by Michael G; 15th Feb 2020 at 09:17 PM. Reason: More thinking
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15th Feb 2020, 10:57 PM #29Member: Blue and white apron brigade
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One way I do it with a manual gas forge/furnace is start heating at high heat and every few minutes measure the temp of the workpiece and plot the Temp and time on the on a graph. After a while it starts to form a fairly smooth heating curve so you can see where its heading from about 700ºC onwards, then start to dial back the gas and air so that it approaches the desired temp.
Unlike the ubeaut "program temp profile and forget" furnaces at work it is a PITA and I haven't done this for some time but I remember usually hitting about a +/- 15ºC window ie if I wanted 950 I would shoot for 975 and typically get 950-980 and then ease it back from there. Temp changes are relatively slow at these high temps so had to monitor it every few minutes and make small adjustments. It takes up a lot of time so when I had to do this regularly I used the programmable furnaces at work.
It's difficult to get good temp control with small work pieces so I'd do them inside a thick section of SHS that acted like a mini oven.
IMG_5998.jpg
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16th Feb 2020, 06:59 AM #30Most Valued Member
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Thanks Michael & Bob.
Both very informative explanations. Michael, I appreciate the time and effort gone into your explanation of that graph. Thanks!
I have always been fascinated by the art of heat treatment of metal. I find knife making quite interesting.
Simon
Sent from my SM-G970F using TapatalkGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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