heat treat oven size

Kevin McGovern

Kevin McGovern

Joined
Jul 31, 2015
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I am starting to gather materials for a heat treat oven build. I am planning on interior dimension of 6.5" wide x 4.5" tall. How deep do most of you think would be appropriate? I can go as long as 24" but I have no current need for anything past 18". I have not made anything exceptionally large, but who knows in the future. I am kind of over building everything, so the only additional cost would be electricity and element. opinions?
 
I made my first ovens 18" long inside. The way I put them together made it easy to build with the length a multiple of the 4 1/2" brick width.

The last one I made was 22 1/2" long.

Using 3000W elements (UK domestic power outlets are 13A at 230V, so 3000W is our limit), there does not seem to be all that much difference in heating time between the 2 sizes and both will easily reach 1205 degC/2200 degF. I think stepping up further to 27" on the 3 kW elements might reduce the maximum temperature to the point where it will not cope with some of the more demanding stainless steels.

I'd built the 18" ones thinking nobody would want to go bigger than a comp cutter at 15" OAL. Everybody now says they'd have liked a bit more length. Including the guy with a 6 kW, 42" oven I built (it used 2 supplies, one for each side element, to overcome the limitations of the UK mains).

On mine, the chamber is 7" wide and 6" tall.
 
Thanks for the input. On another note. I am planning on using one long coil instead of two short ones tied together. Any downside to this?
 
kdogmcg said:
Thanks for the input. On another note. I am planning on using one long coil instead of two short ones tied together. Any downside to this?
Click to expand...

One possible downside is you'll end up taking it around an end which may make that side hotter than the opposite end.

I suggest making the oven larger than you'd think you'd need to, to keep radiant heat from overheating spots on your blades. I wouldn't want any part of the blade within a couple inches of a coil.
 
Thanks Nathan. I was thinking of going with one coil as most builds I've seen has the coil come in at the back, go to the front return to the back and tie to the other one. I'm jus eliminating having to tie the two together by using one long one. If that makes sense.
 
kdogmcg said:
Thanks Nathan. I was thinking of going with one coil as most builds I've seen has the coil come in at the back, go to the front return to the back and tie to the other one. I'm jus eliminating having to tie the two together by using one long one. If that makes sense.
Click to expand...

It seems to me that coils in the back wall, but not in the door, would make the back wall hotter than the opening. I know it's common, and one of my ovens is built that way, but it seems like a design flaw to me. *shrug*
 
I use one coil per side (2 x 110V coils connected in series) and keep the back wall as clear as possible to allow anything long to fit in and not touch, or get too near to, the conductors.

Not heating the back wall so that the front and back behaved the same also seemed fairly obvious. I don't know how much effect heating the back would have on temperature distribution (though I suspect it would be quite small once at temperature) and did not want to do the testing to find out.

I found the easiest way to fit the coils is to pre-stretch them to length, feed them into the grooves, check they are even and staple them in place. They are pretty springy during this process and I have enough trouble wrestling with one side at a time. Trying to do both sides with the bit across the back all at the same time should be quite interesting.

Using the 2 elements ensures the heat from one side matches that from the other and it's only really necessary to keep the coil spacing equal as you go.

The 4 1/2" height seems tight for wall area. In my 6" high by 18" or 22 1/2" wall, I get 4 horizontal grooves and the hairpin bends in which to accommodate the elements. Check with your element supplier how much groove length you'll need and sketch it up to see how it will fit. If they give a range, and you are bent-wire stapling the elements in, bear in mind that it's easier to do and less likely to short out a coil if you stretch to the longer end of the range.

I've had a couple of element failures with 16AWG elements and now fit 1.6 mm (14AWG) elements. They need more groove length than the thinner elements, which is part of the reason I went to the 22 1/2" oven length.
 
Tim:
Any chance you could do a wip? Or photos of what you have done so far? It's always enjoying watching these builds.

Thanks!
 
I always worked on the basis that I'd do a proper writeup once I'd got it about right.

That's still the plan.

In the meantime, here are some pictures of what I've done to date, in more-or-less chronological order.

In the beginning, there was a guy on British Blades who had built a HT oven to Andy Gascoigne's plans http://ftpforge.chez-alice.fr/HEAT_TREATMENT_FURNACE.pdf and was asking for help/advice on the electrics and control. He'd built a couple of nice grinders and lived only 1/2 hr away from me, so I went over with a PID controller and some bits I'd picked up over the years, got his HT oven working and took a good look at his grinders.

I was sufficiently impressed to build my own oven.

http://s667.photobucket.com/user/timmgunn1962/library/Toys?sort=3&page=1

That 1100 degrees is degC: 2012 degF. though I tested it to 1205 degC/ 2200 degF.

I did some testing with it to see how it could be improved: I'm a bit of a geek and I deal with temperature control systems and gas burners at work, so this was in my comfort zone. I had access to some useful measuring and datalogging equipment through work, along with some fairly high-end controllers. My boss was most impressed that I was prepared to spend so much of my own time familiarizing myself with the equipment.

The testing showed a number of things. One was that a ramp/soak controller could make a huge difference to the accuracy of the temperature control. Another was that a short output cycle (2 seconds) helped a lot as well.

The hunt was on for a decent ramp/soak controller at a sensible price. Auber Instruments did not have a presence in the UK, so the SYL2352P was ruled out, leaving the AutomationDirect Solo 4848VRE or the Omega CN7823. These last 2 are, so far as I can tell, the same controller with different badges. I buy whichever is cheapest at the time.

Mk1 was sold to fund the build of MkII.

http://s667.photobucket.com/user/timmgunn1962/library/MarkII Oven?sort=3&page=1

This used a Solo ramp/soak controller and seemed to do pretty well in my testing. It went to a young knifemaker here in the UK who agreed to be my test pilot.

Along with a few other things, his feedback was that it was not conducive to treating multiple blades. The overcentre catch needed both hands to close: pretty nigh impossible when trying to quench a blade and leave the others in the oven. The wiring circuit means that the green start button needs to be pressed after the door is closed. This takes a second or two at best and is easily forgotten: not great.



I was not going to change the wiring circuit. It's as safe as I can realistically make it. There's also a Residual Current Device (Ground Fault Circuit Interruptor?) that's not shown in the diagram. Over here, we have 240V mains Voltage to Ground and I make the ovens as a hobby (I'd quite like to make knives, but it's hard work and ovens are easier). They tend to end up with people I know and like, so I'd rather keep them alive.

I did have a rethink on the door catch though. I wanted one-hand opening and closing with enough closing force to compress the ceramic fiber blanket I use as a gasket.

I came up with a simple F-shaped catch that closed onto a pair of ball bearings (one was too narrow on its own) mounted on a shoulder screw.



It works surprisingly well.

I thought I'd build the oven so that it could be stood on its tail and used with a salt pot. Once I'd built it, I had second thoughts: molten salt and the safety of people I like did not seem to go together. The salt pot (2 1/2" schedule 40 type 310 stainless steel) occasionally gets dragged out from under the bench to use as a former for lining mini forges. It's never seen salt.

http://s667.photobucket.com/user/timmgunn1962/library/Mark 3 Oven with Salt Capability?sort=3&page=1

At about this time, I built a long one using 2 elements each rated at 3 kW and fed from separate mains outlets.

http://s667.photobucket.com/user/timmgunn1962/library/Sword-Length HT Oven?sort=3&page=1

and

http://s667.photobucket.com/user/timmgunn1962/library/Long HT Oven WIP?sort=3&page=1

I was rather lazy with this one and used 1" ceramic fiber board backed up with Calcium Silicate board for the roof. This meant that I could make the floor a full brick-length wide and bridge the roof with the boards, so I only needed to cut 2 bricks for the back of the oven. Everything else used whole bricks. The door was made from the boards.

The last one I built was the 22 1/2" long one. This came about after a discussion with a smith that started after a few beers at a hammerin and was revived some time later when sober. It's basically a normal HT oven with a door on the front and a removable back. I've made all my ovens except the first with separate control boxes, as the first one was a real pain to move. Everything plugs into the box.

The idea with this was that you would have 2 ovens and one control box. Once you've quenched, you plug in the other oven for tempering and avoid the wait while the first one cools down. If you want to make a sword, you bolt both ovens together and use the resulting 45" oven.

I made the first ovenand someone bought it, so I have not actually built and tested the full system.

http://s667.photobucket.com/user/timmgunn1962/library/Mark3 HT oven 23inch?sort=3&page=1

I've also messed about with forges and burners a bit. After building a few burners from scratch, I decided I was never going to get anywhere close to a proper industrial gas mixer for performance and adjustability so settled on Amal Atmospheric injectors from Burlen Fuel Systems here in the UK.

There's a heat-treat Propane forge setup that I've come up with that seems to give near-electric-oven temperature control, together with a reducing atmosphere to minimize scale formation, for long soak times on O1 steel. O1 is the most readily available blade steel in the UK (as Ground Flat Stock). Other steels tend to be hard to find here. I was trying for a cheap-but-good-enough HT setup. The video is from my test-pilot.

https://www.youtube.com/watch?v=1xvWkXBXY6U&feature=youtu.be

816 degC is 1500 degF.
 
More questions. Any advantage to using furnace cement between bricks? I see both two loops of coils per side and one loop per side. Pros and cons or considerations of each?
 
I only tried cement the once. It is not a skill I possess.

When building the first one, I tried it and found that the IFBs sucked all the moisture out of the cement instantly, making it too dry to flow. I really did not want to soak the bricks to overcome this problem, mainly because drying everything out would not have been easy in a Northern English winter.

I found could get a pretty close fit without it and so did not bother with it from then on. The steel angle frame is made as a top frame and a bottom frame. The bricks are assembled and everything is clamped together with ratchet straps before cutting the uprights to fit and welding them in place, so there's not much danger of things moving about. Different IFBs have different tolerances. I have found JM23s from Morgan Thermal Ceramics are both the best-insulating and best-fitting that I've tried. Also the most expensive.

If I was a bricklayer and had the skill, I'd certainly use a jointing compound.

The groove arrangement is simply a case of doing whatever is necessary to accommodate the elements. Element design is pretty complex and needs to be considered together with other aspects of the design. Very few people can do it from first principles, so it's usually a case of looking at what other people/manufacturers have done and not changing too much from that.

I settled on a 3/8" OD element quite early on: it would fit in a 10mm groove and therefore would only reduce the 3" side-brick thickness by a bit over 10mm: say 1/2" to retain as much insulation as possible.

Small coil diameter means lots of loops and a long coil. I think the first 18" oven on 16AWG elements seemed to need a bit over the 36" of groove per side that a 2-groove arrangement would provide. I wanted to keep the electrical connections at the back, so it meant the 4 grooves were needed. For the long oven, 2 grooves were long enough and obviously less effort.

Smaller wire means less of it, so shorter grooves. I regard 16 AWG (1.29mm) Kanthal A1 as the minimum usable wire diameter. There have been element failures (burnouts) on 2 of the ovens I've built with 16 AWG elements and now I've moved to using 1.6mm Kanthal A1 (about 14 AWG or 1/16"). I now buy the elements with doubled, twisted tails and feed through the wall with the tails because I'm not convinced by the stainless through-studs' ability to survive long-term.
 
As far as performance, as long as the total coil length is correct, is there any advantage to two loops per side as apposed to one? I am planning on one per side. The only concern I see is making sure the coil is sufficiently stretched to prevent shorts.
 
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