Heat Treatment Oven

[Dr.Al]

FWIW, the common arrangement is a second thermocouple in the kiln connected to a separate over temperature trip that knocks the power off. It offers protection against T/C shorts, controller faults, etc. as well as SSR failure.

Worth having a programmable controller, I would have thought.

Personally, I wouldn't put sodium silicate anywhere near it - it will start to dissolve those bricks as a fairly low temperature (<<1000°C) and generally make a sticky mess. I really don't think you have to worry much about those bricks - they're easily abraded and become dusty, but put up with a lot of abuse otherwise. (Technical manager at a glass works for ~12 years.)

Thanks, that's useful. I'll plan for the second thermocouple & trip.

Understood that putting sodium silicate in the oven is bad, but what's sodium silicate & why would I be considering putting it near the oven?!

 

[Misterg]

Understood that putting sodium silicate in the oven is bad, but what's sodium silicate & why would I be considering putting it near the oven?!

Sorry, I was picking up on this comment:

A surface coating of alumina grit and sodium silicate binder should make the bricks more durable, as would refractory cement coating.

Sodium silicate is used as a rigidiser for ceramic fibre insulation (amongst other things). It will form a glaze with the insulation bricks that will just make everything sticky at high temps without any appreciable benefit (IMHO).

 

[timgunn1962]

I have built a few (I think 9?) HT ovens for knifemaking, though the last was maybe 7 or 8 years ago. I have also built some gas-fired HT forges specifically for HT.

There was a learning curve. I suspect I've made most of the mistakes you'd like to avoid.

For small and fairly infrequent stuff, I'd strongly suggest going gas for the Austenitizing temperature range and using the domestic oven for tempering.

With a properly-adjustable Venturi mixer, you can get very precise manual control of temperature.

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The precise mixture adjustment allows the air to be progressively throttled, reducing the flame temperature and making the forge atmosphere more reducing, until the desired temperature is reached. Once set, it'll hold that temperature more-or-less indefinitely. The burn is rich and very reducing for Carbon steel Austenitizing temperatures. It MUST be done outside because the Carbon Monoxide produced will kill in a confined space. The forge atmosphere is sufficiently reducing that scaling in the forge is prevented and it even seems to significant limit decarburization.



Domestic ovens usually show some temperature fluctuation. This can usually be damped out by burying the workpiece in dry sand, preferably alongside the thermocouple.

If you really want to go electric:

Whatever you do with the PID controller, don't get too anal about wiring it up neatly on the back. Assuming a 48 x 48mm, 1/16th DIN controller, they all have basically the same connections, just not necessarily in the same layout. Leave an extra 50mm or so on the wiring so you can change the controller later. You can start with a basic one and upgrade to a Ramp/Soak unit if/when you feel it is appropriate.

Ramping to temperature is one way to limit overshoot at tempering temperatures. The bury-in-dry-sand trick is another. It's worth having a good think about whether it's something you need to control for the sort of stuff you do.

Your proposed oven looks very small to me. You need to consider how much surface you have to accommodate the elements. Thinner elements burn out faster. Thicker elements are longer, as well as thicker, for any given power. If you have a decent-sized site tool transformer that'll let you run on 110V, that can halve the length of any element needed.

My first few HT ovens used 16 AWG Kanthal A1 elements (1.29mm diameter) and the ovens were 18" long internally. After a few element burn-outs, I went to 1.6mm Kanthal A1 and a 22.5" internal length to accommodate the longer elements, then to 27" internal length. I've always aimed to stay just under 13A for about 3 kW. I wouldn't go less than 16 AWG for elements, having gained some experience.

Don't let Sodium Silicate anywhere near IFBs.

Genuine JM23 IFBs are worth the money.

If you get a 2.4mm welding and cut it with a pair of non-flush-cutting pliers, you can chuck it in a drill and use it to drill through IFBs, then cut it off and leave it to pin them together. This'll probably hold your brick configuration and save the faff of cutting them. I'd alternate between clockwise and anti-clockwise so the joints are staggered.

Hopefully, pics have attached showing a small forge at temperature (target was 1500 degF, 816 degC) and a couple of in-build pics of an HT oven.

 

[Dr.Al]

Thanks, that's useful advice. Will have a ponder.

 

[Kram]

Sodium sillicate is a binder, and as I understand it, very commonly used to make IFB's as its one of the better binders. Slightly alkaline when wet. Non reactive once set, atleast below its 1200 melting point.

It is also an excellent protective coating for the outside of crucibles (not bottom or inside), I am still using my first and its showing little signs of degrading.

The difference for higher temp bricks is they just have a progressively higher alumina content.
I'm yet to see my perlite mix turn sticky or disolve, but if you get it too hot it will melt back. Perlite is excellent insulation however and reasonably cheap, and easy to repair. My furnace has 1" wall of clay then 2" of perlite and at 950c inside, the outside is about 50-80c.

Perlite is also extremely light. A dustbin full of the stuff might be 1kg or so. If you have space I'd suggest a metal dustbin as container. IFB's as the base, a castable refractory hot face with the element and fill the remaining space with perlite.

 

[oldbikedavey]

Sodium sillicate is a binder, and as I understand it, very commonly used to make IFB's as its one of the better binders. Slightly alkaline when wet. Non reactive once set, atleast below its 1200 melting point.

It is also an excellent protective coating for the outside of crucibles (not bottom or inside), I am still using my first and its showing little signs of degrading.

The difference for higher temp bricks is they just have a progressively higher alumina content.
I'm yet to see my perlite mix turn sticky or disolve, but if you get it too hot it will melt back. Perlite is excellent insulation however and reasonably cheap, and easy to repair. My furnace has 1" wall of clay then 2" of perlite and at 950c inside, the outside is about 50-80c.

Perlite is also extremely light. A dustbin full of the stuff might be 1kg or so. If you have space I'd suggest a metal dustbin as container. IFB's as the base, a castable refractory hot face with the element and fill the remaining space with perlite.

Not sure that the melting point of cured sodium silicate is 1200C...
1200 F, very likely, but not 1200C....

 

[Kram]

 

[Misterg]

atleast below its 1200 melting point.

I'm pretty sure that the 1089°C melting point will be for a specific crystaline compound sodium metasilicate (Na2SiO3). Commercial 'Sodium silicate' aka waterglass, etc. isn't that exact compound - the Si:O ratio isn't an exact number. Further, it is hydrated and mixed with water and doesn't readily crystalise - on drying / heating it forms a glassy substance that doesn't have a well defined melting point. It starts to soften at ~~600°C and gets less and less viscous.

It won't react with the graphite in your crucible, so a coating will stop the graphite burning away; I can believe it won't do too much to perlite because perlite is pretty inert stuff - mostly silica, but it does react with the common '1260' grade insulation bricks (I speculate that this may be because of their calcium content, but it's only speculation).

Horses for courses.

 

[Onoff]

Consider how the element will be retained, as I have blown several where it has fallen out of the slot. With @Onoff 's help we designed a spiral 3d printed former for the refractory clay element housing but I have not got around to it yet ( working space issues).

Some pics from back then for context:

https://flic.kr/p/2n5kwCP

https://flic.kr/p/2n5jioD

https://flic.kr/p/2n5kCqq

https://flic.kr/p/2n5kBC8

https://flic.kr/p/2n5jYVp

 

[Kram]

I'm pretty sure that the 1089°C melting point will be for a specific crystaline compound sodium metasilicate (Na2SiO3). Commercial 'Sodium silicate' aka waterglass, etc. isn't that exact compound - the Si:O ratio isn't an exact number. Further, it is hydrated and mixed with water and doesn't readily crystalise - on drying / heating it forms a glassy substance that doesn't have a well defined melting point. It starts to soften at ~~600°C and gets less and less viscous.

I use an unsupported disc perlite/sillicate as my furnace lid, it has done several brass castings at over 950c, it has not gone soft or floppy yet, supports weight of a full cruicible while hot. I have not seen the issue that you describe.

 

[Dr.Al]

Following some extremely helpful feedback from the folks on here, I've updated the circuit diagram. The main changes are:

1. The addition of a second thermocouple, driving a temperature monitor (exactly what form this takes is still to be determined). The temperature monitor is there to cut the power to the heating element in the event of the temperature getting too high (i.e. if the SSRs fail). I've shown this as driving an electromechanical relay as it won't need to switch rapidly and is more likely to fail open-circuit, but it could also take the form of an SSR.
2. The addition of a silicon-controlled rectifier (SCR), which could be used to reduce the power into the heating element if desired. I'm not sure whether I'll fit this straight away, but it's good to plan for its possible inclusion at a later date.
3. I've changed the schematic symbol for the SSRs to make it clearer that there are two wires from the PID controller.

For now, I've left the second SSR in the circuit. In theory, the over-temperature relay makes the second SSR redundant (and reduces the risk of latent failures where one SSR has failed short and there's no way to tell, meaning I keep using the oven until the second one fails). However, it seems sensible to plan for it even if I don't eventually fit it.

The heating element will be made from "Kanthal A1" resistance heating wire, which is rated for use at temperatures up to 1400°C. I am currently planning on using 1.4 mm wire. Most oven builds I've seen use 1.2 mm or 1.3 mm wire. I couldn't find a source for 1.3 mm wire so the options were 1.2 mm or 1.4 mm. Thicker wire should mean better longevity (i.e. longer before I'll have to take the wire out and replace it). The disadvantages are that it'll take a bit more effort to wind it into a coil and that it's a bit more expensive. The resistance of the 1.4 mm wire is slightly lower than that of 1.2 mm wire (0.92 Ω/m vs 1.28 Ω/m), so this will need to be taken into account: with the 1.4 mm wire I need slightly more wire to get the same overall element resistance (which further adds to the cost over and above the fact that the thicker wire is more expensive).

The overall resistance of the wire is chosen based on the target oven power or current. I'd like the oven to run off a standard 13 A wall plug, although I have a 32 A single-phase socket in the garage, so it wouldn't be the end of the world if I had to go for higher currents. If I aim for 10 A (which I believe is a sensible maximum safe continuous current for most standard UK sockets) then that means I want 25.3 Ω total resistance (the mains RMS voltage in the UK is 230 V ±10%, so the maximum RMS voltage is 253 V and Ohm's Law gives the resistance). That would be 27.5 metres of 1.4 mm wire (or 19.8 metres of 1.2 mm wire). The wire would be formed into a coil, reducing the physical length of the element while increasing its diameter. It's worth noting that this calculation only includes the current in the heating element and the 10 A target may have to be reduced a bit depending on the current consumption of the various bits of control gear (assuming they are all powered from the same plug as the element).

The updated circuit diagram above includes a phase-angle controlled silicon-controlled rectifier (SCR), which could be used to reduce the RMS voltage and thereby reduce the current for a given length of resistance wire. However, if possible, I'd like to design the system to be able to cope with full mains voltage across the element, even if I then decide to lower that voltage for actual use.

The power output of the oven at 10 A and nominal voltage (230 V) would be 2300 W. If I design for a 150 mm × 150 mm × 200 mm internal space, then that's 4.5 litres and hence the power would be 511 watts/litre. By comparison, the commercial Evenheat LB 18 and Paragon DB 18" heat treatment ovens work out as 188 watts/litre and 222 watts/litre respectively, so that suggests I could lower the current without having to worry too much about having enough power. Lowering the current means increasing the length of the wire (to increase the resistance) or reducing the RMS input voltage with the SCR. Those calculations are likely to change after I've revisited the brick layout (which may change the volume of the cavity).

 

[Dr.Al]

I haven't done much more thinking about the heat treatment oven over the last few months (life went a bit sideways as a result of my other half having a bad car accident), but the weather is starting to cool down and that makes the idea of this project a bit more appealing. I'd like to get this built over the winter so I need to make some more progress with the design.

The highest priority outstanding design question was the layout of bricks. I had some feedback on my first design that highlighted the unsupported bricks at the top of the cavity. Removing that issue while keeping roughly the same cavity size (to suit the intended use) and wall thickness (to keep the overall weight down) meant that I had to design it to have a lot more cut bricks. I've never handled insulating fire bricks before but from what I've read / watched it seems that they're relatively easy to cut so hopefully the large number of cut bricks won't cause too much of an issue.

This is the new layout of the body (which should weigh about 14 kg before all the metalwork and insulating wool is wrapped around it):

The middle "slice" is a mirror of the front slice (i.e. the notch in the top brick is on the left rather than the right). In an ideal world, those four corner pieces will all come out of a single brick, but I plan to order quite a few spares in case of problems.

This is the layout of the back (which will also match the door):

If my maths is right (and there's minimal wastage) I think I can make the structure (including door) out of 19 bricks. At £2.76 each that works out as £52.44 for the bricks. In practice I'll order quite a few spares so it'll end up more expensive than that (but then I'll have allowance for breakage and/or non-optimal use of bricks). I'll also need to buy the ceramic fibre blanket to go around the outside, some fireproof rope for the door seal and some sort of mortar (perhaps this stuff or this stuff - I need to do more research here) to stick the bricks together. I think that'll be everything from Vitcas but there are lots and lots of other bits needed from other suppliers. I'll probably order the Vitcas stuff fairly soon as it's often helpful when designing things to have some of the materials to hand.

Speaking of ordering stuff, I have now bought the first part of the oven: the Kanthal A1 heating wire. As described in the previous post, I went for 1.4 mm diameter wire. The cavity in the new design is the same size as in the old so the calculations in the previous post should still be valid. Those calculations suggested 27.5 metres of wire. The disadvantage of using the thick wire and hence having a long length is that it might end up being quite a lot of coiled element to fit in the relatively small cavity I've designed - something I'm putting off thinking about for now! I had the choice of 25 metres or 50 metres so erred on the side of caution and bought 50 metres. This is what it looks like as delivered:

I need to form that into a heating element coil for use, but I'll leave that for a while (as it takes up less storage space in a loop than it will when formed into a coil). I can't put it off too long as it could affect the brick design (if there's simply too much to position in the cavity) or force me to use something to reduce the current (e.g. an SCR or a step-down transformer).

I've had a very kind offer of a very fancy PID controller and some other control gear from a fellow member of the forum, so I'm in the process of working out exactly what the control schematic is likely to look like (in a bit more detail than previously described) and also looking at the capabilities of that controller to see if there's anything else that would be interesting to include. The controller manual is 465 pages long so there's quite a bit of reading to do! The controller has multiple inputs and outputs (to allow monitoring various things as well as the SSR control). I've updated the schematic to try to take advantage of some of those features (click for a bigger view):

The idea of the new schematic is that the controller can monitor temperature at two different parts of the oven. It only has one method of controlling the temperature of the oven (via the heater), so it'll probably use one of the thermocouples for temperature control and the other for logging and tracking. I'd wondered about using the second thermocouple input as a safety shut-off, but I think I prefer the idea of having a completely independent (and much simpler) over-temperature shut-down system so I've left that as a third thermocouple.

As the controller has inputs that can be used as current inputs (I'm guessing they're 4–20 mA but I haven't read the manual enough to know) I should also be able to use a third input to monitor temperature (again just for interest) and a fourth one for a potentiometer input (i.e. a control dial). Combined with the various digital inputs and outputs, I think this should allow me to have four different operating modes (with different programs), such as:

1. Typical heat cycle for hardening (i.e. heat it up and hold the temperature).
2. Typical heat cycle for tempering, possibly with potentiometer based tweaking (i.e. heat it up to a lower temperature and hold it there for a set time, then cool).
3. Typical heat cycle for melting brass (similar to #1 but a different temperature).
4. Heat up to a temperature set by the control dial.

That might be stretching my ability to program the controller but if it can do all of that (and I can figure out how to make it happen) then it could make for quite an easy-to-use but versatile bit of equipment. Of course there will be nothing stopping me reprogramming the controller to do something completely different if needed.

The disadvantage of all that is the number of parts to design / buy or otherwise get hold of (and then do something with) is getting longer and longer, so I reserve the right to dampen my ambitions a bit!

 

[Misterg]

I've never handled insulating fire bricks before but from what I've read / watched it seems that they're relatively easy to cut

They are - a cheap wood hand saw will cut quite a few bricks.

You might want to make your sides symmetrical to make the roof stronger and avoid it cracking like this (the bricks can be quite friable once you start cutting into them):

(It would also mean you need to cut 4 bricks, not 8)

I wouldn't worry too much about the corners (green arrow) - they don't add much to the insulation.

Are you going to clamp it all together with a metal frame?

 

[tigdlo]

I went for 1.4 mm diameter wire.

Did you calculate the surface load of the element?

 

[Dr.Al]

You might want to make your sides symmetrical to make the roof stronger and avoid it cracking like this (the bricks can be quite friable once you start cutting into them):

View attachment 470402

(It would also mean you need to cut 4 bricks, not 8)

Hmmm... okay, I'll have a think. I made it that way to avoid having unsupported bricks. If I make it symmetrical there won't be as much support on each side (38 mm instead of 76 mm), but I guess it would probably be enough.

I wouldn't worry too much about the corners (green arrow) - they don't add much to the insulation.

Are you going to clamp it all together with a metal frame?

Yes, definitely. I haven't designed that bit yet. There will be a frame (probably made out of angle iron, possibly with some sheet of some sort in the gaps) with ceramic wool blanket separating the sides from the bricks.

Did you calculate the surface load of the element?

I didn't, but I will now you've made me look up what "surface load" means . A quick back-of-the-envelope calculation suggests it'll be okay (about 1.9 W/cm² which looks okay if I'm reading the right graph on the Kanthal website), but I'll write up my justification for that in a bit more detail.

 

[Jelly_Sheffield]

I'm pretty sure that the 1089°C melting point will be for a specific crystaline compound sodium metasilicate (Na2SiO3). Commercial 'Sodium silicate' aka waterglass, etc. isn't that exact compound - the Si:O ratio isn't an exact number. Further, it is hydrated and mixed with water and doesn't readily crystalise - on drying / heating it forms a glassy substance that doesn't have a well defined melting point. It starts to soften at ~~600°C and gets less and less viscous.

It won't react with the graphite in your crucible, so a coating will stop the graphite burning away; I can believe it won't do too much to perlite because perlite is pretty inert stuff - mostly silica, but it does react with the common '1260' grade insulation bricks (I speculate that this may be because of their calcium content, but it's only speculation).

Horses for courses.

Based on experience with high temperature incinerator refractory, the presence of group 1 metals or metal salts will act to significantly reduce the melting temperature of high temperature refractories and cause a slagging action.

As far as I know, this starts near the injection lances, due to redox reactions between the binder in the refractory, and free group 1 metal ions/radicals in plasma form (a flame being a very specific form of plasma), but can then proceed away from the flame due dissolved sodium ions in the molten slag.

Whilst not of such interest for a electrically heated furnace, it would certainly be worth bearing in mind for a gas operated one, especially when designing any interfaces between high-alumina brick linings, and RCF secondary insulation using a sodium silicate as a rigidiser.

For small and fairly infrequent stuff, I'd strongly suggest going gas for the Austenitizing temperature range and using the domestic oven for tempering.

With a properly-adjustable Venturi mixer, you can get very precise manual control of temperature.

...

The precise mixture adjustment allows the air to be progressively throttled, reducing the flame temperature and making the forge atmosphere more reducing, until the desired temperature is reached. Once set, it'll hold that temperature more-or-less indefinitely. The burn is rich and very reducing for Carbon steel Austenitizing temperatures.

Out of curiosity what stopped you from taking the next logical step and automating the mixture control using a Flow Control Valve and a PID?

For very high temperature operation fired heating does make economic sense, and you get some of the benefits of a controlled atmosphere furnace (albeit without the ability to nitride or carburise without putting parts into a salt bath/graphite pack).

It MUST be done outside because the Carbon Monoxide produced will kill in a confined space

I found an interesting way round this issue that also reduced fuel consumption when I designed a small cupola that was intended to use a reducing atmosphere like yours some years back (I forget the specific application now, it was some project to do with trying to recover unused feedstock from superalloy production wastes).

The main combustion chamber vented into a secondary combustion chamber at the back, where a secondary air feed would allow it to burn up the CO and release additional energy, then flow through an labrynthine path down an annulus between the main chamber, and an outer refractory wall.

The main air feed for the burners passed through a similar labrynthine path in an annulus between the outer wall, and the outer refractory of the secondary combustion chamber, allowing the hot chamber wall to preheat it to a substantial degree.

Preheated combustion air significantly reduced the energy lost to heating the 70% inert components in air, and gave an effectively higher flame temperature and thermal output once the furnace was up to operating temperature.

I seem to remember that the secondary chamber went to a dedicated flue, and the installation had CO sensors mounted in the room around the cupola which would initially alarm at 20ppm (the 8 hour exposure limit for CO), then shut the gas supply off via a safety relay at 100ppm (the 15 minute exposure limit for CO).

I might even have the CAD drawings somewhere still on a hard-drive.

Whilst building a safety instrumented system would go well beyond most people's idea of DIY, most of the other elements of the design could easily be copied in a home workshop.

 

[Jelly_Sheffield]

It MUST be done outside because the Carbon Monoxide produced will kill in a confined space

...

For small and fairly infrequent stuff, I'd strongly suggest going gas for the Austenitizing temperature range

I found an interesting way round this issue that also reduced fuel consumption when I designed a small cupola that was intended to use a reducing atmosphere like yours some years back (I forget the specific application now, it was some project to do with trying to recover unused feedstock from superalloy production wastes).

The main combustion chamber vented into a secondary combustion chamber at the back, where a secondary air feed would allow it to burn up the CO and release additional energy.

...

I might even have the CAD drawings somewhere still on a hard-drive...

most of the other elements of the design could easily be copied in a home workshop.

Rough sketch of the arrangement from memory.

I forget the specifics about why it used co-current rather than counter-current heat-transfer, I think it was a deliberate attempt at limiting the Pre-Heat of the primary air to allow the use of an off the shelf burner assembly.

If making your own burner to suit, it would be easy enough to move one of the baffles to make it counter-current and get even more combustion air pre-heat.

Edit: Looking at old emails, compared to a non-recouperative design, the thermal efficiency increased from about 26% to around 50%, and we were able to increase the max operating temperature by nearly 200°C (to about 1600°C!).

 

[daleyd]

Rough sketch of the arrangement from memory.

View attachment 470463

I forget the specifics about why it used co-current rather than counter-current heat-transfer, I think it was a deliberate attempt at limiting the Pre-Heat of the primary air to allow the use of an off the shelf burner assembly.

If making your own burner to suit, it would be easy enough to move one of the baffles to make it counter-current and get even more combustion air pre-heat.

Edit: Looking at old emails, compared to a non-regenerative design, the thermal efficiency increased from about 26% to around 50%, and we were able to increase the max operating temperature by nearly 200°C (to about 1600°C!).

It’s kind of how glass furnaces operate - they also reverse air flow every 20 minutes so the heat doesn’t get pushed to the end of the regenerator stack. One bank of burners fire for 20 mins then everything shuts off, the air flow reverses and then the other side burners light and so on.

 

[oldbikedavey]

There is, as my Dear Old Pa used to say , a difference between scratching your bum and tearing the skin off in strips.....

 

[Jelly_Sheffield]

It’s kind of how glass furnaces operate - they also reverse air flow every 20 minutes so the heat doesn’t get pushed to the end of the regenerator stack. One bank of burners fire for 20 mins then everything shuts off, the air flow reverses and then the other side burners light and so on.

That's interesting to know, sort of a cross between a Regenerator in a old-school Blast or Open Hearth Furnaces, and a Recouperator in a Thermal Oxidiser, or Fired Heater.

The glass industry is one of the very few I haven't had very much contact with, so it's cool to learn more about it.