Muffle furnace

[FulmenParticipant @fulmen]

@SOD: Yes, the carbon is dissolved in the steel. But how does the carbon get into the steel? Carbon isn’t volatile so simply being in contact won’t do much. This is where the CO comes into play. It acts as a carrier that can diffuse into the steel and deposit carbon.

And there will be oxygen/CO2 available. First of all there will be air left in the case, and coal and charcoal does contain some chemically bound oxygen. But they will also add carbonates to the case as a source of CO2.

[noel shelley]

On noel shelley Said:

Hey Sonic, The thermocouple you need is an Inconel sheathed K type, 3mm dia will be good and of whatever length you think best. They are rated for the temps you want, I use them in dip pyrometers at 1150c or more. Good Luck. Noel.

Yes, that is exactly the type I was considering. And maybe a grounded version to have faster response.

Meanwhile I ordered this USB thermocouple interface. I liked their API documentation. There are even VB.NET examples. This is great since this is my favorite programming language for Windows. I didn’t used it since 2000’s and it changed a lot. But Copilot, Microsoft’s AI, is very helpful. I even put it to write the PID algorithm 🙂

To control the solid state relay I’ll use the RTS pin from the serial port. Is easy to access it from code and no extra HW is required.

Now I’m abusing the AI to write the code for a nice graphic way to program the ramps and temperature points.

 

 

[Howard LewisParticipant @howardlewis46836]

In Carburising the iron or low carbon steel is packed into a box, sealed and heated at high temperature for a time. The longer the time, the deeper the carbon penetrates into the iron, increasing the carbon content.

The carbon atoms are mixed with the iron atoms. Cast iron, on the other hand has carbon particles amidst the ferrite particles. (Which is why it is easy, but filthy to machine)

Other additives such as Cobalt, Vanadium or Nickel are added to the steel when it is in an open hearth furnace, (Which will have a reducing atmoshere) and samples can be taken to ensure the desired composition is produced.

Howard

[Sonic EscapeParticipant @sonicescape38234]

It looks like three materials are suitable for the furnace. Bellow I gathered their most relevant properties. The numbers might vary depending on specific product. There is no significant difference between them. Except that the vermiculite is heavier. Also there is no significant difference in cost or availability. Hard to decide. Vermiculite is available in a hardware store close to me and I’m going tomorrow to see it.

I incline more towards vermiculite since it has less health hazards. I think the other two create an irritating dust when machined. Also since it is heavier maybe vermiculite is also harder. Also I found a glue for it that works up to 1300°C

 

[jaCK HobsonParticipant @jackhobson50760]

You can make your own insulating refractory with bentonite cat litter, vermiculite, and a little cement.

[Nigel Graham 2Participant @nigelgraham2]

Would the “little cement” be better as lime, or lime-rich, mortar rather than purely Portland cement? I have a vague idea of reading this in connection with domestic chimneys.

[Sonic EscapeParticipant @sonicescape38234]

There are available bags of refractory cement already made. You need only to pour water. And it can be used up to 1300°C. But they have much lover thermal conductivity, in the range of 1W/m*K. I guess this is normal since the density is also much higher at around 2000 kg/ m3.

[FulmenParticipant @fulmen]

My plan is to cast an outer shell of refractory cement filled with either leca, vermiculite or perlite (some testing needed there I guess) and face it with straight refractory.

[BazyleParticipant @bazyle]

The cement needs to be that sold in tubs as ‘fire cement’ not regular builders stuff.

There used to be a firm advertising in ME for hobby level oven supplies. Think the name began with W. Might go look this evening if nobody else comes up with the answer.

There is a staggeringly expensive ceramic paint I think based on Zirconia that reflects heat and is apparently worth the expense for hight temperature work.

[noel shelleyParticipant @noelshelley55608]

Gentlemen ! In the context of high temperature,ie 1150/1200c, been there ! There are 2 types that will work well and not cost the earth, 1 is ceramic fibre and 2 is K23 fire brick. The latter is very easy to work with or form. Intended for pottery kilns Etc it comes in numerous shapes and sizes and can be glued into position with air setting cement. How well does it work = I would run up the furnace to white heat for 20mins and be able to pick it up put it in the back of the car afterwards, the wall is a single brick thick, was made 30 years ago and still has the paint on the outside. The whole furnace weighed about 17Kg. When thinking about the cost of building DO NOT forget the cost of running, a cheaper lining will cost more fuel ! The traditional fire cement lined furnace would use 4 times as much gas as the K23lining did. I costed the bricks a few years ago at about £3 each and 20 would make a furnace to take an A6 crucible. In passing, the body was the casing of a Creda spin drier !

Ceramic fiber is good but needs to be treated carefully, it is fragile even if rigidizer is used.

For electric elements one can use old stock electric fire elements if you can find them to make a small heat treatment oven and for Fulmen and ally it will work but it is important to heat the metal as fast as possible so gas is better.

For smelting to prove the point I set 8 bricks on end in a circle (bit like stonehenge) and used a sievert 2943 (I think) burner in at a tangent and melted the brass to do a casting – you don’t need a furnace per se !

I’ll help if I can, Good Luck. Noel.

[ChrisLHParticipant @chrislh]

Alec Tiranti of Newbury / Thatcham ? Like you said begins with a “W” !

[noel shelley]

On noel shelley Said:

Gentlemen ! In the context of high temperature,ie 1150/1200c, been there ! There are 2 types that will work well and not cost the earth, 1 is ceramic fibre and 2 is K23 fire brick. The latter is very easy to work with or form. Intended for pottery kilns Etc it comes in numerous shapes and sizes and can be glued into position with air setting cement. How well does it work = I would run up the furnace to white heat for 20mins and be able to pick it up put it in the back of the car afterwards, the wall is a single brick thick, was made 30 years ago and still has the paint on the outside. The whole furnace weighed about 17Kg. When thinking about the cost of building DO NOT forget the cost of running, a cheaper lining will cost more fuel ! The traditional fire cement lined furnace would use 4 times as much gas as the K23lining did. I costed the bricks a few years ago at about £3 each and 20 would make a furnace to take an A6 crucible. In passing, the body was the casing of a Creda spin drier !

Ceramic fiber is good but needs to be treated carefully, it is fragile even if rigidizer is used.

For electric elements one can use old stock electric fire elements if you can find them to make a small heat treatment oven and for Fulmen and ally it will work but it is important to heat the metal as fast as possible so gas is better.

For smelting to prove the point I set 8 bricks on end in a circle (bit like stonehenge) and used a sievert 2943 (I think) burner in at a tangent and melted the brass to do a casting – you don’t need a furnace per se !

I’ll help if I can, Good Luck. Noel.

I’m not sure what is K23 fire brick. Here we have JM23 bricks. It might be a similar thing. JM23 works up to 1260ºC. There are available up to JM30 that work above 1500ºC. The higher the temperature, the higher the density and thermal conductivity.

Meanwhile I bought this calcium silicate board. It is interesting. Very light and it can be machined easily. The only thing I don’t like is that is covered in a very fine dust. I suppose if I’ll try to make a groove for resistor I’ll make a big mess. I got also a 1500ºC glue. It works very well with the silicate board. I think I’ll make a combination of bricks and silicate plate.

I was thinking a lot about what resistors to use. I would like to avoid mounting them in the wall. So ideally must be some self supporting type. I found some MoSi2 Heating Elements.

The nice thing is that only two holes are required. The active part the is the smaller diameter. But they came with serious issues. They have very large variation of resistance with temperature. They require low voltage at the beginning. And are expensive. Even on Aliexpress a 2kW one can jump above 200$.

 

Then there are SiC heaters. Again, very easy to mount. And are much cheaper and available locally in many shops. But they have ridiculously low resistance, I’ll need a transformer. And can’t be connected in series because the resistance is changing a lot during lifetime. I think the only advantage of SiC and MoSi2 is the much higher working temperature, up to 1800ºC

In the end I think I’ll just buy some 400W Ni80Cr20 ceramic elements. The cost is ridiculous, 1.5euro each!!! I suppose they will die quickly.  I could rewind them with kanthal wire. Or simply buy more 🙂

After the initial enthusiasm I dropped the idea to write a PID control software. While it would be interesting realistically I just don’t have enough time. Instead I’ll order this PID controller. It has ramp & soak with multiple segments, just what I wanted. It can also drive a SSR with PWM with programmable period.

[noel shelleyParticipant @noelshelley55608]

Hi Sonic, K23 is a very light weight insulating brick, the best seem to be referred to as American K23. JM23 is a type of insulating brick and JM30 is also. The number refers to its max operating temp in hundreds of degrees Fahrenheit. For what you need 2300*F is more than enough. Good luck. Noel.

[Sonic EscapeParticipant @sonicescape38234]

How soft is the steel heated at 1000°C ?  I bought 4 x 500W ceramic resistors and I’m thinking to make a steel frame to hold them in place.

The blue parts will be made from welded 10x10mm square bars (structural steel). And I’ll add 4 x 5mm threaded rods through each resistor to keep everything together. The high temperature steel creep could be an issue?

Also if I’ll connect the wires of the four resistors with screws can I expect any surprises? Maybe some serious oxidation? Should I use some stainless screws?

[jaCK HobsonParticipant @jackhobson50760]

I think you want the electrical connection outside the furnace 🙁

I guess steel will wilt a bit at that temp but my guess is that oxidisation will be a bigger problem – it will turn to scale over time.

 

In my large oil drum oven, I made a grill/shelf out of welded rebar. The rebar did wilt a bit and eventually some welds failed. But not really an issue.

[jaCK Hobson]

On jaCK Hobson Said:

I think you want the electrical connection outside the furnace 🙁

Ideally yes, but the resistors don’t have long enough wires. They are not designed to work at such high temperature. I have to connect them to some temperature resistant conductor material. Maybe a stainless steel bar?

[not done it yetParticipant @notdoneityet]

What is wrong with Nichrome bar?

[SillyOldDufferModerator @sillyoldduffer]

Afraid I don’t like idea of a steel frame inside the furnace supporting the elements!   Five reasons:

1. A metal frame is unnecessary.
2. A heavy steel frame will absorb heat, wasting valuable watts that should be cooking the charge.  The furnace will take longer to get up to temperature and cost more to run than it should.
3. Depending on the alloy, mild-steel melts at between 1200 and 1500°C.   That’s uncomfortably close to the furnace’s top-temperature of 1000°C.   The metal will certainly soften, and although Sonic’s design looks suitably stiff, it’s likely to sag under it’s own weight, or allow the heater fixings to pull out.
4. Cycling the steel from room temperature up to 1000°C will mechanically stress the frame due to thermal expansions and contractions.  I’d expect the frame to distort over time.
5. Steel heated to 1000°C will corrode!   Not dramatically, but…

Supporting the electric elements between insulating firebricks solves all these problems.  A kiln I looked at a few years ago had fluffy fibre insulation inside around which was spiralled a Nichrome wire.   The wire was held in place with short loops of Nichrome anchored in the fluff by bending them.  Very simple, extremely lightweight, and easy to repair.

Dave

[SillyOldDuffer]

I made some calculations to find out what diameter to use (for Kanthal A1 wire). In my understanding the important criteria is the resistor surface load. So I made a spreadsheet to find out the surface load for different diameters given a desired power.

As you can see bellow (c) the maximum surface load for Kanthal A1 at 1000°C is 4W/cm². So the sweet spot for wire diameter is 1.4mm. Larger diameters are better but also the wire length is increased. Surface load varies also a little with the way the wire is mounted. But I’ll pretend I didn’t knew about this.

On SillyOldDuffer Said:

Afraid I don’t like idea of a steel frame inside the furnace supporting the elements!   Five reasons:

Yes, I dropped the idea to add steel inside the furnace to support the resistors. I found a way to support the ceramic rods using only the calcium silicate walls.

[Sonic EscapeParticipant @sonicescape38234]

Today I used the lathe to make a coil from the 1.5mm kanthal wire. I don’t know how else to do it. It is very stiff.

I spread the wire across the garrage, living room an kitchen, almost 30m. There were wire loops everywhere. When the last meters of wire reached the lathe they collected some shoes and other stuff 🙂

One strange thing happened. While coiling the wire ar some 40-50rpm the 30mA house RCCB turned off the power! I thought the moving wire coils touched something in the living room. Maybe I have something that is not grounded and has damaged insulation, but I couldn’t find what. The lathe is not grounded (yes, I know) and has it’s own 10mA RCCB. Weird.

 

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