Heat Treatment Oven

I think it is false economy to try and build a single oven for heat treat and casting and everything else.

Heat treat requires tight temp control. They can be made from soft fire bricks. If you don't go mad, then they can last a long time. If you use them to melt things then you will soon consider them as consumeables.

Most other things don't need such precise control. Most other things can be achieved with cheaply made propane forge. The main challenge is the high heat. Soft firebricks don't last long at casting temps.

The most controllable propane burners can be reasonably cheap - "a 1/2” Amal gas injector costs about £40 from Burlen Fuel Systems in Salisbury." These burners come in a range of sizes and can be adjusted for a massive range of temps. The only problem is that you can't automatically adjust them for temp so you can't have programmed heat treat cycles of the flexibility you can have with electric.

A blown propane burner costs almost nothing and is suitable for melting things. All you _need_ for control is a needle valve, not even a regulator. You don't need jets - just get propane into a tube and blow air through it. Ideal control of mixture is more tricky so the Amal can be better where the atmosphere of the forge/furnace is important.

For melting, you will need hard refractory lining. That is the major cost.

A bag of 1500C castable refractory works out at around £25. I found a very significant amount of high temperature ceramic blanket left over by some one who built a pizza oven. That would have cost rather more but still not out of the question. No idea why he used the high temperature grade. I also have another bag of dubious refractory that was very cheap. Should be ok but have to rely on the honesty of the seller so if I have to use it then it needs to be an easily replaceable panel.

Commercial propane furnace etc torches are very expensive. I've looked at several diy designs. Many seem to be concerned with the size of flame rather than it's colour. The one in the video I linked to looks very good to me. I haven't any information on using other types of commercial burner.

Precise temperature control doesn't seem to be a problem but for some things I can see that the radiated heat getting up to temperature could be a problem. Sand is sometimes used to cure that.

My main interest is casting. If the electrics wont work out I'll have to make a hole in it and use a torch. Element costs of ebay - I now have a fair selection, cost a few quid. To get a feel for it I will probably build something small first just big enough to get small bar to red heat. I have used something like that in the past to forge and harden a rather large screwdriver. To use it, fasten to the bench, plug in and turn on. The hot part was carried on a pole about 30jn long to take care of heat problems. There is a need to be careful what small enameling furnaces are mounted on. I'd guess that the unit I used just had an element similar to those used in electric cookers in it. No control but I would be inclined to power via a dimmer switch.

One problem is weather. Castable refractories have to be dried out slowly. Cool damp times of the year are best. Gives me time to think of a sensible way of casting element grooves in them. The Myfordboy video's I mentioned have some interesting ideas on handling them including adding vibration to remove air pockets etc when they are cast.

John

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Melters usually use induction heating, all the current carrying stuff is cool (even water cooled sometimes). No I'm not offering to design one, way out of my league, but I've been involved in projects using them. Quite fascinating watching the metal melt for no apparent reason. I suspect this is outside amateur use, the power requirements were prodigious

The Amal makes as good a burner as any other commercial furnace burner I know. It really is good. I have a small one. A few friends have bigger ones. They just need a pipe.

But for melting -it would be difficult to make a blown furnace burner that doesn't work. It is easy to make a burner for a furnace, operating in a confined space. The detailed designs only have to be considered if you want a stable flame in free air or you need fine temp and mixture control.

My experience of castable refractory is that you can use it any time of year, Just leave it to dry in the garage for a week then dry it out with a gentle flame for a few hours. I had to buy a pallets worth of supplies so my refractory bill was over £100 delivered.

The knife people really know a lot about heat treat and melting stuff (e.g. to make wootz).

Thanks for the info Jack. The burner shown in the video is stable in air and also when used in the forge he also shows but doesn't give any details on.

I'm hoping that the ceramic blanket material saves on the refractory needed. i'd guess that the volume of a bag of it is roughly equivalent to how much material can be cast. Some people seem to be using blanket for forges and nothing else so it should give pretty good heat insulation.

I have a couple of crucibles that were given to me some time ago which are probably a bit full with 2 lt in them but should be ok for 1 1/2. Not sure what to do size wise yet but I'm thinking in terms of going for kiln style with a front opening door probably with a plug in the top for adding more material. Not what most do but to me that gives a bit more flexibility size wise than looking for suitable barrels. So probably size it on a bigger crucible than I have.

Materials can be found here but I bought my bag off ebay.

**LINK**

Checking my ebay purchases I actually have the 1700C material but didn't pay that much for it. I also have a few insulating fire bricks that have been around for some time. They should be ok to make a base for it if I coat them with the high temperature stuff.

While electric makes temperature control easier it wont be easy to judge the wattage needed. I can get a rough idea by looking at various kilns especially glass furnaces at the sort of size I am likely to finish up with. Rather than go for one element I've gone for several of the same wattage so hopefully I can sort of tune it. eg 5 1kw elements worked out at £9 from a UK seller. 3 800w from the east £5. Should leave some spares. I'm almost bound to burn some out but a type of dimmer switch might prevent that happening while I'm messing about. The initial low resistance might blow these up but I can probably sort that out with a heftier triac.

John

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I'm all for people experimenting and hopefully finding out something new (that's where the knowledge about Amal burners comes from).. So do it!

However, I'd suggest the current 'best practice' for DIY melting iron is blown burner into a top loading cylindrical furnace .... like a gas bottle lined with hard refractory. A cylinder of blanket, and nothing else, will do - seen it done - but you probably throw the blanket away afterwards.

If you get one of those temperature controllers Rod linked to and a 50A SSR (eg Crydom from Farnell for instance), you have a >10kW digital, self-tuning controller for £50. You'd need a thermocouple admittedly but that's not going to be much more. If you are using 3-phase, you only need 2 SSRs, albeit it a higher voltage (500V instead of 240V)

If you are feeling a bit more adventurous, you could try an IR T/C which mimics the output of a thermocouple and allows non-contact temperature measurement. You may be able to get something similar from Ali Express or similar these days. The output characteristic isn't quite the same but for the range you are interested in it's generally not an issue. I used one of these in a furnace to control the temperature of PTFE hoses that we were convoluting and reinforcing and really there weren't many other options.

Merry

Merry,

You will need a high temp IR device as the cheap "normal" ones run out of range about 600/800 deg C

Mark

PS. For accurate heat treatment use we used to use optical hot wire pyrometers

Never worked up there myself but that range extends up to 1100C, apparently ("the OS36-2 and OS36-5 models can be used to target temperatures of 1100°C".

I would have thought the problem with hot wire pyrometers is that it's very difficult (expensive) to make them automatic ie provide a suitable signal for a process controller. The IRTC generates a simple voltage without any active circuitry.

Merry

Merry,

Yes, you are correct and there are IR devices that go way up. The bigger problem is probably matching the emissivity values and targeting accurately but these are all things that can be achieved with thought (they are done industrially, it is just a matter of scaling the cost I suppose to match DIY budgets and aspiration!).

The hot wire was used as a QC check as the furnaces all ran on TC probes which makes it easy to maintain the temperature and log furnace conditions.

I have never worked with steel beyond H/Treatment temperatures but have had some experience with automatic glass processing on optical glass moulding. I also got a a number of (very in-depth) visits to an optical glass production facility that was very interesting.

Mark