How about a second hand pottery kiln?

One extra comment about the final temper you are conditioning it to, based on experience from working with large 6082-T6 weldments.

6082 naturally age hardens, if it is left in the 'O' condition for 20 years, it will be pretty much in the T6 condition. If left for another 15 years, it will be harder & stronger still (as we discovered when trying to use some 35 yo surplus stock – it was &^*(^&*(" difficult to work until we annealed it).

According to an old 'Aluminium Federation' publication we have, the HAZ around a weld is back up to T4 condition within a month, possibly earlier, due to this aging process without any further intervention. Strength testing of boxes at work has validated this.

How sure are you of the weld material strength? For 6082 T6, the recommended alloy is 4040, and using various knockdown factors for voids, porosity, slag inclusion etc, we work to a strength of 101 N/mm^2 for the weld strength based on nominal weld x-section, making it considerably weaker than the 6082-T4 strength. On the basis of this, heat treating the weldment seems to be somewhat redundant, as the weld will be much weaker than the softened base material anyway.

Without knowing the limitations of your design, can I suggest that putting a bit more meat on the sections to cope with not being in the T6 condition, as it will be a lot easier than heat treatment and distortion risk after the welding.

Regards,

Richard.

Hi Carl,

I built one back in the late 80s and wrote it up in Vol. 165 in ME (under a pseudonym!). I was interested in heat treating beryllium copper alloys for springs. Be Cu uses a similar precipitation hardening regime to aluminium alloys.

Nowadays the advent of imports from China has made much of that work redundant. You haven't mentioned the size of your projects but I have now bought one of these. It looks fairly crude but works fine and temperature control seems to be good to +/- 1C. They do, other, larger models. Imported temperature controllers are also very cheap and seem to work OK in my experience.

I don't have much experience of ally alloys but Richard's post makes good sense to me.

Cheers,

Rod

The manufacturer of the unit Rod linked to does a whole range, Note their website is not broken, it is just really reallly reaaaaaaaaaaaaally slowwwwwwwwwwwwwww.

I've set my self the task of building a furnace for metal casting and have started collecting bits. I'm aiming to try electric even having been advised to forget it and go for propane which is easier. The usual problem with elements is having them burn out.

The videos are for casting but Myfordboy on Youtube shows some good construction ideas in his later video's. The materials are easily available.. Of late ebay is cluttered with suitable elements even in the higher temperature range. Only problem is that they are 220v so might have problems at 240 if run flat out. There are all sorts of wattages about. Cheapest usually from China.

Some years ago ME had an article on making a small heat treatment furnace using fire clay. One interesting thing he found was that ordinary fibre glass coated thermocouples were ok as the insulation just burnt off. His elements were buried in the fire clay – that's a lot more difficult to do and he did have problems with them burning out. There are refractory cements available that seem to be suitable for this but I've only seen them in the USA and it's not that clear if they are the right thing.

Rather than having masses of refractory bricks it's better to use ceramic fibre for a lot of the insulation.

Out of interest this is the best I have found on a propane burner. It's impressive. I did keep a llink

**LINK**

The general aspects could be used for other things.

An easy to make set of rolls is also shown in use in Myfordboys video. It uses out of phase cranks to rotate the rolls. Now why didn't I think up that – probably bamboozled by gear driven designs etc.

John

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http://www.ebay.co.uk/itm/12V-DC-DIGITAL-PID-TEMPERATURE-CONTROLLER-4-KILN-FURNACE-OVEN-ESPRESSO-MACHINE-/191605597948?pt=LH_DefaultDomain_3&hash=item2c9c9566fc

Cheap as chips apparently….

That sounds familiar! What are you proposing to melt in your furnace? Nichrome and Kanthal wire melts at around 1400C. The protective oxide coating starts to break down at temperatures a bit below this. In theory therefore you should be able to melt cast iron at around 1200C. The problem is controlling the current input to make sure that the windings don't go above (say) 1300C, bearing in mind that the resistance of the wire increases four fold from room temp to 1000C. You also have little margin for error, so covering the wires and effectively insulating them only makes things worse by retaining the heat and not transferring it to the crucible. I think this is why the little furnace I've linked to in my earlier post has exposed elements in the chamber ( the door has a microswitch to cut power when opened ).

I'd be happy to make an electrical furnace to melt ally but cast iron and bronze are probably too much of a challenge for wire wound furnaces to achieve sufficient superheat for casting. You might stand a better chance with SiC elements. I've had these used professionally but can't comment on their cost and availability for home use.

Good luck!

Rod (aka Tom Dobson)

Edited By Roderick Jenkins on 25/06/2015 18:32:52

On Rod's comments I am no expert so it's been a case of looking around at what is made etc. Kanthal A1 appears to be the one used in kilns that are stated to reach 1200C or so. These are what are being sold on ebay. My main interest is in casting aluminium. People tend to say that a propane furnace will melt much more quickly so for electric the aim as I see it is apply the maximum amount of heat possible to melt it as quickly as possible. The catch with the usual propane burner set up is that there is no control so it would be difficult to use it for heat treatment. Make it too big and it will just take longer to heat up. My aim is casting something like a 12in dia ring with a 1in square section for a telescope. It will take a while to make the mirror so no rush.

I don't think it would be feasible to melt cast iron with this temperature limitation or 1300 really and would guess the iron itself would have to be hotter than this to pour. I did talk to some one who had cast iron and he used a blown charcoal furnace – an old hair drier for the blower. and a cast refractory lined oil barrel for the furnace My grandfather used to burn furnace nuts in his hearth sometime in the winter. It will get white hot just from a chimney draught so could even cast steel. If any one wants to try it burn ordinary coal first and when a good layer has burned down stick the nuts on and open the air vent. The coal and draught prevents the grate from melting.

Kilns generally have exposed elements in slots. Looking at lots on ebay I suspect the positioning is decided by trial and error but the numbers can be used to get some idea of the power needed. I also feel that with ceramic fibre it should be easy to reach higher insulation values without things getting too heavy for moving around easily. Other "heaters" enameling furnaces and some glass ones often use enclosed elements. They are covered with some form of castable refractory. I haven't really found any information on what is used but the ME article just used fire clay but this approach is bound to be more difficult and once an element has burnt out the furnace is useless. Goldsmiths etc use as sort of pot . The element runs round the outside of it so would be replaceable. They are listed on ebay but aren't really big enough for my needs.

It would be nice to be able to harden HSS. I did do that once, part of training. Gas muffle so no problems with oxygen and burning. Quenched in oil. As some kilns get to 1300C it might be possible. Lust really with no real need. I have used disk type thread form cutters. They are easy to sharpen and most of the circumference can be ground away sharpening before they are used up. The same thing could be done with silver steel anyway if needed.

John

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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.

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