Heat Treatment Oven

Has anyone built a simple heat treatment oven? I'd be interested in hearing about it.

I want to have the capacity to heat treat aluminium weldments. As welded, 6082 T6 becomes T0 in the heat affected zone. If I take the part up to about 200 deg C in a kitchen oven, then let it slowly cool, I can get it back to the T4 temper (precipitation hardening). However, to get the whole thing back to T6 I need to heat to in the region of 500 deg C (solution treatment). Then quench in water, then precipitation hardening treatment as above,

I'd be intrigued to find out if anyone has a good hack of an existing bit of kit to do this.

Carl.

You could get 500C in a pyrolytic oven. Search for pizza.

http://forums.egullet.org/topic/103127-hacking-pyrolytic-ovens-to-utilise-high-heat/

Kife makers know about home made heat treat ovens.

http://www.britishblades.com/forums/showthread.php?155732-Building-a-Heat-Treatment-Oven-Information-Database

On that forum, search for posts by TimGunn1962. He is the god of diy heat treat ovens (amongst other things).

How about a second hand pottery kiln?

I have not made one, instead used my end of year bonus at work to contribute towards a kiln for my wife's precious metal clay work. It has a programmable controller and will hold +/- 5C at 800C. This is good for glass sintering, precious metal clay, Raki pottery, silicone mould conditioning for doing small polyester casts AND heat treatment of metals (Shh, don't tell her!). It is one of these **LINK** (didn't get it from them, but it's the same kiln), Ones with programmable controllers are worth getting. Really worth getting.

Regards,

Richard

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.

I have someone promising me an article on an arduino-controlled oven, but it may take a little while.

It's certainly do-able, but the challenge is getting things like hysteresis and time-constants right to make sure the temperature is controlled accurately enough.

Neil

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

Hello all and thank you for the great contributions.

Richard's post does indeed make great sense. I was basing my design on the T4 temper UTS which is about 205MPa. The application involves heat that I can get down to about 150 deg C. According to MIL-HDBK-5J, for this family of alloys this temperature takes the UTS down about 70%. If as you say Richard, you use 101MPa for the weld metal, then derating this for the elevated temperature condition makes it a non starter. I was looking at using aluminium because at first glance it seems like an attractive proposition from the standpoint of excellent thermal conductivity strength and ease of machining. It would also bring considerable cost savings. However, the drawback is the strength deterioration due to welding. I had thought that the weld metal would return to the T4 temper as well, clearly not. I was going to do some testing with a home made tensile tester to verify this.

One last thing, I have been reading an Italian authored report on tensile testing of 6082 welded pieces, using 4043 filler. This is what I was going to use. They report that the UTS on fracture of 6 pieces was around 170MPa, and that of the 6 pieces, 5 failed in the HAZ and only one at the weld. So maybe the weld is stronger than supposed?

Richard, could you possibly give me a reference for the aluminium federation document you mentioned? I might find it online. And lastly, it seems you and your wife have very complimentary hobbies!

Thanks again,

Carl.

Hi Roderick,

I just clicked your link and I reckon that the unit there would be spot on if i was going to do some material testing of heat treated and non heat treated welds. In fact, it is nothing like as expensive as I thought, so I might just do it anyway, out of sheer curiosity.

Oh no, another project.....

Carl.

For completeness, here is the report I alluded to above:-

http://www.teksidaluminum.com/pdf/18-1-3.pdf

It is by the University of Rome Mechanical Engineering Department.

Thanks,

Carl.

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.

Carl,

You are correct. Mind went blunk over the weld alloy number.

We do use a figure of 101 N/mm^2 for the following reason:

'BS 8118: Part 1: 1991. (Structural Use of Aluminium, Code of Practice for Design. BSI.) states that the weld limiting stress is 190 N/mm² for 6082-T6 alloy. A weld efficiency factor of 0.85 and a quality reduction factor of 1.6 is applied for welds not prepared in accordance with BS 4870, giving a weld proof stress of 100.94 N/mm². The Aluminium Federation Publication, “The Properties of Aluminium and its Alloys”. (Eighth (Amended Reprint) Edition, December 1983.), page 72 states that the ultimate strength of the HAZ (the weakest part of a weld) is that of the alloy in the T4 condition for alloys heat treated to the -T6 condition. Thus the ultimate strength of a HAZ in welded 6082-T6 is 190 N/mm².’

It is a bit of a cludge & assumes the welder is to commercial standard. If assuming aerospace standard welding, I'm sure the full strength of the weld material could be used, though with my stress engineer hat on, I always try to assume the welder has an off day.

Regards,

Richard.

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....

John Haine,

That sounds like a simple solution but the SSR (thyristor) end might be a tad more complex?

To all,

Out of interest (and I know this does not apply to the original aluminium application) what is being proposed as atmosphere in this HT oven and what quench material - and more interestingly, how is it getting parts from furnace to oven without exposure to air?

Mark

Meant furnace to quench tank!

Edited By Mark C on 25/06/2015 17:48:22

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

The linked-to purpose built temp controllers work and have appropriate control logic to apply ramps and damping (built in feedback).

I have one almost identical to the linked-to one controlling a gas-fired heat treat oven I made (oil drum size for temps around 800C). It is easy to set up and works perfectly.

More expensive ones (still cheap) can have a cycle programmed in - so a pre-soak, ramp up, soak time, and ramp down.

The arduino project is interesting... I'm a fan but I think you would get a better, cheaper solution from one of these cheap controllers.

I think the most challenging thing for long term reliability for the DIY kilns I linked to is the longevity of the element wire where it makes connections - has a tendancy to break. People have suggested work-arounds.

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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John,

That all sounds do-able to me, well perhaps not the HSS. There is so much more stuff available via ebay and the web than there was available to me when I was messing about in the 80s.

Technicalsupermarket sell spare elements for the furnace I linked to above. A couple of those in parallel would probably heat a crucible with enough ally to cast your ring. A cheap PID controller driving a solid state relay should finish the electrical side - they are self tuning.

Cheers,

Rod