Electric Kiln

[Jo]

As a Xmas pressie I have been given a little Electric Kiln with a usable internal area is 170 by 100 by 150mm 

I suspect with only 1 KW of heating elements it could take a while to warm up and the temperature control leaves a bit to be desired. So to up grade it I have ordered from China  :

• 0 to 1300 deg C Rex C100 Digial PID Controller Regulator with 240V 40A switching relay.
• 1250 degree K type thermos couple ( & spare)

I am not sure how useful it is going to be but further progress will be delayed until the control items turn up 

Jo
--  --  --  --
P.S. A PID (proportional integral derivative)  controller is a special instrument used in industrial control applications to regulate temperature, flow, pressure, speed and other process variables. It uses a control loop feedback mechanism to control process the variables to provide accurate and stable control 

[Jasonb]

Should be a handy addition, a bit of heat treatment, shrink fitting or warming parts to be bent maybe even a little bit of casting.

What does it toast crumpets like

[pgp001]

Nice gift.......

I am still on the lookout for one of those myself.

Phil

[Allen Smithee]

Those thermos couples will be handy to keep the coffee warm...

 

Looks perfect for heat-treating piston rings so I know where to go when I need some! It also looks like you could potentially use it for oven-brazing (well "oven silver-soldering) small parts, which can be a good way of getting very neat, strong joints. And there are times when we could all use a good, strong joint after a hard day in the office.

AS

[10KPete]

And there are times when we could all use a good, strong joint after a hard day in the office.

AS

Alan, Alan, Alan!!  Nailed it!!

Pete

[Mcgyver]

Jo, have you worked with this make of PID before?  I want to replace mine.  I think I know what to expect with the instructions, so I'm a bit scared of getting one....otoh could it possibly be worse than the old Fuji one it replaces?  The stuff of nightmares!

[Jo]

Its all new to me but my Supplier has the same items for his rather larger Kiln so I am assuming they will work.

The parts are cheap enough on Fleabay but you need to make sure that the controller temperature range matches the thermocouple range otherwise it will not indicate the correct temperature.

I am hoping it will just be a case of connecting it up and off we go 

Jo

[b.lindsey]

That will be a nice addition to the shed Jo. Hope the upgrade goes as easy as expected for you. Keep us posted.

Bill

[Zephyrin]

It looks like one of these enamelling kiln for hobby craft. Certainly a useful item.

You have to be very careful when opening it when hot, as the inside of the door, which must be darn hot, is too easily accessible !

[AVTUR]

Can the kiln be purged with inert or reducing gas? I would love to try silver soldering in a kiln.

AVTUR

[gerritv]

Some have suggested burning a bit of wood in the oven to remove oxygen, keeping the door closed.

Gerrit

[gerritv]

Jo, have you worked with this make of PID before?  I want to replace mine.  I think I know what to expect with the instructions, so I'm a bit scared of getting one....otoh could it possibly be worse than the old Fuji one it replaces?  The stuff of nightmares!

I bought this one for my furnace, it works ok but the settings are not entirely intuitive. Also available via Amazon.ca with faster shipping at similar price. I have several sets of instructions at varying levels of good translation to Canadian.

<a href="https://www.youtube.com/watch?v=i8JRYhprZzo" target="_blank">http://www.youtube.com/watch?v=i8JRYhprZzo</a> shows the process and result. I am not happy with my door design, it will become similar to the one Jo has.

Gerrit

[gerritv]

I am not sure how useful it is going to be but further progress will be delayed until the control items turn up 

Jo

It is an incredibly useful addition to the shop for making tools (hardening predictably) and heat bluing. Not used mine for casting yet, but Zamak has been acquired and the first piece of 3D PLA printing to make a plaster mold with has been done. Once up to temperature it doesn't even consume much lekkie.

[steamer]

I know Keith Rucker has a small furnace that is perfect for heat treat, and it's made by one of his friends in the Youtube community....let me look that up...

Perfect for us engine guys and gals....

Dave

[steamer]

Here we go..   found it

<a href="https://www.youtube.com/watch?v=-arlJUuGJBE" target="_blank">http://www.youtube.com/watch?v=-arlJUuGJBE</a>

Dave

[awake]

In addition to noting the temperature range of the PID controller (and of the thermocouple, and ensuring they can talk with each other), pay careful attention to the type of output. Some have a built in relay ... which sounds convenient, but often the maximum amps is pretty low, and mechanical relays will degrade over time. The one in Gerrit's link has an "SSR output" - which is simply a 24v signal that is intended to drive a Solid State Relay (SSR). This would be the best way to go for a kiln. You can find SSR's quite inexpensively, typically handling 30-40 amps.

[Jo]

The SSR driven relays come as part of the kit I have purchased

Jo

[gerritv]

The SSR driven relays come as part of the kit I have purchased

Jo

Best to plan for a heat sink on the SSR, it gets hot as well. Attached is the schematic that I used as a guide for my furnace, from this site. I used a surplus breaker instead of fuses. The controller has a Live Lamp so don't really need the extra one, etc.. for simplification. I generally use the 'make it work first' before embellishments approach.

Gerrit

[bent]

SS 308 Foil wrap works well for getting clean heat treats.  Some people add reducing salts in the packet.

Used to do bright anneals of inconel parts in a tube furnace with pure hydrogen purge...once again in the rocket shop chem lab.  Not a job for faint of heart, but the lead chemist helped out - rigged a small burner to cook off the trickle of H2 coming out the vent.  Nice way to spend a snowy afternoon, warming your hands by the glowing hydrogen fires.  But the wife (who worked there too as a structures analyst) disagreed, because the roads became jammed due to the snow accumulation, and I couldn't leave until the parts had finished, a 3-4 hour process.  The wife and I eventually made it home, dodging spinouts and wrecks, but it was about 4 hours to go some 7 or 8 miles.

[crueby]

SS 308 Foil wrap works well for getting clean heat treats.  Some people add reducing salts in the packet.

Used to do bright anneals of inconel parts in a tube furnace with pure hydrogen purge...once again in the rocket shop chem lab.  Not a job for faint of heart, but the lead chemist helped out - rigged a small burner to cook off the trickle of H2 coming out the vent.  Nice way to spend a snowy afternoon, warming your hands by the glowing hydrogen fires.  But the wife (who worked there too as a structures analyst) disagreed, because the roads became jammed due to the snow accumulation, and I couldn't leave until the parts had finished, a 3-4 hour process.  The wife and I eventually made it home, dodging spinouts and wrecks, but it was about 4 hours to go some 7 or 8 miles.

Pure hydrogen into a hot kiln - did the person who figured out the rate/mix for that still have eyebrows? And fingers?      Though if this was a rocket lab, I'm sure that there was much more dangerous stuff around. Good place to keep the shop gnomes out of!

When I worked at the research labs at Kodak, in the building with all the chem labs, we learned quickly that if we saw someone in a lab coat running down the hall looking paniced to follow them out of the building! Better to watch the smoke from across the street...  It wasn't till after I left that it became public that there was a small nuclear reactor (materials analysis type, not power producing type) in a tunnel under the gate to the parking lot!

[AVTUR]

I have not difficulty in accepting a Hydrogen purge is safe as long as there is a little flame from the escaping gas but how do you shut the furnace down? Do you let the furnace cool and at a set temperature swap over to an inert gas, Nitrogen or Argon, at a higher pressure?

I believe industry uses a gas mixture of Hydrogen and Nitrogen for the reducing atmosphere.

Is this a process that you can safely do in your back garden? Really is it less safe than a propane torch? Remember the housing density in the UK is quite high.

AVTUR

Just a thought - case hardening in a Butane atmosphere. I used to get cams hardened at work, as foreigners, using such a process.

[Hugh Currin]

I built a heat treat oven a few years ago. The electrical circuit looks a lot like Gerrit's. The main difference is the complexity of putting a disable switch on the door.

Hope it's of some help. Thanks.

[bent]

Yes, we had a 3-way valve IIRC, and switched to an argon purge once the heat cycle was finished. 

This was a one-off job...that morphed into a semi-production process.  We did a lot of stuff in that chem lab.

[Don1966]

Jo that’s thee same thing I did with mine the controller and SSR relay. The controller has PWM output to drive the SSR. The controller can be programmed for either J or K thermocouples.



Don

[Jo]

Nice set up  Don.

I am still waiting for the controller but the Thermocouples have arrived 

Jo

[Twizseven]

Jo,

What is the temperature range of the kiln.  What is maximum temperature you would envisage using for heat treatment.

Colin

[Jo]

Hi Colin,

I don't know the temperature range of the Kiln but Eric seemed to think it would do anything I wanted it to do 

Jo

[Twizseven]

I wait with bated breath to see what 'anything' ends up being.

Colin

[Don1966]

Jo,

What is the temperature range of the kiln.  What is maximum temperature you would envisage using for heat treatment.

Colin

If I remember correctly I got 1900 degree F with mine which is 1037 degree C. Your limited to the K thermocouple range.

Don

[Jo]

I have brought 1250 degree C - K type thermocouples 

Jo

[Mcgyver]

Jo, show us how its done!

I bought the same pid, arrived before Christmas.  Cheap. <$20 cdn.  Much to my surprise it arrived with the SCR and thermocoupler!

[propforward]

You could get a bottle of forming gas and have a go at some brazing.

Good for heat treating too - you only need a small percentage of hydrogen to get a nice reducing atmosphere.

[Jo]

You could get a bottle of forming gas and have a go at some brazing.

  I am not sure what you mean by forming gas. Could you explain.

Jo

[propforward]

Sorry about that. Forming gas is a mix of hydrogen and nitrogen. 5% hydrogen in balance N2 is typical(although other mixes are used), and as a result a very safe mixture. It’s just enough to produce the reducing / cleaning effects from heating in Hydrogen, without the safety (BOOM!) risk. Your furnace runs hot enough to melt quite a range of braze alloys, but you need the reducing atmosphere of the forming gas to do it without flux. Reducing atmosphere just means that the hydrogen removes / prevents surface oxides forming.

But admittedly it’s a lot more messing about. Since you have to continuously purge the oven during the run it can use a lot of gas - and I’m not sure what it even costs these days. But I thought - food for thought maybe. Just another way such a kiln could be useful. In reality a lot of faffing about for the home user, but it’s fun to think of the possibilities anyway.

[Jo]

This is all new to me    Maybe that accounts for why there is a hole in the back as well as the front of the kiln.


China post has successfully delivered me all the gadgets I ordered: The correct temperature range controller, which came with a spare thermocouple of a different range, the required relay and two 1250 degree K type 100mm long thermocouples which are long enough that the body can sit outside the kiln and not melt 

The instructions for the controller are in Chinese and some sort of English that is not any more understandable 

So I decided to remove the back of the Kiln to see what it has: It has two separate heating elements and a device for reducing the power to the heater. So first up remove the controller as I am going for electronic temperature control a I also need to look at how that hole in the back could be used to support the thermocouple.

The two power wires for the heaters will need to go to the output on the relay and the entire unit will also require earthing 

Jo

[gerritv]

Would one of these manuals be better suited to your controller? Same model number.

Gerrit

[Jo]

Thanks Gerrit, much better  .

I had read as far as the bit on the instructions that said "Speech inverse action " and "play a positive action" and decided that their translation leaved a bit to be desired 

Jo

[Don1966]

Jo the controller use two devices and RTD OR THERMOCOUPLE tie your thermocouple TO 9 & 10.  Your kiln has two elements in series should be easy enough and the controller has The SSR output well labeled. RTD stands for Resistive temperature device.


Don

[Admiral_dk]

I do really NOT like the fact that the manual from Gerit and the label (second picture) on the unit itself has very different connections   

Especially the fact that the mains is connected to Pins 1 & 2 on the label and Pin 6 & 7 in the manual.

Is there a wrong label on the box itself or is the manual to a different version of the controller ?

Best wishes

Per

[Jo]

Don I was wondering about that as the pin out does not match the pin out on the previous circuit diagrams I will have to do a bit of further reading.

Thanks for pointing that out Per: The manual that Gerrit kindly provided is for units with more pins on the back than the one I have purchased. The important bit is how to program it is in English 

Jo

[jadge]

The acronym RTD stands for resistance temperature detector. Platinum is normally used as the element, although copper and nickel are also used. The coefficient of resistance with temperature is almost, but not quite, linear. Platinum RTDs are limited to around 600°C, above that the platinum tends to get contaminated. Since RTDs are a resistance device they need an accurate voltage or current source in order to work. Whereas thermocouples are inherently a voltage source.

Most standard ovens, including mine, go to about 1100°C. This is limited by the heating element. Before I bought mine (secondhand on Ebay) I looked at ones that would get hot enough to melt iron. That would require a silicon carbide heating element. The problem with SiC elements is that they're not generally used much below 600°C. So I stuck to a standard kiln.

My oven takes several hours to get up to 800°C or so. Although that can include heating the several kilograms of steel in the cavity. For a heating cycle for annealing, or hardening and tempering, I reckon on about 12kWh of electricity.

Andrew

[Jo]

My oven takes several hours to get up to 800°C or so. Although that can include heating the several kilograms of steel in the cavity. For a heating cycle for annealing, or hardening and tempering, I reckon on about 12kWh of electricity.

So best done on a sunny day then 

Jo

[Admiral_dk]

Your comment about the number of pins Jo, made me realize that this is probably yet one of those projects somebody posted on the net in the public domain => a number of Chinese (might be others too) decides to put it into production as their own .... So you're probably also right about the Firmware being the same => same way to operate the units = manual can be shared for almost all parts 

I have other products from China that are copies too - even to the degree that you have to download the drivers from the original maker / company, for it to work 

[Vixen]

Hi Jo,

If you have a generic controller clone, made in China, there may well be some "How to do it" videos on Youtube. It's worth a quick look to check.

Mike

[Mcgyver]

So...I don't think the REX C100 is the right contoller for this, well at least for me.   afaik it doesn't offer ramp and soak programs, the adverts all call it programmable but it looks like that just means you get to set the temp.

something like this might do the trick:

http://www.thermomart.com/index.php?route=product/product&product_id=222

Anyone have a suggestion on whats the best low cost RS (ramp soak) PID?  Or maybe the REX has it and I've missed it?

[gerritv]

https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20200105171512&SearchText=temperature+controller+ramp+soak has better pricing in CA$. XMTG-8 seems the most prolific model.

I use an egg timer for the soak time :-) the bigger challenge is getting the oven cooled off for the tempering before the part cools off too much after quenching. To solve that I will be procuring a used toaster oven.

Gerrit

 

[Jo]

Thank you Dave for the die cast box 

Having finished setting up my 3D printer it is back to working on the Electric Kiln. Time today was short (I have been working with the brown stuff  ) but I did manage to drill and file out the die cast box for all the fittings: Controller, 2 Neon's, a double pole switch and a single pole switch and on the back the relay, its heat sink and two cable glands. Next job will be to wire it up

Jo

[Jo]

Using a very simple wiring diagram the Kiln has been wired up, with Neons showing when power is applied to the control box and when power is applied to the heating elements. It was then bussed out and the orientation of the Thermocouple found to be the wrong way round using hand heat.

Once fully satisfied with the safe operation of the control box I then wired it to the Kiln and mounted the thermocouple through the available hole. The controller is separate from the Kiln and can be used alongside the Kiln or on top (it won't fit where I intend on storing it if it was permanently mounted on top  )

The inside of the Kiln was cooler than my workshop when I started which proved the fire bricks inside it are good insulators. It also means that even when the temperature reaches the correct temperature and the control box turns off the power the temperature is going to continue to rise before it starts to drop back again. You can see when it is heating by the neon on the right 

Using the controller is very simple: When it turns on the top line tells you the temperature range of the controller. After a couple of seconds the screen changes to show the measured temperature on the top line and the demanded temperature on the bottom line. If you press the "set" putting the bottom line flashes and you can raise or lower the demanded temperature, pressing "set" again and the controller starts using the new demanded temperature as its target.

One more job done now maybe I can get on with making model engines again 

Jo

[Vixen]

Hello Jo

You seem to have the kiln and the controller look to be working well.

What are you going to do with it now? Make toast?

Mike

[Jo]

What are you going to do with it now? Make toast?

I fear it will not be any good for reducing just sugars in the surface of the bread but is more likely to cause it to go solid and I would not trust the internal surface to be non toxic

Heat treatment of metal was the plan.

Jo

[Bobsmodels]

Jo

I think you will find the setup really handy.  I use it to anneal every cast iron piece I get for a model or even making a part.  It takes about a day, ie loading it up, running it and letting it cool down slowly.  In making tooling I use A2 so heat it up to harden and an easy tempering.   I made mine virtually the same as everyone else - PID, SSR, K temp probe etc.  Mine is standalone, ie I just plug the kiln into the unit and leave the kiln on HI ie always on.  I also can connect my PID to some software.  I have various programs that get downloaded to it, eg an anneal program which has all the ramps built in so I just let it go.  I would guess most PID's have the interface.  Mine will work manual mode also I just key in the temps.

Have fun with the addition

Bob

[Twizseven]

Bob,

What range of temperatures are you using?  What is the maximum temp you can achieve and usean
Thanks,

Colin

[Dave Otto]

Hi Bob

When you run A2 do you do the recommended double temper?

Dave

[Don1966]

Looks great Jo I am sure you’ll make good use of it.




Don

[gerritv]

Interesting timing, we had an Amber Alert today about an incident at the nuclear plant in Pickering this morning around 0730 EST :-) Turned out to be a mistaken alert though.

I think you need to run it up to 1200F to burn out whatever crud has accumulated. That will also let you know how long things will take to get up there. Might as well put a bit of steel in there as well to see it glow :-)

What you can then do is time the cool down period. I find with my shop made oven this is limiting tempering, it takes too long to get down to 400F without the part getting cold from the quench (it is apparently recommended to not go below 150 or so.) I have only hardened O1 steel so far.

I have however successfully made some gear cutters so it seems not too critical for my purposes.

Gerrit

Gerrit

[Bobsmodels]

Colin

For Annealing grey iron I use the following program  I ramp to 1600 F at a 1 hour rate.  The PID takes it more or less on an even curve to 1600 over a 1 hr period.  I hold at 1600 for 1 hour, that usually guarantees that the part is fully soaked, if the part is real thick I just increase the hold time.  I have 2 ramp down sequences and really have seen no difference in end result.  Both are ramp to the anneal temp. then 1 hr to 1400, 1 hr to 1350, 1 hr to 1280. 1 hr to 1140, and 15 hours to 0.  The other is 2 hr to 900 and 6 hrs to 0.

*******************
Just for fun and information source:
I got those sequences from reading many years ago, at least 30 yrs, from books on the subject - no internet back then-  used a place called a library!  For the younger ones amongst us that was an ancient form of a server, it had many folders (book racks), and the contents of each folder was accessed by your hand retrieving the file ( a book) then opening it, and using your finger rather than a mouse to turn the pages.  You then used a writing instrument like a pencil to take notes on paper to have as a future reference as opposed to saving the information on your PC.  BTW the pieces of paper usually got lost, just like trying to remember what name your gave the file for the PC.   It was kind of fun got you out of the house for a walk in the fresh air.  Probably took 3 or 4 hours, today 10 to 15 seconds to find it, a few clicks to save it, same time to read it.
***********************

Like I said I just assume the process will  take a day and I just let the pc monitor it so I can see all the temps by time on a graph.  Then the next day I "pop" it out of the oven, some bead blasting of the scale, and then off to layout and machining.

I do not know how hot it will get, I take A2 1800 for hardening

What I have always done is just follow manufactures instructions.   When I purchase any A2, O1, or W1 I get the data sheet for that batch, if purchased from a reputable source the data sheet is available.


Bob

[Bobsmodels]

Dave

I am unaware of the double tempering process. 

I put it in the oven and do a 1hour ramp to 1750F to 1800F, hold it for an hour.  Take it out and set on some fire brick to cool so I can barely touch it - about 150F.  I have my oven cooling down and get it set at the RC temp I want. Usually around 56RC which requires about 900F.  It is a bit of tie getting the oven to cool down that fast.  The part stays at the 900F for 4 hours.  seems to work for me, most tools are one off so as long as they cut for the purpose I am happy. 

I can do other RC values just change the temp as you know.

Could you point me to a source for the tempering procedure you follow.  I have always just used the procedures provided in the data sheet from the company I get the material.

Thanks

Bob

[Mcgyver]

I made mine virtually the same as everyone else - PID, SSR, K temp probe etc. 

Bob, what PID did you choose?
thanks

[Hugh Currin]

My circuit is about the same, I got most of it from the Internet. I did put a switch on the door that de-energized the coils when the door opens. I thought it a safety issue. No clue what the codes are, as if they applied here. What are the thoughts on this disconnect, needed or not?

Jo:

I think the overall temp in the furnace must start down when the coils are turned off. What you see must be due to the temp distribution inside the furnace? It could be the coils are hotter than the interior and continue to put out heat when de-energized? The area around the thermocouple must continue up for a short time.

When I bought my controller I thought all contemporary controls had time/temperature set points. When it arrived I found it "only" held the set temp. So, all the above time/temp curves have to be done manually. Not a real problem, still a very useful shop tool. Congratulations on resurrecting this one. I'm sure you'll find it very useful.

Thanks for posting the thread.

The inside of the Kiln was cooler than my workshop when I started which proved the fire bricks inside it are good insulators. It also means that even when the temperature reaches the correct temperature and the control box turns off the power the temperature is going to continue to rise before it starts to drop back again. You can see when it is heating by the neon on the right 

Jo

[Dave Otto]

Dave

I am unaware of the double tempering process. 

I put it in the oven and do a 1hour ramp to 1750F to 1800F, hold it for an hour.  Take it out and set on some fire brick to cool so I can barely touch it - about 150F.  I have my oven cooling down and get it set at the RC temp I want. Usually around 56RC which requires about 900F.  It is a bit of tie getting the oven to cool down that fast.  The part stays at the 900F for 4 hours.  seems to work for me, most tools are one off so as long as they cut for the purpose I am happy. 

I can do other RC values just change the temp as you know.

Could you point me to a source for the tempering procedure you follow.  I have always just used the procedures provided in the data sheet from the company I get the material.

Thanks

Bob

Hi Bob

I don't remember the exact data sheet I was following but if you follow this link and scroll to the bottom there is a PDF data sheet for A2 that indicates a double temper. I remember the one that I was using also had a (do not cool below a certin temperature) before doing the first temper. I might have 150 degrees indicated in this PDF.
https://www.crucible.com/Products.aspx?c=DoList

Dave

[Bobsmodels]

I was asked what PID I used.  Since I was only going to make one of these I spent a little more $ than ebay.  I also wanted to be able to monitor the cycle and have the ability to have programs.  Now I think most PID can do this you just need the spec sheet and some software to interface.  That being said here is what I ordered back in feb 2012 From Omega

CN 7823 PID Controller
SSLRL240DC25   solid state relay
Finned Heat sink for the SSR
Custom K probe - wire length, plug, and length of actual probe ( I did this so it fit into my oven nicely and was certain to be a match for the PID)
RS485 to USB cable to connect the PID to the PC (cost almost as much as the PID, could have got it cheaper elsewhere but I anticipated using OMEGA support so I wanted all components from them)

Software was just a free download, not sure how PID specific it is, maybe you could fake it out with a non OMEGA PID.

I had lots of questions and they got me through everything. 

Bob

[Mcgyver]

Bob, thanks for the detailed answer
Mike

[Bobsmodels]

The inside of the Kiln was cooler than my workshop when I started which proved the fire bricks inside it are good insulators. It also means that even when the temperature reaches the correct temperature and the control box turns off the power the temperature is going to continue to rise before it starts to drop back again. You can see when it is heating by the neon on the right 

Jo

There should be a mode on the PID that lets it learn your oven, on mine it is called Autotune.  Basically the PID figures out your ovens characteristics.   I attached an early sequence I used for annealing, as noted in post 55 above I have changed the anneal sequence.  Before I did the Autotune the PID overshot the temperatures.

Just like any tool took a bit of using it to get it efficient for me.

Bob

[Bobsmodels]

Dave

Thanks for the link.  That site has lots of good information.  The A2 was close to what do.  Clearly  a double temper, I wonder why?

Bob

[nj111]

Only just seen this interesting thread.  Three or four years back I made a heat treat oven as my existing shop one is very small.  I needed to heat treat 300M axle half shafts (about a 1100mm long and 40mm diameter). No one in the UK seemed to want to process them due to the shaft length  and 300M must be oil quenched - of course vertically for a shaft - so I made an oven.  I wound 20 metres of 18 swg Ni Cr wire for each of the three heating elements around a 6mm bar in the lathe and routed grooves in the fire bricks to accommodate it. Used a similar temp controller to Jo.  The oven drew 21 amps (single phase). I think the required temperature for 300M (from memory) was about 870'C then quickly quench. The transfer from oven to vertical quench in oil of such a long shaft (handled manually) and glowing bright red was interesting.  It worked well though. - Anyhow I needed to reduce scale as the shafts (already splined each end) were soaked at the heat treat temperature for a couple of hours. I used what I had available (with no research) which was a gentle Argon Purge at 1 litre per minute and about 5 psi using my TIG gas.  Perhaps there was a down side to using Argon, but the four shafts were perfect, and the vehicle went on to win a European off road race series championship the following year - and still even now those shafts are perfect. Previously half shafts had been a nightmare for this chap as they kept twisting at every event (450Hp V8 and 37" tyres).  What I'd like to know is was I wrong to use Argon only? Nick

[Vixen]

Hello Nick,

Considering you built the kiln, vertically quenched 4 foot long. bright red. glowing haft shafts in oil; which then went on to win a European off road race championship. I think you did remarcably well. The Argon purge seems to have done it's job, you may have been able to get away with using less gas, but , what the hell, it worked. I am constantly amazed at the ingenuity of 'model' engineers in solving full size problems. 

Bravo Bravo

Mike

[nj111]

Thanks Mike.
I've never had great results preventing scale when heat treating things like small camshafts etc in my little oven, (about 8" x 4" x 4" capacity). I previously tried wrapping in stainless foil, also used the brownells powder (glass like powder you sprinkle on to the preheated parts) which was better but a lot of messing around and expensive.  As Argon worked on those big half shafts I may try it in the small oven in due course, unless anyone here can advise that it's not a good idea? That would use very little Argon due to the small capacity of the oven and the fact that small parts are not in there too long. (1 hour of heat treatment soak per inch of thickness being the rule I usually use).  Nick