Electric Kiln

[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