Making a ceramic kiln into a gun parts kiln.

[xtriggerman]

Years ago I bought a used Vulcan 111F on Ebay for about $275 if my memory serves me right. I really didn't know what needed to be done to it to make it work out for a parts hardner and mostly a unit that could color case harden receivers & parts. I knew it could easily provide the 1500 degrees F that is the highest I would need but just what was its failing. Well I found out pretty quick that the thermostat is pretty bogus on a ceramic kiln. It needs constant monitoring and fiddling with the super imprecise thermostat regulator. At the time Vulcan wanted about $475 for a refit controller that could be a set it and leave it unit. So the project languished. Ebay comes to the rescue with a digital PID plug & play controller for kilns for the sum of $168 shipped. So I just finished heat treating a part I made and man this thing works slick now.

Here is the Vulcan in stock form with the mickey mouse regulator knob

Here is the new PID digital regulator unit

 Now I really didn't like having the 2 units as separatee entities so I gutted the PID box and installed the components onto the 111F's control box frame

  The digital controller monitors the temp precisely with a variable pulse width that slows the heating coils gradually so it doesn't over shoot the set temp value. Only down side is it only works the high temp I need in the Celsius scale so I had to print out some C to F tables.

 

  I like the fact that its the perfect size for gun parts and it should be just right to fit a receiver sized crucible for encasing single shot receivers like rolling blocks. Here you see a junk Rem 742 receiver in it.

  So far, so good!

 

 

 

[Grand Torino]

Sweet...heck of a conversion. How long does it take to get up to 1500?

[xtriggerman]

7 hours ago, Grand Torino said:

Sweet...heck of a conversion. How long does it take to get up to 1500?

Im not sure with this controller since I only ran it to the 500F you see there to heat treat a barrel lock on a Mossberg Brownie that I made. It was only about maybe 8 minutes for the 500. Im going to guess about 20-30 minutes to level out at 1500 F.

 500F on 4140 should be about 50 Rockwell and the Safire blue looks cool on my mix master Brownie.

 

[peejman]

Add some soak time. Unless you're heating something really small, the load isn't going to heat up nearly as fast as the air in the furnace. At 8 minutes from ambient to 500, I doubt the load got all the way up to 500.  You need to let it soak and stabilize for a few minutes. 

When you go to higher temps, you need to step up the temp. Pause at 500 for 5 minutes, then pause at 750 for 8 minutes, then 1000 for 10 minutes, and so on.   Also with higher temps, you may need to put some shielding in there to protect the parts from the radiation coming off the heating elements as localized melting is possible.  

What version of 4140 did you start with?   

[xtriggerman]

 Its basic hot rolled plate 4140 with these specs. If there is another designation.... there not saying over at speedy metals where I buy this stuff.

Carbon (C)	Manganese (Mn)	Silicon (Si)	Chromium (Cr)	Molybdenum (Mo)	Phosphorus (P) Max	Sulfur (S) Max
0.35-0.43	0.75-1	0.15-0.35	0.8-1.1	0.15-0.25	0.035	0.04

4140 As-Rolled Plate conforms to ASTM A829

 I was hooked on the idea of 4140 back in the 80's when I got a tour of the Winchester factory by Mr. Stoner the VP. He said 4140 is used exclusively on the Winchesters as barrel and receiver material.

 Thanks for the tip on stepping the temp up, I was thinking to just let it sit toped out for half an hour or so before I put anything thing in just to normalize the kiln but stepping makes sense even on the ware n tare of the kiln.  I only intend to do small parts like adjustable gas blocks and such. Making a crucible that big enough to fit a single shot action yet small enough to be farthest away from the coil heat is the next thing to consider. Im going to size it towards a 1 1/2 rolling block action that I have. Its all trial n error at this point so if you see anything else that can be a potential issue.... by all means let me in on it!  Making stuff you cant buy is always a most enjoyable part of gunsmithing.

Edited March 4, 2017 by xtriggerman

[MacGyver]

You're going to be a lot happier with that PID than you would have been trying to monkey with a replacement thermostat.

[peejman]

I asked about the steel because I was confused about how you were heat treating it. If all you did was heat it to 500 deg and let it cool, you didn't really do anything other than make it change color.  Your kiln barely gets hot enough to do much to 4140.  It takes 1600 deg and then oil quench to harden it. 

[xtriggerman]

34 minutes ago, peejman said:

I asked about the steel because I was confused about how you were heat treating it. If all you did was heat it to 500 deg and let it cool, you didn't really do anything other than make it change color.  Your kiln barely gets hot enough to do much to 4140.  It takes 1600 deg and then oil quench to harden it. 

The kiln and PID will both top out at just over 2000F so its good for anything I need to do. The lock was glass hard before I put it in the kiln. After I machined it and tested it out, I put a map gas flame to it until it lost its magnetic quality then swirled it around in Amsoil ATF for a few seconds. Now I'v read that 4140 will quench harden on small parts at 1475F and some say anywhere 1500-1600. Im guessing it might be associated with warpage? Maybe a complicated part with various thin/thick parts threw out will have a greater percentage of warp at 1600 rather than 1475?  What do you think?  Bownells calls for 1350-1425 for color case hardening and even at those temps dummy blocks n plugs need to be used to contain some warping.  

[peejman]

Ok... so you annealed or tempered it a bit at 500 deg. Keep in mind that thermocouples do degrade over time and no two are the same.  

Yes, 2000F should be plenty hot for most things. Just be very careful and plan accordingly.  You will drop a piece of 1500 deg steel at just the wrong moment, be prepared for the consequences. 

And yes, large differences in section thickness have the potential to cause warpage during quench.  You'll need a good infrared pyrometer to measure the post quench temp of the part.  Some steels are very sensitive to how far you quench them down and then get them into the annealing oven quickly. 

4140 is common enough that you should be able to find phase diagrams and age tempering curves. I have access to the ASM databases through work, so holler if you have questions.