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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 03-23-2009, 11:31 AM
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dbalfa dbalfa is offline
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Question H/T oven control issues

I AM DONE! with my eletric HT oven! And it gets HOT!

But I can't control it!

I used an Auber PID with SSR capability and have played with the parameters quite a bit but haven't hit upon the sweet spot yet. I tend to overshoot sometimes 100 degs. At the higher temps, it can go 300 deg over. It seems like the coils are remaining hot even after the power is shut off and the heat continues to build. The PID turns the output off but that doesn't solve the issue.
Also, the PID is "pulsing" the output on/off. Is that normal? I am using "PID" control mode with dual SSR's on the coil. I have one 3000 watt , 240v coil. I built mine with 3" 23K refrac brick with 3" hard refrac bricks for the floor. It has 5"x4.5"x15" open heated space.

Can anyone assist me on where to start with the PID parameters to get it more consistent?

I think I am going to love this thing......I WANT to love this thing.....IF.....

Thanks-


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Old 03-24-2009, 08:34 AM
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Problem solved! I had burned up one my SSR's. Turns out you MUST INSTALL A HEAT SINK ON ANYTHING MORE THAN 15 AMPS!!! - as I learned to hard way....
Now I have one SSR running the whole thing and it works great- <5deg variation


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Old 03-24-2009, 09:19 AM
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pipecrafter pipecrafter is offline
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You gave me a chuckle, Dennis. Apparently you ended up building with one SSR anyway.

I haven't had a chance to respond to your email yet - mine has the bottom frame welded up and ready to go. I need to pick up the IFB and assemble everything and I'll be ready for final assembly.

How quickly does it get up to heat?


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Old 03-24-2009, 10:11 AM
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ARCustomKnives ARCustomKnives is offline
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I started work on my HT oven last night. Got most of the bricks rough cut. I just need to find some refractory mortar and get some sheet metal for the shell. I can't wait to get this thing going!
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Old 03-24-2009, 03:28 PM
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Yep, I can laugh now...not the case yesterday....

As far as heat up is concerned, it gets to 1200 in about 5 mins, then it slows considerably. It takes about 30 mins to get to 1400, another 20 to get 1900. Then takes about 6 hours to cool down....with the door open....that may make it harder to use as a temper oven than I hoped. It will get to temper oven temps in 90 secs.

Bass, you mentioned refrac mortar- where were you going to use that? I did not mortar my bricks together but I did get some Inswool caulk from the refrac store. It is rated at 23k. I used it in beads between the bricks but don't know that it helped all that much. It did come in handy to seal up the ends of the coil channels. I bought some cheap fireplace caulking to seal around the door bricks (ran out of Inswool) and it seems to be holding up OK. It may give up sooner than later, but it's cheap.

Kurt, I also got some of the high temp staple wire. I used it to make a rack of sorts as well as staples. I thought it just makes sense to suspend the blade rather than lay it on the floor of the kiln. Makes it easier to grab too.


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Old 03-24-2009, 04:16 PM
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Quote:
Originally Posted by dbalfa
Yep, I can laugh now...not the case yesterday....

As far as heat up is concerned, it gets to 1200 in about 5 mins, then it slows considerably. It takes about 30 mins to get to 1400, another 20 to get 1900. Then takes about 6 hours to cool down....with the door open....that may make it harder to use as a temper oven than I hoped. It will get to temper oven temps in 90 secs.

Bass, you mentioned refrac mortar- where were you going to use that? I did not mortar my bricks together but I did get some Inswool caulk from the refrac store. It is rated at 23k. I used it in beads between the bricks but don't know that it helped all that much. It did come in handy to seal up the ends of the coil channels. I bought some cheap fireplace caulking to seal around the door bricks (ran out of Inswool) and it seems to be holding up OK. It may give up sooner than later, but it's cheap.

Kurt, I also got some of the high temp staple wire. I used it to make a rack of sorts as well as staples. I thought it just makes sense to suspend the blade rather than lay it on the floor of the kiln. Makes it easier to grab too.
I'm gonna use refractory mortor to lay a 1/16" joint between all of my bricks and seal them together. I figure this will keep my bricks in place AND help seal the heat in a little better. I also bought a roll of ceramic fiber insulation to wrap in between the bricks and the shell, and to use for a seal on my door. I figure that this will help keep the outside cooler longer and help keep the heat in that much more.

Sounds like we are gonna have a similar control setup. I'm gonna use a 4 wire 240VAC circuit, so that I can run one leg and a neutral to my controller and run it with 120VAC.
I'm also gonna put double pole switch on the coil for safety and a switch for the controller.
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  #7  
Old 03-25-2009, 08:57 AM
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I'm wondering if the heat holding issue may be a detriment instead of a asset. I planned on using my oven as a tempering oven as well but when you get to hardening temps, it takes a couple hours or more to cool back down to tempering temps. I don't typically wait that long. There may be a way to cool it down faster but I haven't came up with one yet. As mine sets, it seems to have no trouble holding 1900 so I don't know if additional insulation would help. I did use 3" brick so if you were using 2.5", then perhaps it would be beneficial. The ouside of mine gets too hot to touch, not an instant burn but you wouldn't want to tarry there.

Anyone have any ideas how to cool this thing down faster?


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  #8  
Old 04-28-2009, 05:31 PM
foxfyre841 foxfyre841 is offline
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What about using a fry-daddy (or something similar/larger) as a marquench bath instead of using a HT oven? Marquenched steel would be less likely to crack as it cools, even if you have to wait longer (at least this is my take on it). As soon as my oven's finished, I had planned on making a marquench bath either using low temp salts, polymers, or special oils.

In terms of quickly cooling the oven, I can't imagine that being good for the fire bricks. I would imagine a shorter life expectancy from them if they were used in such an extreme way.


Regards,
James
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  #9  
Old 04-29-2009, 12:19 PM
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mete mete is offline
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Don't try to cool it quickly - it will shorten the element and brick life !!! You can easily temper in a kitchen oven or toaster oven for the 400 F tempers. If you prefer a 1000 F temper for stainless steels you could temper first at 400 F then later when the oven cools down do the higher temperature temper .
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  #10  
Old 04-29-2009, 11:27 PM
Mike Krall Mike Krall is offline
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I don't know how many blades you might be dealing with at a time, but if there are a few or more, You could snap temper at 300F for an hour or two then leave them laying around until you had a load to temper.

I got an understanding of how to do that from Mete a short while ago...

Mike

Last edited by Mike Krall; 04-30-2009 at 12:48 AM.
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  #11  
Old 05-04-2009, 11:05 AM
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I treated and did the snap temper on three 01 blades this weekend - worked out great! When the oven cooled, I tempered at 425 for 2 hours and got great blades (two cycles Mete)

I plan on my first excursion into stainless this week- wish me luck!

BTW - I used the PBC powder for the first time- and I don't remember how much it costs, but (nobody tell Tracy M.) I would pay twice that much! Man that stuff is WONDERFUL! Cut my finish sanding in half! Slag/powder fell off like potato chips after quench and the blade looked just like it did prior to HT! IF you work oil hardening steels- GET YOU SOME!!!

(Tracy- I'll invoice you later for the advertising.)


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  #12  
Old 05-04-2009, 11:28 AM
Mike Krall Mike Krall is offline
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Dennis,

And there is this for anti-scale http://www.brownells.com/aspx/NS/sto...=anti-scale&s= ... don't know if a person can get it directly from the company or from other outfits, or not. I've been using it recently after using PBC powder for a while. I like the way the liquid goes on a lot better than I like the way PBC goes on. I'm not sure I like the way it comes off compared to the PBC... I'll have to use it more, first.

Mike
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