Pyrometric cones for determining HT temp in junkyard gas forges?

L

Line_of_weakness

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Feb 8, 2014
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Hi all. New knifemaker here. I'll post some pics for my knifeforums background check ASAP.

Because I'm new to this process, I'm doing my own heat treating in a propane forge. Being that I'm a control freak, I hate not knowing whether I'm on the mark for the particular steel I'm using during the soak phase. And being that I'm stubborn and broke, I haven't invested in a PID or even a high temp ceramic probe and thermometer.

I did invest in a few of these.

orton-self-supporting-cones


Pyrometric cones are used when firing ceramic in a kiln. They come in a huge range of temps, from tempering to forge temps IFRK.

These pyrometric cones sit in a firebrick cone holder I whipped up. I use three, one that's too cold, one that's too hot, and one that's just right.
As soon as the just right cone begins to droop, I dial the propane flow back a tiny bit and set my timer for the soak.

I've gotten pretty damn good results on A-2, 1095, and 0-1 steels (all foil wrapped) and my latest A-2 knife has proven ridiculously difficult to damage during torture tests.

They are really more a measurement of temperature/time rather than ambient temp in the moment, but has anyone experimented with these? Tell this new guy why he's wrong, with as many insults as possible.
 
What is the accuracy of the cones typically?

I considered using cones, then I realize they are consumable items and went with a pyrometer instead.
 
I've considered this route before as well. With regards to Karburized's question, cones are extremely accurate at measuring when ceramics have reached the proper temperature because they're essentially made of the same stuff. I would guess that if you know your HT protocol well and the cone chart even better it should work fine.
 
Thanks guys, sorry for the double post. Weird computer ghosts probably. This is my first post and I've gotten nothing but helpful replies. You guys are pretty cool. I have budgeted for a pyrometer but I'm really in the dark as to what kind of thermocouple to get. It seems that its not just temperature range, but deterioration of certain types of thermocouples that makes it tricky.

What type of thermocouple probe do you use Karburized?

ALSO: Found out that pyrometric cones are used to evaluate the accuracy of thermocouple-based temp monitoring systems. They can give you a tremendous amount of information based on the level of droopiness. May be useful to check the accuracy of your PID setups after a few years. Either way, I've got three more pieces to finish and collect on before I can buy that probe.
 
Karburized, the cones are very, very accurate. Each step up is 10-50 degrees F (I know, it's stupid and makes no sense). They are used to verify digital pyrometers. There's a very confusing chart available online for the temp breakdowns, but it takes some time for the cones to vitrify, and take on the plasticity of molten glass, allowing gravity to do it's thing. The range is all over the place, in a confusing format. the Orton brand starts at 1000F if I remember correctly, to well beyond the hottest forge temps.

If I'm doing a HT on say, 1095, I use one rated for 1400 or so, then one rated just below 1500, then one rated an immediate step up, which in my case is about 1530. I let them sit until the first cone just starts to droop. I maintain propane pressure for another five minutes or so, or until the next one, just below crit temp droops from 12 o clock to 3 o clock. I throw my foil packet in, drop the gas down a tiny bit, and soak. A short soak time like 1095 is a pain. Longer soak times for a-2 and 0-1 and such require variation of course. At $7 for 50 cones, it's a decent solution for a poorly-funded newbie like me. I haven't had a lot of HT go wrong on me. Just D2.

Let's have a moment of silence for all the new knifemakers melting their girlfriends' hairdryer over a pile of hot coals and Mexican Nicholson files.
 
Do they really? That's probably the origin of the raw mild steel they were made from anyway. I noticed that they changed brands, but not the item, at my local OSH hardware store. Another cooper industries brand name, but same product as far as I can tell, and still from Mexico. I love my files, and use them in a filing jig as a final step before hand sanding, to get a better plunge line and because my belt grinder is woefully home-made and not suitable for finishing touches. I do all flat grinds for now, so it works out well.

disaster struck last night. I had some A2 in the forge and knocked.the.pyro.cones.over. The brick supporting the pins holding the SS foil packet upright must've had a crack in it. It was just one piece of steel fortunately, but I need a proper cone holder, or better yet a proper shielded thermocouple. I don't need to do any more HT for at least a week, so hopefully I can afford the goods by then.
 
For a thermocouple, I use a Mineral Insulated type K thermocouple assembly with a handle, curly cable and miniature (flat-pin) plug. The probe diameter is 6mm or 1/4" and the sheath is type 310 stainless steel. I get 24" of thermocouple beyond the handle, which keeps the handle and the hand holding it well out of the hot zone. I use the long, handled one so that I can check the temperature is even throughout the forge. If you can use a short fixed probe through the casing, the costs can come down considerably.

The type K thermocouple itself good to 2500 degF/1370 degC. The 310 SS sheath is generally considered good to only 1100 degC, but this is for industrial applications and it will usually survive quite well at higher temperatures for fairly short periods. I have just killed one as a result of using it to tune some burners to run at welding temperatures (possibly plus a bit). It had probably been over its maximum range of 1370 degC/2500 degF fifteen or twenty times and spent ten to fifteen hours above 1250 degC/2282 degF.

I buy locally to avoid shipping charges. It is worth checking out local thermocouple suppliers; there are more than most people expect and most make to order.

If you have no choice but to eat the shipping, Omega have a proprietary sheath material, Omegaclad XL, that is claimed to be good to 1335 degC/2435 degF, which should be good enough for welding temperature and does not add much to the cost.

http://www.omega.com/pptst/KHXL_NHXL.html

The part number I would suggest is KHXL-14G-RSC-24 ; put the part number in the search box to get the price, as the page doesn't give the option of 24" length.

The Pyrometer I usually use is a TM902C from ebay. Only reads in degC, but all the way up to 1369 degC (2500 degF). It is as accurate as any branded instrument I've tried and costs around $5 delivered.
 
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