*Safely* tempering in oil?

S

Surgo

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I've seen the folks on Man at Arms Reforged temper their larger pieces in oil. I also know from watching Alton Brown that this is a big cause of American house fires, especially around Thanksgiving. IIRC one of the contestants on FiF actually burned down their shop doing this. So, yeah, easy to get wrong.

But is there a way to get it right? I wouldn't want to have any open flames near the tank, but we can all remember high school chemistry class with the hot plates. Maybe an induction burner with some sort of stirring apparatus, provided that doesn't take too long to heat up a volume of oil? I don't know, just thinking out loud. I'd probably have to keep some automated system to turn off the heat source to make sure the oil doesn't get too hot, because depending on the oil that could also be a big danger.

Plus, I'm not even sure what oil would be usable. Most mineral oils seem to have a flash point around 350 degrees. Doesn't really leave much room for safety.

I guess I talked myself out of this idea while writing this.
 
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There are special oils that are used for quenching.
The only acceptable cooking oil is canola oil.
As long as the blade is immersed quickly and completely, there is little danger of fire.
 
Bill, he is talking about tempering in 400F oil.

Tempering in oil is very dangerous. Industry has equipment for this, but heating a metal cylinder filled with peanut oil is not the same.

DO NOT TEMPER IN HOT OIL.
 
Stacy E. Apelt - Bladesmith said:
Bill, he is talking about tempering in 400F oil.

Tempering in oil is very dangerous. Industry has equipment for this, but heating a metal cylinder filled with peanut oil is not the same.

DO NOT TEMPER IN HOT OIL.
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I would say that it is calling for disaster :D BW Stacy , is there any advantage to temper blade like this ?
 
there are some fryers on the market that could accept a 10" or smaller blade, like a FRY DADDY. are you talking about a room temperature blade or a red hot one? biggest issue is a home fryer is not designed to stay on for hours at a time. a big pot of oil on a hot plate is an accident waiting to happen.
 
Think of it as keeping your car engine rpm in the red for extended periods ... can the engine do it? Yeah, kinda like a ticking time bomb, though.
 
The Baltimore knife and sword guys do it because of the size of the projects they are working on. A giant sword just doesn’t fit in the toaster oven that well. They’ve also got a pretty purposeful setup for it that isn’t going to be in your average garage. If I remember correctly, there was actually a forged in fire episode where a guy tried doing this. His oil tank tipped and he basically burnt his shop down.
 
Sorry, I misread "tempering" as "quenching."
Long night on stage last night and I was tired.
Thanks for the correction.
 
OK, I'll elaborate more on oil tempering:
Advantages - Even tempering. Slightly faster tempering. No surface oxidation ( funny colors).
Disadvantages - HUGE fire risk. Big health and safety risk of anyone around it.

To get the same advantages with lower safety risk, build a low temp salt pot. You can buy commercial units for two thousand dollars, but you can build one for $400-500. It is a stainless steel tube about 4" in diameter with a heat source to heat it to 350-450°F. A surplus pottery kiln with a hole in the top for the tube, a Monel clad TC to read the salt temp, and a PID control will do it all. Tubes can be welded by any good welder or welding shop, or bought from places like Evenheat or High Temp Tools and Refractory. For a low temp salt pot, I would build my own tube, as it isn't nearly as critical as a high temp salt pot. Salts can be purchased from several sources.

Low temp salt pots have a second really big advantage - they can be used as marquenching/martempering pots. A blade that is 1500°F can be quenched in the molten salt and the quench is almost instant. Because the salt conducts the heat so fast … with no vapor jacket … the drop (especially the edge) below the pearlite nose is faster than even water. Because the blade only cools to the Martensitic start temperature, the core has time to catch up before conversion starts. This dual cooling rate in conventional quenching is what causes warp. Then the blade just sits there comfortably at the Ms for a while until you take it out and wipe it off to cool in the air. Martempering can virtually eliminate warp. Once equalized then cooled to room temp, standard tempering can be done by simple putting the blade back in the salt pot for an hour, twice. Again, the tempering is absolutely even with no surface colors or overheated areas.

If quenched into salt around 450-500°F and left in the salt for several hours, you would be austempering to form bainite.
martempering.jpg
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The new trend in semi-pro bladesmithing is getting a pair of the new Evenheat salt pots and doing the austenitization, marquench/martemper, and subsequent temper cycles in them. The price has come down drastically to where they are nearly equivalent to a HT oven. They are not amateur tools, but in a good size shop run by an experienced smith, they can step your game up a big notch.
evenheat-sb-718_600px.jpg

https://usaknifemaker.com/evenheat-sb-718.html
 
I thought about setting up a low temp salt pot until i read a few guys comments stating that it is only a matter of time before you get severely burned. What are your thoughts on the safety of low temp salt pots, stacy?
 
There's very little risk with low temp salt pots. The big risk is with high temp pots. And saying it's only a matter of time before you're burned is like saying it's only a matter of time before your buffer throws a knife across the room or you catch your thumb between your work rest and belt or having a negligent discharge with a firearm.
 
I would consider a properly built and stable low-temp salt pot safer than most of the power tools in the shop. It is certainly no more dangerous than a HT oven or a tempering oven.

Now, sticking a 12" tall 3" pipe filled with salts on top of a electric hot plate (as I saw an ABS Master Smith do once at a hammer-in) is just asking for a problem.
 
Thanks. I made a special tempering oven from a toaster oven that is insulated and pid controlled. I can hit 650 degrees in it for tempering slip joint springs. I like being able to quench a spring, wipe it off, then go directly to tempering. I have to have a fan blowing on the tempering oven and have it set up correctly to not get too hot. I may try the salt pot and if it works well just toss the oven and reuse the control box. I like the salt over lead since salt has a good working range for both blades and springs.
 
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For small blades and parts on folders, a small salt pot is really simple ... and the perfect tool. A pawn shop or Craig's list small pottery kiln, old jewelers/dentist oven, etc. will all work to hold a 2'3" tube through the top. The tube should stick out about 3" above the top of the kiln.

For work on folders, a high temp salt pot is also much less of a safety issue as a larger knife/sword salt pot.
 
I'm using a little Lee lead pot for my nitre bluing salts. You could do a lot of slip joint parts in one of them.
 
I would say that you could absolutely make a relatively "safe" oil tempering bath. It would just cost a ton more than doing it in a less terrible medium. For instance, you could pour a slab and or pilings offset from anything that could burn, build a non flammable structure around it, and secure the burner and pot to said slab or pilings so that they couldn't tip over. You would also need air evacuation and probably somethign to protect you from splash back when inserting the blade.

Or you could use a low temp salt pot or just on oven. If you don't care about the discoloration, use or build a tempering oven with a deep tray of brass (or aluminum) chips. Bring the oven to temp (put your TC in the chip tray) and just submerge the part fully. Either brass or aluminum is sufficiently conductive that you will get very even heating, and your TC will tell you precisely what temp you are running. And your setup is no more dangerous than a household oven.

I do love that the safety threshold in the smithing community is far shifted from most other hobbies. Running a nitre bluing operation is often considered pretty dangerous amongst gunsmiths (or at least to require significant precaution) - here we propose it as the safe alternative.
 
Niter bluing - Hot bluing a gun barrel
I here those terms misused regularly.

There is a big difference between Niter bluing salts ( molten salts at 400-500F) and Gun bluing salts (corrosive liquid at 280F).

Niter salts are pretty easy to handle, very hot lye is something that scares brave men.
 
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Thanks for all the info on salt pots, Stacy. I had no idea about the anti warping benefitsb for quenching in them. I had worried a lot over their safety but I do suppose that since I have zero interest in a high temperature salt pot, I have a bit less to worry about and could probably work with my own mediocre welding skills.

I do worry about how ridiculously corrosive molten salt is and the upkeep that requires, but everything in life has a downside.
 
So marquenching in a low temp salt pot would be considered a very fast quench medium i take it? Would it be appropriate to quench steels that only require an 11 second oil in this setup?
 
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