forging temperatures, soaking, heat treat...

[Daniel Koster]

Of all the forging demos I have attended, I can't remember anyone having a pyrometer. Most just said to pay attention to the colors, etc. I agree that this is a good skill - don't misunderstand.

That said, I bought a digital pyrometer.

Which leads me to a question....

Let's say I have some 1095 in the forge to heat treat and I've dialed in the forge around 1475-1500 F. How long can the steel stay in there before having problems? Technically, it's not overheating....I want it to heat as evenly as possible...so I'd like to err on the "soak" side of things.

Anybody use a pyrometer and have any suggestions?

 

[Matt Shade]

I loaned my charts out and can't remember the exact numbers off the top of my head
But the basic idea is that the crystal structure of steel/iron changes from BCC to FCC (or do I have that backwards?) when you get above a certain temp. I think its the same temp you start forming austenite in the steel, but don't have a phase diagram handy :grumpy:
FCC is a much more flexible structure and lets you move the metal around by forging. You want to work the steel above that temperature, but in the case of knives you don't want to go too high above that temp due to increased scale and decarb. You don't want to try and hold it right at that temp because you won't have any working time before it falls below.

You really need to learn to watch the color because the pyrometer won't tell you when to stop hammering. If you try to work the steel too cold, it will develop cracks that may not show themselves until you heat treat, or even start using the knife.
As far as how long to leave it in the forge, as little as possible. Holding the steel at that temperature causes scaling and decarb. If your going to set up with a pyrometer to hold the forge at your upper working temp, then you should try to get the steel out and start working it as soon as its showing the same color/temperature as the forge. It won't burn up overly fast. If you need a minute or two to set up something the knife won't be ruined, but you should try to focus your attention on getting the knife shaped and cooled down again. Its amazing how much scale will form if you leave something just sitting in the forge.

 

[Matt Shade]

Oops. I thought you were talkin forging, your talkin heat treat. Sorry about that.

For heat treat, you wait as long as you have to to get even heating. If you've got the forge locked in at that temp, then wait until all the shadows on the steel have gone away. It should be uniformly the same (or awful close) color as the forge.
The main problem your going to have since your not over heating and causing grain growth or anything is scaling and decarb. It will still happen at HT temps. There really isn't any reason to hold the steel a long time if its not one that requires a soak, and its evenly heated.
Best thing is just to start experimenting and getting some experience with it. You'll have ot figure out how much scale you want to deal with, which will partly be determined by how close to final shape you want to be before heat treat. If you want the edge a certain thickness after finish grind, you've got to allow for some scaling before when you grind before HT.
I've found that if I'm doing multiple cycles normalizing or anything like that, that a brass brush is handy to knock the loose scale off before putting it back in the forge. Loose scale can make it a little bit harder to judge temp.

 

[Burchtree]

The only problem with using a pyrometer in a forge is that it only gives the temperature at the spot where it is placed. Either way, don't just sit the blade in the forge and wait for it all to come to heat. Go slow, bring the tip up to heat, bring it out, put it back in the until a little more is up to heat, etc. The thin areas heat up fast, but they also lose the heat quicker. If you keep doing this, you won't be overheating the tip and since the thicker areas hold heat in better, sooner or later you're going to meet in the middle and you'll be bringing the blade up to critical all at once. Makes for good grain structure too. And since you're going slow, you aren't decarbin' stuff up and losing those precious carbon points.

BTW -- got that lil tidbit o' info about slow heating from Longrifle.

 

[Daniel Koster]

excellent advice Matt and Michael.

I like your "taking it slow" policy...makes perfect sense.

Thanks, guys.

 

[Bruce Bump]

I use a pyrometer in my forge but rarely HT in the forge anymore. I use an Even Heat oven. When I do use the forge I put a thin walled pipe in the forge and stabilize the heat and put the blade in the pipe. This makes a much more even temp. I dont use a pyrometer in the pipe, I just use a magnet and turn out the shop lights.

 

[NickWheeler]

Dan-

Great advice here, all of it.

I use my salts for most everything, but I still do my clay-hardened blades in the forge.

I do like Bruce, and use a pipe inside of the forge chamber. It's pretty amazing how much more of an even, consistent heat you'll get inside of a pipe.

I do it with all the lights off, preferably at night.

There's a lot to be said for watching the shadows in the steel.

If you can dial the forge in, so that it will hover around that temp, you'll mainly just have to worry about an even, thorough heat.

I usually put the tang in first and allow the thick cross sections to heat up first, and when they start to get a fair amount of color, I'll flip the blade around.

-Nick-

 

[Stacy E. Apelt - Bladesmith]

I'm with Nick.I mostly use an oven to HT,but when I need the forge I put the tang in first and let the heat run toward the point.When it seem close to the right color I push the blade the rest of the way in the pipe.