Heat treating question

[Micdanson88]

I built a propane forge to heat treat O1 knives I am trying to finish. I cant get it below 1550* and O1 needs 1200 preheat and 1500 before the quench. the regulator is set as low as it can go and the choke plate is mostly closed. The gas jet is drilled .040" I think it may be too big. I really need to finish this knife for a friend of mine so I would appreciate your help greatly! Thanks

 

[Overmountain]

I built a propane forge to heat treat O1 knives I am trying to finish. I cant get it below 1550* and O1 needs 1200 preheat and 1500 before the quench. the regulator is set as low as it can go and the choke plate is mostly closed. The gas jet is drilled .040" I think it may be too big. I really need to finish this knife for a friend of mine so I would appreciate your help greatly! Thanks

Have you tried using a baffle? With a propane atmospheric O1 is gonna be a bear. Too hard to hold steady temp IMHO. I had some issues with O1 on a blown PID controlled forge, ramping and short overtemp issues, etc. Maybe find a mate who has a HT furnace?

My best guess is to try a baffle and a smaller tip. If you mentioned pressure I missed it. The pressure of the gas is as/more important than the tip size. The venturi effect drives these burners, sure, but that is helped/initiated by the flow of the gas. Even shooting off center can effect a burner... You can have a smaller volume of gas moving at a faster velocity through a smaller tip and maybe get a lower temp... The baffle will effectively reduce the volume of the forge or at least the part the burner 'sees' so it will be trial and error I'm afraid.

Cheers.
-Eric

 

[timgunn1962]

What is the burner? And what is the general design of the forge?

Is it all built exactly to a "known good" design, or have you built something that to the untrained eye looks not overly dissimilar to some other design that someone somewhere on the internet built?

TBH, there are so many variables that it's very difficult to give definite advice. If you've built to a "known good" design, your best bet is to contact the designer and find out whether they had the same problem and what was done to overcome it.

I've played with a few burners and (mostly Kaowool-lined) forges. A couple of general tips that I have picked up through personal experience (YMMV):

If you can't get it cool enough, you are probably running too lean. The "nearly closed" choke plate is probably still too wide open.

Usually, it's not getting hot enough for welding that's the concern. When I tell someone to fit a smaller gas jet to lean off the mixture, they look at me like I'm stupid. It usually works though.

For O1 HT temperatures, you are likely to need to run a yellow smoky flame. It may not even light at the correct mixture and you may find you have to start leaner and get the forge temperature up, then richen it off to get the temperature you want.

I may be teaching Granny how to suck eggs. If so, I apologize, but not everyone gets that there are 2 adjustments that control how hot a Naturally Aspirated gas forge runs.

First is the gas pressure. Everyone gets this. Not everyone gets the pressure: gas flow relationship though. The pressure and flow are related by a square law. To double the gas flow (and therefore the BTU input), the pressure needs to be increased by a factor of two squared: 4 times the pressure.

Second is the air:fuel ratio. This is a bit more complex. I found a website that will calculate the adiabatic flame temperature for gas:air mixtures and plotted the results. It's highly theoretical (rough translation: it's not reality), but it does show the basic shape of the curve. Top trace is in degF, bottom one in degC.



Left of the chart is rich (reducing), right is lean (Oxidizing).

A picture of one of my forges at 797 degC (1467 degF) showing the yellow flame. I was trying for 800 degC (1472 degF) in a general-purpose forge that would also reach welding temperature.



In the UK, O1 is readily available as Ground Flat Stock from engineers suppliers. Anything else is much harder to find. As a result, it gets used a lot more over here than it probably should, given the need to soak at a constant temperature.

I built another forge specifically for HT with O1 and asked an aspiring knifemaker to test it for me. He made a 4-minute video. It seems a lot longer to me, but it's never going to be thrilling, making a video of the temperature not changing.

816 degC is 1500.8 degF 818 degC is 1504.4 degF. He'd put the blade in with the forge at about 700 degC (1292 degF) and ramped it up to 816 degC with the mixture adjustment.

Presetting the temperature before putting in the workpiece would have allowed a more stable temperature and is what I had in mind when I built the forge. I think he was looking ahead to Stainless Steels and practising ramp-and-soak on the cheap stuff. I'm quite impressed that the control is good enough to do it.

https://www.youtube.com/watch?v=1xvWkXBXY6U&feature=youtu.be

 

[Overmountain]

Not to hijack/undercut Tim here and I know your query was a short term concern but honestly if you are planning on doing a lot of HT in the future, especially stainless, a forge is a poor setup. A HT furnace/oven is the right choice for several reasons.

If you can only afford one or the other, and plan on doing a lot of hot work/forging, etc, then build a forge. If you are only planning on doing thermal cycles, i.e. normalizing, annealing, HT etc, build or buy an oven. Yes, a good PID controlled blown forge can be very versatile, but an oven is just good clean and easy to control heat.

Just my $.02

Cheers.

 

[phorizt]

I had this same issue when I built my forge and went out and got a good burner for it. I could get it really hot but could not get it low enough to do any of the simple HT I wanted to do without risking overheating. I ended up going down to HF and buying one of their torches to try out and see what would happen. It works perfectly now and the hottest I can get it is about 1550 with that torch. I have a muffle pipe inside of the forge and put a thermocouple inside of that to get the most accurate reading and can maintain temps within a few degrees of my desired target. If I need it hotter I use the burner but for anything under 1550-1600 I just use the torch.

 

[stezann]

If you need to do complex annealing operation i agree the oven is the way to go...
A good gas forge equipped with needle valves and a good TC is able to do very precise hardening operations. Installing a PID on a gas forge IMHO is adding excessive complexity, since at least i won't leave a gas forge unattended under the PID control, and since i'll be there i would do the PID work

 

[timgunn1962]

LucyCustomKnives
No question, an electric HT oven is the better option for most people intending to HT their own knives, but the OP was specifically asking for help with a forge.

Phorizt,
Using a muffle tube can certainly help to even out the temperature when used in an uneven forge, but is not necessary if the forge can hold temperature evenly and accurately.

When a muffle is used, the inside of the tube usually contains air and the scale/decarb is the same as in an electric oven.

A really good burner, like the one in the youtube link above, will hold its temperature stable to within a couple of degrees once set. By running a rich (reducing) mixture, both scaling and decarb are substantially reduced (though not completely eliminated; an anti-scale compound still helps).

If your good burner would not run down to HT temperature, I suspect it was not designed to do so.

Did it claim to work at HT temperatures? Most of the hype around burners seems to be bigging up their ability to reach welding temperatures, whilst ignoring HT temperatures altogether. My guess is most buyers either assume that the low temperature is the easy bit, or that what will do a lot will do a little, and don't ask the right questions. In my experience, hot is actually pretty easy, it's getting the wide range that is difficult.

Stezann,
Annealing of air-hardening steels is certainly where the forge falls short. I'm not sure that is necessarily a major limitation though. Most of the air-hardening steels seem to be available annealed.

I agree that a PID-controlled gas forge is unnecessarily complex.



The 1/2" burner in the youtube link will run that HT forge from about 700 degC (about 1300 degF) to about 1200 degC (about 2200 degF): there is only 1" of insulation and a pretty big chamber.

In a smaller and better-insulated forge (the blue one in the photo), the same burner gives a range from under 800 degC (1472 degF) to something over 1370 degC (2500 degF), which is the upper range limit of a type K thermocouple.

The Venturi mixer is the clever bit and is a commercial product costing about 35 GBP ($54) as a walk-in customer. I've not seen a DIY Naturally Aspirated forge burner design that comes anywhere near the performance of the commercial unit. It's not really practicable to make a "proper" Venturi mixer without machining.


I think the forge may actually be better for O1 than an electric oven because of the atmosphere control (I have built a few electric ovens too, and don't think I am biased towards either gas or electric).

With foil, I think it could probably even do a decent job on Stainless.

I'd not really thought about using it for stainless before seeing Marks video due to a lack of imagination on my part, combined with a lack of appreciation of just how bloody-minded and focussed knifemakers tend to be.

All my own burner testing had been trying to set a temperature and hold it indefinitely. Most of it had involved the general-purpose forge in the picture in post #3.

I had been impressed by a 55-gallon drum HT forge for swords I'd seen at a hammerin and wanted to scale it down for knives. I was fairly certain that the size of the chamber was a large part of the reason it worked so well, so scaling it down looked like being quite a challenge and it took me a while to get reasonable results. The original (big) one had been a set-the-temperature-beforehand job and I seem to have had tunnel vision about making it smaller.

The small HT forge design I came up with is the one in the youtube video. Mark obviously spent some time test-driving it and marking-up the adjustment points. Now he's getting more out of it than I'd ever expected when I built it.

 

[stezann]

Also i use commercial venturi.... my forge has 2 ports and i have 2 sizes of venturi, 4 total....that gives me a broad range of combinations to setup a proper athmosphere for the different ttemperatures required.
For austenitizing carbon steels i always use only 1 venturi, the smaller size.
Usually i don't anneal air hardening steels, 'cause as you said they come annealed, but since i do forge high carbon steels, i soften hypereutectoids by cycling under ac1 a couple of time, after normalizing....that allows me to hand file the steel easily enough.
I don't know, if i would settle on stainless i would probably...definitely get me an electric kiln, but for carbon if i would have money and guts i would invest in a molten salt setup.
For the simple gas forge i suggest to shield the blade from the direct flame....i use a piece of iron pipe as a muffle, the tc goes inside the pipe

 

[Overmountain]

LucyCustomKnives
No question, an electric HT oven is the better option for most people intending to HT their own knives, but the OP was specifically asking for help with a forge.

Phorizt,
Using a muffle tube can certainly help to even out the temperature when used in an uneven forge, but is not necessary if the forge can hold temperature evenly and accurately.

When a muffle is used, the inside of the tube usually contains air and the scale/decarb is the same as in an electric oven.

A really good burner, like the one in the youtube link above, will hold its temperature stable to within a couple of degrees once set. By running a rich (reducing) mixture, both scaling and decarb are substantially reduced (though not completely eliminated; an anti-scale compound still helps).

If your good burner would not run down to HT temperature, I suspect it was not designed to do so.

Did it claim to work at HT temperatures? Most of the hype around burners seems to be bigging up their ability to reach welding temperatures, whilst ignoring HT temperatures altogether. My guess is most buyers either assume that the low temperature is the easy bit, or that what will do a lot will do a little, and don't ask the right questions. In my experience, hot is actually pretty easy, it's getting the wide range that is difficult.

Stezann,
Annealing of air-hardening steels is certainly where the forge falls short. I'm not sure that is necessarily a major limitation though. Most of the air-hardening steels seem to be available annealed.

I agree that a PID-controlled gas forge is unnecessarily complex.



The 1/2" burner in the youtube link will run that HT forge from about 700 degC (about 1300 degF) to about 1200 degC (about 2200 degF): there is only 1" of insulation and a pretty big chamber.

In a smaller and better-insulated forge (the blue one in the photo), the same burner gives a range from under 800 degC (1472 degF) to something over 1370 degC (2500 degF), which is the upper range limit of a type K thermocouple.

The Venturi mixer is the clever bit and is a commercial product costing about 35 GBP ($54) as a walk-in customer. I've not seen a DIY Naturally Aspirated forge burner design that comes anywhere near the performance of the commercial unit. It's not really practicable to make a "proper" Venturi mixer without machining.


I think the forge may actually be better for O1 than an electric oven because of the atmosphere control (I have built a few electric ovens too, and don't think I am biased towards either gas or electric).

With foil, I think it could probably even do a decent job on Stainless.

I'd not really thought about using it for stainless before seeing Marks video due to a lack of imagination on my part, combined with a lack of appreciation of just how bloody-minded and focussed knifemakers tend to be.

All my own burner testing had been trying to set a temperature and hold it indefinitely. Most of it had involved the general-purpose forge in the picture in post #3.

I had been impressed by a 55-gallon drum HT forge for swords I'd seen at a hammerin and wanted to scale it down for knives. I was fairly certain that the size of the chamber was a large part of the reason it worked so well, so scaling it down looked like being quite a challenge and it took me a while to get reasonable results. The original (big) one had been a set-the-temperature-beforehand job and I seem to have had tunnel vision about making it smaller.

The small HT forge design I came up with is the one in the youtube video. Mark obviously spent some time test-driving it and marking-up the adjustment points. Now he's getting more out of it than I'd ever expected when I built it.

I know Tim. That's why I gave him burner advice first. (Frankly, I think just the change from atmospheric to blown is a good progressive step even before an oven...) Then suggested that for a future project. It is a natural step if all you are doing is heat treatment operations for stock removal blades.

And PID operation on a properly set up blown forge is actually fairly simple and quite nice to have. Will hold a nice even temp within a very narrow band actually.

Cheers.
-Eric