getting lower temp in forge for HT

phorizt

phorizt

Joined
Aug 26, 2013
Messages
358
I have been trying to HT with my small forge and I wanted to be as accurate as possible so I got a temp sensing setup from Auber Ins(kiln thermometer, type k thermocouple, ceramic sheath etc) and got it all set up today and started testing it out. The problem I have is that I can't keep the temp in the forge below 1650-1700F and all of my current steel is 01 and 1084 so I need lower temps. I can make the 1084 work for now but can't get low enough for a soak with the 01. If I try to turn down the burner any lower to decrease the temperature the burner sputters and goes out. I figure I have a few options but wanted to see if anyone had any suggestions since I have very little experience with any of this:

-increase the size of the openings on the front and back of my forge to release more heat and decrease the temp. I think if I did this I'd still have a hot spot in the middle and cool spots near the front and back.

-Build a bigger forge for the same burner. I really don't want to do this at this time as I have a long list of other gear/equipment I need to get.

-Get a different burner. Don't want to do that b/c that may be the only decent part of my forge since I didn't make it. I'm using one of Zaph's Atlas 30k burner w/0-20 psi regulator sets. As the product description says I can get it "hot enough to forge weld" but unfortunately not cool enough to heat treat.

-Figure out if it's possible to decrease the output from this burner so the current setup will work and produce lower temps. Obviously this would be ideal and why I'm posting this here to find out if there's a simple way to do this that I haven't thought of. Anyone have recommendations on how I might be able to do this?

Here is my forge setup:
Housing- 12" long 6" diameter stove pipe
Insulated with 1" thick Inswool with satanite coating. I believe it has 1 layer of 1' thick Inswool all the way around and then another piece on the bottom to form the floor. Inside diameter of the forge is about 3.5" vertical and 4" horizontal. About 130 cubic inches volume.
Front entrance of the forge is 2.5"x2.5". Back opening is 1.5" x 2.5".

Ultimately I need a better forge and a HT oven but for now if anyone has any suggestions on how I can make what I have work better and get lower temps in the forge I would appreciate it.

-Aaron
 
How are you regulating the pressure on your burner? Just with the propane regulator?

Most venturi's I've played with can be run pretty low, but it's important to balance fuel input with air flow, since too little fuel and too much air will cause it to go out. You need to regulate air flow in conjunction with lowering fuel PSI, or you may have to put a needle valve before your burner, to keep high pressure, but lower fuel input, since venturi's rely on pressure to draw air in.


Looking at the one picture I can find of the burner I think you're using, I don't see a way to regulate the air flow easily, and the large gap between the fuel nozzle and the burner tube definitely makes me think you're going to need to maintain pressure but lower flow.


Maybe Zaph will chime in here since he's a forum member.


Otherwise, consider buying or building a t-rex style burner, which I know from playing with a few can easily reach low even temps in a small forge.


Still, in many cases you can HT simple steels (like 1084 as you mentioned), without a soak, by austenizing at higher temps. However, it becomes more of an art at that stage, and may require more trial and error than simply "hit temp, hold, quench".
 
Thanks that really helps me better understand how these burners work a little better and what is causing it to go out.

the propane regulator is the only way I have of controlling anything.

I will try tomorrow to see if blocking some of the air flow where it enters the burner tube might allow me to keep the flame going at a lower temp. If so I can probably rig up some type of little adjustable gadget to reduce airflow. If Zaph doesn't stumble across this I'll try to email him to see if he has any suggestions too.

-Aaron
 
Well I'm no venturi expert, but hopefully you've got a starting point to mess with.


I know there's a point where you lower the pressure enough and it simply wont draw in enough air, since the gas pressure exiting the fuel delivery port causes a vacuum that draws air into the burner tube mixing with fuel (or whatever). However, I'd suspect, if your pressure was too low to draw air in, it would simply burn at the hole in the fuel delivery area, with a dirty orange flame. Which I'm guessing you see for a second once you cut the propane tank off, and should be familiar with.
 
I find most venturi burners will sputter out or backflash when trying to run them at low ( 1400-1500F) temps. A blown burner can be run lower.

Building a PID controlled burner will allow precise temp control, but a small forge may not allow good control. It works best in a 12-16" forge with a 4-6" or larger chamber.
 
I was waiting for Stacy to say to "pump" the blade in and out of the forge while heat treating it. You don't leave it sitting on the bottom unless you're using an oven. This will also keep from overheating the tip.

If you set the forge temp to 1450°, which it isn't designed to do, it will take a long time for the blade to get to that temp. After maybe a 20 minute soak on the tip while the edge has only been up to temp for maybe 2-3 minutes. Every time I've seen a knife HT in a forge, the temp wasn't at soak temp.
 
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The other possibility of too low a pressure is the gas igniting inside the venturi. Ungood. You need enough gas pressure to prevent this.
 
Yes, backflash is not a good thing. The sound of it squealing is enough to suggest that something is wrong. You usually have to momentarily shut off the gas valve at the forge and re-open it to stop the backflash, and then raise the pressure a bit.



Here is how I use a venturi forge to do HT on a simple carbon steel
. It works well for 1084, and with experience, will work on other carbon steels. Steels that need a soak time will not work as well. A few minutes of soak is about the max you can get in a forge. 1095, O-1, W2, and similar steels may get slightly less hard because the excess carbon and alloys won't have the needed time to get into proper solution, but will still make a reasonably good knife. Sometimes you just have to learn to work with what you have. (Stainless steels, High allow, and most all Air Hardening steels can not be done in a forge).

Let the forge run at normal heat for about 5-10 minutes to fully soak the refractory, then adjust it to run as smoothly as possible at low pressure. You may have to use more pressure than you want to keep the burner running and not sputtering or backflashing. Most likely it will be higher than 1500F, but that will have to do if it can't run any lower. It may require additional adjustment as it runs for a while, so be prepared to make little increases in the pressure if needed.

Once the forge is soaked and running at low, place the blade in the forge while holding the tang in tongs. Keep it moving and turning so all sides get exposed evenly. Move the blade in and out as needed to keep the tip from getting hotter than the rest of the blade. Point the edge and tip toward the bottom or sides to keep them out of the flames as much as possible.Sometimes you need to push some of the blade out the back of the forge to keep the tip from going bright red before the rest of the blade even glows. The goal is to have the blade evenly heat up slowly to non-magnetic. When it gets red, start checking it with a magnet placed next to the forge door/port. When it stops sticking to the magnet, you are at 1414°F. Heat about one shade of red hotter to get to 1475°F. You probably can't keep it there for long, or it will get too hot, so once the entire blade is evenly colored and at 1475°F temperature, pull out and quench.

Sometimes I call the movement used to move the blade in and out of the forge "pumping the blade". On long blades and swords you can pump the blade in and out of the forge to heat a 30" blade in a 16" forge.
 
phorizt, burners have whats called a "turndown ratio" it means exactly what it says, it's how much you can turn the burner down from full rated power. Some burners are 25:1 some are 2:1. You need to size the burner for the job your doing, or waist a lot of heat.
 
I have a similar situation and I have been removing the blade from the furnace briefly and blowing on the tip. As many times as necessary. I watch the color and blow on the tip as many times as necessary to bring the body of the blade up to temp at the same rate as the tip. Crude method I realize but my little furnace only runs at one speed, full tilt. I do not have any means of hardness testing and I have not made that many blades but my low tech whetstone test tells me the blades are fairly hard because they can be a bear to sharpen.
 
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Thanks everyone for the great info. I wasn't able to do anything yesterday due to a couple of my kids being sick so i'll try again this evening. However, based on all of the feedback I'm not expecting to be able to make it work with my current setup but I'm going to try to make a few adjustments and see what happens. I've seen videos of a small forge where they're able to maintain a 1500F constant temp for HT.

I have been using a muffle pipe in the forge to get a more even heat on the blade which has helped a lot. At this point if I can't get it to work I'll probably just stick with 1084 until I can make some more modifications.
 
I'm surprised no one suggested using a piece of pipe as a muffler to reduce hotspots/create an area with even heat. You can still pump the blade but the muffler will make it easier to bring it up to temp evenly. It may also reduce your temp some by "absorbing" some of the residual heat, but don't quote me on that one. Just be sure it's a black iron pipe and not galvanized.

Phorizt, you posted while I was typing. Lol.
 
zaph1 said:
If you set the forge temp to 1450°, which it isn't designed to do, it will take a long time for the blade to get to that temp. After maybe a 20 minute soak on the tip while the edge has only been up to temp for maybe 2-3 minutes. Every time I've seen a knife HT in a forge, the temp wasn't at soak temp.
Click to expand...

Is that any different than a HT oven? I mean if they're both at a constant temp then the blade inside of it is going to react the same way right? 1450 in a forge is the same as 1450 in an oven as long as you're not in the direct path of the flame?
And with a blade in a muffle pipe in a forge I would think that most of the heat is traveling from the spine, which is in contact with the pipe, out to the tip as the spine comes up to temp. At least so far from my limited experience that is what is happening and it prevents that tip from burning up in the time it takes the main body of the blade to warm up.
 
I haven't seen the situation Zaph reports, so I can't comment on that. My observations is that the tip and edge heats much faster than the thicker body no matter what the forge is set at.

Broomhead, thanks for reminding me to add - a MUFFLE ( different from a muffler) is a good way to make the blade heat more evenly in a forge. Use thick walled stainless pipe if you have it around. 2.5-3" is great, but any size that will hold the blade is better than a blade exposed directly to the flames.
 
I'm agreeing with Stacey entirely on this.

I guess I shouldn't have stated a "soak" on the tip. I was trying to point out that while you might put a blade in an oven and not touch it for 20 minutes to get a 10 minute soak, you can't do that in a forge. Every forge, regardless of design or burner, will have temperature variations while ovens do not. The Atlas Mini forge has a hot spot about 4" long of 2350° and will be around 1950° at the back of the chamber. At lower temps with a piece of inswool blocking the opening, the hotspot is around 1700° with 1550° in the back.
 
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ok thanks. I need to move my thermocouple around and check what the temps are in the different areas of my forge.
 
An open thermocouple in a running forge is likely to read off by a fair bit. Placing it in a ceramic TC sheath is how you avoid reading the flame temperature, and read what is more likely the forge temperature.


As a check on how your forge is adjusted for soaking, place the TC in a ceramic sheath, and place the sheath in a pipe muffle. After the forge has been adjusted and has run for 5-10 minutes, place the TC/sheath/pipe in the forge where a blade would sit. Let the assembly heat up for about three minutes. The reading should very close to what a blade sitting in the pipe would be. Watch the temp for a five or ten more minutes to see how steady the temp says steady or if it climbs significantly.
 
I do have the thermocouple in a ceramic sheath. The other day I did test it in the sheath in the muffle pipe and the lowest I could get it was 1650 in that setup and like you said I figured that was probably the most accurate reading for the temp of the blade steel. It was closer to 1700F outside the muffle pipe.
 
As you are finding out, it is not easy to get a Propane burner to give the full range of forge temperatures needed by most Bladesmiths.

There are essentially two things that can be adjusted to vary the temperature.

The first is how much gas goes in. Every burner I have seen has the facility to adjust this, usually with the gas regulator. It might be worth pointing out that pressure vs gasflow through a jet follows a square law; to double the gas flow, you need 4 times the pressure.

The second is how much air goes in. Some burners have the facility to control this very well, others can be adjusted to some degree. A few (very few) have progressive air adjustment over a wide range.

The temperature of the flame varies with the ratio of Air to Fuel. It is not a straight-line thing and I find it quite difficult to explain, but I'll try.

Propane burns with Oxygen, releasing energy as heat.

The Propane was originally made up of Carbon and Hydrogen. After burning, the products are Carbon Dioxide and Water (vapor).

The Oxygen was provided by the air. Air is about 21% Oxygen and 79% other stuff (mostly Nitrogen), so the other stuff has had to come along for the ride.

The heat raises the temperature of the Water vapor, Carbon Dioxide and Nitrogen and this results in a temperature. For Propane burning in air, the Adiabatic flame temperature is generally given as 1980 degC/3896 degF. "Adiabatic" basically means without heat transfer to or from the system (in this case the flame). The temperature given is the maximum temperature and it occurs when the air:fuel ratio is at, or very close to, the "stoichiometric" ratio. The Stoichiometric ratio is where all of the Oxygen reacts with all of the gas, leaving no Oxygen and no Propane unburnt.

When we are at the stoichiometric ratio, the amount of the “other stuff” not actually contributing to the burn is at a minimum; effectively just the Nitrogen from the air. All of the heat released by the burn goes into this minimal amount of “stuff” (Carbon Dioxide, Water vapor, Nitrogen) and heats it to the 1980 degC/3896 degF given by the textbooks.

If we burn Propane with pure Oxygen, we do not have Nitrogen present, so the heat of combustion only has to raise the temperature of the Carbon Dioxide and Water vapor. The adiabatic temperature of a Propane/Oxygen flame is usually given as 2820 degC/5108 degF; with less “stuff” to heat and the same amount of heat energy, the temperature is higher.

If we feed in extra air, more than is needed for stoichiometric combustion, the Oxygen it contains has no gas to react with. Both the excess Oxygen and its associated Nitrogen are effectively just more “other stuff” and the heat produced by the burning gas is absorbed by more “total stuff”, giving a lower temperature. The more extra air we add, the lower the flame temperature gets.

Because we are only interested in what is happening inside the forge, we can instead add extra gas as “other stuff” and get a lower flame temperature that way. For most other applications, this would be an unthinkable waste of gas. However, in a forge, it gives the not insignificant advantage of a reducing atmosphere.

When we add extra gas (or choke down the air to achieve the same effect), the gas that is unable to burn within the forge due to lack of Oxygen there, finds the Oxygen needed to complete the burn as it exits the forge and produces the well-known “dragons breath”.

The chemistry of burning Propane is not a simple all-or-nothing process. As we start to run rich/reducing, Carbon Monoxide is produced in increasing amounts. As we go even further into the rich/reducing region, we start to see Carbon being produced, and as we get close to the limit of combustion, we see the flame as yellow and luminous, like a candle flame, due to tiny particles of Carbon incandescing.

Here is a chart I put together after a discussion about Venturi burners, temperatures and forge atmospheres with a blacksmith/bladesmith (I was trying to work out whether an automotive UEGO wideband air:fuel ratio sensor would have a wide enough measuring range to be useful to "us"; I'm still not sure). Hopefully, it shows the link between temperature, mixture and composition of the forge atmosphere.




The Y scale is temperature and the X scale is Equivalence ratio, Lambda. When Lambda = 1, the mixture is stoichiometric, so at Lambda = 0.5, there is only half the air that there would be at stoichiometric

“We” are never going to have an adiabatic system, primarily because the whole point of a forge is to transfer heat to the workpiece, but also because we will lose heat to the forge and the shop.

“Our” temperature curve on the graph is always going to be below the “ideal” curve shown, but the principle is still clear still clear; we can follow the curve and vary the forge temperature if we can vary the air:fuel ratio.

The range needed to get both HT and welding temperatures is very wide. It can be achieved with a reasonably well-executed blown burner. It can also be achieved with a very well-executed Venturi burner with an adjustable choke. I use commercial Venturi mixers from Amal and they are excellent.







The temperature is in degC. 781 degC is 1438 degF.

That forge was intended for HT only, though it easily reaches forging temperatures. It is 8" inside diameter and around 18" long, with only 1" of kaowool insulation; maybe 900 cu in on a 1/2" burner.

The next forge I built used the same 1/2” burner. It only has a .020” diameter gas jet and can be run up to 60 PSI. The plan was to build a general-purpose forge able to do everything from HT to welding (though the kaowool lining is just rigidized and coated with a mixture of porcelain slip and Zirconium Silicate, so would be unlikely to survive welding flux in its present form).

I wanted to see how wide the adjustment range was just on the choke, so set the gas pressure to 20 PSI.



The choke (air gap) was adjusted to get to 800 degC/1472 degF.



The flame was very smoky and yellow.

Temperature missed the target by 3 degC: between 5 and 6 degF. I think we can live with that.



After half an hour, it was still within a couple of degrees of the set temperature.



I opened up the air gap, still at 20 PSI, and waited a few minutes.



Temperature reached 1346 degC/2455 degF. Easily enough for welding.



This forge is only 5" inside diameter and 15" long; about 295 cu in.

The full range of useful temperatures was obtained only by adjusting the air. The gas pressure stayed the same throughout.

Although at 20 PSI, the pressure will seem high to some, the gas consumption is low because of the small jet size. Many smiths use .035 MIG tips as gas jets, which are around .040"-.042" internal diameter. At 20 PSI, my .020" gas jet flows the same amount of gas as the .035" MIG tip at 1.5 PSI.

To get the full useful temperature range, I am pretty certain you will need to be able to adjust the air: fuel ratio.

This almost certainly means either a blown burner or a really good Venturi burner. I have never seen a shop-built Venturi that I feel comes anywhere close to a commercial Venturi mixer in terms of performance and ease of use. The ones I use are from Amal (an old British motorcycle carburettor maker) and are probably not readily available on your side of the pond.



Stacy generally advises either a blown burner or a PID-controlled system.

I certainly agree that a blown burner is a great way to go; fine adjustment of both gas and air is easy to build in.

I am much less sure about PID control for a small forge; in my experience, it is very difficult to avoid temperature oscillation unless you scale things up to get enough thermal mass to damp any oscillation out. Then you no longer have a small forge.

I am a big fan of Venturi burners and suspect that spending a few hours working with a really good Venturi setup would bring him round. It certainly did for a couple of guys I know.

Apologies for the length and pic-heaviness of the post.
 
Tim,
Thanks for some great info. I usually avoid suggesting changing the injector, as most guys don't grasp how a venturi works, and fuel/gas mix ratio is a foreign language. I have suggested building smaller and larger burners for those who need both welding and HT in the same forge. Swap the burner when you switch the task.

On the PID control:
The need to avoid the cycling and large chamber temp swings was why I invented my two-stage burner ( which I am sure others have invented, but am not aware of it).
It cycles between a manually set low and high flame, instead of on and off. Once soaked on HI, you adjust the HI for a desired top range, say 1500F. Then you switch to the low control and set the burner to hold the desired LOW temp, say 1400F. When switched to PID, it will cycle the burner between the two adjusted flame settings. Thus, it will be very easy to hold the forge at 1450F without big swings in temperature. By setting the hysteresis, you can have the cycle range 3 degrees+ and 3 degrees - , and the temp in the chamber will change slowly up and down only a few degrees. The learning program of the PID will learn to set the burn time so it holds as close as +/- 1 degree. Even though the chamber may vary this tiny amount, the actual blade will stay more centerline, as it has more mass than the TC.


I purchased a superb lab use triple PID and controller with multiple readouts a year or so back. I plan on using it to make the setting of the two burner stages digital instead of manual.
Set HI
Set LOW
Set Target
Press "Start Program"
 
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