Gas forge build advice

[Gerhard Gerber]

I've decided I have to use part of my December break to build a gas forge that will reach welding temp, and I have a volunteer who is a slightly better welder than myself to assist.

 

Been gathering what I need for several years, and recently got some refractory cement, one of the last outstanding ingredients.

My attempt at building burners were a complete flop despite all the advice I got here and from fellow knife makers locally, nobody can understand why they refuse to work, least of all me.....
I have two burners, gift from one of the locals, I know they can get a forge up to welding heat, but they are heavy on gas.

So I have the following:
8 x fire bricks, the crumbly, white real thing

6m of Kao wool, basically enough for 2 forges I think.

25l refractory cement rated for 1600C

2 x burners.

Tried but I can't find old LPG bottles to use for the chassis, maybe not a bad thing.
Might be able to get sheet steel rolled (but not yet welded) into tubes.

I'm basically starting with a clean slate and no restrictions at this stage, my questions are many......

Firstly, do I incorporate the fire bricks or leave them for something else?  I was thinking I could use them along the sides for extra insulation.

Do I build a square forge, or round on top with a flat floor?

I want to build doors front and back using fireplace bricks.

I would really appreciate opinions on the basic design, and especially how to best utilize what I have.

No use asking the people working where I purchased the refractory cement, and tips on how to mix that up, what consistency to aim for etc?

Thank you all....... 

[Matt Walker]

4 hours ago, Gerhard Gerber said:

I'm basically starting with a clean slate and no restrictions at this stage

Have you considered a blown forge? Just one opinion, Blown forges are simpler to run, easier to control the atmosphere, and generally less fiddly. This one has made lots of good clean damascus https://www.youtube.com/watch?v=kmunsYUvA_Q . Fire bricks make good doors. An air curtain is a wonderful thing. https://www.bladesmithsforum.com/index.php?/topic/25573-blown-burners-the-care-and-feeding/#comment-241196 I prefer an impeller blower over a squirrel cage for this application.

[Pieter-Paul Derks]

I use a vertical forge with a blown burner and I really like it, but it is more complicated than a venturi forge.

 

With the materials you have I would go for a smaller sized forge, I do not know the size of your burners, but sometimes one burner is enough for a welding forge.

I would go for a round forge no bigger than you need with the burner at a tangent to make a nice swirling flame, making a flat floor is not neccesary in my opinion.

 

My main advice is to not overthink it, gas forges are quite simple and will not last forever, and you will probably want to make adjustments after trying it a few times, like making the doors bigger and such.

 

 

 

 

[Gerhard Gerber]

On 11/11/2022 at 5:34 PM, Matt Walker said:

Have you considered a blown forge? Just one opinion, Blown forges are simpler to run, easier to control the atmosphere, and generally less fiddly. This one has made lots of good clean damascus https://www.youtube.com/watch?v=kmunsYUvA_Q . Fire bricks make good doors. An air curtain is a wonderful thing. https://www.bladesmithsforum.com/index.php?/topic/25573-blown-burners-the-care-and-feeding/#comment-241196 I prefer an impeller blower over a squirrel cage for this application.

I got the two venturi burners from a guy who converted his forge to a blown setup with a ribbon burner, runs better and uses less gas according to him.

Problem is I've been unable to find an appropriate blower, would say not for love or money, but actually money is exactly the problem.  Everything I found locally was ridiculously expensive, same reason why my neighbour brought one in in his suitcase after a visit to the UK.

So round is best?

Any use incorporating the fire bricks, or is the kaowool with refractory good enough? 

[Alan Longmire]

The wool with refractory should be enough.

[Kreg Whitehead]

I just went simple ...old propane bottle....2 burners.....straight down flame....no ribbon burners...blowers ect.

Now that I have 20 /20 hindsight vision my front burner will get moved and inch or 2 fwd when I re do it

Mine is all wool and cement....but the floor is pretty much flat.

I initially kinda kicked myself for not doing the angle burner dealio to try to avoid hot spots.

Having said that I just work with/around them.

The way blades are shaped and beveled ect......they arent going to heat up evenly even in a perfect environment.

Soooooo if you dont have a hot spot.....how do you deal with that?

Its an honest question and just throwing it out there......I kinda like my hot spot(s).

Edit....my forge will hit forging temps no problem.

I melted part of one of the bells on my burner off first time I tried to weld something.

Edited November 14, 2022 by Kreg Whitehead

[Matt Walker]

33 minutes ago, Kreg Whitehead said:

Its an honest question and just throwing it out there....

For me having mostly worked with making damascus long even heats are my friend and scale is my enemy. Always being hotter in the middle I used my long hot spot to get the ends matching the center of the billet before a forging cycle.

 

 

10 hours ago, Gerhard Gerber said:

So round is best?

Again this is just one opinion. Yes round is better if you bring the burners in tangent to the ID. My reasoning is more time for the oxygen to be consumed before the heat reaches the metal. I'm neither a physicist or a chemist so I can't prove it but it's another one of those faith things. Like wearing dirty socks because you won the last game. Yes we accept dragon breath indicates a reducing atmosphere. But think a second, that is at the door, we don't have a good way to consider the atmosphere at the workpiece.

 

Sorry about the blower issues. Keep your eye open anyway because you'll want one for an air curtain eventually. Gas forges are terrible for heating your work holding handle or tongs too far back. An air curtain is a luxury thing that lets you hold closer to the work and controls the heat hitting you. 

[Dan Hertzson]

There are a wide variety of castable refractories as well as a variety of refractory insulation blanket.  How well they will work for making a forge for pattern welded billets varies quite a bit depending on what you chose and, to some extent, whether you use flux.  For the typical borax fluxed billet it is important to have a refractory surface that is resistant to the molten glassy material that the flux creates.  Usually that is a high alumina castable.  I like Kastolite 30 because it is a high alumina insulating castable material that has a heat transfer coefficient almost as good as insulating firebrick.  I like a 3/4" thick cast liner made out of this material, though you can get away with 1/2" if you do a good job casting and you have the correct refractory blanket.  It is easy to mix too much water into the castable, which should be mixed to a consistency where if you form a handful into a small ball and toss it gently up above your hand it stays together.  Too little water and it will break apart quickly.  Too much and it won't even form a good ball.  Then it should be packed into a form with removable sides...

 

Note: refractory cement is not the same thing as castable refractory.  Refractory cement is make for mortaring bricks together (like Satanite).  If it is rated for 3000 deg. F it can be used (applied in very thin layers like paint till you build up at least 1/8" thickness), but in my experience cracks pretty easily and has limited to no flux resistance.

 

The refractory blanket comes in different flavors as well.  My recommendation would be for a 2" thick layer of 2600 degree blanket at 8# density.  You can use (2) 1" layers if that is what you have.  

 

Also note: not all insulating (soft) firebrick is the same either.  I used some rated for 2300 deg. F on my first forge and melted them with the flame from my burner.  Try to keep the flame (which is in excess of 3000 deg. F) from directly impacting the forge liner unless you have a castable rated appropriately.

 

Tangential burner orientation is a good idea to keep the flame dwelling inside the forge as long as possible (for maximum heat transfer) and to avoid having the flame directly hitting your stock.  The tips of these flames usually have a bunch of superheated uncombined air which will increase scale production on your stock surface. 

Edited November 15, 2022 by Dan Hertzson

[Gerhard Gerber]

Thanks Dan, Matt, Kreg & Alan

The Kaowool I have was a combined purchase with another maker, he gets welding heat, but that's with the blown ribbon burner.
The cement I got is a complete unknown quantity apart from the temperature rating which is in the ball park.

Can't find a gas bottle to buy, but I'll know this week about the rolled sheet steel.

I hate getting good advice and not being able to follow it, the only blower I could source locally is used on Jacuzzis and very expensive, 3rd world joke I guess....at my expense 

[Dan Hertzson]

One potential budget source for burner blowers is to contact local HVAC repair shops or scrap yards.  Blowers used in the burners for gas fired furnaces and boilers with the same heat output as your burner design (say 150-250 MBH) should be able to work.  Note: not the furnace fans for distributing the air, the combustion blowers.  As a bonus, the newer ones may also have speed controls, though you will have to play with the controller board a bit.  These can sometimes be available very cheap from a scrapper or HVAC repair shop where they are discarding the unit due to a cracked heat exchanger...  Years ago I got a great radial centrifugal blower from a liquidator for $25.

 

I use one of these in my shop (now $575, but around $350 when I bought it years ago).  It supplies more than enough air for two or three forges:

[Gilbert McCann]

This may not be popular, but how about a hair dryer I have a friend thats been using one for a while. Julian Antunes made or maybe still makes some incredible stuff with what appears to be a hair dryer. I think just don't restrict the flow. It should be way more efficient than a venturi set up. 

[Sean Blum]

If you have access to automotive shops or suppliers you could see if they have any old or new portable air tanks. They can come in a few sizes but are generally cheaper new than LP tanks if that's necessary and safer to modify if used than LP.

[vlegski]

 Just an aside. FYI ... A gentleman ( Frosty) on the  I forge iron forum,  built what he calls a naturally aspirated ribbon burn, NARB. I don't remember if he got welding heat from it.  It's an older thread but he details his build. It might be of use in your search for a ribbon burner. I use his burner head details with a small  blower in my forge. 

[Dan Hertzson]

 Also, over here a lot of folks use the helium tanks that are supplied with party balloon kits.  They are a little thinner than propane tanks, but a lot safer since they only held an inert gas.

[Alan Longmire]

Another potential cheap blower source: old photocopiers.  They have a strong blower, but are a pain to remove without destroying.

[Welsh joel]

I actually use an inflatable bounce house blower in my forge. I got it free from someone who had a rental company- and had had one destroyed.

 

It's really too much air for mine, and I forge weld with it- with a valve open, reducing air.

Edited November 16, 2022 by Welsh joel

[Matt Walker]

I'm also a fan  of the bouncy house blowers. 

[Dan Hertzson]

11 hours ago, Welsh joel said:

inflatable bounce house blower

I've heard good things about those also.  When I purchased my light industrial blower I needed it rated for continuous service, which I doubt the bounce blowers will do.  For a hobby smith that is not likely to be an issue.

[Gerhard Gerber]

We have a bouncy house blower at work used for some advertising equipment, threatened to steal it more than once! 

They are very expensive, about 5000 in local monopoly money.

Jacuzzi blowers were 3500 last time I checked, expect them to be 4000-4500 now.

Thanks for the ideas, NARB is sounds interesting.
I should know about the rolled steel plate by the end of the week, I assume 2mm thickness is sufficient?

[Alan Longmire]

2mm is more than fine.

[John Page]

I just finished up a ribbon burner build, the first time I have really spend time using them. The forge build itself was designed for huge billets for a project on the near horizon that requires hundreds of pounds of steel broken into ~15kg billets. Really massive project. Anyway, I made the mistake of using too much castable refractory and although it is good to have the thermal mass when adding that much cold metal, it takes almost an hour to get up to temp and stays hot for more than 6 hours after turning it off even keeping the fan running. I tried the smallest bouncy house blower I could find (around 130cfm, nor sure what that translates to) and even with an inline speed control to reduce the power to less than half it was still far too much. In the end I got a three way valve marketed for pool supply and vent almost 90% of the air out the side and, with the burner in a smaller forge I can get to an extreme welding heat in maybe 15min. Moral of the story for me was having as small of a forge chamber as you can get away with, more kaowool than castable, and devoting a solid amount of time to getting the fuel/air right. It's probably simpler with a blown forge that uses a single or double orifice rather than a ribbon burner but I have otherwise only ever used venturi forges which by comparison are easy. 

[Jaro Petrina]

Why not go to scrapyard and get a car turbo. 

[Gerhard Gerber]

Took a friend over to my "neighbour" for a forging lesson yesterday afternoon, most of the forge-side chat was about the new forge.

We fixed the size of the steel plate so I should get a price for the rolled steel today, but neighbour Mike has an exploded hot water geyser, and the steel tank is about the perfect size.

I'm not one to ask advice and not take it, I know the venturi burners are second place, so Mike's going to try bring back more fans from the UK.

Plan now is to start building in December, get the forge and ribbon done, then do the plumbing mid-January when he gets back.

 

My current small gas forge is incredibly noisy, it was on for about 6 hours Saturday and it's a relief on the ears when you turn it off.
The two monster venturis I planned on using is even louder, so even if the fan import flops for whatever reason, better to build a ribbon burner forge and keep scratching for a fan.

[Gerhard Gerber]

Got my fan from the UK, we had to pay import duties so it was quite a bit more than I planned. 

On second thought it was the same price as a jacuzzi pump several years ago, so not bad considering inflation. 

Please tell me it wil do the job? 

[Alan Longmire]

That will work fine.