School Shop - Bad Forge - Burnt Knife

D

Don Nguyen

Joined
Oct 4, 2011
Messages
1,043
I have access to the shop at my community college again. They have a forge and power hammer there which I like to use. Two questions:


  • The forge runs really nasty. There is always gunk build up. I have no idea what it is, but apparently it's something to do with the crap lining the ceramic lab tech's put in the forges. I can do nothing about it. Is this threatening to the steel, possibly oxidizing it too much or doing other stuff?
  • -Yesterday I used the forge for the first time in about a year. It normally does not run that hot, maybe 1900 degrees F max. When I used it yesterday though, it was going way, way hotter than what I was used to and I carelessly burned a blade in half (I think it was due to a rainy day, or something). The tip crumbled off in the forge. How much if this blade is recoverable? Not all of it burned, but you can see how it was affected in the pictures.

voqrk50.jpg

sTZ5WgV.jpg

66HGYu2.jpg

BgsEUC6.jpg
 
I've had that happen before when doing outdoor demos, rain will screw stuff up and the forge will burn too hot. I think the atmosphere becomes much more oxidizing at these times, and it sounds as if the forge chronically burns oxygen rich. It's a venturi burner, right? If you can, add a choke plate to the burner and tune it as needed so that your work doesn't scale up harshly. I've used a bent piece of slotted tin in a pinch, over the venturi. Or, maybe you could retro it to have a blown burner...

I'd scrap the blade, it looks as if you may have massive grain growth in much of it, and steel is cheap at any rate.
 
The forge is actually a blower, but it still scales pretty bad.

And I'll just throw this one out, or give it to a classmate to destroy it.
 
Bummer? I'd ditch it that mess and start over with a new piece. Looks like you got some good hammer control and shaping done though!

Do you have the option or right to make adjustments on the burner when using the forge? Lowering the temp (less gas volume and/or pressure) and reducing the oxygen (slower blower or more choke) would be prudent if you can make adjustments like that. You might also want to consider bringing a piece of pipe you could put in the forge that you could then put your knife steel within to help buffer the negative impact on your steel of an inadequately managed forge chamber.

How are you doing or planning to do heat treating?
 
I can adjust the temp, but can't do anything about the blower. I'm not sure if I can put a pipe in there, because it would cramp up the forge (there's a bunch of other people using it at the same time).

HT I do it at my friend's shop who has some kilns, or I just send them out.
 
Just rig a flap over the intake of the blower, or really partially obstruct it with almost anything- it should help.

Whoever is the overseer of that workspace should be happy with you modding or fixing that forge in any way- a blown forge should have blast control of some sort.
 
To reiterate:
There are two things to adjust in a blown forge -
Air....The blower should have some sort of speed control ( a light dimmer works often); or a valve in the air supply line to restrict the air output; or a choke plate to restrict the air intake.
Gas ..... The pressure should be regulated to allow the burner to run at the desired temperature. From less than one pound to five pounds pressure is the normal range for a blown burner.

Both together should be adjusted together so the flame runs neutral or slightly reducing.

Your forge right now is running very oxygen rich and is probably making white fuzzy stuff grow on the insides as it burns up the lining and the burner flare. Any blades forged in it right now will probably be very poor. Fix it before you make any more.
 
Thanks Stacy, that's exactly what I was concerned about.

How bad does it effect the steel overall if it's oxygen rich?
 
In the photos you can see that oxygen was attacking the $%!# out of the surface, from the outside inward. As long as the steel was not overheated in the process, under the gnarly scale it should be salvageable if you had to... if the blade is burnt in half though, that's a good indication of massive grain growth. I had an embarrassing moment once at a trade fair demo, forging out a bullet cutting knife (think straight razor type blade) for a guy standing right there... a little rain was falling, and in the daylight I couldn't judge the forge heat spot on. I knew it was running junky, too much scale and all... I finish the tang and do a little side tap at a black heat to correct it straight, and the blade just snaps like a toothpick. Astonished, I picked it up and looked at the grain- kind of looked like cast iron. The guy stuck around and bought the next one though, it's been 6 years and he tells me it still works great for him...
 
That was cool of him to stick around for the next one :)

I'm curious though - if forge welding, the steel is brought to a really similar temperature isn't it? Wouldn't the grain size be large at that point as well?
 
Grain growth is time AND temperature dependent . When the forging begins the grains have a chance to reform to new grains..as recrystallization. Grain boundaries at one point start to oxidize [ they are more chemically active ] Then they will liquify then it's over !!
 
You must log in or register to reply here.
Back
Top