Coal forge welding and to much scale

[Karim]

Was working on a new project today and tried to weld bigger then i normally do, instantly noticed the extra time it takes the metal to come to forging heat but also the scale that was formed in the process of heating. Once the heat was right the scale already formed prevented to set the weld. It was formed before the first flux and only a flapdisk worked.. so i restacked and will try again soon.

Meanwhile i could use some pointers on the fire management department (eq http://www.bladesmithsforum.com/index.php?showtopic=29698) to prevent this heavy scale and burning the thinner layers the next time which was my last resort

Edited July 6, 2016 by Karim

[M. Cochran]

If you're getting scale before you get hot enough to flux you definitely have a problem. Lace your billet on the very top of the fire for a few minutes to start warming. After a few minutes you should be able to flux and place it in the fire. Be sure you don't place it close to the tuyere or you'll get too much air on the billet and it'll cause it to scale excessively or it won't get hot enough.

[Dan Waddell]

I've fluxxed cold mixing borax with kerosene, and a couple times with vodka even. That seemed to help a bit to widen the window for fluxxing hot. It sounds like you may be running your blast to high and rushing the heat, but take what I say with a grain of salt. What I think is a success now might be considered a mistake by someone with more experience.

[Dan P.]

Hold back on the blast- oxygen = scale.

Also, there is not only one welding heat. Get it hot enough and the scale will melt, but at these temperature wastage and burning become a hazard if you are not careful.

[Matthew Parkinson]

the shape and depth of the fire will need to be adjusted for larger work, build the fire up a bit let it coke up and try for a "cave , make sure you have enough burning coke BELLOW the work to burn off all the O2, and have plenty of coke above to insulate and block O2 from the top. same for a charcoal fire just deeper and most likely you would need firebricks to support the deeper fire.

MP

[Al Massey]

Make sure your coal is decent quality too. Too much sulphur and other crap and you're screwed from the get-go.

[Alan Longmire]

All of the above!

 

The big question other than the coal itself is, how deep is your fire above the air inlet? You must have at least 10cm (15cm is better, especially if you're using wood charcoal) of coke between the air and the steel, and a few cm of coke above the steel to seal out air. Go for as little air blast as you can to arrive at heat slowly. It should take a few minutes to bring the steel up to red, which is when you need to flux. Then it should take a few more minutes to bring up to welding heat. Also remember the higher the carbon in the steel, the lower the welding heat required. If you are welding high carbon to low carbon such as wrought iron, weld at the correct temperature for the high carbon steel. It will weld when it is ready even if the low carbon is not where it usually should be.

 

For instance, wrought iron usually needs a full white heat to weld to itself, but with a high carbon steel it will weld at a bright yellow heat.

 

That said, since you are asking about scale I think Dan P. is correct, you are using too much air and possibly not a deep enough fire.

[Karim]

Thanks all for the advices

 

The theories fit the example, will try more coal and more heat before sticking the metal in.

My pot is ab 7/8cm deep, air fully adjustable, coals are fine

 

[Alan Longmire]

Seeing your setup, too much air is definitely the problem. You do not need the whole floor of the firepot full of holes, just a couple of slots about 12mm x 40mm 12mm apart in the very center is all you need. The idea is to add just enough oxygen to make a hot fire with no excess.