After realising that it was 22 years ago I had last done any brazing/welding of cast iron, I decided to read up a bit on the subject. Basically the advice was to:
1) Use a flame with oxygen surplus.
2) Preheat the parts to 350 - 400 dgC.
And off course the usual stuff about chamfering and how to hold the flame (15-30 degree angle).
I mounted two pieces of angle bar in a vise on the deck. One would hopefully keep the sole falt, and the other should keep the plane aligned sideways.
A fairly important trick for cast iron is to first use a flame (oxygen surplus) directly on the crack, to burn away any carbon deposits.
After this I did my best to preheat evenly to 400 dgC. I then started brazing with a special filler material intended for cast iron. The biggest challenge was that I couldn't flip the plane on the side to do the major parts of the crack, and this particular type of filler works best if applied on a horizontal piece.
I did the best I could, and at a point I decided that I couldn't do much more.
The next suggestion from the welding handbook was to let the item cool slowly packed in dry sand or in some insulation material. I elegantly skipped that part and just reduced the intensity of the flame and heated a bit on the plane now and then till I reached 150 dgC.
After that I just let it cool down.
Once cold I rinsed it off in water and took it to the workshop.
The sole had a cup in the middle, so I flattened it using a file and later some emery paper. It still has a small area behind the mouth where it is possible to slip a piece of paper under. But I think it might be OK despite that. If not I'll just flatten it some more later.
Sideways it was also a bit off, but it was mostly on the forward most part of the plane. I flattened until I could see that the nicker was in line with the aft part of the plane, and then I stopped. I have never used the plane with the blade in the forward position, and I doubt I ever will, so no need to fuss to much over it.
The conclusions to the project:
By using the cast iron filler instead of the bronze filler, the repair job is a little bit more hidden, and a bit stronger.
The structured original paint of the plane is destroyed. And the entire plane needs to be washed again because it makes your hands dirty when you try to hold it.
It is possible to repair a broken cast iron plane. BUT it is better if you don't drop it in the first place!
Plane positioned on angle bars.
Burning away the carbon deposits.
How I checked the temperature (this is cooling down).
First shot of the repair job.
Flattening the sole, pen marks show my progress.
I did flatten it more, but didn't take more pictures.
Light colour shows where it is flat.
Repair job on the inside.