Posted by David Sherman on Tuesday, June 20, 2006 at 10:49AM :
In Reply to: Cast iron.... posted by Peter Sprouse on Monday, June 19, 2006 at 9:43PM :
Broken ast iron can be brazed so it's as good as new. The brazing material is actually stronger than the iron, so if you can get the entire crack wetted by the brazing, it will be as strong as it was before it broke. The important things are to get all dirt and oil out of the broken surfaces(tough if it's a gearbox housing, but do-able for everything else) and to heat the whole part relatively evenly before you start brazing. The shape of the part has a lot to do with how much you have to heat. If the break's in the middle of a long thin piece, just heating the broken ends will be good enough because it can expand and contract freely. If the crack only goes part way across or it's more than a couple inches long, you'll need to heat the whole cracked area to a dull red before you commence brazing. Use extra powdered flux beyond what's on the brazing rods. Grinding cracked surfaces can cause trouble with adhesion because it can smear the graphite across the iron, but sometimes I've done it without a problem, maybe because I use a coarse fast-cutting wheel. Once the part it brazed, before it cools below a red heat, bury it in dry sand or vermiculite so it can cool slowly and evenly. I've done this with many broken cast-iron parts and so far they've all stayed fixed.