I strongly advise drilling a hole either end of the crack, seems to help stop it cracking more when you weld it. You didnt say if your rods had been pre-heated, im guessing not, and therein lies your problem. They need to be hot.

I've never welded cast iron, but I thought preheating the rods and the part, plus peening the weld as it cooled was best practice? Hopefully someone with first hand experience can provide some tips!

I got a bit over ambitious with my chinese sheet metal folder and cracked the casting clean in half……. I was going to replace it with solid bar, but it woudln't hurt to have a go at welding it – it would save me £30 and a fair amount of swarf….

Edited By Russ B on 29/10/2019 10:10:51

I've never bothered with proper cast iron welding rods for home shop repairs. There is no doubt that the proper rods in skilled hands produce a stronger job but if you don't have the experience and don't use the correct procedures they can be serious trouble.

My method is to use a 1.5 mm ordinary general purpose rod at the lowest practical current to build up thin layers of weld on the two sides of the crack. Each layer should be peened with the chipping hammer before it cools off. This reduces cooling stresses but means you can only do the weld in short strips. Make sure everything is cool before adding the next layer. Once you have 3 or 4 layers each side do the final fusion in the normal way with a larger rod but don't use too thick a weld. Most stuff I seen needs three or more runs. Peen between runs as before and let everything cool off before the next run.

Yup it takes ages with all the waiting around for cool off but it does work.

Theory is that it sidesteps the two major issues of carbon migration and cooling contraction leading to cause failure if you simply weld cast iron like its steel. Carbon migration produces very hard, brittle joint lines which fail easily under stress. Cooling contraction guarantees plenty of stress. Carbon migration needs high temperatures. The thin layer build up process minimises temperatures and, once the first layer or two is done blocks the weld region from further migration. Peening helps counteract contraction stresses. Being thin layer means less stress anyway. Once you have a few layers of weld on the iron the final joint is weld metal to weld metal so that behaves just like an ordinary joint. Multiple thinner runs peened when hot again keep the cooling stresses down.

This is an official, albeit old fashioned, technique for field expedient repairs. That said I'd be chary about using it on anything liable to take serious tension loads. Frankly I'd be worried about serious tension loads on any home repair.

Lots easier with an inverter welder than buzz box 'cos you can turn the current right down.

Clive

Back when I was doing repairs off all types we often got broken castings to repair.

we always discussed the job with the client to make sure that welding was the best (or only!) option.

If a weld repair was all that could be done then we would usually use a high nickel content rod as it was easier to machine after the welding. What we always did is use a welder with a high voltage which made it easier to strike and maintain an arc. A really old Murex/BOC welder was the favorite tool with up to 120 volt AC or DC. Probably illegal now!

A modern inverter welder works really well when set at a high frequency and with the proper rods.

If you do it right it will be as good as the original casting.

I've tried welding CI over the years; some were ok others were not, I would recommend you need to preheat the area & weld similar to Vintage Engineer's method. As there seems to be no pressure or liquids involved, & for the size you say I would simply cold stitch it.

George.

I needed to do a quick repair many years ago on the oil tank on a Ellison starter as I remebr I vee'd the joint preheated both tank and rods. The weld looked really c*ap but the tank did not leak and although I purchased a replacment tank I never fitted it.

H

Unless you're really on top of your game a better option might be brazing. Even with nickel rods, welding CI often produces brittle carbides at the edges of the weld. Brazing avoids that as the temperature is so much lower.

The last time I welded cast iron it was in the form of an ornamental clock face some three feet across, after many cracks appeared in places other than the areas being welded I resorted to a degree of pre and post heat and ultimately covering the whole casting in agricultural lime (slaked) keeping this is hot as I could with a spreader torch for two hours after welding had finished. The shape of the casting, many interconnecting segments, may also have caused uneven stresses to build up in adjacent areas not subject to welding heat. As this casting was more of an ornament than substance my efforts were acceptable I feel.

If pressed again to tackle a similar job I would concentrate more attention on pre and a longer post heat.

Guy