Nothing makes a welder wince like a cracked weld. The good news, if you can call it that, is that cracks are rarely random. They come from a short list of causes, and every one of them is preventable when the person holding the torch knows what they are doing.
This is the sneaky one. Moisture, from damp rods, humid air, or a wet surface, breaks down in the heat and leaves hydrogen dissolved in the weld. As it cools, that hydrogen tries to escape and tears tiny cracks on its way out, sometimes hours or days later. The fix is boring and it works: dry rods, clean metal, right technique.
Steel that heats up to a liquid and then cools too quickly can turn hard and brittle, especially higher-carbon and thicker steel. Brittle metal cracks under stress. The answer is often preheating the piece before welding and letting it cool slow and even afterward, so the steel stays tough instead of turning glassy.
Not all steel is the same, and the filler rod has to match what it is going into. Use the wrong one and you build a weld that was fighting itself from the first pass. Knowing the metal you are working with is step one.
Welding over rust, paint, oil, or mill scale, or jamming two badly-fit pieces together and trying to bridge the gap, builds weakness right into the joint. Grinding it clean and fitting it right first is unglamorous and completely essential.
Here is the thing: the cheap weld that cracks is never actually cheap. You pay once to have it done wrong, again when it fails, and a third time to have it fixed right. We fix a lot of other people's cracked welds, and almost every one traces back to one of these four. Not sure how to tell? See how to spot a bad weld.
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