Multipass welding

You do multi-pass welding to improve the weld metal structure. Each new pass reheats the one below, refining the grain. The result is a tougher, more stable weld that won't crack as easily. Make sure the interpass temperature doesn't get too high, otherwise you'll get coarse grain again and lose the benefit. So wait a bit until the previous pass cools down some, but don't let it cool completely. If you build the weld cleanly in narrow, even passes and clean each pass well, you'll have much less risk of cracks and lack of fusion.