propagation unless there is a large thermal shock or enough stress from thermal growth over cycles.. however, the cycles are at most a couple a day, and are slow, and even over years wont amount to anything considered high cycle. You could try shot peening the outside surface or a heat treatment to relax the stresses induced from welding... I just bought a set of these manifolds and will eventually do this to them, do you know the specific metal composition? You say carbon steel, what standard?
