Google butt weld (flat end-to-end) and scarf weld (beveled ends overlapping) for illustrations
re: decarburization - 'splained by Steve Culver
In this photo of the defects in the barrel pattern, the condition was caused by decarburization of material that had been exposed to the fire for an extended period of time. Decarburization would have occurred over the entire surface of the barrel tube during the forge welding and shaping process of the tube. The decarburized outer material would be ground away during the finishing process on the tube. In the area of the flaw however, the exposed edges of the material extended below what would become the outer surface of the finished tube. Once the welds were closed on the decarburized material, there was no possibility to grind them away in the finishing process.
Another example which looks somewhat like Bro. Tom's barrel; at the tube segment weld
Threaded Mode