Ok, as a Material Science and Engineering grad, lemme try and straighten this out a little. In a metal such as these handle bars (I'm assuming low carbon steel) the only reason a weld would be stronger would be more material i.e. a weld bead that is thicker than the material. If there was a failure of the material post welding, it will be in the Heat Affected Zone (HAZ). This is the area adjacent to the weld where the material properties change due to the heating of the metal. All of the area that changes color next to the weld is the HAZ. The negative effects of the HAZ can be treated by annealing the entire piece post welding. This is typically done in a heat treating oven and IIRC is refered to as normalizing the material.
The reasson that welded handle bars will fail later down the road is that the HAZ is softer and will flex more during operation. This will propogate any microcracks in the material that are the result of the rearangement of the matrix of the material and the inclusion of impurities during welding. Eventually, the microcracks will grow and the material will fail. Usually under load---like cornering or hitting a bump.
Most of the clubmans that I have seen ARE welded. I don't know if they are normalized post welding, I doubt they are. How do they get away with that? Well, I think they use thicker material than stock bars. Typically, the materials used in bikes like ours are comprimise between cost, weight, and durability. The load on stock roadbike bars is not that high so factor in as light as possible and crappy steel and...well you get the point. They serve the purpose they were designed for but try to push the envelope and...
Hope that makes sense. ;D
