niggle wrote:Or is it about the strength? Stonger steel can be made thinner and therefore lighter of course....
up to a point...but the strength and toughness of a bike frame is normally limited by the post weld/post braze strength of the tubing next to the joints. Although the tube manufacturers make much of the pre-braze/pre weld strength values, they don't shout as much about the post-joining values, or if they do, they sometimes even fudge the data.
Fudge the data? Yes, definitely. If you drop hardness indents in once every 1mm or so you can make a pretty graph but you are liable to miss the narrow regions of hardness that can and do exist in low heat input weldments. Worse still the HAZ can have regions of low toughness in it as well.
So if you see a hardness plot across a (single pass) weld and there is no horizontal axis marked (or there is, but there are not many data points), be suspicious. A pretty sure sign that something is screwy is when the peak hardness and/or minimum hardness values on one side of the weld seem different to the other side; unless there is a good reason for it to be otherwise, they should be the same.
Similarly if you see a hardness plot for a brazed joint, it needs to extend as far as the HAZ does, which can be several inches. Again the data can be fudged, this time by omission.
Weld hardness is a good guide to strength but an unreliable indicator of toughness.
Furthermore most of the steels used in bike frames are sensitive to cooling rate; you can get substantial variations in post-weld strength and toughness with variations in cooling rate.
So I'm not saying 'don't believe the hype' but I am saying 'take it with a pinch of salt'....
cheers
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see here:- 
