Another difference between bicycle tubing and general plumbing pipe is the finishing that it gets before being made into a frame. While the lower cost Hi Ten bike tubing (and some pretty nice 4130 bike tubing) is usually seamed (like generic plumbing tubes are), the after welding the seam finishing is FAR more then plumbing tubes get. So the roundness and evidence of the welded seam is pretty much gone (on the outside). The residual stresses from the seam will be much more "worked out".


All steels are graded/defined with SAE numbers, here's a link to a description https://en.wikipedia.org/wiki/SAE_steel_grades


Remember that these qualities are prior to any actual fabrication (being done on test samples) and after welding, brazing, bending that a bike frame goes through when made. So the real differences between frames of different grades of steel are more about weight, resiliency then strength (as a structure). Some might say that the lower grades of steel (and with their thicker walls) make for a frame with greater ability to survive the test of time. Andy.