As told by one of my predecessors who was taught to build by a Phd metallurgist said the steel alloy did not mean much in the overall strength, but said it was all in the heat treatment. The only way to preserve the heat treatment was to keep the joining temp. under 1350 degrees. As for the best range for strength, I had a long talk with Lon at Nova he told me about some independent testing they had done and the tubing that faired the best was in the 175,000 range.