>>1556548Steel is very forgiving with dents, Alu is not.
When buying a steel bike I check the lugs, if they are tight and all the brazing intact, check for rust inside the frame and check if the frame is straight.
Steel is either lugged (mechanically the best), fillet brazed (most beautiful imho, hardest to make) or welded. Welding is only possible with certain steels and welded frames are normally a little heavier and a little cheaper than lugged/brazed frames because it is easier to make them with automated technology and because steel suitable for welding is a little less strong than steel for brazing.
The big 3 steel tube makers are Reynolds (UK), Tange (Japan) and Columbus (Italy), see any of their stickers on a frame, you know it is quality. There are, or were, more manufacturers like Dedacciai, Mannesmann etc. but those are rare.
The most basic quality tube steel is 25CroMo4 alloy, known as CroMo and comes butted (single, double or triple), non butted tubes are known as leadpipe for their weight and many off brand bike manufacturers will use such steel
Alu is more complicated, an invisible hair fissure can kill the frame, normally I don't buy used Alu, and if I do I only buy perfect frames, preferably bikes that have not many miles on them. I'd only buy top notch Alu frames, as most alu frames are industrially made by welding robots and QC is always an issue.
