About 1045|Medium carbon steel. This is where you step up from mild steel territory. 0.45% carbon gives it significantly more strength and hardness than
1018, and it can be through-hardened (unlike 1018 which can only be case hardened). Machines noticeably better than 1018 because the higher carbon content produces shorter, cleaner chips that break instead of forming bird's nests. Feels more like cutting real steel instead of fighting taffy. The tradeoff is weldability. You can weld 1045, but it needs preheat and careful technique to avoid cracking in the heat-affected zone. Common for shafts, gears, bolts, and any part that needs moderate strength without going to an alloy steel. Can heat treat to roughly 50-55 HRC. Compare to
4140 (stronger, more hardenable, but more expensive) and 1018 (weaker but easier to weld and cheaper).
