About 4130|The weldable chromoly. Same chromium-molybdenum family as
4140 but with 0.30% carbon instead of 0.40%. That lower carbon makes a real difference in weldability and ductility while still giving good strength after heat treatment. This is the aerospace tube and race car chassis steel. 4130 tube is everywhere in aircraft frames, roll cages, and structural fabrication where you need to weld chromoly and trust the joint. Machines well in the annealed condition. Not as strong as 4140 when heat treated because of the lower carbon, so if you're making a shaft or gear that needs maximum hardness, go with 4140 or
4340. But if the part gets welded, 4130 is almost always the better choice. Compare to 4140 (stronger but harder to weld) and
1045 (cheaper, similar strength range but no alloy content for hardenability in thick sections).