About 4150|4140's stronger sibling. Same chromium-molybdenum alloy system but with 0.50% carbon instead of 0.40%. The extra carbon gives higher achievable hardness (up to HRC 55+ vs 50-55 for 4140) and better wear resistance, but at the cost of reduced toughness, ductility, and weldability. Used for parts that need to be harder than what 4140 can deliver: heavy-duty gears, high-strength shafts, and tooling. More difficult to machine than 4140, especially in the hardened condition. Needs careful heat treatment to avoid cracking. If you're specifying 4150, make sure you actually need the extra hardness. Most applications are fine with 4140 at the right temper. Compare to 4140 (tougher, easier to work with, usually sufficient) and
ETD-150 (which is essentially elevated-temperature-drawn 4150 chemistry).
