About 4140|The workhorse alloy steel. Chromium-molybdenum with 0.40% carbon gives you excellent strength, toughness, and wear resistance through a wide range of heat treatments. Available annealed, normalized, or pre-hardened (Q&T to 28-32 HRC). Pre-hardened 4140 machines well with carbide at moderate speeds. Don't dwell or take light cuts. It will work-harden if you rub instead of cut, so keep the feed up (0.006-0.010 IPR minimum) and maintain a productive depth of cut. Chips break cleanly. Weldable with preheat and proper procedure, but it's not casual. The 4140 you buy at the distributor is not all the same. 'Pre-hard' is vague. Ask for the actual hardness and MTRs. Compare to
4340 (stronger, tougher, more expensive),
1045 (cheaper but less hardenable),
8620 (case-hardening steel, different use case), and
ETD-150 (pre-hardened alternative that skips heat treat).