4340 adds about 1.8% nickel to the chromium-molybdenum formula. This gives 4340 much deeper hardenability, thick sections harden all the way through and better toughness at high strength levels. 4140 is cheaper and sufficient for medium-strength parts. 4340 is chosen for large, critical, high-impact components.

After quenching and tempering, 4340 typically reaches 28–40 HRC depending on tempering temperature. It can be surface-hardened by induction or flame to 55–60 HRC. Nitriding achieves surface hardness up to 65 HRC with excellent wear resistance and no quenching distortion.

In the annealed or normalized state, 4340 has fair-to-good machinability. Pre-hardened 4340 (30–36 HRC) machines with carbide tools at moderate speeds and feeds. Fully hardened 4340 requires rigid setups and carbide or ceramic inserts. Most shops machine it annealed, then heat-treat, then finish-grind critical dimensions.

4340 can be welded but is more difficult than 4140 due to higher alloy content. It requires preheating (250–350°C), low-hydrogen electrodes, and post-weld stress relief to prevent cracking. For welded structures, lower-alloy steels are usually preferred. Welded 4340 should always be stress-relieved after welding.

Yes, 4340 is not stainless and will rust when exposed to moisture. Finished parts are protected with oil, paint, powder coating, cadmium plating (aerospace), or black oxide. Nitriding also slightly improves corrosion resistance. Store raw and finished parts in dry conditions with a protective oil coating.