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4340 Alloy Steel (Q&T)

4340 is a nickel-chromium-molybdenum alloy steel supplied here quenched and tempered to about 40 HRC, delivering 1280 MPa tensile and 1100 MPa yield with 12% elongation. That combination of very high strength and genuine toughness is why it is the classic choice for heavily loaded shafts, gears, and aircraft landing gear.

How 4340 Alloy Steel (Q&T) machines

At 2.5/5 in the Q&T condition, 4340 is harder to machine than mild steel because it is already hardened to roughly 40 HRC. Use rigid setups, sharp carbide tooling, moderate speeds, and good coolant. Where extensive machining is needed, parts are sometimes cut in a softer pre-hardened state, then finished after heat treat.

Manufacturing & processing

4340 is supplied as forgings and barstock and is defined by its deep hardenability — it through-hardens in thick sections via quench and temper. It can be nitrided for a wear-resistant surface. Corrosion resistance is low (1.5/5), so finished parts need plating, oiling, or paint for protection.

Typical applications

The standard for high-strength, high-toughness rotating and structural parts: power-transmission shafts, gears, crankshafts, connecting rods, landing gear, and high-stress fasteners. Best where the part sees heavy cyclic and impact loading and needs strength well beyond what medium-carbon steel can provide.

When to choose it

Choose 4340 Q&T when you need maximum strength combined with toughness in a heat-treated steel — heavily loaded shafts and gears especially. If loads are moderate, 4140 or 1045 are cheaper. If you want high strength without in-house heat treat, 1144 Stressproof avoids the quench-and-temper step.

Suitable surface finishes

Common finishes for 4340 Alloy Steel (Q&T): zinc plating, black oxide, powder coating, nickel plating. Use the finish selector →

FAQ

Why choose 4340 over 4140?
4340 adds nickel for greater toughness and deeper hardenability, letting thick sections through-harden to high strength while staying tough. It is the choice for the most demanding shafts, gears, and landing gear. 4140 is cheaper and adequate when loads and section sizes are more moderate.
Should I machine 4340 before or after heat treatment?
It depends on tolerances. Roughing in a softer condition then finishing after quench and temper minimizes distortion issues, but at 40 HRC machining is harder (2.5/5) and may need grinding for tight features. Many shops finish-machine in the hardened state with carbide tooling.
Is 4340 corrosion resistant?
No, it rates 1.5/5 like most alloy steels and will rust without protection. Landing gear and shafts in 4340 are typically plated (such as cadmium or chrome), nitrided, or painted. Where corrosion resistance is essential, a high-strength stainless like 17-4 PH is an alternative.

Property values are typical/nominal for early guidance and vary by temper, grade, supplier and heat treatment. Confirm critical specs against a certified datasheet or with an mfgiq engineer.