4340 Steel Properties

4340 is a medium-carbon nickel-chromium-molybdenum alloy steel (AISI/SAE 4340). It belongs to the ultra-high-strength structural steels and is valued for deep hardenability, high toughness at high strength, and strong fatigue performance. Typical uses are aircraft landing gear, heavy-duty shafts and axles, gears, crankshafts, and other parts that carry high stress in thick sections.

It helps to set expectations early. 4340 is not a tool steel. It is a structural alloy steel and is not designed for the wear resistance or hot hardness required for die and cutting work. If you are choosing material for a die, punch, or hot-work tool, see the comparison with tool steels further down.

International Equivalents

These are close equivalents rather than exact matches. SNCM439 tracks AISI 4340 most closely in terms of nickel content. GB 40CrNiMoA sits at the lower end of the nickel band, at 1.25 to 1.65 percent, compared with 1.65 to 2.00 percent for 4340, and 817M40 carries higher chromium. Confirm the exact specification when cross-substituting.

Chemical Composition

Values in percent. The combination of nickel, chromium, and molybdenum is what gives 4340 its character. Nickel raises toughness, especially at low temperatures, and works with chromium and molybdenum to push hardenability deep into the section. Molybdenum also reduces susceptibility to temper embrittlement and helps the steel hold strength during tempering.

Mechanical Properties and Heat Treatment Response

4340 reaches very high strength while keeping usable toughness, and that balance is its defining property. The figures below are typical ranges for an oil quench followed by tempering. Actual values depend on section size, exact tempering temperature, and the testing standard.

As-quenched hardness is roughly 55-57 HRC before tempering. Tempering trades hardness and strength for toughness and ductility, so the working point is chosen to match the load case.

Tempering Ranges to Watch

Two forms of embrittlement matter when tempering 4340. Tempering in the range of 230°C to 370°C can cause embrittlement of tempered martensite, lowering impact toughness even though hardness remains high. Separately, slow cooling through roughly 375°C to 575°C can cause reversible temper embrittlement, driven by phosphorus segregation at prior austenite grain boundaries. For impact-loaded parts, fabricators usually keep tempering below about 230°C or above about 425°C, and cool quickly from the high-tempering range. The molybdenum in 4340 reduces this sensitivity but does not remove it.

Hardenability: The Main Advantage

Deep hardenability is the reason engineers reach for 4340 over leaner alloy steels. The nickel-chromium-molybdenum system allows 4340 to transform to martensite throughout thick sections during an oil quench, so a large shaft or landing-gear component can achieve near-uniform hardness from surface to core. Leaner grades such as 4140 harden well in smaller bars but tend to soften in the center of heavy sections, leaving a softer, weaker core. Where full properties are needed in sections beyond roughly 75-100 mm, 4340 is often the practical choice.

4340 vs 4140

4140 and 4340 share the chromium-molybdenum base. The difference is nickel. 4140 has essentially none, while 4340 carries 1.65 to 2.00 percent, and that nickel is what separates them in practice.

Choose 4340 over 4140 when full hardness is needed in heavy sections that 4140 cannot harden through, when maximum toughness is required at high strength, when the part sees fatigue loading, such as aircraft components and heavy-duty shafts, or when low-temperature toughness matters. For lighter sections at moderate strength, 4140 is usually the more economical answer.

4340 vs Tool Steel

This is the distinction that brings many buyers to this page, so it is worth being direct. 4340 is not a substitute for tool steel in die and cutting work.

4340 wins where the part is structural and requires extreme strength and toughness at room or moderate temperatures. Tool steel wins when the part must resist wear, maintain a cutting edge, or remain hard at elevated temperatures. A cold-work die calls for D2 or A2. A hot-work die or extrusion tool calls for H13 or H11. Using 4340 in those roles results in premature wear and deformation.

Common Applications

4340 is found in aircraft landing gear and structural fittings, helicopter and powertrain shafts, heavy-duty axles and drive shafts, gears, crankshafts, high-strength bolting, and gun barrels. Fatigue-critical aerospace parts are often made from vacuum-remelted (VAR or ESR) 4340 to reduce inclusions and lift fatigue life, since cleaner steel resists crack initiation under cyclic load.

• 4140 Steel Properties
• 4140 vs D2 Tool Steel
• 4140 vs H13 Tool Steel
• What Is Tool Steel
• Tool Steel Grade Selection Guide