4340 Steel: Properties, Hardness and Heat Treatment

What Is 4340 Steel?

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 parts are aircraft landing gear, heavy-duty shafts and axles, gears, crankshafts and other components that carry high stress in thick sections.

The nickel is what separates it from a chromium-molybdenum steel such as 4140. At 1.65 to 2.00 percent, nickel raises toughness, works with chromium and molybdenum to push hardenability deeper into the section, and keeps impact properties useful at low temperature.

One point matters for material selection: 4340 is an alloy structural steel, not a tool steel. It is not designed for the wear resistance or hot hardness that die and cutting work needs. If you are choosing material for a die, punch or hot-work tool, the comparison with tool steels further down is the relevant one.

4340 at a Glance

Chemical Composition

Values in percent. The electric furnace grade E4340 uses the same ranges with tighter residuals, phosphorus and sulfur at 0.025 max each, and manganese at 0.65 to 0.85. It is the version specified where inclusions and fatigue life matter, such as aerospace forgings.

The nickel band is the number to check when a customer asks for 4340 and the mill offers 40CrNiMoA. At 1.25 to 1.65 percent nickel the Chinese grade sits at the bottom of the 4340 band, so a heavy section can miss the hardness the drawing assumes even though every element is inside its own range.

Tempering Response and Mechanical Properties

4340 reaches very high strength while keeping usable toughness, and that balance is its defining property. The table below is the standard tempering response for oil-quenched 4340, held at temperature for at least two hours.

Read the impact column, not just the hardness. Between 205 °C and 540 °C the hardness falls by about 160 HB while the absorbed energy rises more than twice over, which is why 4340 is specified where a part must survive shock at high strength.

Two embrittlement effects matter when tempering 4340:

• Tempered martensite embrittlement — properties trough in the 230 to 370 °C band, where impact toughness drops while hardness still looks healthy.
• Reversible temper embrittlement — slow cooling through roughly 375 to 575 °C after high-temperature tempering allows phosphorus to segregate at prior austenite grain boundaries. Fast cooling and double tempering limit it; the molybdenum in 4340 reduces but does not remove the sensitivity.

Because of the second effect, tempering below 455 °C is not recommended for 4340 where the part sees impact or fatigue loading. Where maximum strength is the requirement, such as some aircraft components, the low-temperature window is used with double tempering and a qualified process.

Hardenability and Section Size

Deep hardenability is the reason engineers reach for 4340 over leaner alloy steels. Oil quenching hardens round sections through 75 mm in diameter; larger sections need a water quench, which raises the cracking risk. Leaner grades such as 4140 harden well in small bars but soften in the core of heavy sections.

The loss across that range is about 5 HRC. Run the same comparison on 4140 and the drop is closer to 8 HRC with the core falling away much faster past 75 mm, which is the practical difference between the two grades.

After tempering the advantage holds. Oil quenched and tempered at 425 °C, 13, 38 and 75 mm bars of 4340 hold 1460, 1450 and 1420 MPa. Water quenched and tempered at 650 °C, 75, 100 and 150 mm bars hold 1055, 1035 and 1000 MPa at 340, 330 and 322 HB.

Equivalent Grades

DIN 1.6565 (40NiCrMo6) and BS 817M40 (EN24) belong to the same family and are the designations most often quoted alongside 4340 in Europe. These are nearest grades rather than interchangeable parts: the nickel band, the sulphur limit and the hardenability band all move between standards, and a grade that meets the chemistry can still miss the hardness at the section size the drawing assumes.

Heat Treatment

Two practice points that differ from 4140 and cost money when missed. First, the austenitizing range is lower, 800–845 °C against 830–870 °C, and the oil should be warm rather than cold. Second, 4340 is normally double tempered: the first temper converts retained austenite, the second restores toughness.

Welding 4340

4340 is not a prequalified welding steel. A weld procedure has to be written and qualified for the joint.

• Preheat above the martensite-start temperature, 290 °C (550 °F), so the heat-affected zone transforms to bainite instead of untempered martensite.
• Hold the preheat as the interpass temperature and reheat the part if it drops below.
• A low-hydrogen process and low-hydrogen consumables are mandatory.
• Post-heat no higher than 28 °C (50 °F) below the tempering temperature the part was given.

Machining 4340

In the annealed condition at 187 to 241 HB, 4340 machines at about 50% of the rate of AISI 1212 taken as 100%. That is slower than 4140 at 65%, and the nickel is the reason. In the quenched and tempered condition at 255 to 363 HB the rating falls further: expect lower speeds, more tool wear and a stiffer setup, but no distortion from a later heat treatment.

Parts commonly machined from 4340 include piston pins, bearings, gears, machine tool arbors, die blocks and cutting tool shanks, alongside the structural uses listed below.

Toughness and Fracture Toughness

Toughness is the property 4340 is actually bought for, and it trades directly against hardness. The table below is the same steel at three working hardnesses, with plane-strain fracture toughness measured alongside the impact energy.

Read across the row: from 380 HB to 550 HB the steel gains about 60 percent in strength and gives up half its fracture toughness, from 110 to 53 MPa·m½. Strength values in that table are estimated from hardness, so treat them as a mapping rather than measured data. Where a drawing calls for 4340 at 550 HB, it is calling for a part that tolerates little damage, and the design has to account for that.

A related grade worth knowing: 300M is a silicon-modified 4340, about 1.6 percent silicon with slightly higher carbon and molybdenum and a small vanadium addition. The silicon allows a higher tempering temperature at the same strength, which improves toughness. It is also more prone to decarburization during heat treatment, so surface protection matters more than it does on plain 4340.

4340 Against 4140

4140 and 4340 share the chromium-molybdenum base, and the difference is nickel. 4140 has essentially none, 4340 carries 1.65 to 2.00 percent, and that single addition is what separates them in a drawing review.

Choose 4340 over 4140 when full hardness is needed in a section 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 the more economical answer.

4340 Against 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 needs extreme strength and toughness at room or moderate temperature. Tool steel wins when the part must resist wear, hold a cutting edge, or stay hard when hot. 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 produces premature wear and deformation, not a cheaper die.

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 4340, VAR or ESR, to reduce inclusions and lift fatigue life, since cleaner steel resists crack initiation under cyclic load.

Standards and References

4340 is covered by ASTM A29/A29M and SAE J404 for bar stock, ASTM A322 for billets and bars, and AMS specifications for aerospace bar and forgings. The equivalent grades sit in JIS G4053 (SNCM439) and GB/T 3077 (40CrNiMoA). Certified mechanical properties should always be tied to a stated standard, a section size and a test location.

Data on this page:

• Tempering response, mass effect and heat treatment practice — ASM Handbook Vol. 4, Heat Treating, 4340 section.
• Annealed, normalized and tempered hardness bands, hardness by section size, machinability rating and typical parts — Cutting Data for Turning of Steel, AISI 4340 chapter.
• Equivalent grades and chemistry — Handbook of Comparative World Steel Standards (ASTM DS 67B), Ni-Cr-Mo steels table.
• GB 40CrNiMoA chemistry — 通用钢铁材料 (Chinese steel materials handbook).
• Welding practice — ASM Handbook Vol. 6, Welding, Brazing and Soldering.
• AISI 4340 reference card, PDF, 2 pages, 412 KB. The same tables, printable.

Related reading:

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