Steel 4340M · heat-treated state
Steel 4340M Q&T
Properties of the Q&T condition, compared with the other 4340M tempers.
The standard 300M service condition (AMS 6419): highest strength/toughness balance for landing gear, actuator rods and shafts.
What is 4340M Q&T?
4340M Q&T is 4340M heat treated to Q&T — the standard 300M service condition (AMS 6419): highest strength/toughness balance for landing gear, actuator rods and shafts. 4340M Q&T has a yield strength of 1,620 MPa (235 ksi) and a tensile strength of 1,930 MPa (280 ksi) — stronger than 98% of steel grades. Elongation at break is 10% and the elastic modulus is 205 GPa (29.7 Msi). 4340M Q&T has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F). HR is the default condition FabDigit quotes; Q&T is available on request or by drawing note.
Advantages
- High strength — 1,620 MPa (235 ksi), better than 98% of steel grades
- Polishes to a fine finish — 75/100, better than 94% of steel grades
Limitations
- Difficult to machine — 12/100, worse than 99% of steel grades
- Difficult to weld — 10/100, worse than 99% of steel grades
- Limited formability — 8/100, worse than 97% of steel grades
- Limited ductility — 10%, worse than 91% of steel grades
- High embodied carbon — 2.6 kg CO₂/kg, worse than 86% of steel grades
4340M Q&T properties
Typical room-temperature values for 4340M Q&T — 15 properties are specific to this condition; the rest are grade-level values shared by every 4340M temper. Each bar shows where the value sits among the steel grades in FabDigit's library — further right is higher.
Physical3
4340M Q&T has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
4340M Q&T has a yield strength of 1,620 MPa (235 ksi) and a tensile strength of 1,930 MPa (280 ksi) — stronger than 98% of steel grades. Elongation at break is 10% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
4340M Q&T is rated for continuous service to 290°C (554 °F). It conducts heat at 33 W/m·K (19.1 BTU/hr·ft·°F), worse than 80% of steel grades. Thermal expansion is 11.7 µm/m·K (6.5 µin/in·°F).
Electrical3
4340M Q&T conducts electricity at 7 % IACS, worse than 74% of steel grades.
Chemical & Environmental3
Corrosion resistance is not recommended (14/100), better than 70% of steel grades.
Sustainability4
Producing a kilogram of 4340M Q&T takes about 36 MJ of energy and emits 2.6 kg of CO₂ — more than 88% of steel grades. Typical recycled content is 25%.
Manufacturability8
4340M Q&T's machinability is not recommended (12/100, worse than 99% of steel grades); weldability not recommended (10/100); formability not recommended (8/100).
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Other 4340M tempers and conditions
4340M is also supplied in 8 other conditions. The full side-by-side table is on the 4340M overview.
- HRDefaultMill product as rolled or forged — air-hardened martensite, too hard and brittle to machine; always annealed or normalized before use.1,150 MPa yield · 47 HRC
- Q&T-2070Maximum-strength condition for weight-critical rocket motor cases and springs; accept lower toughness and high notch sensitivity.1,720 MPa yield · 56 HRC
- VARConsumable-electrode vacuum remelted stock — choose for fatigue- and fracture-critical aerospace parts needing best transverse ductility and low inclusion count.1,640 MPa yield · 54 HRC
- Q&TThe standard 300M service condition (AMS 6419): highest strength/toughness balance for landing gear, actuator rods and shafts.1,620 MPa yield · 54 HRC
- Q&T-1790Slightly higher temper for parts where fracture toughness and ductility matter more than peak strength.1,520 MPa yield · 51 HRC
- NormalizedNormalize ~925 °C then temper for uniform grain and moderate hardness — a machining/stock condition and a grain-refining step before final hardening.965 MPa yield · 40 HRC
- CDSmall-diameter annealed bar/wire cold drawn to size for tight tolerance and better surface before heat treatment; rarely stocked in this grade.860 MPa yield · 31 HRC
- AnnealedSoftest supply condition — pick for rough machining, deep drilling and forming before final quench and temper.690 MPa yield · 26 HRC
Working with 4340M Q&T
Is 4340M easy to machine?
Rough machine in the annealed condition (≈40% of B1112) with carbide and rigid setups; at 52–56 HRC only hard turning with CBN, creep-feed or plunge grinding is practical — grind gently to avoid re-tempering burn and grinding cracks.
Can 4340M be welded?
Not weldable in the hardened state; if welding is unavoidable use electron-beam or GTAW with matching filler, 260–320 °C preheat/interpass, and immediately re-austenitize, quench and temper the whole part.
Can 4340M be formed, bent or molded?
Hot forge/upset at 1050–1200 °C with finish above 900 °C, then normalize and anneal; no meaningful cold forming above the annealed condition — bend and straighten only warm and re-stress-relieve.
What surface finishes work on 4340M?
Shot peen highly stressed fillets, then low-hydrogen-embrittlement cadmium or IVD aluminium plating (or HVOF WC-Co on journals) with a 190 °C/23 h bake within 4 h of plating; chrome plating requires strict bake and peening control.
4340M chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| C | 0.4 – 0.46 | 0.43 |
| Sikey modification vs 4340 — raises temper resistance | 1.45 – 1.8 | 1.6 |
| Mn | 0.6 – 0.9 | 0.75 |
| Cr | 0.7 – 0.95 | 0.82 |
| Ni | 1.65 – 2 | 1.83 |
| Mo | 0.3 – 0.45 | 0.38 |
| Vgrain refinement | 0.05 – 0.1 | 0.07 |
| P | ≤ 0.01 | 0.01 |
| S | ≤ 0.01 | 0 |
| Cu | ≤ 0.35 | 0.1 |
| Fe | balance | — |
4340M Q&T — frequently asked
What is 4340M Q&T used for?
4340M Q&T is typically used for aircraft landing gear cylinders and axles, arresting hooks, high-strength airframe fittings, rocket motor cases, torsion bars and high-strength drive shafts. In short: high-strength aerospace bar.
What is the yield strength of 4340M Q&T?
4340M Q&T has a typical yield strength of 1,620 MPa (235 ksi) and a tensile strength of 1,930 MPa (280 ksi) — stronger than 98% of steel grades. Strength varies by condition: see the 9 listed tempers.
Is 4340M Q&T easy to machine?
Not especially — machinability is rated not recommended (12/100, worse than 99% of steel grades). Rough machine in the annealed condition (≈40% of B1112) with carbide and rigid setups; at 52–56 HRC only hard turning with CBN, creep-feed or plunge grinding is practical — grind gently to avoid re-tempering burn and grinding cracks.
Can 4340M Q&T be welded?
Not readily — weldability is rated not recommended (10/100). Not weldable in the hardened state; if welding is unavoidable use electron-beam or GTAW with matching filler, 260–320 °C preheat/interpass, and immediately re-austenitize, quench and temper the whole part.
What surface finishes work on 4340M Q&T?
Shot peen highly stressed fillets, then low-hydrogen-embrittlement cadmium or IVD aluminium plating (or HVOF WC-Co on journals) with a 190 °C/23 h bake within 4 h of plating; chrome plating requires strict bake and peening control.
Can 4340M Q&T be formed, bent or molded?
Hot forge/upset at 1050–1200 °C with finish above 900 °C, then normalize and anneal; no meaningful cold forming above the annealed condition — bend and straighten only warm and re-stress-relieve.
What is the maximum service temperature of 4340M Q&T?
4340M Q&T is rated for continuous use to about 290°C (554 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between 4340M-HR and 4340M-VAR?
HR is the default condition — mill product as rolled or forged — air-hardened martensite, too hard and brittle to machine; always annealed or normalized before use. VAR: consumable-electrode vacuum remelted stock — choose for fatigue- and fracture-critical aerospace parts needing best transverse ductility and low inclusion count. Yield strength is 1,150 MPa in HR versus 1,640 MPa in VAR.
Can FabDigit make parts in 4340M Q&T?
Yes — 4340M Q&T is available for CNC machining and sheet metal with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- AMS 6419 — Steel Bars, Forgings, Tubing 1.6Si-0.82Cr-1.8Ni-0.40Mo-0.08V, Consumable Electrode Melted, 1930 MPa — SAE International (2020)
- AMS 6417 — 300M Steel Bars and Forgings, Normalized and Tempered / Annealed — SAE International (2019)
- MMPDS-2023, Chapter 2 (Low-Alloy Ultrahigh-Strength Steels) — Battelle / FAA (2023)
- Aerospace Structural Metals Handbook — 300M — CINDAS (2015)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels and High-Performance Alloys — ASM International (1990)
- 300M (UNS K44220) alloy steel bar data — Carpenter Technology / Aubert & Duval (2022)
- 40CrNi2Si2MoVA (2015)
- 300M/40CrNi2Si2MoVA — 1688 / Mysteel (2026)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.