Steel 4340M · supply condition · default condition
Steel 4340M-HR
Properties of the HR condition, compared with the other 4340M tempers.
Mill product as rolled or forged — air-hardened martensite, too hard and brittle to machine; always annealed or normalized before use.
What is 4340M-HR?
4340M-HR is 4340M supplied in the HR condition — mill product as rolled or forged — air-hardened martensite, too hard and brittle to machine; always annealed or normalized before use. 4340M-HR has a yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,550 MPa (225 ksi) — stronger than 94% of steel grades. Elongation at break is 6% and the elastic modulus is 205 GPa (29.7 Msi). 4340M-HR 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).
Advantages
- High strength — 1,150 MPa (167 ksi), better than 94% of steel grades
- Polishes to a fine finish — 75/100, better than 94% of steel grades
Limitations
- Limited ductility — 6%, worse than 98% of steel grades
- Difficult to machine — 15/100, worse than 98% of steel grades
- Limited formability — 8/100, worse than 97% of steel grades
- Difficult to weld — 15/100, worse than 91% of steel grades
- High embodied carbon — 2.6 kg CO₂/kg, worse than 86% of steel grades
4340M-HR properties
Typical room-temperature values for 4340M-HR — 8 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-HR 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-HR has a yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,550 MPa (225 ksi) — stronger than 94% of steel grades. Elongation at break is 6% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
4340M-HR 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-HR 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-HR 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-HR's machinability is poor (15/100, worse than 98% of steel grades); weldability poor (15/100); formability not recommended (8/100).
Common Calculations5
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-HR
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-HR — frequently asked
What is 4340M-HR used for?
4340M-HR 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-HR?
4340M-HR has a typical yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,550 MPa (225 ksi) — stronger than 94% of steel grades. Strength varies by condition: see the 9 listed tempers.
Is 4340M-HR easy to machine?
Not especially — machinability is rated poor (15/100, worse than 98% 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-HR be welded?
Not readily — weldability is rated poor (15/100). 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-HR?
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-HR 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-HR?
4340M-HR 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-HR?
Yes — 4340M-HR 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.