Engineering · Machining guide
How to machine 4340 alloy steel.
4340 is the high-strength alloy steel for landing gear, aircraft structural fittings, and high-load shaft work. The added nickel and molybdenum vs 4140 give it tougher mechanical performance and slightly tougher machining. The 300M variant pushes UTS even higher and shows up on the most demanding aerospace landing-gear programs.
01 · Why 4340 is different from 4140
More nickel, more strength, slightly tougher to cut.
4340 differs from 4140 mainly in its 1.8% nickel content, which gives better deep-hardenability and toughness in heavy sections. UTS in Q&T condition is 20–30% higher than 4140 at the same hardness. The trade-off is slightly tougher machining — chip control is more difficult and tools wear faster on continuous cuts.
The 300M variant (also called 4340M, AMS 6417 / AMS 6419) pushes UTS to ~285 ksi (~1965 MPa) for landing-gear applications. Same machining strategy as 4340 but with another ~15–20% reduction in cutting speed.
02 · Cutting parameters
Numbers by condition.
| Operation | Condition | SFM | IPT / IPR |
|---|---|---|---|
| Roughing turn | Annealed (22 HRC) | 250–350 | 0.010–0.018 IPR |
| Roughing turn | Normalized (30 HRC) | 200–280 | 0.008–0.015 IPR |
| Roughing turn | Q&T 38 HRC | 120–180 | 0.006–0.010 IPR |
| Roughing turn | Q&T 44 HRC | 80–120 | 0.005–0.008 IPR |
| Hard turn | Q&T 50+ HRC (CBN) | 350–450 | 0.004–0.007 IPR |
| Finishing turn | Annealed | 350–450 | 0.005–0.010 IPR |
| Finishing turn | Q&T 44 HRC | 150–200 | 0.003–0.006 IPR |
| End mill rough | Annealed | 250–350 | 0.003–0.005 IPT |
| End mill rough | Q&T 44 HRC | 120–160 | 0.001–0.003 IPT |
| Drilling | Annealed | 80–120 | 0.004–0.008 IPR |
| Drilling | Q&T 44 HRC | 30–60 | 0.002–0.004 IPR |
For 300M (4340M), drop SFM another ~15–20% from the values above. AMS 6417 and AMS 6419 cover 300M bar; premium double-vacuum-melt material is AMS 6257.
03 · Practical notes
Aerospace landing-gear quality discipline.
- Tooling: coated carbide (PVD AlTiN) for soft conditions, CBN for ≥50 HRC hard-turning. Sub-micron grain inserts for finishing.
- Coolant: water-soluble flood, EP additives. Through-tool coolant for deep holes — 4340 holes deeper than 5×D need it to evacuate chips.
- Heat-treat sequence: rough in annealed, send out for Q&T, finish on hardened only when tolerance demands it. Distortion 0.002–0.005″/in is typical for thin sections.
- Hydrogen embrittlement: 4340 above ~52 HRC is sensitive. Plating shops run a 24h bake-out post-plate. Don’t skip it on aerospace work.
- Common AMS: 6415 (normalized bar), 6414 (forgings). See AMS spec cross-reference for the full list.
Keep exploring
Related tools & references
Mechanical properties, UNS, AMS specs, and machinability at a glance.
Convert these SFM and IPT figures into spindle RPM and feed rate.
Recommended chip load by tool diameter for high-strength alloy steel.
4340 density for part weight and stock estimates.
Compare 4340 and 300M against other high-strength steels.
Technically reviewed by the True Precision Machining engineering team · Last reviewed June 2026
Standards referenced: AMS 6415, AMS 6414, AMS 6417, AMS 6419
Need 4340 steel parts machined?
Send your drawing — 4340 steel is a material we run regularly. Aerospace, medical, and defense work welcome.
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