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. At the same hardness the two reach about the same UTS (strength tracks hardness); 4340’s advantage is that heavy sections harden through and stay tougher. 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 (~217 HB) | 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 |
IPT (chip-load) values assume roughly ½–1″ end mills; scale down for smaller tools — see the chip-load chart.
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: post-plate relief bake applies to steel at ≥1000 MPa (~145 ksi) tensile, about 31 HRC and up, per ASTM B850 (aerospace bake practice: AMS 2759/9) — that includes 4340 at 38–44 HRC, not just the hardest conditions. Plating and the bake are done by the plating partner; call the bake out on the drawing or PO and don’t skip it on aerospace work.
- Common AMS: 6415 (bar, forgings, tubing; air-melted), 6414 (same forms, vacuum-arc-remelted premium quality). 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.
Need 4340 steel parts machined?
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