4140 Alloy Steel vs 4340 Alloy Steel (Q&T)
4140 and 4340 are both chromium-molybdenum alloy steels, but 4340 adds nickel for deeper hardenability and higher strength. As listed, 4140 is ~655 MPa tensile / 415 MPa yield in a general condition, while 4340 in quench-and-tempered form reaches ~1280 MPa tensile / 1100 MPa yield. 4340 is the premium high-strength, deep-hardening choice; 4140 is the versatile, lower-cost alloy-steel workhorse.
The verdict
Choose 4140 for general high-strength shafts, gears, and tooling where its versatility, lower cost, and good heat-treat response suffice. Choose 4340 (Q&T) for the most demanding high-strength, large-section parts — landing gear, heavy shafts — where its nickel-boosted hardenability and ~1100 MPa yield are essential.
Side-by-side data
| Property | 4140 Alloy Steel | 4340 Alloy Steel (Q&T) |
|---|---|---|
| Category | Steel | Steel |
| Density (g/cm³) | 7.85 | 7.85 |
| Tensile strength (MPa) | 655 | 1280 |
| Yield strength (MPa) | 415 | 1100 |
| Elongation (%) | 20 | 12 |
| Hardness | 197 HB | 40 HRC |
| Max service temp (°C) | 425 | 450 |
| Machinability | ●●●●● | ●●●●● |
| Corrosion resistance | ●●●●● | ●●●●● |
| Relative cost | ●●●●● | ●●●●● |
| Thermal cond. (W/m·K) | 42 | 44 |
| Typically used for | High-strength heat-treatable shafts/gears | High-strength shafts & landing gear |
Which should you choose?
Choose 4140 Alloy Steel when…
- General-purpose high-strength shafts, gears, and machine parts
- Lower cost matters (~1.7 vs 4340's ~2.0 index)
- Section sizes are moderate, within 4140's hardenability range
- Good all-around strength and toughness after Q&T is sufficient
- Tooling, axles, and fixtures needing heat-treatable alloy steel
- Easier machining and broad availability are priorities
Choose 4340 Alloy Steel Q&T when…
- Maximum strength is required (~1100 MPa yield, ~40 HRC)
- Large or thick sections must harden fully through (deep hardenability)
- Aircraft landing gear, heavy-duty shafts, and high-stress forgings
- Superior toughness-to-strength balance from nickel alloying is needed
- Fatigue-critical rotating parts under severe loading
- The higher cost is justified by performance demands
Key differences that matter
- 4340 adds nickel (~1.8%) to the Cr-Mo base of 4140, giving deeper hardenability and higher attainable strength
- Strength: 4340 Q&T ~1280 MPa tensile / 1100 MPa yield (~40 HRC) vs 4140 as-listed ~655 MPa / 415 MPa
- Hardenability: 4340 hardens through much thicker sections, making it the choice for large forgings and heavy shafts
- Both are quench-and-temperable Cr-Mo steels; 4340's nickel improves toughness at a given strength level
- Cost: 4340 (~2.0 index) is pricier than 4140 (~1.7); 4140 is the more economical general alloy steel
- Machinability: both are moderate (4140 ~2.5, 4340 ~2.5), best machined in the annealed/pre-hardened state
- Corrosion resistance is poor for both (1.5/5); they require coatings or plating in exposed service
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What is the difference between 4140 and 4340 steel?
Both are chromium-molybdenum alloy steels, but 4340 adds about 1.8% nickel, giving deeper hardenability and higher strength. In quench-and-tempered form 4340 reaches around 1100 MPa yield versus 4140's lower as-supplied strength. 4340 is chosen for large, highly stressed parts; 4140 is the cheaper, versatile general-purpose alloy steel.
Is 4340 stronger than 4140?
Yes. Quench-and-tempered 4340 reaches roughly 1280 MPa tensile and 1100 MPa yield at about 40 HRC, well above typical 4140. More importantly, 4340's added nickel lets it harden fully through much thicker sections, so the strength advantage is biggest in large parts where 4140 wouldn't harden all the way to the core.
When should I use 4340 instead of 4140?
Use 4340 when you need maximum strength in large or thick sections, or superior toughness at high strength — aircraft landing gear, heavy power-transmission shafts, and high-stress forgings. For moderate sections and general high-strength parts, 4140 delivers similar performance at lower cost, so reserve the more expensive 4340 for the demanding cases.
Can 4140 and 4340 be welded?
Both can be welded but require care because they're hardenable alloy steels prone to heat-affected-zone cracking. Preheat, controlled interpass temperature, and post-weld heat treatment are typically needed, especially for 4340. Welding is usually done before final heat treatment. For heavily welded assemblies, lower-carbon steels are generally easier to fabricate.
Property values are typical/nominal figures for early-stage guidance only and vary by temper, grade, supplier and heat treatment. Confirm critical specifications against a certified datasheet or with an mfgiq engineer before production.