Author: Site Editor Publish Time: 2026-10-07 Origin: Site
Both 4140 and 4340 are Cr-Mo low-alloy steels in the AISI 4xxx family, both are oil-hardening, both are sold to ASTM A29 / ASTM A322 / EN 10083-3 / DIN 1.6582-type specs, and both are hardened and tempered in roughly the same temperature windows. On a chemical analysis, the only meaningful difference is one element — nickel — at 1.65–2.00% in 4340 and absent in 4140. That single addition roughly doubles the hardenable section size and lifts the impact toughness at any given strength level, which is why 4340 is the grade called out for landing gear, transmission shafts and pressure vessels, while 4140 covers the bulk of medium-strength industrial shafting. This article lays out where the line between "medium strength" and "high toughness" actually falls.
Both grades share the same carbon window (0.38–0.43%), the same manganese range, and a chromium content that sits at roughly 0.8–1.0%. They both austenitize at 815–870 °C, oil quench, and temper in the 425–650 °C range. Both are weldable with low-hydrogen consumables and proper preheat. Both are magnetic and forgeable. The overlap is real, which is why the comparison is fair: in thin to medium sections hardened to the same tensile target, the difference in service performance can be marginal, and 4140 wins on cost.
Where the comparison stops being fair is in three regimes: thick sections (above ~75 mm), high impact loading, and parts that must combine ultra-high tensile strength with acceptable notch toughness. In each of those, 4340 is the grade to specify, and 4140 will not meet the spec regardless of how it is heat-treated.
| Element (%) | 4140 (AISI / ASTM A29) | 4340 (AISI / ASTM A29) | DIN Equivalent |
|---|---|---|---|
| Carbon (C) | 0.38–0.43 | 0.38–0.43 | 1.7225 (42CrMo4) |
| Manganese (Mn) | 0.75–1.00 | 0.60–0.80 | 1.6582 (34CrNiMo6) |
| Silicon (Si) | 0.15–0.35 | 0.15–0.35 | — |
| Phosphorus (P), max | 0.035 | 0.025 | — |
| Sulfur (S), max | 0.040 | 0.025 | — |
| Chromium (Cr) | 0.80–1.10 | 0.70–0.90 | — |
| Molybdenum (Mo) | 0.15–0.25 | 0.20–0.30 | — |
| Nickel (Ni) | — | 1.65–2.00 | — |
The two differences that matter are nickel (added in 4340) and a slight tightening of P and S in 4340, which reflects its aerospace heritage. Note also that 4340 carries slightly more molybdenum (0.20–0.30%) and slightly less chromium (0.70–0.90%) than 4140; the balance is designed around the nickel addition, which alters the carbide morphology during tempering.
The numbers below are typical for bars in the 25–50 mm range, oil-quenched and tempered at ~450 °C. Both grades can be tempered harder or softer; the comparison is at a strength level where both grades are commonly used.
| Property | 4140 (Q&T ~450 °C) | 4340 (Q&T ~450 °C) |
|---|---|---|
| Tensile strength, MPa | 1100–1300 | 1280–1450 |
| Yield strength (0.2%), MPa | 950–1100 | 1170–1320 |
| Elongation in 4D, % | 14–18 | 12–16 |
| Reduction of area, % | 45–50 | 40–50 |
| Hardness, HRC | 35–40 | 38–45 |
| Charpy V-notch at 20 °C, J | 25–35 | 35–55 |
| Charpy V-notch at −40 °C, J | 12–20 | 20–35 |
The pattern to read is this: 4340 reaches higher tensile and yield strength in the same section, but its real advantage is impact toughness — both at room temperature and, decisively, at low temperature. At equal tensile strength (say, both tempered to 1100 MPa), 4340's Charpy values run roughly 30–60% higher than 4140's. That is the metallurgical payoff of the nickel addition: nickel lowers the ductile-to-brittle transition temperature and slows carbide coarsening during temper.
Hardenability, expressed as the ideal critical diameter (the largest round bar that through-hardens in oil), is where 4340 decisively pulls ahead.
4140: ideal critical diameter (oil) ≈ 50–90 mm. Above this, the core hardness drops and toughness suffers.
4340: ideal critical diameter (oil) ≈ 100–180 mm. Sections up to ~150 mm through-harden in oil with a uniform hardness profile.
The 1.65–2.00% nickel shifts the TTT (time-temperature-transformation) curves to the right, delaying the pearlite and bainite noses and giving martensite time to form at slower cooling rates. In practical terms, a 4340 bar 150 mm in diameter will have a hardness profile within 2–4 HRC across the section after oil quench, while a 4140 bar of the same size will show a soft core at 22–28 HRC surrounded by a hard case at 40+ HRC. For a part designed to carry bending fatigue through its full section, that is the difference between a part that lasts and a part that fails.
Normalizing: 870 °C, air cool.
Austenitizing: 845–870 °C, oil quench. (Water quench is possible for large sections, but raises crack risk; only used with care.)
Tempering: 425–650 °C. At 425 °C → ~1100 MPa UTS, ~950 MPa YS, ~15% elongation, ~38 HRC. At 650 °C → ~800 MPa UTS, ~660 MPa YS, ~23% elongation, ~25 HRC.
Stress relief: 550–650 °C.
Normalizing: 840–870 °C, air cool.
Austenitizing: 815–845 °C, oil quench. (For very thick sections, marquench in hot oil or salt at 200 °C to reduce distortion and quench-crack risk — 4340's high hardenability tolerates the slower cooling.)
Tempering: 425–650 °C. The grade is sensitive to temper embrittlement if held in the 375–575 °C window for extended periods; cool through this range rapidly after tempering, or use a one-step temper above 575 °C.
Stress relief: 550–650 °C.
A note on temper embrittlement: 4340 contains nickel and is therefore more prone to reversible temper embrittlement than 4140. Avoid soaking in the 375–575 °C range; if intermediate toughness is needed, use a single temper above 575 °C and cool quickly. 4140 is less sensitive but not immune — the same caution applies.
Nickel is the cost driver. As a planning figure, expect 4340 bar stock to run 1.4–1.8× the price of 4140 in common sizes, with larger premiums in diameters above 150 mm where mills run 4340 less frequently. Lead times for 4340 in stock sizes are comparable to 4140; for non-standard sections, expect longer mill cycles and higher minimum order quantities.
For 4140 round bar in normalized-and-peeled, Q&T, or black bar condition, Qilu Metal holds stock from 16 mm to 350 mm under AISI 4140 alloy steel bars. For 4340 in aerospace-grade or industrial Q&T bar, see AISI 4340 Ni-Cr-Mo alloy steel bar. For applications between the two — slightly more hardenability than 4140 but without the cost of 4340 — the AISI 4145 grade is the intermediate choice used heavily in oilfield drill collars.
| Application | Section size | Loading type | Recommended grade | Reasoning |
|---|---|---|---|---|
| General industrial shafting, 25–75 mm | Medium | Bending + torsion | 4140 | Through-hardens; cost-effective |
| Hydraulic cylinder rods | 30–100 mm | Surface wear + tensile | 4140 | Q&T base + chrome plate |
| Grade 8 fasteners, spline shafts | Small | Tensile + fatigue | 4140 | Holds 880–1080 MPa YS |
| Oilfield drill collars, heavy-wall tubing | 75–200 mm | Tensile + impact | 4145 / 4140 | Higher C improves wear; cost-effective |
| Landing gear, flap tracks | 25–100 mm | High impact + fatigue | 4340 | Toughness at 1380+ MPa UTS |
| Helicopter and rotor transmission shafts | 50–150 mm | Torsion + impact | 4340 | Through-hardens in oil to 150 mm |
| Gun components, breech parts | Medium | Impact + pressure | 4340 | Holds toughness at 45–50 HRC |
| Pressure vessels, high-pressure flanges | 75–200 mm | Pressure + embrittlement risk | 4340 | Lower ductile-brittle transition |
| Crankshafts, propeller hubs | 75–200 mm | Bending + torsion + impact | 4340 | Toughness at large section |
| Mining and crusher shafts | 100–250 mm | Heavy impact | 4340 (or 4330V/300M) | Through-hardens to center |
| Lightly loaded shafts, pins under 50 mm | Small | Low | 4140 or 1045 | 4340 is overkill |
| Welded assemblies, brackets | Variable | Welded | 4140 (with PWHT) | Both grades weld poorly; 4140 slightly easier |
A common specification mistake is jumping to 4340 whenever a part is "critical." For sections below 50 mm that see steady loads and no impact, 4140 is the right engineering and economic choice; 4340's extra toughness is bought but not used. The same is true for parts that will be case-hardened or induction-hardened on the surface only — there, the core strength of 4340 adds little because the surface carries the load.
The reverse mistake — specifying 4140 where 4340 is needed — is more dangerous. On a 100 mm shaft carrying shock loads, 4140 will develop a soft core that quietly absorbs impact energy and then fails by low-cycle fatigue. The failure is invisible until the shaft breaks; the cost difference over the life of the part is trivial.
Section above 100 mm and through-hardened? Yes → 4340.
Service temperature below −20 °C or impact loading is significant? Yes → 4340.
Strength target above 1200 MPa UTS while still needing ≥30 J Charpy? Yes → 4340.
Aerospace, defense or pressure-retaining application per spec? Yes → 4340 (or a vacuum-melted variant such as 4330V / 300M).
Everything else — typical industrial shafts, gears, fasteners in the 25–80 mm range? Yes → 4140 is the cost-effective answer.
Section 75–100 mm, moderate impact? This is the gray zone — choose 4340 if the part is fatigue-critical, 4140 if wear or steady load dominates.
Q: Is 4340 always stronger than 4140?
At the same temper temperature, 4340 develops higher tensile strength and hardness. But both grades can be tempered to overlap in a 800–1100 MPa tensile range, where the meaningful difference is toughness, not strength.
Q: Why is 4340 more prone to temper embrittlement?
The nickel addition promotes impurity segregation (P, Sn, Sb) to prior austenite grain boundaries in the 375–575 °C window. Cool through this range quickly after tempering, or specify a vacuum-melted, low-P variant for aerospace service.
Q: Can 4140 be substituted for 4340 in a 100 mm shaft?
Not safely. 4140 will not through-harden at that diameter, and the soft core will compromise fatigue and impact performance. The substitution is the classic failure mode in shafting.
Q: Are 4140 and 4340 weldable?
Both are weldable with low-hydrogen consumables (E7018 / ER80S-D2 for 4140, E11018-M for 4340), preheat of 200–315 °C, and immediate post-weld stress relief at 550–650 °C. Neither should be specified for welded structural assemblies where post-weld heat treatment is impractical.
Q: What is the relationship between 4340 and 300M?
300M (4340M) is a modified 4340 with added silicon (~1.6%) and vanadium, designed for higher toughness at very high strength (1900+ MPa UTS). It is the aerospace upgrade path beyond 4340.
If you have a shaft, gear or pressure-retaining part where the grade choice between 4140 and 4340 is not obvious, send the drawing, the section size, and the load and temperature profile. Qilu Metal supplies both grades in normalized, Q&T, peeled and black bar condition to ASTM A29, ASTM A322, EN 10083-3 and DIN specs, with full mill test reports and full traceability for aerospace and industrial customers. Email enquiry@qilumetal.com and our metallurgy team will come back with a quotation and a heat-treatment recommendation matched to your section and load case.
