RFQ Today
Certifications: EN 10204 3.1 / 3.2 material test certificates, Jominy hardenability data, hydrogen embrittlement relief certification (coated fasteners), and complete export documentation packages.
Alloy 4340
Steel
A world-class technical reference for EPC contractors, aerospace and mechanical engineers, procurement heads, and TPI inspection agencies specifying Alloy 4340 (AISI/SAE 4340) nickel-chromium- molybdenum steel — covering deep-hardenability metallurgy, the critical tempering temperature/strength-toughness relationship, temper embrittlement and hydrogen embrittlement risk at very high strength levels, and the QC and documentation discipline required for critical aerospace and high-strength fastener supply.
Hardenability
& Selection Logic
Alloy 4340 is a nickel-chromium-molybdenum low-alloy steel renowned for its exceptional hardenability — the ability to develop a fully martensitic, uniformly hardened structure through thick sections on quenching — combined with an excellent strength-to-toughness ratio across a wide range of heat-treated conditions, making it the benchmark high-strength structural steel for aerospace and critical mechanical component applications.
1.1 — What Alloy 4340 Steel Is
Alloy 4340 is a low-alloy steel with a nominal composition of approximately 0.38–0.43% carbon, 1.65–2.00% nickel, 0.70–0.90% chromium, and 0.20–0.30% molybdenum, with the balance comprising iron and standard residual elements (manganese, silicon). This specific alloying combination — nickel for toughness and hardenability, chromium for hardenability and moderate corrosion/ wear resistance, molybdenum for hardenability and resistance to temper embrittlement (discussed in detail in Section 3.1) — gives 4340 exceptional “deep hardenability,” meaning the steel can be through-hardened to a fully martensitic structure even in relatively thick sections (unlike plain carbon steel, which can only be surface-hardened or hardened in thin sections before the cooling rate at the core becomes too slow to form martensite). This deep hardenability is 4340’s defining practical advantage, allowing large, thick-section components (landing gear struts, large shafts, drill collars) to achieve uniform, high strength throughout their cross-section rather than only at the surface.
1.2 — Key Engineering Properties
Exceptional Deep Hardenability
4340’s alloy content allows oil quenching (a less severe, more distortion-tolerant quenchant than water) to still achieve full martensitic transformation through substantial section thicknesses — verified and quantified using the Jominy end-quench hardenability test (ASTM A255), which measures hardness at increasing distance from a quenched end to characterise the alloy’s hardening depth for a given section size.
Excellent Strength-to-Toughness Ratio
Through correct quench and temper heat treatment (Section 3), 4340 achieves an unusually favourable combination of very high tensile strength (up to approximately 1900–2000 MPa in the highest-strength tempered condition) with good fracture toughness and ductility — a combination that makes it the benchmark comparison alloy against which many other high-strength structural steels are measured.
Good Fatigue Resistance
Well-documented, favourable fatigue performance across its heat-treated strength range, supporting demanding cyclically loaded aerospace and mechanical applications including landing gear components, shafts, and gears subject to millions of load cycles over their service life.
Moderate Cost Relative to Performance
As a low-alloy steel (rather than a stainless or nickel superalloy), 4340 offers its exceptional strength and toughness combination at substantially lower material cost than the corrosion-resistant and superalloy materials discussed throughout RR Hydraulic’s other material references — though 4340 itself has only modest corrosion resistance and typically requires a protective coating (per RR Hydraulic’s surface treatment references) for anything beyond dry, controlled-environment service.
1.3 — Comparison to Related Low-Alloy Steels
| Grade | Key Alloying Additions | Relative Hardenability | Typical Use |
|---|---|---|---|
| 4340 | Ni-Cr-Mo | Very high — deep hardening in thick sections | Aerospace landing gear, high-strength bolting, large shafts, drill collars |
| 4140 | Cr-Mo (no nickel) | Good, but lower than 4340 in thick sections | General-purpose high-strength shafts, gears, moderate-section fasteners |
| 8620 | Ni-Cr-Mo (lower carbon, carburizing grade) | Good — optimised for case hardening | Carburized gears and components requiring a hard wear surface with a tough core |
| 300M | 4340 chemistry + silicon and vanadium additions | Similar to 4340, improved temper resistance | Ultra-high-strength aerospace landing gear where maximum strength/toughness is required |
Strength Levels
& Mechanical Reference
Alloy 4340 is manufactured primarily as bar and forging stock, governed by specific ASTM and aerospace material specifications, and supplied against a specified heat-treated strength/hardness level rather than a single fixed mechanical property set.
Submit form, strength level, size, and quantity to sales@rrhydraulics.com for a certified offer.
2.1 — Governing Standards
ASTM A322 — Standard Alloy Steel Bars
Governs the chemical composition of standard alloy steel bar grades including 4340 — the base compositional specification for hot-rolled and cold-finished 4340 bar stock.
ASTM A331 — Cold-Finished Alloy Steel Bars
Governs cold-finished (turned, ground, or polished) 4340 bar with tighter dimensional tolerance than hot-rolled bar — used for precision machined components and fasteners.
AMS 6414 / AMS 6415 — Aerospace Bar Specifications
The primary aerospace material specifications for 4340 bar (AMS 6414 for normalized-and-tempered condition; AMS 6415 for annealed condition), imposing considerably more rigorous chemistry, cleanliness (inclusion rating), and mechanical testing requirements than general industrial ASTM specifications — the standard reference for aerospace-critical 4340 landing gear and high-strength fastener supply.
ASTM A255 — Jominy End-Quench Hardenability Test
The standard test method for determining and verifying a specific 4340 heat’s hardenability — hardness measured at defined distances from the quenched end of a standardised test specimen, plotted as a hardenability curve and compared against the specification’s required minimum hardenability band.
AMS 2759 — Heat Treatment of Steel Parts
Governs the qualified heat treatment process (normalizing, annealing, austenitizing, quenching, and tempering) parameters for aerospace steel parts including 4340, ensuring documented, controlled heat treatment cycles are applied to achieve the specified mechanical property condition.
2.2 — Mechanical Properties by Tempering Temperature
| Tempering Temp. | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HRC) | Typical Application |
|---|---|---|---|---|---|
| 205°C (400°F) | ~1900 | ~1600 | ~10 | ~50 | Maximum strength — highest-load aerospace fasteners |
| 315°C (600°F) | ~1700 | ~1500 | ~10 | ~45 | Approaching the temper embrittlement risk zone — see Section 3.1 |
| 425°C (800°F) | ~1500 | ~1350 | ~12 | ~40 | High-strength structural components |
| 540°C (1000°F) | ~1250 | ~1150 | ~14 | ~35 | Balanced strength/toughness — general high-strength applications |
| 650°C (1200°F) | ~950 | ~860 | ~18 | ~28 | Maximum practical toughness, moderate strength |
Values are indicative and section-size dependent — always verify the specific supplier’s heat treatment process qualification data and the current ASTM/AMS specification revision for the exact mechanical property guarantee applicable to a specific ordered condition and section size.
Hydrogen Embrittlement Risk
& Fabrication Guidance
Alloy 4340’s very high achievable strength brings two distinct and specification-critical embrittlement risks that must be actively managed — temper embrittlement within a specific tempering temperature range, and hydrogen embrittlement sensitivity that increases sharply with strength level.
3.1 — Temper Embrittlement (“500°F Embrittlement”)
3.2 — Hydrogen Embrittlement Sensitivity Increases Sharply with Strength
3.3 — Fabrication and Welding Guidance
Machining — Preferably in the Annealed Condition
4340 is considerably easier to machine in the annealed (soft) condition than in the quenched-and-tempered high-strength condition — standard manufacturing practice is to machine components to near-final dimensions in the annealed condition, then perform the final quench-and-temper heat treatment, followed only by finish grinding or minimal final machining in the hardened condition, minimising tool wear and machining cost.
Welding Difficulty
4340’s alloy content and hardenability, while beneficial for through-hardening, make the alloy difficult to weld reliably without careful process control — the heat-affected zone can form untempered, brittle martensite unless preheat (typically 150–260°C), controlled interpass temperature, and mandatory post-weld heat treatment (tempering) are correctly applied. Welding of 4340 in critical high-strength applications is generally avoided in favour of mechanical joining or machining from solid stock where practical, given the difficulty of reliably matching the base metal’s heat-treated properties in a welded joint.
Section Size and Hardenability Verification
For critical applications, verify the specific 4340 heat’s hardenability (via Jominy testing per ASTM A255) is adequate for the actual component section size before committing to production — a heat at the lower end of the alloy’s permitted composition range may not achieve full through-hardening in the thickest sections of a large component, even though it meets the nominal chemical composition specification.
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from certified alloy steel heat to finished, tested, and packed Alloy 4340 component shipment. Chemical composition, hardenability, mechanical, and hardness verification are standard on all project-grade supply.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 / Documentation Requirements
| Certificate | Content | EPC Requirement | When Mandatory |
|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Not acceptable for critical aerospace/high-strength supply | Never for aerospace or critical high-strength fastener supply |
| 3.1 (EN 10204) | Heat-traceable chemical + mechanical test report | Mandatory — all EPC supply | All industrial and general high-strength component supply |
| Jominy hardenability report | ASTM A255 hardenability curve per heat | Mandatory — large-section critical components | Aerospace and critical structural applications |
| Hydrogen embrittlement relief certificate | ASTM B850 baking process record (coated fasteners) | Mandatory — plated high-strength fasteners | Any electroplated 4340 fastener above the applicable strength threshold |
| 3.2 (EN 10204) | 3.1 + TPI countersign | Critical / owner-specified critical items | Aerospace-critical and safety-critical high-strength components |
4.3 — Applications by Industry
Aerospace Landing Gear and Structural Components
Alloy 4340 (per AMS 6414/6415) for aircraft landing gear struts, axles, and high-strength structural components where the alloy’s deep hardenability supports large-section, uniformly high-strength parts, and its excellent fatigue and fracture toughness performance provides reliable service life under the highly cyclic, high-impact loading landing gear experiences.
High-Strength Fasteners and Mechanical Components
4340 studs, bolts, shafts, and gears for aerospace, industrial machinery, and heavy equipment applications requiring very high strength combined with good toughness — the benchmark material choice wherever this specific strength-toughness combination is the governing design requirement, subject to the hydrogen embrittlement precautions discussed in Section 3.2 for any coated/plated fastener application.
Oil Field Drill Collars and Tool Joints
4340 drill collars and drill string tool joint components for oil and gas drilling operations, leveraging the alloy’s high strength and toughness combination to withstand the severe cyclic bending, torsional, and impact loading experienced during drilling operations.
4.4 — Export Packaging Specification
- Bar and finished component stock protected with rust-preventive oil (per the plain/self-colour finish practice discussed in RR Hydraulic’s dedicated reference) or an applied coating, given carbon/low-alloy steel’s lack of inherent corrosion resistance
- Heat/lot number stamped or tagged on each item, cross-referenced to the accompanying material test certificate including the specific heat treatment condition, hardness result, and Jominy hardenability report
- High-strength fastener components packed to prevent surface damage (nicks, scratches) that could act as stress concentration points and fatigue crack initiation sites in service
- Documentation in a waterproof pocket: EN 10204 3.1/3.2 MTC, chemical composition report, mechanical properties report, hardness report, Jominy hardenability report, Charpy impact test report, hydrogen embrittlement relief certificate (coated fasteners), and packing list with form/condition/size breakdown per item
- ISPM-15 timber or export cartons for international shipment, with country of origin and HS tariff code documentation matched to the alloy steel product category
Submit your form, strength level, size, and quantity to RR Hydraulic for a complete, certified commercial offer.