Si vous concevez des composants qui doivent résister à des contraintes extrêmes, impact élevé, ou des charges lourdes, comme un train d'atterrissage aérospatial, arbres de grue industrielle, or high-performance automotive parts—you need a material that delivers exceptional strengthet toughness.AISI 4340 acier allié is the premium solution: comme nickel-chrome-molybdène (Ni-Cr-Mo) alliage, it offers higherrésistance à la traction, limite de fatigue, and low-temperature toughness than lower-alloy grades like AISI 4130 or AISI 4140. Ce guide détaille ses propriétés, applications du monde réel, processus de fabrication, and material comparisons to help you solve the most demanding high-load design challenges.
1. Material Properties of AISI 4340 Acier allié
AISI 4340’s performance comes from its quadruple-alloy design: nickel boosts toughness, chromium enhances corrosion resistance and hardenability, molybdenum improves high-temperature strength, and controlled carbon balances strength and ductility. Let’s explore its key properties in detail.
1.1 Composition chimique
AISI 4340 adheres to ASTM A29/A29M standards, with precise control over alloy elements to prioritize high strength and toughness. Below is its typical composition:
| Élément | Symbole | Gamme de contenu (%) | Key Role |
|---|---|---|---|
| Carbone (C) | C | 0.38 – 0.43 | Enables heat treatment; delivers baserésistance à la traction |
| Chrome (Cr) | Cr | 0.70 – 0.90 | Enhancesrésistance à la corrosion et trempabilité; improves wear resistance |
| Molybdène (Mo) | Mo | 0.20 – 0.30 | Raiseslimite de fatigue et stabilité à haute température; prevents creep under heavy loads |
| Nickel (Dans) | Dans | 1.65 – 2.00 | Core toughness booster; maintainsimpact toughness at low temperatures (-40 °C) |
| Manganèse (Mn) | Mn | 0.60 – 0.80 | Refines grain structure; enhancesductilité without reducing strength |
| Silicium (Et) | Et | 0.15 – 0.35 | Aids deoxidation; supports structural stability during heat treatment |
| Phosphore (P.) | P. | ≤ 0.035 | Minimized to avoid brittle fracture in low-temperature or high-stress conditions |
| Soufre (S) | S | ≤ 0.040 | Controlled to balanceusinabilité et la ténacité (lower S = better impact resistance) |
| Vanadium (V) | V | ≤ 0.03 | Trace element; refines grains for uniform strength across thick sections |
| Cuivre (Cu) | Cu | ≤ 0.30 | Trace element; adds mild atmospheric corrosion resistance for outdoor parts |
1.2 Propriétés physiques
These traits make AISI 4340 suitable for extreme environments—from sub-zero aerospace conditions to high-heat industrial machinery:
- Densité: 7.85 g/cm³ (same as standard steels)—simplifies weight calculations for heavy-load parts like crane shafts
- Point de fusion: 1,425 – 1,450 °C (2,597 – 2,642 °F)—compatible with forging and heat treatment for complex shapes
- Conductivité thermique: 42.0 Avec(m·K) à 20 °C; 38.0 Avec(m·K) à 300 °C—ensures even heat distribution during quenching (reduces distortion)
- Coefficient de dilatation thermique: 11.5 × 10⁻⁶/°C (20 – 100 °C)—minimizes stress from temperature swings (par ex., aerospace takeoff/landing cycles)
- Propriétés magnétiques: Ferromagnetic—enables non-destructive testing (CND) like ultrasonic phased array to detect internal defects in thick parts.
1.3 Propriétés mécaniques
AISI 4340’s mechanical performance is unmatched among mid-range alloy steels, especially after heat treatment. Below are typical values for common conditions:
| Propriété | Méthode de mesure | Recuit (Soft Condition) | Quenched & Tempered (300 °C) | Quenched & Tempered (600 °C) |
|---|---|---|---|---|
| Dureté (Rockwell) | CRH | 20 – 23 CRH | 52 – 55 CRH | 30 – 33 CRH |
| Dureté (Vickers) | HT | 190 – 220 HT | 500 – 530 HT | 290 – 320 HT |
| Résistance à la traction | MPa (ksi) | 700 MPa (102 ksi) | 1,800 MPa (261 ksi) | 1,050 MPa (152 ksi) |
| Limite d'élasticité | MPa (ksi) | 450 MPa (65 ksi) | 1,600 MPa (232 ksi) | 900 MPa (130 ksi) |
| Élongation | % (dans 50 mm) | 22 – 26% | 7 – 9% | 16 – 18% |
| Résistance aux chocs | J. (à -40 °C) | ≥ 70 J. | ≥ 30 J. | ≥ 55 J. |
| Fatigue Limit | MPa (rotating beam) | 350 MPa | 800 MPa | 500 MPa |
1.4 Autres propriétés
AISI 4340’s traits solve high-load design challenges:
- Weldability: Moderate—requires preheating to 250–300 °C and post-weld heat treatment (PWHT) to avoid cracking, but produces strong joints for load-bearing parts.
- Formabilité: Fair—best forged (not bent) in the annealed condition; formes complexes (par ex., ébauches d'engrenages) are created via hot forging to maintain strength.
- Usinabilité: Good in the annealed condition (20–23 HRC); heat-treated parts need carbide tools (due to high hardness) but still cut cleanly.
- Résistance à la corrosion: Moderate—resists mild rust and chemicals; pour environnements difficiles (par ex., marin), add chrome plating or ceramic coating.
- Dureté: Exceptional—nickel content keeps it tough at -40 °C (critical for aerospace and cold-climate industrial parts), even at high strength.
2. Applications of AISI 4340 Acier allié
AISI 4340’s high strength-toughness balance makes it ideal for components that can’t fail under extreme loads. Here are its key uses:
- Composants aérospatiaux: Landing gear struts, engine crankshafts, and helicopter rotor shafts—handles takeoff/landing impacts and sub-zero temperatures.
- Pièces automobiles: High-performance racing engine crankshafts, engrenages de transmission, and differential housings—tolerates high torque and engine heat.
- Composants mécaniques: Heavy-duty shafts (crane, excavator), hydraulic press rams, and turbine rotors—supports loads up to 100+ tons without bending.
- Machines industrielles: Mining equipment gears, steel mill rolls, and power generator shafts—resists wear and cyclic loading for 10+ années.
- Construction Equipment: Crane hooks, bulldozer axles, and pile driver rods—absorbs impact from heavy lifting and ground contact.
- Defense Components: Tank tread pins, artillery recoil mechanisms, and missile launcher parts—tough enough for military-grade stress.
3. Manufacturing Techniques for AISI 4340 Acier allié
Producing AISI 4340 requires precision—especially in heat treatment—to unlock its full strength-toughness potential. Here’s the step-by-step process:
- Sidérurgie:
- AISI 4340 is made using an Four à arc électrique (AEP) (recycles scrap steel) ou Four à oxygène de base (BOF). Nickel (1.65–2.00%), chrome (0.70–0.90%), and molybdenum (0.20–0.30%) are added during melting to ensure uniform alloy distribution.
- Forgeage & Roulement:
- Most AISI 4340 parts start as Hot Forged blanks (1,150 – 1,250 °C)—forging aligns grain structure, renforcer la force. After forging, blanks are Laminé à chaud to rough shapes (barres, assiettes) or left as-forged for near-net-shape parts (par ex., vilebrequins).
- Traitement thermique (Critical for Performance):
- Recuit: Heated to 815–845 °C, held 3–4 hours, slow-cooled to 650 °C. Softens the steel (20–23 HRC) for machining and forging.
- Trempe: Heated to 845–870 °C (austenitizing), held 1–2 hours (plus long pour les pièces épaisses), cooled in oil (not water—reduces cracking risk). Hardens to 58–60 HRC.
- Trempe: Reheated to 200–650 °C (based on needs):
- 300 °C: Max strength (1,800 traction MPa) pour pièces soumises à de fortes charges.
- 600 °C: Balanced strength-toughness (1,050 traction MPa) for impact-prone parts.
- Usinage:
- Annealed AISI 4340 is machined with HSS or carbide tools for turning, fraisage, ou perçage. Heat-treated parts (52–55 HRC) need coated carbide tools (par ex., TiAlN) to reduce wear. For precision (par ex., bearing seats), finish grinding is used.
- Traitement de surface:
- Placage: Chromage (résistance à l'usure) for shafts; nickelage (résistance à la corrosion) pour pièces aérospatiales.
- Revêtement: Ceramic coating (high-heat resistance) pour pièces de moteur; revêtement époxy (résistance chimique) for industrial machinery.
- Nitruration: Optional—heats to 500–550 °C in ammonia gas to harden the surface (60–65 HRC) without distortion, ideal for gears and bearings.
- Contrôle de qualité:
- Chemical Analysis: Mass spectrometry verifies nickel, chrome, and molybdenum levels (per ASTM A29/A29M).
- Mechanical Testing: Traction, impact (-40 °C), and hardness tests confirm performance.
- CND: Ultrasonic testing checks for internal defects; magnetic particle inspection finds surface cracks.
- Microstructural Analysis: Optical microscopy ensures uniform grain structure (no large grains that cause weakness).
4. Études de cas: AISI 4340 in Action
Real projects highlight AISI 4340’s ability to handle extreme loads.
Étude de cas 1: Aerospace Landing Gear (U.K.)
An aircraft manufacturer needed landing gear struts that could handle 120 kN impact loads and -40 °C temperatures. They chose AISI 4340, traité thermiquement pour 300 °C (52 CRH) pour la force. Après 10,000 landing cycles, the struts showed no fatigue cracks—outperforming AISI 4140 struts (which failed at 6,000 cycles). This extended the landing gear’s lifespan by 67%, économie $200,000 per aircraft in maintenance.
Étude de cas 2: Industrial Crane Shaft (Allemagne)
A steel mill needed a crane shaft to lift 150-ton steel coils. They replaced the AISI 4140 shaft with AISI 4340 (traité thermiquement pour 450 °C for toughness). The new shaft lasted 8 years—double the lifespan of the old one—because its nickel content prevented fatigue from repeated lifting cycles. The mill saved $150,000 in replacement costs and avoided 3 production shutdowns.
5. AISI 4340 contre. Autres matériaux
How does AISI 4340 compare to lower-alloy steels and premium materials?
| Matériel | Similarities to AISI 4340 | Différences clés | Idéal pour |
|---|---|---|---|
| AISI 4140 | Cr-Mo alloy steel | No nickel; lower toughness (-40 °C impact: ≥20 J vs. 30 J.); 25% moins cher | Medium-load parts (par ex., arbres de pompe) |
| AISI 4130 | Low-alloy steel | Lower carbon/nickel; weaker (1,450 MPa max tensile); better weldability; 40% moins cher | Welded, low-to-medium load parts |
| 304 Acier inoxydable | Résistant à la corrosion | Excellent rust resistance; weaker (515 traction MPa); 3× pricier | Corrosive environments, low-load parts |
| Alliage de titane (Ti-6Al-4V) | Haute résistance/poids | Plus léger (4.5 g/cm³); similar strength; 8× pricier | Aerospace parts where weight is critical |
| Fibre de carbone | Haute résistance/poids | Plus léger; pas de corrosion; poor impact toughness; 10× pricier | Non-load-bearing high-performance parts |
Yigu Technology’s Perspective on AISI 4340 Acier allié
Chez Yigu Technologie, AISI 4340 is our top pick for high-load, high-toughness components. Its Ni-Cr-Mo composition solves the biggest pain point for clients: getting strength without sacrificing toughness—critical for aerospace, industriel, and defense projects. We supply AISI 4340 in forged blanks, barres, or plates, with custom heat treatment (300–650 °C) to match project needs. For clients upgrading from AISI 4140 ou titane, AISI 4340 offers unbeatable value: 2x the toughness of 4140 et 1/8 the cost of titanium, with enough strength for 90% of extreme-load applications.
FAQ About AISI 4340 Acier allié
- Can AISI 4340 be welded for load-bearing parts?
Yes—but it requires careful preheating (250–300 °C) and post-weld heat treatment (600–650 °C) to reduce residual stress. Use low-hydrogen electrodes (par ex., E9018-B3) to avoid cracking, and test welds with ultrasonic inspection to ensure strength. - Is AISI 4340 suitable for low-temperature applications?
Absolutely—its nickel content maintains impact toughness à -40 °C (even when heat-treated to 52 CRH). For temperatures below -40 °C (par ex., arctic machinery), choose a nickel-enriched variant (AISI 4340Ni) for extra toughness. - What’s the maximum thickness for AISI 4340 parties?
AISI 4340 can be used for parts up to 200 mm thick—its high hardenability ensures uniform heat treatment across thick sections. For parts thicker than 200 mm, extend quenching hold time (2–3 heures) and use oil cooling to avoid core softening.