86717956 Montabert Front Guide

TECHNICAL DATA SHEET: SAE 4340 (AISI 4340) ALLOY STEEL

General Description:
SAE 4340 is a deep-hardening, ultra-high-strength nickel-chromium-molybdenum alloy steel. It belongs to the medium-carbon, low-alloy steel classification. It is renowned for its exceptional responsiveness to heat treatment, offering a superb combination of high tensile strength, excellent toughness (even in low-temperature environments), ductility, and fatigue resistance. Its primary distinguishing feature compared to other alloys like SAE 4140 is its superior hardenability, which allows for uniform martensitic transformation and consistent high strength throughout the core of large cross-sections.

Chemical Composition (Weight %):
(Typical range according to SAE J404, ASTM A29, or AMS standards)

Carbon (C): 0.38 – 0.43 % (Primary element for hardness and strength)

Nickel (Ni): 1.65 – 2.00 % (Critical element for toughness, ductility, and deep hardenability)

Chromium (Cr): 0.70 – 0.90 % (Increases hardenability and wear resistance)

Molybdenum (Mo): 0.20 – 0.30 % (Enhances hardenability, improves creep resistance, and reduces temper brittleness)

Manganese (Mn): 0.60 – 0.80 %

Silicon (Si): 0.15 – 0.35 %

Phosphorus (P): ≤ 0.035 % (Impurity)

Sulfur (S): ≤ 0.040 % (Impurity)

Iron (Fe): Balance

Physical Properties (At Room Temperature):

Density: 7.85 g/cm³ (0.284 lb/in³)

Modulus of Elasticity (Young's): 205 GPa (29.7 x 10^6 psi)

Poisson's Ratio: 0.29

Thermal Conductivity: 44.5 W/m·K

Mean Coefficient of Thermal Expansion (20-100°C): 12.3 µm/m·K

Melting Point: ~1427°C (~2600°F)

Typical Mechanical Properties (Heat Treatment Dependent):
(Values vary drastically based on tempering temperature. Example values for a 25mm diameter standard test specimen, oil quenched.)

Condition: Annealed

Yield Strength (0.2% Offset): ~470 MPa (68 ksi)

Ultimate Tensile Strength (UTS): ~745 MPa (108 ksi)

Elongation (in 50mm): ~22%

Hardness: ~200 HBW (15 HRC)

Condition: Normalized

Yield Strength (0.2% Offset): ~860 MPa (125 ksi)

Ultimate Tensile Strength (UTS): ~1280 MPa (185 ksi)

Hardness: ~360 HBW (38 HRC)

Condition: Hardened & Tempered (@ 315°C / 600°F)

Yield Strength (0.2% Offset): ~1550 MPa (225 ksi)

Ultimate Tensile Strength (UTS): ~1800 MPa (260 ksi)

Hardness: ~500 HBW (52 HRC)

Toughness (Charpy V-Notch @ RT): Low (~20 J)

Condition: Hardened & Tempered (@ 540°C / 1000°F)

Yield Strength (0.2% Offset): ~1140 MPa (165 ksi)

Ultimate Tensile Strength (UTS): ~1240 MPa (180 ksi)

Hardness: ~360 HBW (39 HRC)

Toughness (Charpy V-Notch @ RT): Very High (>60 J)

Condition: Hardened & Tempered (@ 650°C / 1200°F)

Yield Strength (0.2% Offset): ~860 MPa (125 ksi)

Ultimate Tensile Strength (UTS): ~965 MPa (140 ksi)

Hardness: ~280 HBW (30 HRC)

Toughness (Charpy V-Notch @ RT): Maximum (>80 J)

Technical Applicability Analysis:

Machinability:

In the annealed condition, machinability is good and comparable to SAE 4140.

In the hardened condition (e.g., 30-35 HRC), machining is difficult but feasible using advanced carbide or ceramic tooling, often required for precision finish machining after heat treatment to avoid distortion.

Weldability:

Difficulty is high. The high carbon and alloy content result in high hardenability, making the steel extremely susceptible to cold cracking (hydrogen cracking) in the heat-affected zone (HAZ).

Requirements include strict pre-heating (typically 200-300°C), specific low-hydrogen filler metals, and immediate post-weld heat treatment (PWHT/Stress Relieving) to temper brittle martensite formed during welding.

Key Advantages:

Uniform and deep hardenability, ideal for large components.

Excellent balance of toughness and hardness, superior to Cr-Mo steels (like 4140) at high hardness levels.

Very high fatigue resistance, suitable for high-cyclic loading.

Retains good performance and toughness in sub-zero environments.

Critical Disadvantage:

High susceptibility to hydrogen embrittlement, especially when treated to high strength levels (>1300 MPa). Strict controls are required during chemical cleaning or electroplating processes (baking treatment is mandatory immediately after plating).

Common Applications:
SAE 4340 is preferred where standard alloy steels cannot achieve the required through-hardening or toughness in large sections under heavy loads.

Aerospace: Landing gear components, structural airframe parts, high-strength bolts, helicopter rotor shafts.

Automotive & Heavy Equipment: Heavy-duty crankshafts, camshafts, axle shafts, connecting rods, transmission gears, differential components.

Oil & Gas: High-strength drill collars, tool joints, valve components, blow-out preventers (BOP) requiring high sour-gas resistance (when tempered to lower hardness).

General Engineering: Large diameter gears, hydraulic machinery shafts, turbine bolts, extrusion dies.