By World Material

SAE AISI 4340 Steel (UNS G43400)

AISI 4340 steel (UNS G43400) is an ultra-high strength medium carbon low alloy steel, which combines deep hardenability, high ductility, toughness and strength, and has high fatigue resistance and creep resistance. It is not particularly affected by tempering and embrittlement, and exhibits good strength retention at high temperatures and is not easily softened. In thin sections, this steel is air-hardened. In practice, it is usually oil quenched.

4340 Alloy Steel Data sheet

The following tables and lists give 4340 alloy steel data sheet and specification including chemical composition, physical properties and mechanical properties, heat treatment, welding, etc.

Chemical Composition

AISI 4340 steel material chemical composition is listed in the following table based on cast analysis.

AISI 4340 Steel Properties

The tables below list AISI 4340 steel properties, including physical and mechanical properties.

Physical Properties

4340 alloy steel physical properties are given in the following lists, including density, thermal expansion, elastic modulus, thermal conductivity, specific heat capacity and electrical resistance.

Notes:

· 10-6·K-1 = 10-6/K

· 1 Ω·mm²/m = 1 μΩ·m

· 1 g/cm3 = 1 kg/dm3 = 1000 kg/m3

· 1 GPa = 1 kN/mm2

· 1 MPa = 1 N/mm2

Mechanical Properties

4340 steel mechanical properties are listed in the following tables.

4340 Steel Heat Treatment

Alloy steel 4340 heat treatment including: normalizing, annealing, hardening, tempering, spheroidization, stress relief, etc. (4340 heat treat)

· Normalizing: Heat to 845 to 900 °C (1550 to 1650 °F) and hold for a period of time, which depends on the thickness of the section; air cooling.

· Annealing: Heating to 830 to 860 °C (1525 to 1575 °F) and maintaining it for a period of time, which depends on the thickness of the section or furnace load; furnace cooling.

· Spheroidization: The preferred schedule is to preheat to 690 °C (1275 ° F) for 2 h, increase the temperature to 745 °C (1375 °F) for 2 h, and cool to 650 °C (1200 °F) ) And hold for 6 hours, cool the furnace to about 600 °C (1100 °F), and finally air-cool to room temperature. Another arrangement is to heat the temperature to 730 to 745 °C (1350 to 1375 °F), hold it for several hours, and then cool the furnace to room temperature.

· Hardening: Heat to 800 to 845 °C (1475 to 1550 °F) for 15 minutes (minimum 15 minutes) per 25 mm (1 inch) thickness; oil quench to below 65 °C (150 °F), or quench in fused salt at 200 to 210 °C (390 to 410 °F), hold for 10 minutes, and then air-cool to below 65 °C (150 °F).

· Tempering: Hold at 200 to 650 °C (400 to 1200 °F) for at least 1/2 hour; air cooling. The temperature and the time at the temperature mainly depend on the required final hardness.

· Stress Relief: After straightening, forming or processing, the stress of the part can be released at a temperature of 650 to 675 °C (1200 to 1250 °F).

· Baking: To avoid hydrogen embrittlement, the plated parts must be baked at least 8h at a temperature of 185 to 195 °C (365 to 385 °F) as soon as possible after plating.

· Forging: 4340 steel is usually forged at a temperature of 1065 to 1230 °C (1950 to 2250 °F); after forging, the parts can be air-cooled in a dry place, or preferably with a furnace.

Welding of AISI 4340 Steel

AISI 4340 steel has good weldability. It can be easily gas or arc welded, but electrodes of the same composition should be used. Since 4340 alloy steel is air hardened, the welded parts should be either annealed or normalized and tempered shortly after welding.

The high hardness of 4340 alloy steel requires that the material be welded in the annealed or tempered state and then heat treated to resist martensite formation and cold cracking. However, as in motor shaft applications, under quenching and tempering conditions, 4340 steel typically use high preheating in low hydrogen processes. Preheating or interpass heating of weld metal and heat affected zones is recommended. Hydrogen control is also essential to prevent cracks in the weld. Extremely clean vacuum smelted steel is the first choice for welding.

Machinability and Machining

Applications

SAE AISI 4340 steel is usually used in places where severe conditions exist and heavy parts requiring high strength, and can be widely used as billet, bar, bar, forging, sheet, pipe and welding wire. AISI 4340 steel can also be produced as light plates and castings. Typical applications include bolts, screws and other fasteners. Gears, pinions, shafts and similar mechanical components; crankshafts and piston rods of engines; and landing gear and other key structural components of aircraft.

Equivalent

ASTM AISI 4340 steel equivalent to SAE, European (German DIN, British BSI, France NF), Japanese JIS and Chinese GB standard (for reference).