Legovaná ocel 4340 | 34CrNiMo6 | 1.6582

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4340 alloy steel is a prominent type of medium-carbon low-alloy steel and is recognized as an ultrahigh-strength steel. It is often considered the standard against which other ultrahigh-strength steels are compared. The UNS (Unified Numbering System) designation for 4340 alloy steel is G43400. Internationally, it corresponds to standards like SNB23-1-5 or SNB24-1-5 in Japan, and 34CrNiMo6 in Europe. It is also identified by the AMS (Aerospace Material Specification) number 6414.

1. Aplikace

Aircraft and Aerospace: Aircraft parts. Aircraft landing gear. Airframe parts. Connecting rods
Machinery Components: Gears. Pinions. Shafts. Crankshafts and piston rods for engines. Splines
Fasteners: Bolts. Screws. Studs
Structural Components: Pressure vessels. Mountain bicycle frames

2. 4340 Steel Composition¹

3. 4340 Alloy Steel Properties

3.1 Mechanické vlastnosti

4340 alloy steel exhibits excellent strength, good ductility, and outstanding toughness, as well as resistance to fatigue and creep. Its temperování properties after kalení are roughly equivalent to those of carbon steel. Mechanical properties are greatly influenced by the tempering temperature after oil quenching.

The following table lists the 4340 steel material properties in different oil quenching and tempering conditions.

After quenching and tempering, AISI 4340 alloy steel forms a lath martensite microstructure. During rapid cooling, the formation of martensite is accompanied by volume expansion. Depending on the cooling rate, other microstructures, such as bainite, may also form.

3.2 Obrobitelnost

The machinability of steel AISI 4340 is generally rated at 55% for cold-drawn material and 45% for annealed material, relative to B1112 steel (rated at 100%). For optimal machinability, a partly spheroidized microstructure, achieved by normalizing followed by tempering, is recommended.

3.3 Weldability

4340 steel has excellent weldability and can be joined using gas or arc welding processes. However, as an air-hardening steel, it also requires certain protective measures. AISI 4340 steel is more difficult to weld than ordinary carbon steel, and preheating the material and controlling the interpass temperature are necessary during welding. Additionally, annealing or normalizing treatment followed by tempering should be performed as soon as possible after welding. 

3.4 Nitriding

AISI 4340 can be readily nitrided, and this process often improves its fatigue life. While 4140 steel, because of its higher chromium content and nickel-free composition, generally exhibits superior nitriding characteristics, 4340 still develops a heavier nitrided case than 8640 steel during the initial 24 hours of nitriding. Typical nitriding temperatures for 4340 steel range from 510 to 550 °C (950 to 1020 °F). Based on our customers’ applications of nitrided 4340 steel for gear manufacturing, the core hardness is typically 38-42 HRC.

3.5 Potential Issues and Processing Precautions

Users of material 4340 by si měl být vědom potenciálních problémů:

• Praskání: Kalení vodou výrazně zvyšuje riziko praskání.
• Vodíkové křehnutí: This can be a concern when the steel is heat-treated to very high tensile strengths (above approximately 1400 MPa / 200 ksi). We suggest baking after processes like pickling or plating.
• Praskání v důsledku koroze v důsledku napětí: Ocel AISI 4340 vykazuje slabou odolnost proti praskání v důsledku koroze v důsledku napětí, zejména při popouštění na vysoké pevnosti v tahu (1500 až 1950 MPa / 220 až 280 ksi).
• Mezikrystalová zlomenina: Citlivost je ovlivněna nečistotami a přítomností vodíku.
• Zkřehnutí za stálého působení: To je u této slitiny známý jev.

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4. Tepelné zpracování

The heat treatment of 4340 steel involves several key processes, each influencing its final microstructure and mechanical properties:

4.1 Kování

The forging temperature is 1065 to 1230°C (1950 to 2250°F). After the forging process, parts can be air-cooled in a dry environment or, preferably, furnace-cooled. The flow stress of 4340 steel can increase by approximately 15% with a 55 °C (100 °F) drop in hot forging temperature. 

4.2 Normalizace

Normalizing can improve the microstructure of 4340 steel after uneven cooling at high temperatures. This process is also applicable to alloy steel forgings, rolled products, and castings.

Heat 4340 steel to a temperature 55 to 85°C (100 to 150°F) above its phase transformation temperature (845 to 900°C, or 1550 to 1650°F), at which the original microstructure is completely dissolved into austenite. The material is then cooled to room temperature in air.

4.3 Žíhání

The annealing temperature for 4340 steel is 790-845 °C (1450-1550 °F), with a maximum 4340 steel hardness of 223 HB after annealing.

4.4 Kalení (kalení)

Heat 4340 steel to its austenitic transformation temperature of 800 to 845°C (1475 to 1550°F). We usually select an austenitizing temperature of 845 °C (1550 °F). Soak for approximately 15 minutes per 25 mm (1 inch) of section size, with a minimum hold time of 150 minutes, followed by annealing. For thin-section 4340 steel, annealing is performed by air annealing. For circular sections with a diameter not exceeding 75 mm (3 inches), oil quenching may be used.

 Isothermal transformation diagram for a 4340 steel containing 0.42% C, 0.78% Mn, 1.79% Ni, 0.80% Cr, and 0.33% Mo. [Source: From The Making, Shaping and Treating of Steels, 9th ed., ibid.]

4.5 Temperování

Tempering effectively reduces the brittleness of untempered martensite. Tempering temperatures range from 200 to 650°C (400 to 1200°F) to achieve the required strength levels, and can even reach as high as 705°C (1300°F).

For AISI 4340 steel oil-quenched at 845°C (1550°F), we strongly recommend a tempering temperature of 425°C (800°F), while for water-quenched 4340 steel, we strongly recommend a tempering temperature of 650°C (1200°F).

Variation in hardness with tempering temperature of 4340 steel. All specimens oil quenched from 845 °C (1550 °F) and tempered 2 h at temperature. AQ, as-quenched. [Source: ASM International. (1991). Příručka ASM, svazek 4: Tepelné zpracování (p. 506). ASM International.]

4.6 Uvolnění stresu

This step is designed to reduce residual stress generated by processes such as straightening, forming, or machining. Stress relief for 4340 steel is performed before hardening at a temperature of 650 to 675 °C (1200 to 1250 °F). For hardened 4340 material, the stress relief temperature should not exceed the previous tempering temperature. 

4.7 Sféroidizace

The purpose of spheroidizing treatment is to form a spheroidal carbide microstructure in the ferritic matrix, which softens the steel. The maximum temperature for spheroidizing treatment of 4340 steel is 760-775°C (1400-1425°F). A semi-spheroidized structure is obtained by normalizing, followed by tempering at 650°C (1200°F).

5. Ekvivalentní jakosti oceli 4340

1. Mezinárodní ASM. (1991). Příručka ASM, svazek 4: Tepelné zpracování (p. 496). ASM International. ↩︎

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