SAE 4340 Alloy Steel

 What is SAE 4340 Alloy steel?

 Due to their high tensile strength and low costs, steels are widely used in construction. Hardening agents such as carbon, other elements, and inclusions in iron prevent dislocations from occurring in the crystal lattices of iron atoms. Typical steel alloys contain up to 2.1% carbon. A steel’s hardness, ductility, and tensile strength are controlled by the amount of alloying elements that are present as solutes or precipitated phases, which retard the movement of those dislocations that make iron so ductile and weak, and thus control their hardness, ductility, and tensile strength. The strength of steel is only possible at the expense of ductility, which is abundant in iron.

While steel had been produced in bloomery furnaces for thousands of years, its use expanded after more efficient production methods, blister steel and then crucible steel, were devised in the 17th century. With the invention of the Bessemer process in the mid-19th century, a new era of mass-produced steel began.

This was followed by the Siemens-Martin process and then the Gilchrist-Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron. Further refinements in the process, such as basic oxygen steelmaking (BOS), largely replaced earlier methods by further lowering the cost of production and increasing the quality of the metal. Today, steel is one of the most common materials in the world, with more than 1.3 billion tons produced annually. It is a major component in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons. Modern steel is generally identified by various grades defined by assorted standards organizations.

Some of the common types of SAE 4340 Alloy steel are: 

1. SAE 4340 Forged steel:
   In the production of forged steel, steel is heated to a specific temperature and then hammered or pressed into a desired shape after it has been heated to the desired temperature. The SAE 4340 forged steel is commonly used in mechanical components with a high degree of stress, such as crankshafts, gears, and axles.
2. SAE 4340 hot-rolled steel:
   In contrast to forged steel, hot-rolled steel is produced by rolling the steel at high temperatures, resulting in a less precise shape compared to forged steel as a result. Steel SAE 4340 is generally used in applications that require high strength and toughness, such as aircraft landing gear and engine parts, because of its high strength and toughness.
3. SAE 4340 cold-drawn steel:
   As opposed to hot-rolled steel, cold-drawn steel is produced by drawing or pulling the steel through a die, which results in a smoother, more precise shape compared to hot-rolled steel as a result. It is commonly used in applications that require high surface quality and dimensional accuracy, such as hydraulic cylinders and bearings, to use SAE 4340 cold-drawn steel.
4. SAE 4340 normal steel:
   The normalization of steel is a heat treatment process that involves heating the steel to a specified temperature, then cooling it in air to produce a microstructure and mechanical properties that are specific to the steel. As a result of its toughness and fatigue resistance, SAE 4340 normalized steel is commonly used in applications that require high toughness and fatigue resistance, such as landing gear and engine parts for aircraft.

Thermal Properties of SAE 4340 Alloy Steel

Physical Properties Of SAE 4340 Alloy Steel

Applications

Commercial and military aircraft, automotive systems, forged hydraulic and other machine tool applications, forged steel crankshafts.

Forging

Forging should be carried out between 2250 and 1800 o F (1230 and 980 o C). After forging in ash or sand, parts should be slowly cooled.

Heat Treatment

Steel is heat treated after forging to make it suitable for machining, and to meet the mechanical property limits specified for its application. It is only through practice that the optimum temperature and conditions for heat treating a steel part can be determined. Please take the following information as a guide only.

• Stress Relieving

For pre-hardened steel stress relieving is achieved by heating steel 4340 to between 500 to 550°C. Heat to 600 °C – 650 °C, hold until temperature is uniform throughout the section, soak for 1 hour per 25 mm section, and cool in still air.

• Annealing

A full anneal may be done at 844°C (1550 F) followed by controlled (furnace) cooling at a rate not faster than 10°C (50 F) per hour down to 315°C (600 F). From 315°C 600 F it may be air cooled.

• Tempering

AISI 4340 alloy steel should be in the heat-treated or normalized heat-treated condition before tempering. The tempering temperature depends upon the strength level desired. For strength levels in the 260 – 280 ksi range temper at 232°C (450 F). For strength in the 125 – 200 ksi range temper at 510°C (950 F). And don’t temper the 4340 steels if it is in the 220 – 260 ksi strength range as tempering can result in degradation of impact resistance for this level of strength.

Tempering should be avoided if possible within the range of 250 °C – 450 °C due to temper brittleness.

• Flame or Induction Hardening

As mentioned above, pre-hardened and tempered 4340 steel bars or plates can be further surface hardened by either the flame or induction hardening methods resulting in a case hardness in excess of Rc 50. AISI 4340 steel parts should be heated as quickly as possible to the austenitic temperature range (830 °C – 860 °C) and required case depth followed by an immediate oil or water quenching, depending upon hardness required, workpiece size/shape, and quenching arrangements.

Following quenching to hand warm, tempering at 150°C – 200°C will reduce stresses in the case with minimal effect on its hardness.

All de-carburized surface material must first be removed to ensure the best results.

• Nitriding

Hardened and tempered 4340 alloy steel can also be nitrided, giving a surface hardness of up to Rc 60. Heat to 500°C – 530°C and hold for sufficient time (from 10 to 60 hours) to develop the depth of the case. Nitriding should be followed by slow cooling (no quench) reducing the problem of distortion. The nitrided grade 4340 materials can therefore be machined to near final size, leaving a small grinding allowance only. The tensile strength of the 4340 steel material core is usually not affected since the nitriding temperature range is generally below the original tempering temperature employed.

Surface hardness achievable is 600 to 650HV.

Annealing

4340 should be annealed at a nominal temperature of 1525 o F (830 o C), cooled to 1350 o F (730 o C), and furnace cooled to 1130 o F (610 o C) at a rate of 20 o F (11 o C) per hour, then air cooled. As full annealing involves slow cooling over the entire temperature range from the austenitizing temperature to a temperature well below that at which transformation is complete, it is very time-consuming.

Obtaining a spheroidized structure in 4340 grade requires austenitizing at 1380 o F (750 o C), furnace cooling to 1300 o F (705 o C), and then cooling to 1050 o F (565 o C) at a rate of 3 o C per hour. It is likely that this structure will be more machinable than the coarse lamellar pearlite structure obtained by full annealing.

Normalizing

During this process, steel is heated above its ferrite to austenite transformation temperature, then cooled in the air to a temperature well below it. As a conditioning treatment prior to heat treatment, the treatment can be applied to forged products. Moreover, normalizing refines the structure of forgings that may have cooled unevenly during their forging process. For 4340 grade, the nominal normalizing temperature is 1500 o F (815 o C), but production experience may require a temperature of 50 o F (10 o C) above or below this value. As a rule of thumb, when forgings are normalized before, say, carburizing or hardening and tempering, the upper range of normalizing temperatures is used. When normalizing is the final heat treatment, the lower temperature range is used.

Hardening

After quenching, martensite forms, increasing hardness and tensile strength. Direct hardening of 4340 degrees requires an austenitizing temperature of 1500 – 1550 o F (815 – 845 o C). Due to its hardenability, 4340 is normally quenched in oil.

Tempering

During tempering, stresses from hardening are relieved, but the primary purpose is to obtain the mechanical properties required for the final application. In many cases, it will be a matter of trial and error to determine the appropriate tempering temperature. This grade 4340 can also be hardened by nitriding, flame hardening, or induction hardening.

Machinability

This grade is readily machinable, with either a coarse lamellar pearlitic structure or a spheroidized structure being best advised, depending upon section size and complexity and the amount of machining to be carried out. If there is doubt about the suitability of any other structure, then a spheroidized structure should be aimed for in heat treatment.

Welding

This grade is readily welded in the annealed condition, but welding in the hardened and tempered condition should be avoided where possible because of the effect on mechanical properties. Welding in nitrided or flame or induction-hardened conditions is not recommended.

Guidance details for welding include the use of low-hydrogen electrodes and a preheat from 400 – 570 º F (200 – 300 º C,) if possible to be maintained during welding. Parts should be slowly cooled after welding in ashes or sand and stress relieved if possible.

Applications:

• Aircraft Landing Gear
• Automotive,
• Oil and Gas Drilling,
• Forging,
• Warm and Cold Forming,
• Machine Building,
• Transfer Systems, like power transmission gears and shafts.
• General engineering industries and structural use applications, such as: heavy duty shafts, gears, axles, spindles, couplings, pins, chucks, molds etc.