SAE 4340

What is SAE 4340?

SAE 4340 is a quenched and tempered nickel-chromium-molybdenum alloy steel. It contains 0.38 to 0.43 percent carbon and 1.65 to 2.0 percent nickel. It combines high strength, toughness and deep hardening for aerospace and heavy machinery components. Nickel extends the chromium-molybdenum hardening response farther into a section and increases impact energy at a given strength relative to 4140.

SAE 4340 and EN 24 are near-equivalent grades that many industries specify interchangeably. Their composition limits differ. SAE 4340 sets a nickel minimum of 1.65 percent, which is above the middle of the EN 24 range. EN 24 permits up to 1.4 percent chromium, while SAE 4340 permits up to 0.9 percent. SAE 4340 also has tighter sulphur and phosphorus limits. The corresponding continental designation is 34CrNiMo6, with material number 1.6582. Air-melted SAE 4340 has the UNS designation G43400.

SAE 4340 retains greater toughness than lower grades as strength increases.

Laxcon Steels lists SAE 4340 in its grades reference as an alloy steel grade. The same steel is written 4340, 34CrNiMo6, EN 24, 1.6582 and G43400.

Nickel chromium molybdenum steel per SAE J404, close to EN 24

What is the chemical composition of SAE 4340?

Composition limits in weight percent are carbon 0.38 to 0.43, manganese 0.6 to 0.8, sulphur 0.04 maximum, phosphorus 0.035 maximum, silicon 0.15 to 0.35, chromium 0.7 to 0.9, nickel 1.65 to 2.0 and molybdenum 0.2 to 0.3.

What is SAE 4340 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

Specifications and melting routes

ASTM A29 covers hot-wrought bars. ASTM A322 covers alloy steel bars supplied to composition or hardenability requirements. Aerospace specifications distinguish the grade by melting route. AMS 6415 covers air-melted 4340 bar and forging stock. AMS 6414 covers the vacuum-melted and consumable-electrode remelted form of the same chemistry, which has a separate UNS number.

Remelting reduces non-metallic inclusions and dissolved gases. Inclusions can initiate fatigue cracks at the strength levels used in aircraft structures. Dissolved hydrogen can permit static failure. Air-melted and remelted bars with identical certificates of analysis can therefore have different fatigue behaviour.

Mechanical properties

SAE 4340 does not have one fixed strength because tempering temperature controls its properties. ASTM A320 Grade L43 covers quenched and tempered 4340 bolting for low-temperature service. It requires a minimum tensile strength of 125 ksi, or 860 N/mm squared, and a minimum yield strength of 105 ksi, or 725 N/mm squared. It also requires 16 percent elongation, 50 percent reduction of area and a maximum hardness of 35 HRC. The minimum tempering temperature is 1100 degrees Fahrenheit, or 593 degrees Celsius. The required Charpy impact value is 20 foot pounds force, or about 27 joules, at minus 150 degrees Fahrenheit, or minus 101 degrees Celsius.

ASTM A320 Grade L7 is the chromium-molybdenum equivalent. It has the same strength requirements but is limited to a diameter of 2.5 inches. Grade L43 covers 4340 diameters up to 4 inches. Nickel enables the larger section to meet the strength and low-temperature toughness requirements together.

Structural components can receive heat treatment to strengths above the bolting range. Lower tempering temperatures increase tensile strength. They also reduce fracture toughness, notch tolerance and resistance to hydrogen.

Heat treatment and hardenability

The normal heat-treatment route uses austenitising at about 830 degrees Celsius, followed by an oil quench and tempering at a temperature selected for the required strength. Double tempering is common at high strength. Retained austenite transforms after the first cycle and receives tempering during the second cycle instead of remaining in the component. Annealing and normalising occur above the austenitising range. Annealing uses furnace cooling, while normalising uses air cooling.

Nickel, chromium and molybdenum delay transformation and give SAE 4340 high hardenability for its cost. Martensite can form through sections in which a chromium-molybdenum steel would develop partial pearlite at the centre. This property supports its use in large forgings, thick plate and heavy shafts as well as bar. The H version is supplied to a guaranteed Jominy end-quench hardenability band rather than relying only on composition.

Hydrogen embrittlement and surface treatment

Hydrogen embrittlement controls the processing of high-strength SAE 4340 parts and requires definition on the drawing. Acid pickling, electroplating, phosphating and some cleaning processes introduce atomic hydrogen into the steel. Hydrogen diffuses out without damage at low strength. At high strength, it can collect at internal stress concentrations and cause delayed fracture under static load. Failure can occur hours or days after installation without preceding plastic deformation.

Post-coating baking is the accepted control. ASTM B850 provides guidance for steels whose properties are not damaged by baking at 190 to 230 degrees Celsius. It treats parts with an actual tensile strength of at least 1000 N/mm squared, corresponding to about 31 HRC or 300 HV, as requiring treatment unless another specification states otherwise. Baking must follow plating within a short period because hydrogen becomes more difficult to remove after it reaches a crack tip.

High-strength SAE 4340 is also sensitive to residual process damage. Shot peening, controlled fillet radii and avoidance of grinding burns therefore affect component performance. Failure can begin at defects left by processing.

Machining and welding

Machining normally occurs in the annealed condition or at a moderate temper. Complex features are cut before final heat treatment where tolerances permit. Grinding after hardening requires controlled wheels, dressing and coolant. A grinding burn can produce a locally re-austenitised and untempered layer that initiates cracking in a fatigue-loaded component.

Welding requires specialist control. At the midpoints of the SAE composition ranges, the International Institute of Welding carbon equivalent is about 0.85. This value is nearly twice the 0.45 level at which preheating and low-hydrogen practice become obligatory. Welded joints in high-strength 4340 also reintroduce hydrogen risk. An unavoidable joint therefore requires preheating, low-hydrogen consumables, interpass control, post-weld tempering and re-inspection as part of the design.

Applications

Applications include aircraft landing gear, undercarriage components, airframe fittings, helicopter rotor shafts, gears, gearbox shafts, crankshafts and connecting rods in high-output engines. Other uses include heavy drive shafts, torsion bars, drilling and downhole tools, forging dies, die holders and low-temperature bolting to ASTM A320 Grade L43. Very high strength landing gear applications commonly use a modified form of the same chemistry with added silicon and vanadium instead of changing to another alloy family.

Where SAE 4340 is not the right choice

SAE 4340 is unsuitable where its hardenability and toughness are not required. In light sections at ordinary strength levels, 4140 can achieve the same centre properties at lower cost. The additional nickel provides no property benefit in that case.

Very high strength reduces the tolerance of SAE 4340 to flaws, notches and surface damage. Fracture toughness decreases as tensile strength increases. The permissible defect size can become smaller than routine inspection can detect. Hydrogen embrittlement increases this limitation. Plating, pickling and baking procedures must therefore form part of the specification rather than remain unspecified process details.

SAE 4340 is not a case-hardening steel. Wear surfaces instead require a carburising grade such as SAE 8620 or EN 36C. SAE 4340 also does not permit casual welding. It has no corrosion resistance. Protective coatings are therefore necessary in corrosive conditions, but the coating processes can introduce the hydrogen to which high-strength 4340 is vulnerable.

Other alloy steel grades