MIL Specification 4140 Alloy Steel Alloy Steel

MIL-S-5626

Chrome-Molybdenum (4140) Alloy Steel – Bars, Rods & Forging Stock for Aircraft Application

Steel, Chrome-Molybdenum (4140), Bars, Rods, and Forging Stock (For Aircraft Application) – Superseded by SAE-AMS6382 and SAE-AMS6349

AS9100D / ISO 9001

Mill Test Reports

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Quick Specification Facts

Spec Number MIL-S-5626 (MILS5626)

Material Chrome-Molybdenum (4140) Alloy Steel

Spec Type Military (MIL) Specification – Department of Defense

Category Alloy Steel / 4140 Steel

Composition 4140 (Fe-0.4C-1Cr-0.2Mo; with Mn, Si)

Condition Annealed, Normalized, or per specification requirements

Product Forms Bars, Rods, and Forging Stock

Superseded By SAE-AMS6382 and SAE-AMS6349

Certification AS9100D / ISO 9001 with full MTR traceability

Specification Overview

MIL-S-5626 (also referenced as MILS5626) is a U.S. Department of Defense Military Specification covering Chrome-Molybdenum (4140) Alloy Steel in the form of Bars, Rods, and Forging Stock for Aircraft Application. This specification has been superseded by SAE-AMS6382 and SAE-AMS6349, but continues to be widely referenced in legacy aerospace and defense programs. The 4140 alloy steel is a versatile chromium-molybdenum steel that provides an outstanding combination of strength, toughness, hardenability, and fatigue resistance suitable for critical aircraft structural and mechanical components.

The 4140 alloy steel specified under MIL-S-5626 contains approximately 0.38–0.43% carbon for hardness, 0.80–1.10% chromium for hardenability and corrosion resistance, and 0.15–0.25% molybdenum for improved strength, toughness, and hardenability at elevated temperatures. Manganese and silicon additions further enhance strength and deoxidation. The combination of these alloying elements gives 4140 an exceptional balance of mechanical properties across a wide range of heat treatment conditions—from annealed for machinability to quenched and tempered for maximum strength—making it one of the most widely used alloy steels in aerospace, defense, and industrial applications worldwide.

Available Product Forms

Bars

Rods

Forging Stock

MIL-S-5626 material is supplied as Bars, Rods, and Forging Stock per specification requirements. Round bar, hex bar, flat bar, and rod forms are available in a wide range of diameters and cross-sections. Forging stock and custom-cut lengths are available from our in-house processing facility. Contact our materials specialists for non-standard dimensions or custom forging requirements.

Nominal Chemical Composition

Element	Symbol	Range / Nominal %	Notes
Iron	Fe	Balance	Base metal
Carbon	C	0.38–0.43%	Primary hardening element; controls strength and hardness
Chromium	Cr	0.80–1.10%	Hardenability and corrosion resistance
Molybdenum	Mo	0.15–0.25%	Hardenability, high-temperature strength, and toughness
Manganese	Mn	0.75–1.00%	Hardenability and strength; deoxidizer
Silicon	Si	0.15–0.35%	Deoxidizer; improves strength and toughness
Phosphorus	P	0.035% max	Controlled impurity; limits set to preserve toughness
Sulfur	S	0.040% max	Controlled impurity; limits set to preserve toughness and ductility

Chemical composition ranges are nominal. Consult the current superseding specifications SAE-AMS6382 and SAE-AMS6349 for precise minimum/maximum requirements. Aero-Vac provides full mill test reports (MTRs) with certified chemistry for every order.

Key Properties & Specification Details

Property / Attribute	Details
Specification Number	MIL-S-5626 (MILS5626)
Issuing Organization	U.S. Department of Defense (DoD)
Material Type	Alloy Steel – Chrome-Molybdenum (4140) Bars, Rods, and Forging Stock
Alloy Name / Grade	4140 (Chrome-Molybdenum Steel)
Nominal Composition	Fe-0.4C-1Cr-0.2Mo (with Mn, Si)
Melt Practice	Electric Furnace or as specified
Heat Treatment Condition	Annealed, Normalized, or Quenched & Tempered per specification requirements
Available Forms	Bars, Rods, and Forging Stock
Superseded By	SAE-AMS6382 and SAE-AMS6349
Tensile Strength	Up to 150 ksi+ depending on heat treatment condition
Machinability	Good in annealed condition; sharp tooling and proper feeds/speeds recommended
Weldability	Good with appropriate pre- and post-weld heat treatment
Magnetic Properties	Magnetic (ferritic/martensitic)
Certification Available	AS9100D / ISO 9001, Certificate of Conformance, MTR

Typical Applications

Aircraft Structural Components: Airframe structural members, bulkheads, fittings, and brackets requiring high strength, toughness, and fatigue resistance for demanding aircraft structural applications.

Landing Gear Components: Landing gear struts, actuator components, and related structural members that demand high tensile strength and excellent impact resistance under cyclic loading conditions.

Aerospace Fasteners & Hardware: High-strength bolts, studs, pins, shafts, and other aerospace fasteners and hardware requiring elevated mechanical properties and reliable performance in critical applications.

Hydraulic & Mechanical Components: Hydraulic cylinder bodies, fittings, valve bodies, and mechanical transmission components where strength, hardness, and wear resistance are essential.

Defense & Military Applications: Weapon system components, vehicle drivetrain parts, and other defense hardware requiring a well-characterized, high-strength alloy steel with proven performance in military service.

Frequently Asked Questions – MIL-S-5626

What is MIL-S-5626 and what does it cover?

MIL-S-5626 (also written MILS5626) is a U.S. Department of Defense Military Specification covering Chrome-Molybdenum (4140) Alloy Steel in the form of Bars, Rods, and Forging Stock for Aircraft Application. The specification defines chemistry, mechanical property requirements, and inspection criteria for 4140 steel used in critical aircraft and aerospace structural components. MIL-S-5626 has been superseded by SAE-AMS6382 and SAE-AMS6349, but continues to be actively referenced in legacy aerospace and defense programs. Aero-Vac can supply material to MIL-S-5626 or the superseding AMS specifications depending on your program requirements.

What is 4140 Chrome-Molybdenum Steel and what makes it suitable for aircraft applications?

4140 is a chromium-molybdenum alloy steel containing approximately 0.38–0.43% carbon, 0.80–1.10% chromium, and 0.15–0.25% molybdenum. The chromium provides hardenability and corrosion resistance, while molybdenum increases strength, toughness, and through-hardening capability. Combined with the carbon content for hardness and the manganese addition for hardenability, 4140 offers an excellent balance of tensile strength, fatigue strength, and impact toughness across a wide range of heat treatment conditions. Its versatility—from annealed for machining to quenched and tempered for maximum strength—makes it one of the most widely specified alloy steels for aircraft structural components, landing gear, fasteners, and mechanical hardware.

What specifications supersede MIL-S-5626?

MIL-S-5626 has been superseded by SAE-AMS6382 and SAE-AMS6349. For new design and procurement applications, SAE-AMS6382 (covering 4140 steel bars, forgings, and rings) or SAE-AMS6349 (covering related product forms) should be used as the primary specification reference. However, MIL-S-5626 remains a valid procurement reference for legacy programs, existing component drawings, and repair/overhaul applications that specify this MIL spec by number. Aero-Vac can supply certified material to MIL-S-5626, AMS6382, AMS6349, or other equivalent specifications depending on your application and program requirements. Contact our team to discuss which specification best fits your needs.

Does Aero-Vac supply MIL-S-5626 with full mill test reports and certifications?

Yes. All MIL-S-5626 material supplied by Aero-Vac is fully traceable to mill test reports (MTRs) confirming chemical composition and mechanical properties per the specification. We are AS9100D/ISO 9001 certified with a 99.6% quality rating. Every order ships with certificates of conformance, material certifications, and chemical/mechanical test reports. Electronic delivery of all documentation is available for fast turnaround. If your program requires additional special process certifications or source-controlled documentation, our quality team can coordinate as needed.

What sizes of MIL-S-5626 bars, rods, and forging stock does Aero-Vac stock?

Aero-Vac maintains inventory of 4140 Chrome-Molybdenum alloy steel in round bar, hex bar, flat bar, rod, and forging stock forms across a wide range of diameters and cross-sections. We carry standard bar diameters from small rounds up to large-diameter stock and can provide custom-cut lengths from our in-house processing facility at the Port of Kalama. For forging stock or near-net-shape requirements, our team can source from our global supplier network. Contact us with your specific size, quantity, and certification requirements for a prompt quote with availability and lead time.

Can Aero-Vac provide value-added processing for MIL-S-5626 4140 alloy steel?

Yes. Aero-Vac operates a complete in-house processing facility at our Port of Kalama location. For MIL-S-5626 4140 alloy steel bars, rods, and forging stock, we offer precision saw cutting, surface grinding, turning, milling, and heat treatment services including annealing, normalizing, and quench-and-temper processing. We also provide comprehensive mechanical and non-destructive testing services in-house. Having all capabilities under one roof allows for faster turnaround, tighter quality control, and reduced overall cost compared to using multiple vendors. Contact us to discuss your specific processing requirements and how we can support your program.

Need MIL-S-5626 material or a custom quote?

Our alloy steel specialists can provide fast pricing, availability, and technical guidance for your MIL-S-5626 / 4140 Chrome-Molybdenum Steel requirements.