MIL Specification 303 Stainless Steel 303 Series

MIL-S-7720

303 Stainless Steel – Corrosion Resistant (18-8), Aircraft Quality

Steel, Corrosion Resistant (18-8) Bars, Wire and Forging Stock (Aircraft Quality)
18Cr – 8Ni (18-8) | Superseded by SAE-AMS-S-7720

AS9100D / ISO 9001

Mill Test Reports

2–4 Week Delivery

877-237-6822

Quick Specification Facts

Spec Number MIL-S-7720 (MIL S 7720)

Material 303 Stainless Steel

Spec Type Military Specification (MilSpec / Department of Defense)

Category 303 Series Stainless Steel

Composition 18Cr – 8Ni (18-8)

Condition Aircraft Quality

Product Forms Bars, Wire, Forging Stock

Superseded By SAE-AMS-S-7720

Certification AS9100D / ISO 9001 with full MTR traceability

Specification Overview

MIL-S-7720 (also referenced as MIL S 7720) is a Military (Department of Defense) specification covering 303 stainless steel in the form of bars, wire, and forging stock for aircraft quality applications. This specification has been superseded by SAE-AMS-S-7720, though MIL-S-7720 remains widely referenced in legacy aerospace and defense programs. The nominal composition of 18% Chromium and 8% Nickel (18-8) delivers excellent corrosion resistance combined with superior machinability compared to standard austenitic stainless steels.

Type 303 stainless steel is a free-machining grade of the austenitic 18-8 family, achieved through the addition of sulfur (and sometimes phosphorus) to the alloy. This makes MIL-S-7720 material particularly well-suited for precision machined components, screw machine products, and aircraft hardware where tight dimensional tolerances and high production rates are required. The "Aircraft Quality" designation ensures elevated cleanliness standards, tighter chemistry controls, and enhanced mechanical property requirements compared to commercial-grade material.

Available Product Forms

Bars

Wire

Forging Stock

All product forms are supplied to Aircraft Quality standards per MIL-S-7720 / SAE-AMS-S-7720 requirements. Bars are available in round, flat, and hex configurations. Custom sizes, lengths, and pre-machined forms are available upon request. Contact our materials specialists for non-standard dimensions or specialty forging stock requirements.

Nominal Chemical Composition

Element	Symbol	Nominal %	Notes
Chromium	Cr	17–19%	Primary corrosion resistance
Nickel	Ni	8–10%	Austenite stabilizer
Iron	Fe	Balance	Base metal
Carbon	C	0.15% max	Controls carbide formation
Sulfur	S	0.15% min	Key free-machining addition – enhances machinability
Manganese	Mn	2.0% max	Austenite stabilizer, aids hot workability
Silicon	Si	1.0% max	Deoxidizer
Phosphorus	P	0.20% max	May be added to further improve machinability

Chemical composition ranges are nominal. Consult the current issue of MIL-S-7720 or the superseding SAE-AMS-S-7720 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-7720 (MIL S 7720)
Issuing Organization	U.S. Department of Defense (Military / DoD)
Superseded By	SAE-AMS-S-7720
Material Type	Corrosion Resistant Stainless Steel – Free Machining
Steel Grade	303 Stainless Steel (AISI / UNS S30300)
Nominal Composition	18Cr – 8Ni (18-8)
Condition	Aircraft Quality
Available Forms	Bars, Wire, Forging Stock
Machinability	Excellent – highest machinability of the austenitic stainless grades due to sulfur addition
Corrosion Resistance	Good – suitable for mildly corrosive environments; slightly lower than 304/316 due to sulfur content
Weldability	Not recommended – sulfur content promotes hot cracking during welding
Magnetic Properties	Non-magnetic in annealed condition
Certification Available	AS9100D / ISO 9001, Certificate of Conformance, MTR

Typical Applications

Aerospace & Aircraft Hardware: Precision machined aircraft fasteners, bolts, studs, nuts, fittings, and brackets requiring tight dimensional tolerances and moderate corrosion resistance in aircraft quality applications.

Screw Machine Products: High-volume precision turned components produced on automatic screw machines, including shafts, bushings, valve stems, and connectors where superior machinability is critical for production efficiency.

Precision Machined Components: Instrument housings, gears, pinions, pump components, and fittings requiring close tolerances and smooth surface finishes achievable through the excellent chip-breaking characteristics of 303 stainless steel.

Defense & Military Applications: DoD-specified components for military aircraft, ground vehicles, and defense systems where MIL-S-7720 or its superseding specification SAE-AMS-S-7720 is required by drawing or contract.

Industrial & Commercial: Pump shafts, valve bodies, food processing equipment fittings, and general industrial components where free-machining stainless steel is preferred over standard austenitic grades for cost-effective manufacturing.

Frequently Asked Questions – MIL-S-7720

What is MIL-S-7720 and what does it cover?

MIL-S-7720 (also written MIL S 7720) is a U.S. Department of Defense Military Specification covering 303 stainless steel in the form of bars, wire, and forging stock for aircraft quality applications. The nominal chemistry of 18% Chromium and 8% Nickel (18-8), combined with a minimum sulfur addition, delivers excellent corrosion resistance along with the superior machinability that defines the 303 grade. This specification has been superseded by SAE-AMS-S-7720 but remains referenced in many legacy aerospace and defense programs. It is widely used for precision machined aircraft hardware, screw machine products, and components requiring tight tolerances and high production rates.

What is the difference between MIL-S-7720 and SAE-AMS-S-7720?

SAE-AMS-S-7720 is the direct superseding specification for MIL-S-7720, issued by SAE International to replace the original Department of Defense military specification. Both specifications cover the same material – 303 stainless steel (18-8) in bars, wire, and forging stock for aircraft quality use – and the technical requirements are essentially equivalent. The primary difference is the issuing authority: MIL-S-7720 was a DoD document, while SAE-AMS-S-7720 is maintained by SAE International. For new designs and procurement, SAE-AMS-S-7720 is the current preferred reference. However, material certified to MIL-S-7720 is generally considered interchangeable, and Aero-Vac can supply material certified to either or both specifications as required by your program.

Does Aero-Vac supply MIL-S-7720 with full mill test reports and certifications?

Yes. All MIL-S-7720 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 DoD-specific documentation, our quality team can coordinate as needed.

What forms and sizes of MIL-S-7720 does Aero-Vac stock?

Aero-Vac maintains inventory of MIL-S-7720 / SAE-AMS-S-7720 in multiple product forms including bars (round, flat, and hex), wire, and forging stock. We carry a range of standard sizes and can provide custom-cut lengths, discs, and blocks from our in-house processing facility at the Port of Kalama. For non-standard dimensions or specialty forging stock requirements, our team can source from our global supplier network. Contact us with your specific size requirements and we will provide a prompt quote with availability and lead time.

What are the typical lead times for MIL-S-7720 material?

In-stock MIL-S-7720 material is typically available for same-day or next-day shipment. For custom-cut or specially processed orders, our standard lead time is 2–4 weeks—significantly faster than the industry average of 8–12 weeks. Specialty forging stock items or quantities requiring mill production may require 4–8 weeks depending on quantity and form. We prioritize urgent aerospace and defense requirements and will always provide honest, upfront delivery estimates. Rush processing is available on select items.

Can Aero-Vac provide value-added processing for MIL-S-7720 material?

Yes. Aero-Vac operates a complete in-house processing facility at our Port of Kalama location. For MIL-S-7720 stainless steel, we offer precision saw cutting (including disc and ring cutting), surface grinding, trepanning, turning, milling, and heat treatment. We also provide comprehensive mechanical and non-destructive testing services in-house. The excellent machinability of 303 stainless steel makes it particularly well-suited for precision turning and screw machine operations. 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.

Need MIL-S-7720 material or a custom quote?

Our stainless steel specialists can provide fast pricing, availability, and technical guidance for your MIL-S-7720 / 303 stainless steel requirements.