Military Specification NIT135 Mod Wrought Low Alloy Steel Alloy Steel

MIL-S-6709

Chrome-Molybdenum-Aluminum Steel – Bars, Rods, Billets and Forging Stock (For Nitriding)

Steel, Chrome-Molybdenum-Aluminum; Bars, Rods, Billets and Forging Stock (For Nitriding) (Aircraft Quality) (S/S by SAE-AMS6470 and SAE-AMS6472)

AS9100D / ISO 9001

Mill Test Reports

2–4 Week Delivery

877-237-6822

Quick Specification Facts

Spec Number MIL-S-6709

Material NIT135 Mod Wrought Low Alloy Steel

Spec Type Military Specification (MilSpec) – Department of Defense

Category Alloy Steel / Nitriding Steel

Composition Chrome-Molybdenum-Aluminum

Condition Bars, Rods, Billets and Forging Stock (For Nitriding)

Product Forms Bars, Rods, Billets, Forging Stock

Superseded By SAE-AMS6470 and SAE-AMS6472

Certification AS9100D / ISO 9001 with full MTR traceability

Specification Overview

MIL-S-6709 is a Department of Defense Military Specification covering chrome-molybdenum-aluminum wrought low alloy steel (NIT135 Mod) in the form of bars, rods, billets, and forging stock intended for nitriding applications. This aircraft-quality specification ensures consistent chemistry and mechanical properties for components that will undergo surface hardening via the nitriding process. MIL-S-6709 has been superseded by SAE-AMS6470 and SAE-AMS6472, though it remains referenced in legacy programs and legacy procurement documents across the aerospace and defense industries.

The chrome-molybdenum-aluminum composition of this steel makes it particularly well-suited to gas nitriding, producing an extremely hard case with excellent wear and fatigue resistance while maintaining a tough core. The aluminum content is a key alloying element that reacts with nitrogen during nitriding to form aluminum nitrides, contributing to a very hard and stable surface layer. This combination of properties makes MIL-S-6709 / NIT135 Mod an important material for high-performance aerospace gears, shafts, dies, and other precision components where exceptional surface hardness and dimensional stability are required.

Available Product Forms

Bars

Rods

Billets

Forging Stock

MIL-S-6709 material is supplied as Bars, Rods, Billets, and Forging Stock per specification requirements for nitriding (aircraft quality) applications. Standard and custom sizes are available. Custom lengths and pre-machined forms are available upon request. Contact our materials specialists for non-standard dimensions or special processing requirements.

Nominal Chemical Composition

Element	Symbol	Nominal %	Notes
Carbon	C	0.38 – 0.43%	Medium carbon – enables heat treatment to high core strength
Chromium	Cr	1.40 – 1.80%	Enhances hardenability; promotes nitride formation
Molybdenum	Mo	0.30 – 0.40%	Improves hardenability and elevated-temperature strength
Aluminum	Al	0.95 – 1.30%	Key nitriding element – forms hard aluminum nitrides in case
Manganese	Mn	0.50 – 0.70%	Deoxidizer; improves hardenability and strength
Silicon	Si	0.20 – 0.40%	Deoxidizer; improves strength
Iron	Fe	Balance	Base metal

Chemical composition ranges are nominal. Consult the current issue of MIL-S-6709 or the superseding specifications SAE-AMS6470 / SAE-AMS6472 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-6709
Issuing Organization	U.S. Department of Defense (Military Specification / MilSpec)
Material Type	NIT135 Mod Wrought Low Alloy Steel – Chrome-Molybdenum-Aluminum
Steel Grade	Nitralloy 135 Modified (N135 Mod)
Nominal Composition	Chrome-Molybdenum-Aluminum (Cr-Mo-Al)
Available Forms	Bars, Rods, Billets, Forging Stock
Intended Process	For Nitriding (gas or liquid nitriding to achieve hard case)
Surface Hardness (Nitrided)	Very high – typically 65–70 HRC case hardness after nitriding
Hardenability	Excellent – responds well to quench and temper prior to nitriding
Machinability	Good – best machined in pre-hardened/tempered condition prior to nitriding
Fatigue Resistance	Excellent – nitrided case produces compressive surface stresses enhancing fatigue life
Dimensional Stability	Outstanding – nitriding causes minimal distortion vs. carburizing
Magnetic Properties	Magnetic (ferritic/martensitic)
Superseded By	SAE-AMS6470 and SAE-AMS6472
Certification Available	AS9100D / ISO 9001, Certificate of Conformance, MTR

Typical Applications

Aerospace & Defense: Aircraft gears, camshafts, crankshafts, actuator components, and precision aerospace structural parts requiring extreme surface hardness, wear resistance, and dimensional stability after nitriding.

Gears & Power Transmission: Transmission gears, splines, gear shafts, and pinions for aerospace and high-performance industrial applications where hard, wear-resistant tooth surfaces and tough cores are required.

Dies & Tooling: Die casting dies, extrusion dies, forging dies, and precision tooling components that benefit from the exceptional hardness and heat resistance imparted by nitriding.

Shafts & Spindles: Precision ground shafts, spindles, and other rotating components requiring hard, wear-resistant surfaces with minimal dimensional change during surface hardening treatment.

High-Wear Structural Applications: Components subject to sliding contact, fretting, or abrasive wear in demanding aerospace and industrial environments where long service life is critical.

Frequently Asked Questions – MIL-S-6709

What is MIL-S-6709 and what does it cover?

MIL-S-6709 is a U.S. Department of Defense Military Specification covering chrome-molybdenum-aluminum wrought low alloy steel (NIT135 Mod) in the form of bars, rods, billets, and forging stock intended for nitriding applications at aircraft quality. The chrome-molybdenum-aluminum composition makes this steel exceptionally responsive to gas nitriding, producing an extremely hard case layer with outstanding wear resistance and fatigue life. MIL-S-6709 has been superseded by SAE-AMS6470 and SAE-AMS6472, but remains referenced in legacy aerospace and defense programs.

What is the difference between MIL-S-6709, AMS 6470, and AMS 6472?

MIL-S-6709 is the original Department of Defense Military Specification for chrome-molybdenum-aluminum nitriding steel (NIT135 Mod). It has been formally superseded by SAE-AMS6470 and SAE-AMS6472, which are the current SAE Aerospace Material Specifications covering the same material with updated requirements. AMS 6470 and AMS 6472 differ in their specific heat treatment conditions and mechanical property requirements. For new designs, the AMS specifications are preferred; however, material supplied to AMS 6470 or AMS 6472 is generally considered equivalent for programs originally calling for MIL-S-6709. Aero-Vac can supply material to any of these specifications and will help you determine the correct one for your program.

Does Aero-Vac supply MIL-S-6709 with full mill test reports and certifications?

Yes. All MIL-S-6709 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 sizes of MIL-S-6709 material does Aero-Vac stock?

Aero-Vac maintains inventory of MIL-S-6709 chrome-molybdenum-aluminum nitriding steel in a range of bar diameters, rod sizes, and billet dimensions. We carry standard sizes and can provide custom-cut lengths from our in-house processing facility at the Port of Kalama. For non-standard dimensions or 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-6709 material?

In-stock MIL-S-6709 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 items requiring mill production may require 4–8 weeks depending on quantity and dimensions. 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-6709 material?

Yes. Aero-Vac operates a complete in-house processing facility at our Port of Kalama location. For MIL-S-6709 chrome-molybdenum-aluminum nitriding steel, we offer precision saw cutting, surface grinding, turning, milling, and heat treatment services. 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, including pre-nitriding heat treatment and rough machining.

Need MIL-S-6709 material or a custom quote?

Our alloy steel specialists can provide fast pricing, availability, and technical guidance for your MIL-S-6709 requirements.