AMS 6440 vs AMS 6444

Updated July 29, 2026  ·  Reading time ~9 min  ·  Material class High-carbon chromium bearing steel  ·  UNS G52986  ·  Also known as SAE / AISI 52100

Buyers and design engineers run into this pairing constantly, and the two callouts look almost interchangeable on a datasheet. Both AMS 6440 and AMS 6444 point to the workhorse of the bearing world — SAE 52100, a roughly 1% carbon, 1.5% chromium through-hardening steel that has anchored rolling-element bearings for a century. If the chemistry is the same, why do two specifications exist, and why does one cost noticeably more? The answer is not in the alloy. It is in how the steel is made, and how clean the finished structure has to be.

01What AMS 6440 covers

AMS 6440 specifies 52100 steel supplied as bars, forging stock and tubing. It is the baseline aircraft-quality version of the alloy: fully deoxidized, controlled chemistry, verified macrostructure and a defined micro-inclusion ceiling, but produced by conventional melting practice without a mandatory vacuum-remelt step. In service it hardens deeply and predictably, holding a fine, uniform carbide distribution that gives excellent wear resistance and a high, stable hardness.

Because it delivers most of what 52100 is famous for at a sensible cost, AMS 6440 is the default for the majority of forged bearing and wear components — bearing rings, races, rollers and balls, plus mill rolls, roll sleeves, shear blades, spindles, cold-forming dies and cutting tools where the duty is severe but not fatigue-limited at aerospace levels.

02What AMS 6444 covers

AMS 6444 starts from the identical 52100 chemistry and then raises the bar on purity. The defining requirement is consumable-electrode vacuum melting (CEVM, commonly delivered as vacuum arc remelting, VAR). An ingot from the primary melt is remelted under vacuum into a water-cooled mold. As it drips through the arc, dissolved gases escape, volatile tramp elements are driven off, and the ingot solidifies directionally with far fewer and smaller non-metallic inclusions.

That cleaner structure is the whole point. Non-metallic inclusions act as internal stress raisers, and under the millions of load cycles a bearing sees, a single oxide or sulfide of the wrong size can seed a spall. By shrinking and dispersing those inclusions, AMS 6444 raises the practical rolling contact fatigue life of the same alloy — which is exactly why it is the grade written into demanding aircraft main-shaft and gearbox bearings.

03The one difference that matters: melt route & cleanliness

Strip away the labels and the distinction reduces to a single axis — how clean is the steel. Everything downstream follows from the melting route.

Note what does not change: the alloy content, the way it hardens, the achievable surface hardness, and the machining behavior in the annealed condition are all effectively the same. You are not buying a stronger steel with AMS 6444 — you are buying a cleaner one, and paying for the process control and testing that proves it.

04Shared chemistry (nominal 52100)

Both specifications draw on the same nominal composition. Actual ranges are set by the governing spec revision and the mill's practice, but a representative 52100 analysis looks like this:

Values in weight %. Confirm the exact range against the current AMS revision on your purchase order.

05Side-by-side comparison

* Hardness is a property of the alloy and heat treatment, shared by both grades — it is not a differentiator.

06Mechanical behavior & hardness

Because the alloy is identical, mechanical performance is governed by heat treatment rather than by which specification you buy. Through-hardened and tempered 52100 typically finishes around 60–67 HRC, with most bearing races and rollers held near 60–64 HRC to balance hardness against toughness. In the fully hardened condition the steel reaches very high tensile strength — on the order of 1,900 MPa — paired with limited ductility, which is exactly the profile a rolling-element surface wants.

What the vacuum-melted 6444 grade buys you is not a higher number on a tensile chart. It is consistency and endurance: fewer internal defects to nucleate sub-surface fatigue cracks, and therefore a longer and more predictable life under repeated Hertzian contact stress.

07When to use each

The decision is almost always driven by one question: is the part fatigue-critical under rolling or cyclic contact? If yes, and failure carries safety or high-cost consequences, specify the vacuum-melted grade. If not, the standard grade delivers the same hardness and wear resistance for less.

A useful rule of thumb: if a customer drawing or an airframe/engine specification already calls for premium aircraft quality, do not substitute down to 6440 to save cost — the vacuum melt is there for a reason. Conversely, for a general-purpose forged ring, paying the 6444 premium buys cleanliness the application will never use.

08Forging & heat-treatment notes

Both grades forge and heat-treat the same way. 52100 is hot-worked from roughly 1040–1150 °C, then slow-cooled and spheroidize annealed to soften it for machining and to condition the carbides. Parts are machined near-net in this soft state, then through-hardened — austenitized, quenched, and immediately tempered — to reach final hardness, followed by grinding and finishing.

One caution that applies to both: 52100 is neither corrosion-resistant nor a high-temperature steel. It tempers back and loses hardness with sustained heat, so service is generally kept below about 120–175 °C. If your application needs heat or corrosion resistance, 52100 is the wrong family regardless of whether you pick 6440 or 6444.

09Typical forged applications

10How to specify it correctly

When you place an order for forged 52100, a clean specification prevents costly rework. State the following on the drawing and purchase order:

• Specification and revision — e.g. AMS 6440 or AMS 6444, with the applicable revision letter.
• Melt route — confirm CEVM / VAR where 6444 is required.
• Cleanliness — the governing inclusion rating (ASTM E45, Method A) and any customer-specific limits.
• Delivery condition — spheroidize annealed for machining, or hardened and tempered to a target HRC.
• Documentation — a Mill Test Certificate (EN 10204 3.1 or 3.2) for full material traceability.

For a full breakdown of the forged products we produce in these grades — including bearing rings, valve components, pump parts, rolls and gears — see our dedicated page on AMS 6440 & AMS 6444 forged forging parts.

11Frequently asked questions

Sourcing these grades

This article is an educational comparison of the two specifications, written to help engineers choose between them. If you are sourcing finished components rather than researching the difference, browse our 52100 forged parts in AMS 6440 and AMS 6444 — including bearing rings, valve and pump components, rolls and gears.

Standards & sources referenced

• SAE AMS 6440 — Steel, Bars, Forgings, and Tubing (SAE 52100), standard quality.
• SAE AMS 6444 — Steel, Bars, Forgings, and Tubing (SAE 52100), premium aircraft quality, consumable-electrode vacuum melted.
• ASTM E45 — Standard Test Methods for Determining the Inclusion Content of Steel (Method A).
• ASTM E381 — Standard Method of Macroetch Testing Steel Bars, Billets, Blooms, and Forgings.
• EN 10204 — Metallic products, types of inspection documents (Mill Test Certificate 3.1 / 3.2).