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ASTM B777: The Four Classes of Tungsten Heavy Alloy

Updated September 26, 2026 · Reviewed September 26, 2026

ASTM B777 is the reference most shops and mills use when they talk about tungsten heavy alloy class. If your drawing or RFQ says “heavy alloy” without a class, expect a shop to ask which one you mean, since class affects density, minimum properties and price. This guide explains what the standard actually covers and how to pick a class without already being a metallurgist.

What ASTM B777 defines

ASTM International publishes B777 as the specification for tungsten base, high-density metal, most commonly used in radiation shielding, balance weights and similar dense-metal parts. The standard sets out four classes, each defined by a minimum tungsten content and a corresponding density range, along with minimum tensile strength, yield strength and elongation for each class. It does not specify a shape or a machining process; it defines the material itself, the same way a steel alloy specification defines composition and mechanical properties without dictating how a part is cut.

The four classes at a glance

Class	Tungsten content	Typical density (g/cm³)
Class 1	Lower end of the heavy alloy range	Around 17.0
Class 2	Mid range	Around 17.5 to 18.0
Class 3	Higher tungsten content	Around 18.0 to 18.5
Class 4	Highest tungsten content	Around 18.5

These figures are typical, not exact for every mill’s material. The binder, usually nickel with iron or nickel with copper, makes up the remainder and is what determines whether the alloy is magnetic. Your mill or shop confirms the exact composition and density for the material they supply against a given class.

How the classes trend on mechanical properties

B777 does not just set density. It also sets minimum tensile strength, yield strength and elongation for each class, and these tend to move in opposite directions as tungsten content rises. The table below shows the general trend, not specific values, since exact minimums are defined in the standard itself and should be confirmed against the current edition or with your mill.

Class	Tungsten content and density	Typical strength trend	Typical ductility trend
Class 1	Lowest	Lower	Higher, more ductile
Class 2	Mid	Moderate	Moderate
Class 3	High	Higher	Lower
Class 4	Highest	Highest	Lowest, most brittle of the four

This is the practical reason a higher class is not automatically the “better” choice. A part that needs to absorb shock or bend slightly under load may perform better at a lower class with more ductility, even if a higher class would pack more density into the same envelope.

Why the class matters beyond density

Density is the number most buyers focus on, since it drives how much mass fits in a given envelope, which is the whole point of choosing tungsten heavy alloy over steel or lead. But B777 also sets minimum tensile strength, yield strength and elongation for each class, which matter for parts that see mechanical load rather than sitting as static ballast or shielding. A counterweight bolted into a rotating assembly, for example, has different mechanical demands than a static shielding block, and the class should reflect that, not just the density target.

Class and machinability

All four classes machine in broadly the same way: they turn and mill on standard CNC equipment with carbide tooling, closer in behavior to hard gray cast iron than to a difficult exotic metal. Small differences in binder ratio across classes can shift tool wear rates and surface finish slightly, but this is a secondary effect. Shops choosing tooling and cutting parameters plan primarily around whether the part is heavy alloy or pure tungsten, not around which B777 class the heavy alloy falls into.

W-Ni-Fe versus W-Ni-Cu across classes

Each B777 class can be supplied with either a nickel-iron binder or a nickel-copper binder, and this choice, not the class number, is what determines magnetism. W-Ni-Fe grades carry mild magnetism from the iron content. W-Ni-Cu grades are effectively non-magnetic. If your part sits near MRI equipment, sensors or any system sensitive to stray magnetic fields, specify W-Ni-Cu explicitly, since a mill defaults to W-Ni-Fe unless told otherwise, and the two are not interchangeable once a part is in service.

Choosing a class when you do not know one yet

Most buyers arrive at this decision from the application, not from the standard. A practical way to work backward into a class:

1. Start from the functional requirement. A shielding thickness constraint or a mass target within a fixed envelope translates into a required density.
2. Check whether the part carries mechanical load. A static shielding block has different needs than a bolted counterweight or a part that sees repeated stress.
3. Pick the lowest class that meets the density requirement, rather than defaulting to the highest, since lower classes are generally more ductile and easier to machine.
4. Confirm with the shop or mill, since they can tell you what is readily available and whether your target sits comfortably within a class or right at its boundary.

Common mistakes when specifying a B777 class

• Naming a class without checking it against a real density need. Some buyers copy a class number from a similar past project without confirming it still fits the current part’s envelope and mass target.
• Assuming every mill’s Class 3, for example, is identical. The class sets minimums, and material from different mills can sit at different points within the allowed range. If your tolerance for variation is narrow, say so.
• Picking the highest class “to be safe.” A higher class costs more and can be less ductile and slightly harder to machine cleanly. Match the class to the requirement, not to a general instinct that more density is always better.
• Leaving out the binder system. A class number alone does not tell a mill whether you need W-Ni-Fe or W-Ni-Cu. State both.

How to specify a class on your RFQ

A drawing or RFQ that names the class clearly avoids back-and-forth with the mill and shop:

1. State the class number (1 through 4) if you know it, or the target density if you do not.
2. State the binder system, W-Ni-Fe or W-Ni-Cu, especially if magnetism is a constraint.
3. Note any mechanical requirements, such as minimum yield strength, if the part carries load beyond its own weight.
4. Ask for material certification referencing ASTM B777 if your customer or application requires traceable documentation.

If you know your target density but not the class, that is enough to start: tell us the application and the density you need when you send your RFQ, and we match you with shops that source the right class from their mill. For how heavy alloy actually cuts once the class is chosen, see tungsten heavy alloy machining. For a side-by-side of heavy alloy against pure tungsten and carbide, see what each grade allows.

See also: the ASTM B777 reference page, with the full class table, edition history and certificate lines.

Frequently asked questions

Sources

• ASTM B777 - Standard Specification for Tungsten Base, High-Density Metal
• Wikipedia - Tungsten heavy alloy
• Wikipedia - ASTM International

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• Doped Tungsten: Lanthanated and Potassium Doped Grades and Their Parts