GB/T 8320 vs ASTM B702 vs RWMA Class 10/11/12: Copper Tungsten Grade Cross-Reference
A drawing from a European OEM says "WCu 80/20". A drawing from a North American switchgear builder says "RWMA Class 12". A Chinese supplier's material certificate says "CuW80 per GB/T 8320". Are these the same material? Nearly, but not exactly, and the differences matter when the material certificate has to satisfy the drawing. This article lays out the three main designation systems side by side and explains where they line up and where they do not.
The Three Systems
GB/T 8320 (China): "Copper-tungsten and silver-tungsten electrical contact materials". Grades are named by tungsten weight percent: CuW50, CuW55, CuW60, CuW65, CuW70, CuW75, CuW80, CuW85, CuW90. For each grade the standard sets minimum density, minimum hardness (Brinell) and minimum electrical conductivity (% IACS), plus composition tolerance. Most Chinese-produced CuW is certified to this standard.
ASTM B702 (USA): "Standard Specification for Copper-Tungsten Electrical Contact Material". Material is designated by nominal tungsten content, covering roughly the same 50 to 90 percent range, with minimum density, hardness and conductivity per composition. Check the current edition for exact class labels; the structure is the same as GB/T 8320, grade by tungsten percent with property floors.
RWMA Group B (Resistance Welding Manufacturers Association, now part of AWS): Classes 10, 11 and 12 are copper-tungsten; Class 13 is tungsten, Class 14 is molybdenum. The classes were written for resistance welding electrodes, not switchgear contacts, but North American electrical drawings use them by habit. The important difference: RWMA classes are defined by minimum hardness and conductivity, not by a fixed tungsten percentage. The tungsten content is whatever the producer needs to hit the property floors.
No EN or ISO standard exists for copper tungsten contact material composition. European drawings either reference a supplier data sheet designation such as "WCu 80/20", or the OEM's own material specification, or occasionally an ASTM or RWMA class.
GB/T 8320 Grades: Industry-Typical Minimums
| Grade | W (wt%) | Density (g/cm³) min | Hardness (HB) min | Conductivity (% IACS) min |
|---|---|---|---|---|
| CuW50 | 50 | 11.85 | 115 | 54 |
| CuW55 | 55 | 12.30 | 125 | 49 |
| CuW60 | 60 | 12.75 | 140 | 47 |
| CuW65 | 65 | 13.30 | 155 | 44 |
| CuW70 | 70 | 13.80 | 175 | 42 |
| CuW75 | 75 | 14.50 | 195 | 38 |
| CuW80 | 80 | 15.20 | 220 | 34 |
| CuW85 | 85 | 15.90 | 240 | 30 |
| CuW90 | 90 | 16.75 | 260 | 27 |
These are the minimums the standard sets. A good infiltrated lot exceeds them; certified values are per lot. For switchgear arcing contacts, CuW70, CuW75 and CuW80 cover almost every drawing. CuW85 and CuW90 appear on EDM electrodes and some specialised arcing tips. Below CuW70 is rare in HV apparatus.
RWMA Group B: Defined by Properties
| Class | Typical composition | Hardness min (HRB) | Conductivity min (% IACS) | Typical density (g/cm³) |
|---|---|---|---|---|
| Class 10 | ~72 W / 28 Cu | 72 | 45 | ~14.2 |
| Class 11 | ~75 W / 25 Cu | 94 | 40 | ~14.5 |
| Class 12 | ~78 to 80 W / 20 to 22 Cu | 98 | 35 | ~15.0 |
Two things to notice. The minimum hardness for Class 10 is much lower than a producer's typical value, because the class was set to be achievable across producers. And the composition is approximate; one producer's Class 12 is 78 percent tungsten, another's is 80. If a drawing says "Class 12" and the material certificate says "CuW80, 220 HB, 34 % IACS", the buyer has to decide whether 34 % IACS satisfies a 35 % IACS class minimum. Usually the producer's typical lot value is above 35 and the certificate shows it, but the standard minimums do not line up exactly.
Cross-Reference Table
The mapping most buyers and producers work to:
| GB/T 8320 | Nominal W% | RWMA Group B | European data-sheet designation | Typical switchgear use |
|---|---|---|---|---|
| CuW70 | 70 | Class 10 (approx.) | WCu 70/30 | MV arcing contacts, load-break, thin sections |
| CuW75 | 75 | Class 11 | WCu 75/25 | 72.5 to 245 kV arcing contact rods |
| CuW80 | 80 | Class 12 | WCu 80/20 | 245 kV and above arcing tips, generator breakers |
| CuW85 | 85 | (none) | WCu 85/15 | Specialised high-erosion tips |
| CuW90 | 90 | (none) | WCu 90/10 | EDM electrodes, rarely contacts |
Where the mapping is approximate, we have said so. CuW70 to Class 10 is the loosest: Class 10 is nearer 72 percent tungsten and its property floors are set differently. CuW75 to Class 11 and CuW80 to Class 12 are close enough that most OEMs accept them as equivalent once a lot certificate shows the actual values meet both sets of minimums.
Hardness Units: HB vs HRB
GB/T 8320 uses Brinell (HB). RWMA uses Rockwell B (HRB). There is no exact conversion for a tungsten-copper composite, and the ASTM E140 conversion tables for copper alloys are only a guide. As a rough working reference for infiltrated CuW:
- 175 HB is in the region of 88 to 90 HRB
- 195 HB is in the region of 92 to 94 HRB
- 220 HB is in the region of 97 to 99 HRB
If a drawing requires hardness in one unit and the certificate reports the other, the sensible route is to test in the drawing's unit for first article and agree the equivalence in writing. Do not rely on a conversion table to settle an acceptance dispute.
What Usually Goes Wrong
The drawing names a producer's trade designation. "Elkonite 30W3" or "Plansee WCu 80/20" is a product name, not a standard. Ask the OEM whether an equivalent grade to a public standard is acceptable, and get it on the drawing or in the purchase order. Most will accept "CuW80 per GB/T 8320 or ASTM B702 equivalent" once the first-article certificate is reviewed.
The certificate reports composition but not properties. Composition alone does not prove infiltration quality. A CuW80 skeleton with poor infiltration has the right tungsten percentage and the wrong density. Insist on density, hardness and conductivity per lot.
The drawing specifies hardness only. Hardness is the easiest property to hit by adjusting tungsten content and the least informative on its own. Add density and conductivity.
Different minimums, same grade name. A GB/T 8320 CuW80 certificate showing 34.5 % IACS meets that standard. If the drawing cites an OEM specification with a 36 % IACS floor, the lot fails the drawing while passing the standard. Always read the drawing's referenced document, not just the grade name.
Recommended Drawing Callout
The callout that avoids all of the above:
Material: Copper tungsten, 80 % W nominal, infiltrated. Density ≥ 15.2 g/cm³, hardness ≥ 220 HB, conductivity ≥ 34 % IACS. Per GB/T 8320 CuW80 or ASTM B702 equivalent. Certificate per lot: composition, density, hardness, conductivity.
Grade by tungsten percent, production route, three property floors, two acceptable standards, and the certification requirement. Any producer in any system can quote against it and any inspector can check it. More on this in How to Specify Copper Tungsten Contacts on a Drawing.
Summary
GB/T 8320 and ASTM B702 both grade copper tungsten by tungsten percent with property minimums, and map to each other closely at the same nominal composition. RWMA Group B classes are defined by property floors rather than composition; Class 11 and Class 12 correspond in practice to CuW75 and CuW80, Class 10 loosely to CuW70. European "WCu 80/20" style designations are supplier data-sheet names with no standard behind them.
For a drawing, state the nominal tungsten percent, the production route, and density, hardness and conductivity minimums. Then the certificate can be checked against the drawing regardless of which standard the producer works to.
Related Reading
- Copper Tungsten Properties Guide — Density, hardness, conductivity and what they mean in service
- How to Choose Copper Tungsten Grade — Grade selection by application
- CuW70 vs CuW80 for 145 kV, 245 kV and 420 kV — Grade by voltage class
- Copper Tungsten Series — CuW70/75/80 made to drawing