Precious Metal B631 Class A
ASTM B631 Class A
ASTM B631 Class A is a powder-metallurgy silver-tungsten composite with the lowest tungsten content of the B631 classes, biasing the balance toward conductivity rather than maximum arc erosion resistance.
Reference datasheet — B631 Class A is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the INF condition
B631 Class A tempers and conditions
B631 Class A is supplied in 3 conditions, soft to hard. INF is the default FabDigit quotes; the values on this page are for it.
Compare all 3 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Pressed and sintered, low density | Use the lowest-cost single press-and-sinter parts (≈93-96 % TD) for light to medium duty switch contacts where residual porosity and 56 % IACS keep temperature rise acceptable. | 200 | 280 | 2 | 80 HB | 56 % IACS |
Pressed, sintered and repressed/coined, high density | Choose when sintering is followed by repressing/coining to ≥97 % TD: low porosity, good dimensional accuracy and 65 % IACS give medium-duty contacts stable contact resistance and temperature rise. | 260 | 380 | 5 | 100 HB | 65 % IACS |
INFDefault Tungsten skeleton infiltrated with silver | Default choice: a sintered tungsten skeleton infiltrated with silver to ≥98.5 % TD is virtually pore-free, reaches 250 W/m·K and resists welding, suiting breaker main and arcing contacts. | 290 | 390 | 2.5 | 130 HB | 60 % IACS |
Supply forms, finishes and certifications
Ways B631 Class A is supplied — form, process route, surface or paperwork — without a datasheet of their own. The property values above apply.
| Option | What it is | Type |
|---|---|---|
INF-AGB | Infiltrated, with brazed/co-sintered silver backing layer | Surface finish |
What is B631 Class A?
ASTM B631 Class A is a powder-metallurgy silver-tungsten composite with the lowest tungsten content of the B631 classes, biasing the balance toward conductivity rather than maximum arc erosion resistance. It is used for moderate-duty switching contacts, supplied as pressed-and-sintered (low and high density) or infiltrated blanks, including silver-backed forms for brazing; typical properties are 13.6 g/cm³ density, 390 MPa tensile, 290 MPa yield, 130 HB (78 HRB), 150 GPa modulus, 60 % IACS, 250 W/m·K and service to about 350 °C. Elongation is only 2.5 % and the formability score is 8, so parts must be produced near net shape. For higher arc erosion resistance at lower conductivity, move to a higher-tungsten B631 class. B631 Class A has a density of 13.6 g/cm³ (0.491 lb/in³), heavier than 71% of precious metal grades. It melts at 961°C (1,762 °F). B631 Class A has a yield strength of 290 MPa (42.1 ksi) and a tensile strength of 390 MPa (56.6 ksi) — stronger than 89% of precious metal grades. Elongation at break is 2.5% and the elastic modulus is 150 GPa (21.8 Msi).
What is B631 Class A used for?
- circuit breaker contacts
- contactor tips
- motor starter contacts
Advantages
- High strength — 290 MPa (42.1 ksi), better than 89% of precious metal grades
- Stiff — 150 GPa (21.8 Msi), better than 87% of precious metal grades
Limitations
- Limited formability — 8/100, worse than 99% of precious metal grades
- Limited ductility — 2.5%, worse than 97% of precious metal grades
- Hard to polish — 55/100, worse than 97% of precious metal grades
- Difficult to machine — 25/100, worse than 94% of precious metal grades
- Difficult to weld — 20/100, worse than 86% of precious metal grades
B631 Class A properties
Typical room-temperature values for B631 Class A-INF; bars are the position among precious metal grades.
Physical3
B631 Class A has a density of 13.6 g/cm³ (0.491 lb/in³), heavier than 71% of precious metal grades. It melts at 961°C (1,762 °F).
Mechanical12
B631 Class A has a yield strength of 290 MPa (42.1 ksi) and a tensile strength of 390 MPa (56.6 ksi) — stronger than 89% of precious metal grades. Elongation at break is 2.5% and the elastic modulus is 150 GPa (21.8 Msi).
Thermal6
B631 Class A is rated for continuous service to 350°C (662 °F). It conducts heat at 250 W/m·K (144.4 BTU/hr·ft·°F), better than 73% of precious metal grades. Thermal expansion is 13 µm/m·K (7.22 µin/in·°F).
Electrical3
B631 Class A conducts electricity at 60 % IACS, better than 73% of precious metal grades.
Chemical & Environmental3
Corrosion resistance is very good (78/100), worse than 56% of precious metal grades.
Sustainability4
Producing a kilogram of B631 Class A takes about 950 MJ of energy and emits 90 kg of CO₂ — less than 77% of precious metal grades. Typical recycled content is 45%.
Manufacturability7
B631 Class A's machinability is poor (25/100, worse than 94% of precious metal grades); weldability poor (20/100); formability not recommended (8/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the precious metal family
Yield strength against density for every precious metal grade FabDigit runs. B631 Class A is highlighted; hover a dot for its name, or open the interactive family chart.
Working with B631 Class A
Is B631 Class A easy to machine?
Parts are used mainly near net shape; where machining is needed, grind with diamond or CBN wheels or cut by wire EDM. Carbide turning tools wear extremely fast and feeds must be kept light.
Can B631 Class A be welded?
Not suitable for fusion welding. Contacts are brazed with silver-based filler or resistance welded to copper carriers, preferably using a pure-silver backing layer.
Can B631 Class A be formed, bent or molded?
Produced by powder pressing and sintering or by infiltrating a tungsten skeleton. The finished material has almost no cold formability — do not bend it.
B631 Class A chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| Agsilver matrix, introduced by infiltration or powder blending | 48 – 52 | 50 |
| Other (total impurities)includes Fe, Ni, Mo, C, O; total capped by the standard | ≤ 0.5 | — |
| Wtungsten skeleton / tungsten powder phase | balance | — |
What surface finishes work on B631 Class A?
Cannot be anodized. Surfaces may be silver plated to resist sulphidation or used bare; degrease before assembly and keep contact faces flat.
How much does B631 Class A cost?
B631 Class A is a high-cost precious metal ($$$$), pricier than about 78% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
B631 Class A — frequently asked
What is B631 Class A used for?
B631 Class A is typically used for circuit breaker contacts, contactor tips and motor starter contacts. In short: highest-conductivity Ag-W class; arc-erosion resistant contact; near-net shape only.
What is the yield strength of B631 Class A?
B631 Class A has a typical yield strength of 290 MPa (42.1 ksi) and a tensile strength of 390 MPa (56.6 ksi) — stronger than 89% of precious metal grades. Strength varies by condition: see the 3 listed tempers.
Is B631 Class A easy to machine?
Not especially — machinability is rated poor (25/100, worse than 94% of precious metal grades). Parts are used mainly near net shape; where machining is needed, grind with diamond or CBN wheels or cut by wire EDM. Carbide turning tools wear extremely fast and feeds must be kept light.
Can B631 Class A be welded?
Not readily — weldability is rated poor (20/100). Not suitable for fusion welding. Contacts are brazed with silver-based filler or resistance welded to copper carriers, preferably using a pure-silver backing layer.
What surface finishes work on B631 Class A?
Cannot be anodized. Surfaces may be silver plated to resist sulphidation or used bare; degrease before assembly and keep contact faces flat.
Can B631 Class A be formed, bent or molded?
Produced by powder pressing and sintering or by infiltrating a tungsten skeleton. The finished material has almost no cold formability — do not bend it.
What is the maximum service temperature of B631 Class A?
B631 Class A is rated for continuous use to about 350°C (662 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between B631 Class A-INF and B631 Class A PS-LD?
INF is the default condition — default choice: a sintered tungsten skeleton infiltrated with silver to ≥98.5 % TD is virtually pore-free, reaches 250 W/m·K and resists welding, suiting breaker main and arcing contacts. PS-LD: use the lowest-cost single press-and-sinter parts (≈93-96 % TD) for light to medium duty switch contacts where residual porosity and 56 % IACS keep temperature rise acceptable. Yield strength is 290 MPa in INF versus 200 MPa in PS-LD.
How much does B631 Class A cost?
B631 Class A is a high-cost precious metal ($$$$), pricier than about 78% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- ASTM B631 Standard Specification for Silver-Tungsten Electrical Contact Material — ASTM International (2018)
- ASM Handbook Vol. 2: Properties and Selection — Nonferrous Alloys and Special-Purpose Materials (Electrical Contact Materials) — ASM International (1990)
- Silver-Tungsten (AgW) Contact Material Data — Contact material producers (Metalor / AMETEK / ) (2023)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.