Precious Metal B631 Class A-INF
Heat-treated state · default condition
ASTM B631 Class A
INF = 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.
Values for the INF condition
Other B631 Class A tempers and conditions
B631 Class A is listed in 3 conditions; see the grade overview at B631 Class A.
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 |
INFDefaultThis page 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 |
What is B631 Class A-INF?
B631 Class A-INF is B631 Class A heat treated to INF — 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. B631 Class A-INF 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). B631 Class A-INF 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).
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-INF properties
Typical room-temperature values for B631 Class A-INF; 18 are specific to this condition. Other listed values are B631 Class A reference data and may not represent this condition. Bars are the position among precious metal grades.
Physical3
B631 Class A-INF 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-INF 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-INF 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-INF 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-INF 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-INF'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. Confirm any required mill or lot documentation when quoting.
Working with B631 Class A-INF
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.
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.
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 | — |
B631 Class A-INF — frequently asked
What is B631 Class A-INF used for?
B631 Class A-INF 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-INF?
B631 Class A-INF 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-INF 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-INF 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-INF?
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-INF 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-INF?
B631 Class A-INF 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.
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. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.