Precious Metal B631 Class B
ASTM B631 Class B
ASTM B631 Class B is a sintered silver-tungsten contact composite at mid tungsten content, produced by pressing and sintering or by infiltration (conditions INF, PS-HD, PS-LD).
Reference datasheet — B631 Class B 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.
What is B631 Class B?
ASTM B631 Class B is a sintered silver-tungsten contact composite at mid tungsten content, produced by pressing and sintering or by infiltration (conditions INF, PS-HD, PS-LD). It is chosen where arc erosion resistance must be balanced against conductivity — 50% IACS, 215 W/m·K and 150 HB — and is supplied as pressed tips, buttons and blanks, usually pre-bonded to a copper carrier. Elongation is 1.5%, so the composite cannot be bent or drawn and is not fusion weldable; service is limited to about 400 °C. Class C adds tungsten for severe interrupting duty at lower conductivity. B631 Class B has a density of 14.6 g/cm³ (0.527 lb/in³), heavier than 73% of precious metal grades. It melts at 961°C (1,762 °F). B631 Class B has a yield strength of 330 MPa (47.9 ksi) and a tensile strength of 450 MPa (65.3 ksi) — stronger than 93% of precious metal grades. Elongation at break is 1.5% and the elastic modulus is 190 GPa (27.6 Msi).
What is B631 Class B used for?
- air circuit breaker contacts
- load-break switch contacts
- arc chute contact tips
Advantages
- Stiff — 190 GPa (27.6 Msi), better than 94% of precious metal grades
- High strength — 330 MPa (47.9 ksi), better than 93% of precious metal grades
- Low embodied carbon — 55 kg CO₂/kg, better than 86% of precious metal grades
Limitations
- Limited ductility — 1.5%, worse than 99% of precious metal grades
- Difficult to weld — 10/100, worse than 99% of precious metal grades
- Limited formability — 8/100, worse than 99% of precious metal grades
- Difficult to machine — 25/100, worse than 94% of precious metal grades
- Hard to polish — 60/100, worse than 92% of precious metal grades
B631 Class B properties
Typical room-temperature values; bars are the position among precious metal grades.
Physical3
B631 Class B has a density of 14.6 g/cm³ (0.527 lb/in³), heavier than 73% of precious metal grades. It melts at 961°C (1,762 °F).
Mechanical12
B631 Class B has a yield strength of 330 MPa (47.9 ksi) and a tensile strength of 450 MPa (65.3 ksi) — stronger than 93% of precious metal grades. Elongation at break is 1.5% and the elastic modulus is 190 GPa (27.6 Msi).
Thermal6
B631 Class B is rated for continuous service to 400°C (752 °F). It conducts heat at 215 W/m·K (124.2 BTU/hr·ft·°F), better than 70% of precious metal grades. Thermal expansion is 10.5 µm/m·K (5.83 µin/in·°F).
Electrical3
B631 Class B conducts electricity at 50 % IACS, better than 71% of precious metal grades.
Chemical & Environmental3
Corrosion resistance is very good (82/100), better than 53% of precious metal grades.
Sustainability4
Producing a kilogram of B631 Class B takes about 840 MJ of energy and emits 55 kg of CO₂ — less than 81% of precious metal grades. Typical recycled content is 40%.
Manufacturability7
B631 Class B's machinability is poor (25/100, worse than 94% of precious metal grades); weldability not recommended (10/100); 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 B is highlighted; hover a dot for its name, or open the interactive family chart.
Working with B631 Class B
Is B631 Class B easy to machine?
Grind, lap or wire/sinker EDM wherever possible; if turned or milled use sharp carbide or PCD tooling, light feeds and flood coolant — the W skeleton is abrasive and the material chips rather than shears.
Can B631 Class B be welded?
Not fusion weldable; attach with Ag-Cu-P or BAg brazing filler to a Cu or Cu-alloy carrier, or buy pre-bonded Cu/AgW bimetal blanks with a silver-rich braze layer.
Can B631 Class B be formed, bent or molded?
Net-shape is set by pressing and sintering/infiltration; only minor coining or riveting of a soft backing layer is possible — do not bend or draw the composite.
B631 Class B 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 phase, infiltrated or co-sintered | 38.5 – 41.5 | 40 |
| Other (total impurities)Fe, Ni, Co, C, O residuals per ASTM B631 | ≤ 0.5 | — |
| Wtungsten skeleton, balance | balance | — |
What surface finishes work on B631 Class B?
Contact face is ground/lapped and left bare (arcing cleans it); braze face is commonly silver- or nickel-plated for wetting and storage; degrease and avoid sulfur-bearing packaging to limit tarnish.
How much does B631 Class B cost?
B631 Class B is a high-cost precious metal ($$$$), pricier than about 78% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
B631 Class B — frequently asked
What is B631 Class B used for?
B631 Class B is typically used for air circuit breaker contacts, load-break switch contacts and arc chute contact tips. In short: mid-tungsten B631 grade; balanced erosion and conductivity; braze-on tips only.
What is the yield strength of B631 Class B?
B631 Class B has a typical yield strength of 330 MPa (47.9 ksi) and a tensile strength of 450 MPa (65.3 ksi) — stronger than 93% of precious metal grades.
Is B631 Class B easy to machine?
Not especially — machinability is rated poor (25/100, worse than 94% of precious metal grades). Grind, lap or wire/sinker EDM wherever possible; if turned or milled use sharp carbide or PCD tooling, light feeds and flood coolant — the W skeleton is abrasive and the material chips rather than shears.
Can B631 Class B be welded?
Not readily — weldability is rated not recommended (10/100). Not fusion weldable; attach with Ag-Cu-P or BAg brazing filler to a Cu or Cu-alloy carrier, or buy pre-bonded Cu/AgW bimetal blanks with a silver-rich braze layer.
What surface finishes work on B631 Class B?
Contact face is ground/lapped and left bare (arcing cleans it); braze face is commonly silver- or nickel-plated for wetting and storage; degrease and avoid sulfur-bearing packaging to limit tarnish.
Can B631 Class B be formed, bent or molded?
Net-shape is set by pressing and sintering/infiltration; only minor coining or riveting of a soft backing layer is possible — do not bend or draw the composite.
What is the maximum service temperature of B631 Class B?
B631 Class B is rated for continuous use to about 400°C (752 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
How much does B631 Class B cost?
B631 Class B is a high-cost precious metal ($$$$), 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)
- Electrical Contacts: Principles and Applications (Slade) — CRC Press (2014)
- Silver-tungsten (AgW) contact material datasheets — AMETEK / Metalor / Plansee / (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.