What Is CuSn7Pb15-B (CB496K)?
CuSn7Pb15-B is a copper-tin-lead alloy supplied under the European designation system, in which CB496K is the material number quoted on drawings and inspection certificates. The grade is covered by EN 1982:2008, Copper and copper alloys - Ingots and castings. The letter B added to the designation identifies material supplied in the form of ingots rather than finished castings. With 13.2 to 17 percent lead and 6.2 to 8 percent tin, the alloy combines the load carrying capacity of tin bronze with the self-lubricating behaviour of lead, which is why it is one of the most widely specified bearing bronzes in heavy machinery.
The lead phase is distributed as fine isolated particles inside a copper-tin matrix. This structure tolerates boundary lubrication, short periods of dry running and shaft misalignment far better than lead-free bronzes, and it machines freely. Bearing shells that have worn in service can be re-machined or re-lined economically, an important factor for large mill, marine and turbine bearings where replacement cost is high.
Chemical Composition to EN 1982:2008
The limits below apply to the cast analysis. Copper is stated including nickel; all other values are mass percent.
| Element | Requirement, % | Element | Requirement, % |
|---|---|---|---|
| Cu (including Ni) | 74.0 - 79.5 | Pb | 13.2 - 17.0 |
| Sn | 6.2 - 8.0 | Zn | max 2.0 |
| Ni | 0.5 - 2.0 | Sb | max 0.5 |
| Fe | max 0.2 | Mn | max 0.2 |
| P | max 0.1 | S | max 0.08 |
| Si | max 0.01 | Al | max 0.01 |
Impurity control is the part of the specification that decides bearing life. Silicon and aluminium are held at 0.01 percent because both form hard, abrasive oxides that score shafts; sulphur and phosphorus are restricted because they embrittle the matrix and reduce fatigue resistance under cyclic loading.
Mechanical Properties
Property values depend on the casting route, because cooling rate controls grain size and porosity. EN 1982:2008 lists separate minima for sand cast and for centrifugal or continuously cast material; the sand cast figures are the ones used for heavily sectioned housings.
| Property | GS (sand cast) | GZ (centrifugal cast) | GC (continuous cast) |
|---|---|---|---|
| Tensile strength Rm, MPa | 170 | 200 | 200 |
| 0.2 % proof strength, MPa | 80 | 90 | 90 |
| Elongation at fracture A, % | 8 | 7 | 8 |
| Brinell hardness, HBW | 60 | 65 | 65 |
Hardness in this range, roughly 60 to 65 HBW, is deliberately moderate. A soft, conformable bearing surface beds into the shaft, spreads load over a wider contact patch and embeds stray particles instead of ploughing them through the journal. Designers therefore pair CuSn7Pb15-B shells with a hardened and ground shaft rather than with a soft counterpart.
Casting Processes and Material Conditions
GS - sand casting. Lowest tooling cost, used for large housings, brackets and one-off bearing blocks where section thickness varies widely.
GM - permanent mould casting. Gravity die casting for repeated medium-size parts with a finer grain than sand castings.
GZ - centrifugal casting. Preferred for plain cylindrical bushings and sleeves; the highest tensile values of the grade are reached because centrifugal force feeds the section and drives porosity to the bore, which is then machined away.
GC - continuous casting. Long bar and tube stock that is subsequently cut and machined into bearing shells and wear plates.
GP - pressure die casting. High volume, thin wall components where dimensional accuracy matters more than absolute strength.
Material condition R is designated by the minimum tensile strength requirement, while condition H is designated by the minimum hardness requirement. For bearings subject to shock loading, condition R is normally specified so that elongation is guaranteed as well as strength.
Typical Applications and Machining Notes
The combination of lead and tin makes the alloy a general purpose sliding bearing material. Typical uses include plain bearings and bushings for rolling mills and crushers, worm wheel rims, spindle sleeves, pump and valve bushings, marine stern tube bearings, railway axle box components and wear plates in lifting equipment.
Machining is carried out with sharp, high rake tooling; leaded bronze tends to form discontinuous chips, so positive rake angles and generous clearance prevent smearing of the bearing surface. Rough boring should leave 0.3 to 0.5 mm for finish passes to avoid work hardening beneath the surface. Oil grooves and lubrication holes are cut from the bore side, and any burr must be removed because a raised edge will scrape the journal film.
Supply, Testing and Traceability
Ingots designated with the -B suffix are supplied to the cast analysis of EN 1982:2008 with a certificate of analysis from each heat. Castings are supplied after machining when required, with dimensional inspection reports and hardness tests taken at agreed positions. Spectrochemical analysis is used for the verification of copper, tin, lead, nickel and zinc, while carbon and sulphur combustion or optical emission methods cover the low-level impurities. Where the application is safety related, tensile specimens are taken from integrally cast coupons so that the reported Rm and elongation represent the same metal as the component.
Frequently Asked Questions
Q: What does the -B suffix in CuSn7Pb15-B mean?
Under EN 1982:2008 a suffix -B is added to the designation to identify material supplied in the form of ingots, distinguishing ingot stock from the equivalent casting designation.
Q: Can CuSn7Pb15-B be used for potable water fittings?
No. With 13.2 to 17 percent lead the alloy is not intended for drinking water contact. Bearing and machinery applications where the material is not exposed to the water supply are the correct field of use.
Q: Which equivalent designations exist for this alloy?
The grade is listed in the German DIN / WNr system as CuSn7Pb15-B, 2.1183, and the closest Chinese designation is CuPb15Sn. Equivalents are for cross reference only and the ordered specification governs.
Q: Why do sand cast and centrifugally cast values differ?
Centrifugal and continuous casting produce a denser section with finer grains, so the minimum tensile strength rises from 170 MPa to 200 MPa and hardness from 60 to 65 HBW. Sand castings that must meet the higher figures cannot be substituted without re-testing.
Q: How is the alloy lubricated in service?
The lead phase acts as a solid lubricant when the oil film is momentarily lost, which protects the journal during start-up and short overloads. Continuous operation still requires an oil or grease film, correctly sized clearance and clean lubricant.
Q: What hardness should be specified for a bearing shell?
Specify hardness by casting route, 60 HBW for sand cast and 65 HBW for centrifugal or continuous cast material, and keep the shaft at least 30 HBW harder than the shell so that wear is confined to the replaceable part.
