What are Silicon Bronze Fasteners?
Looking back in history, copper has been a staple material choice in human creations across most of our civilized past (and even before). In fact, copper was the only metal known to humankind for over five thousand years, serving as the gateway discovery that catapulted us from our stone-age to bronze-age eras. Today, we certainly have countless more natural and synthetic materials to choose from, but copper and bronze still play a very important role in our everyday lives.
Bronze is created by adding trace amounts of other chemical elements (namely tin) to copper, improving upon copper’s utility by bolstering its physical strength and durability. (For comparison, brass is another similar alloy made of copper with added zinc). Taking the high corrosion resistance, low friction, high workability, and aesthetically-pleasing appearance of bronze, engineers sought to evolve the alloy by adding another element, silicon, with the intent to gain advanced strength and low wear in mechanical applications. The result was Silicon Bronze, which toted improved strength, corrosion resistance, and electrical conductivity, paving the way for use in industrial and electrical applications of the modern era.
Do you have a silicon bronze or general bronze fastener application but are unsure of which specialty fastener might be best to use? E&T Fasteners offers expert support and product selection between the above options and more! Contact us to discuss your application, see our Screw and Bolt catalog here, or to see our entire fastener catalog, visit our product selection here.
Silicon Bronze Technical Specifications
Silicon Bronze fasteners can be traced back to two categories of technical standards – one governing their material composition, and another governing their fabrication.
Beginning with material standards, ASTM B98 specifies Silicon Bronze (officially, copper-silicon alloy) material compositions in their raw stock rod, bar, and flat shapes. Alloys in this standard are classified as numbers C65100, C65500, and C66100. For stocks that end up as fasteners, typically C65100 and C65500 are used, otherwise known in shorthand as Bronze Alloys 651 and 655. This standard also specifies several heat treatment tempers available, including soft annealing, hard tempering, hot rolling, and more.
- Alloy 651 is known as Low-Silicon Bronze B, consisting of 0.05% Lead, 1.5% Zinc, 0.8% Iron, 0.7% Manganese, 0.8-2% Silicon, and the rest Copper.
- Alloy 655 is known as High-Silicon Bronze A, consisting of 0.05% Lead, 1.5% Zinc, 0.8% Iron, 0.6% Nickle, 0.5-1.3% Manganese, 2.8-3.8% Silicon, and the rest Copper.
Generally, Alloy 651 provides lower tensile strength, yield strength, and hardness, as well as higher elongation / elasticity, than 655, but this is partially dependent on the heat treatments being compared.
When fabricating fasteners and hardware from Alloys 651 and 655, additional ASTM standards apply.
- ASTM F468 applies to non-ferrous threaded bolts and screws
- ASTM F467 applies to non-ferrous threaded nuts
- ASTM B18 generally applies to all other fasteners
While the above standards cover the raw material and unit fabrication details of silicon bronze fasteners, please remember that there are a number of additional standards that govern the applications in which these fasteners are used. There are many separate standards that users must consult when placing these fasteners into code-regulated applications, such as in structural, electrical, marine, aerospace, and similar arenas.
Most common fastener types are available in silicon bronze materials, many even found locally at retail hardware stores and commercial distributors. Wood screws, lag screws, machined screws, hex cap bolts, flat washers, nuts, lock washers, and lock nuts are readily available in most markets.
Generally speaking, alloy 651 is your ‘common’ selection used for silicon bronze fasteners sold in bulk, whereas alloy 655 is mainly found as engineered, product-specific hardware serving a particular function.
Silicon bronze fasteners are popular choices for applications that can benefit from high corrosion resistance, high strength, and a heightened degree of visual aesthetics. Woodworking, fencing, decking, and high-end furniture projects benefit from silicon bronze hardware, as do similar architectural and commercial construction projects where the bronze finish provides a desirable appearance.
Maritime, marine, and other applications involving fresh- and seawater benefit heavily from silicon bronze hardware’s ability to withstand aggressive environmental conditions, even edging out stainless steel materials.
By the same token, silicon bronze fits well into many industrial applications, being able to withstand chemical, high heat, and complete submersion in water and other fluids. In fact, bronze is naturally resistant to microbial activity thanks to a passive attribute known as anti-biofouling, making it great for plumbing and potable water equipment.
Electrical and electronic applications often turn to silicon bronze fasteners for their high electrical conductivity and self-lubricating, highly galling-resistant material composition. In particular, jewelry and decorative electronics such as wrist watches are good examples of devices that seek to drawn on these combined traits. On the complete other end of the spectrum, heavy power distribution gear in outdoor electrical substations take advantage of the same benefits.
Silicon bronze performs poorly in the presence of oxidizers such as certain acids, sulfides, and chlorides. These potential risks combined with typically high material costs make up the main detractions from choosing silicon bronze fasteners, from which users may examine stainless steel and polymeric fasteners as close alternatives. That said, for most applications where a combination of strength, conductivity, water-driven corrosion resistance, and aesthetic factors reign, silicon bronze is likely the most dominant contender.
One thing to note about any copper and bronze materials is that the exterior finish is expected to change over time. Copper-based materials form a thin layer on their surface due to age and exposure, known as a patina (or as the result of tarnishing). These chemical compounds rise from the interaction between the copper material and atmospheric gases. This patina alters the color and texture of the copper surface, ranging from lighter golds to darker browns, though they are often found desirable to many for their unique visual appearance.
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About E&T Fasteners
With sales and support teams spanning the Continental US, E&T Fasteners is here to serve your hardware, fastening, and component needs both domestically and internationally. We are a stocking distributor of metallic, plastic, and exotic alloy fasteners, backed by our technical staff directly experienced in your unique applications. We can assist your specifying and selection efforts, providing engineered solutions, record-fast lead times, and stellar customer service from qu