bimetal

A bimetal is a composite strip or object made from two different metals bonded together along their entire length or surface, almost always steel and a non-ferrous metal such as brass, bronze, or invar. The bond is permanent and metallurgical, achieved through rolling, brazing, or explosive welding depending on the application and required strength. The two metals retain their distinct material properties while functioning as a single unit.

The fundamental purpose of a bimetal is to exploit differential thermal expansion. When heated, the two metals expand at different rates because they have different coefficients of expansion. This mismatch creates mechanical stress that causes the strip to bend or deflect in a predictable direction and magnitude. Steel expands less than brass, so a steel-brass bimetal will curve toward the steel side when warmed. This property makes bimetals invaluable as temperature sensors and automatic switches.

Common applications and construction

Bimetallic thermostats are the classic use case: the strip is formed into a coil or cantilever beam and connected to an electrical contact. As temperature rises, the bimetal flexes and breaks or closes a circuit. Industrial ovens, water heaters, refrigeration units, and motor protection devices all rely on this principle. The metal pairing and strip thickness are chosen so deflection occurs within a specific, narrow temperature range. A bimetal designed to trip at 65 degrees Celsius will not trip at 64 degrees.

Less familiar but equally important are bimetals used in compensation applications. In precision instruments, clocks, and gauges, bimetal strips are incorporated to counteract unwanted thermal expansion in other parts of the mechanism. A steel balance wheel in a mechanical clock will expand with heat and lose accuracy; a bimetal compensation system curls inward just enough to restore the effective radius and keep timekeeping stable across temperature swings.

The strength of the bond between the two metals is critical. Poor adhesion leads to delamination under thermal cycling or mechanical stress, destroying the component's utility. Industrial bimetals must also resist corrosion at the interface, especially in humid or chemical environments. Thickness ratios and alloy selection vary widely; a thermostatic bimetal might be 0.5 mm thick and contain invar (a nickel-steel alloy with minimal expansion) paired with brass, while structural applications may use thicker stock with different pairings.