Beryllium copper fingerstock gaskets offer several advantages over similarly shaped fabric gaskets. They are more resistant to compression set and can be used in higher temperature applications. While a typical maximum operating temperature for fabric EMI gaskets is in the 150°F (65°C) range, beryllium copper material can function in an environment several times hotter. This heat can come from an ambient source, like a furnace, or directly from electrical current flowing through a contact.
What gives beryllium copper gaskets their unique spring properties is a precipitation heat-treat process. After the metal has been cold-worked to the desired shape, it is heated to 600-675°F (315-357°C) for two hours (ASTM B194-15). During the heat-treat process, the beryllium molecules within the copper alloy align to form a stronger crystal structure and prevent plastic deformation. This results in increased hardness, tensile strength, and yield strength. The sacrifice is reduced elongation. Beryllium Copper Alloy 25 in the 1/4 Hard condition has an elongation range of 20-45%. Once the material has been heat treated to the 1/4 HT (heat treat) condition, the elongation is 3-10% (Materion Brush Inc., 2015). This means that the metal will have higher spring properties but will be more prone to fracture once pushed past the yield strength.
The spring properties of beryllium copper are desirable because of their ability to create a constant contact force. Once the material is initially heat treated, these
spring characteristics are locked in. However, operating the gasket or contact at temperatures that approach the original heat treat temperature can have an effect of “undoing” the heat treat process. This loss of spring properties is known as stress relaxation and results in a reduced contact force. The mechanism by which this process works is a diffusion of the previously linked beryllium particles back to their original amorphous state within the alloy (ConnectorSupplier.com).
The chart below shows the reduction in yield strength (YS) and ultimate tensile strength (UTS) of Beryllium Copper Alloy 25 in Annealed (AT) and Full Hard (HT) heat-treated conditions:
In conclusion, the operating temperature of heat-treated beryllium copper plays a significant role in its performance and properties. Using parts in an environment at or near the original heat treat temperature can have adverse effects. Therefore, designers should ensure that applications for beryllium copper gaskets or contacts are kept below 500°F (260°C).
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