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114 THE DISTRIBUTOR’S LINK SPIROL THE DIFFERENCE BETWEEN ALLOY STEEL & CARBON STEEL COILED SPRING PINS from page 32 Theoretically, the goal of heat treating Spring Pins is to achieve a 100% martensitic microstructure. While this is not feasible in practice, an effective heat treat process will optimize the martensitic composition within the material. Martensite is formed when the material is quenched rapidly enough to prevent carbon diffusion. If the quenching process is too slow, the carbon atoms escape the desired atomic geometry, and less desirable microstructures will form (pearlite, bainite). The final step of heat treatment is tempering. This is performed after the pins have been quenched to ambient temperatures. Tempering occurs at temperatures under 1000°F (540ºC) (below the critical eutectoid temperature). Tempering improves the ductility and toughness of martensite because the pins are brittle immediately after being quenched. The tempering process must be below the temperature at which the metal transforms into austenite. These low temperatures relieve internal stress, decrease brittleness, and maintain high hardness. Springs (i.e. Coiled Spring Pins) tempered at higher temperatures result in greater loss of hardness and strength, but they display improved elasticity. Therefore, the heat treat recipe is integral for the Coiled Spring Pin manufacturing process. Heat treatment is one of the most critical processes in the production of Spring Pins, as it directly impacts the performance and lifetime of the assembly using the pin. Seemingly minor variations in time (minutes) and temperature (± 10°F (± -12ºC)) can have a drastic impact on the quality of a Spring Pin. For this reason, it is critical that fastener manufacturers have effective control measures in place. Carbon vs Alloy Steel Carbon steel Coiled Pins must be quenched to ambient temperatures within several seconds in order to achieve a high martensitic composition. Conversely, alloy steel allows for a much longer quenching time (~1 minute) in order to achieve a high martensitic composition. The quench time is adversely effected as the Spring Pin gets larger and has more mass. Specifically, the outside of the pins will achieve a high martensitic composition, but the inside of the pins will not. Coiled Spring Pins at diameters of Ø.500” / Ø12mm (or less) are able to quench quickly enough to use carbon steel. However, Coiled Pins with larger diameters require the use of alloy steel so that the entire composition of the pin has the opportunity to achieve optimal martensitic composition. Field Impact – Ineffective Heat Treat Static Applications If a Coiled Pin does not achieve the desired metallurgical microstructure, the pin carries a risk of failure in the field after being exposed to applied loads. This may present itself in the form of bending or shear failure. Dynamic Applications If a Coiled Pin does not achieve the desired metallurgical microstructure, the pin’s fatigue life will be sacrificed. This limits the number of cycles the pin can withstand in the field, reducing the assembly’s functional lifetime. Conclusion Designers should view carbon steel and alloy steel Coiled Spring Pins as roughly equivalent when looking through a Spring Pin manufacturer’s catalog. However, be wary of large diameter (>Ø.500” / Ø12mm) Coiled Pins made from carbon steel, as they carry mechanical risks. Heat treat is one of the most critical steps in the manufacturing of Spring Pins, so it’s recommended that fastener manufacturers heat treat in-house for full control. Although this article offers general design guidelines, it is recommended that Application Engineers who specialize in the design and manufacturing of Coiled Spring Pins be consulted to ensure the proper material is selected for each specific assembly. SPIROL INTERNATIONAL CORP.

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