Effect of microstructure on the hysteresis performance of force transducers using AISI 4340 steel spring material

The application of heat treatments on spring elements of transducers is a very effective method for attaining good performance in force measurements. Heat treatments change the microstructure of the spring material, which plays a major role in the improvement of performance characteristics of force transducers, particularly in terms of hysteresis error. The present study covers attempts at changing the microstructure of AISI 4340 steel through the application of different heat treatments, and the subsequent measurement of the hysteresis performance of force transducers in relation to the change in the structural characteristics of the spring elements. Some of the specimens were quenched and tempered to 35, 45 and 55 HRC. Some of the other specimens were austempered to obtain a bainitic structure with 45 HRC. The remaining specimens were austenitized at a high temperature for a long time to obtain a coarse tempered martensitic structure with 45 HRC hardness. The transducer type strain gages were bonded properly onto the heat-treated column type tension specimens and a Wheatstone bridge was constructed for each transducer. The hysteresis characteristics of the force transducers were determined using a dead weight, force standard machine. The results have shown that the hysteresis characteristics of quenched and tempered specimens improved with increasing hardness. Bainite exhibits better hysteresis performance over tempered martensite at the same hardness level, while a coarse martensite structure has a detrimental effect on the hysteresis characteristics of force transducers.