Abstract
Three Si-killed 55SiCrA suspension spring steels with different boron contents (0, 23 ppm and 43 ppm) were obtained by vacuum induction levitation melting furnace. Through hot tensile, torsional Bauschinger and rotating bending fatigue tests, the microstructure, precipitate, tensile strength, percentage of area reduction, elastic sag resistance and fatigue life of spring steel were analyzed, respectively. Boron accelerated the dynamic recrystallization at 800 ºC and prevented the formation of reticular proeutectoid ferrite and coarse pearlite in steel, then made the tempered troostite of heat-treated spring steel become fine and uniform. However, much BN and boron carbide formed along prior austenite grain boundaries in spring steel containing 43 ppm boron and was retained after quenching and tempering, leading to the decrease of ductility and fatigue life of spring steel. Therefore, 23 ppm boron was the best content for increasing hot ductility and ductility at room temperature of spring steel, and improved its tensile strength and elastic sag resistance, finally, the fatigue life increased from 5.3 × 107 cycles to 8.2 × 107 cycles.
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