Development of lead-free crankshaft for motorcycle 2011-32-0649

Lead-added free-cutting steel has been used by many parts which need high machinability because lead improves chip friability and drill life. However, the demand of lead reduction increases in recent years, because of environmental impact substance reduction. Therefore, we developed lead-free crankshaft for motorcycle.

Until now, crankshaft for motorcycle has been manufactured with lead-added free-cutting steel by a following process; Hot-Forging - Quenching and Tempering (QT) - Prior Machining - Nitrocarburizing - Finishing process because of strength and machinability. When we tried to change steel to lead-free, we examined to change to sulfur-added free-cutting steel. However, chip friability of sulfur-added free-cutting steel is inferior to lead one. Thus, we concerned about increase in machining expense. Then, heat-treatment after forging was examined to change from QT to normalizing for reducing the heat-treatment expense. As a result, total production cost achieved equivalent to conventional.

Deterioration of fatigue strength was feared because of a decrease of hardness by changing heat-treatment. Thus, we paid attention to that crankshaft for motorcycle needs nitrocarburizing. We adjusted the alloying elements so that hardness near the surface after nitrocarburizing which influences especially fatigue strength might become higher than conventional. Therefore, fatigue strength became equivalent to conventional.