Development of Nitrocarburized High Strength Crankshaft Through Controlling Vanadium Carbonitride Precipitation by Normalizing

In addition to the requirements of high power output and compactness, further reduction of weight is being required for motorcycle engines from the standpoint of fuel economy and reduction of CO₂ emissions. For this purpose, it is important to reduce crankshaft weight, which is the heaviest rotating part in the engine. The crankshaft has to be strong enough to bear loads, as the demands of weight reduction are increasing. Yet, productivity has to be considered at the same time even when increasing crankshaft strength. In this report of crankshaft material studies that feature high fatigue strength, machinability and distortion correct-ability, attention is given to the fact that the amount of vanadium, which is known as an element that enhances the strength with its precipitation, accelerates deposition, dissolved in the steel depends on the heating temperature. The material that allows for the increase of strength by control of the normalizing temperature, for which processing conditions can be set easier than the control of cooling after hot forging, has been developed. By setting the normalizing temperature at the optimum level, the fatigue strength is improved by more than 30% from conventional alloy steel while maintaining machinability and distortion correct-ability.