曲轴的工作条件极差，在工作中受交变载荷所引起的弯曲和剪切应力的作用，因而对曲轴的疲劳耐磨和强度等性能均有极高的要求。随着载货车向大功率重型载重车发展，轿车向速、轻量化发展，对发动机的各项性能要求也越来越高，铸造曲轴不得不面临严峻考验。因为曲轴大部分是由平衡块曲面分模曲轴于弯曲疲劳而损坏，锻钢曲轴的最大优点是钢材具有较高的弯曲疲劳强度，而且基本上与抗拉强度成正比。而球墨铸铁曲轴由于存在石墨孔穴、磷化物与合金元素的晶界偏析，因此其疲劳强度较低，且不与抗拉强度成正比。据美国福特汽车公司试验证明，对于同样的曲轴和连杆颈支撑面宽度，铸造曲轴的寿命只有锻造曲轴的一半。
随着高功率低油耗、低排放的要求，发动机的爆发压力从8MPa~9MPa上升到12MPa,甚至14MPa,球墨铸铁曲轴难以承受如此高的爆发压力，所以近年来国内外铸造曲轴的比例在下降而锻造曲轴的比例在上升，曲轴铸改锻的趋势日趋明显。例如国内某柴油机厂由原100%使用铸造曲轴，现改为70%使用锻造曲轴。
Crankshaft casting and forging are becoming increasingly obvious
The working conditions of the crankshaft are extremely poor. The bending and shear stress caused by the alternating load during the work, so the fatigue wear resistance and strength of the crankshaft have extremely high requirements. With the development of trucks to high-power and heavy-duty trucks, and the development of cars to speed and light weight, the performance requirements of engines have become higher and higher. Casting crankshafts have to face severe tests. Because most of the crankshaft is damaged by bending fatigue due to the curved block split mold crankshaft, the biggest advantage of the forged steel crankshaft is that the steel has a higher bending fatigue strength, which is basically proportional to the tensile strength. Ductile iron crankshafts have low fatigue strength and are not proportional to tensile strength due to grain boundary segregation of graphite voids, phosphides, and alloying elements. According to the test of Ford Motor Company, for the same crankshaft and connecting rod neck support surface width, the life of the cast crankshaft is only half of that of the forged crankshaft.
With the requirements of high power, low fuel consumption and low emissions, the explosion pressure of the engine has risen from 8MPa to 9MPa to 12MPa, or even 14MPa. Ductile iron crankshafts cannot withstand such high explosion pressures, so the proportion of cast crankshafts at home and abroad has declined in recent years. The proportion of forged crankshafts is increasing, and the trend of crankshaft casting to forging is becoming increasingly apparent. For example, a domestic diesel engine factory changed from 100% of cast crankshafts to 70% of forged crankshafts.
