曲軸鑄改鍛日趨明顯

發(fā)動機曲軸的作業(yè)標(biāo)準(zhǔn)偏差，在工作上受交替變化負載所造成的變形和剪應(yīng)力的功效，因此對傳動軸的疲憊耐磨損和抗壓強度等特性均有較高的規(guī)定。伴隨著載貨汽車向功率大的超重型載重車發(fā)展趨勢，小汽車向速、輕量發(fā)展趨勢，對汽車發(fā)動機的各類特性需求也愈來愈高，鑄造發(fā)動機曲軸迫不得已遭遇嚴(yán)峻形勢。由于發(fā)動機曲軸絕大多數(shù)是由配重塊斜面分模發(fā)動機曲軸于彎折疲憊而毀壞，鍛鋼發(fā)動機曲軸的最大優(yōu)勢是不銹鋼板材具備較高的彎折疲勞極限，并且大部分與抗壓強度正相關(guān)。而球墨鑄鐵發(fā)動機曲軸因為存有高純石墨主骨、磷化處理物與鋁合金因素的位錯縮松，因而其疲勞極限較低，且不與抗壓強度正相關(guān)。據(jù)美國福特汽車公司實驗證實，針對相同的發(fā)動機曲軸和曲軸頸支撐點面總寬，鑄造發(fā)動機曲軸的使用壽命僅有煅造發(fā)動機曲軸的一半。

伴隨著大功率油耗低、低排放量的規(guī)定，汽車發(fā)動機的出現(xiàn)工作壓力從8MPa~9MPa升高到12MPa,乃至14MPa,球墨鑄鐵發(fā)動機曲軸承受不住如此高的出現(xiàn)工作壓力，因此近些年世界各國鑄造發(fā)動機曲軸的占比在降低而煅造發(fā)動機曲軸的占比在升高，發(fā)動機曲軸鑄改鍛的發(fā)展趨勢日趨顯著。例如我國某柴油機廠由原應(yīng)用鑄造發(fā)動機曲軸，現(xiàn)改成70%應(yīng)用煅造發(fā)動機曲軸。

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 of cast crankshafts to 70% of forged crankshafts.

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