Fracture failure analysis and process improvement of agricultural pump shaft

Abstract: It is necessary to possess fine comprehensive mechanical properties because of the severe working conditions of agricultural pump shafts. Therefore, the qualities of the materials, the structural design and the machining methods have strong effects on the using life of the pump shafts. The 40Cr steel, a typical alloy structural steel, shows some advantages such as high strength and toughness. After treated by quenching-tempering process, the 40Cr steel is used to manufacture mechanical parts which bear medium loads and medium speeds, for example, it is machined as the rear axles of cars and the spindles of machine tools. The 40Cr steel can be the best choice of agricultural pumps to manufacture pump shafts. The 40Cr pump shafts, which usually work under the conditions of the twisting moment and impact loads, and the pump shafts also are the important moving parts and load supporting parts which ensure the safe working of pump. The pump shafts need to be strengthened in order to satisfy the requirements of hardness, strength and toughness of the pump. The 40Cr steel uses quenching and high tempering heat treatment processes to obtain sorbitic structures, and its hardness can be ranged from 280 to 320 HBW. For some reasons, the pump can not work normally which is caused by impurities and abnormal microstructures existed in the steel. Parts of agricultural pump shafts present early failure phenomenon in shafts shoulder which is produced by one manufacturing company. It is considered that many impurities and unreasonable structural design should be responsible for the early failure. To find out the reasons of agricultural pump shafts fracturing in the early using life and avoid the appearance of early failure of the pump shafts again, the chemical compositions, the macroscopic fracture figures, the microstructures, the hardness and the impact ductility of the pump shafts were analyzed by using the direct reading spectrometer, scanning electron microscope, optical microscope, the hardness testing machine and impact testing machine. The results showed that the chemical compositions and impact energy of materials met the requirements of the pump. And the main reasons causing the fatigue fracture of pump shafts in the early using life were that the unreasonable structure designs resulted in stress concentration, the oxide film existed in the sample surface decreased the strength, and the inappropriate heat treatment processes produced the ferrites with widmanstatten structure. According to the analytic results, some optimized and improved methods were taken on pump shafts, such as enlarging the fillet sizes of shafts shoulder, using the nitrogen protection during heat treatment process, improving the quenching temperature and changing the cooling medium (replace oil cooling with brine cooling) in heat treatment process. By observing the quenching and tempering organization, the pump shafts reached the requirements after the improvement processes, and fatigue fracture did not appear during the early using life. The long-term durability was improved. The research in this article can provide important reference for improving the control of agricultural mechanical product quality and designing and processing of the key parts of agricultural machinery.