Numerical and experimental investigation of the effect of cold extrusion process on residual stress and fatigue life of internal thread of high-strength steel

In order to improve the fatigue performance of the internal thread of the connecting rod for marine diesel engine, the internal thread cold extrusion technology is introduced into the bolt hole position. Based on the three-dimensional finite element model, the residual stress distribution around the cold extrusion internal thread of 42CrMo4 high-strength steel plate hole structure under different edge distance ratios is analyzed in detail. The multiaxial fatigue life prediction model of thread is established based on the stress-strain method; the accuracy and feasibility of the prediction model are further verified by fatigue experiments. The thread surfaces are examined by utilizing the scanning electron microscope and coordinate with the X-ray diffraction method to measure the residual stress of thread. The relationship between the residual stress distribution, surface structure, and fatigue life of the extrusion thread is revealed. The research shows that the improved fatigue prediction model gives a satisfactory accuracy in predicting the fatigue life. Both the tangential residual stress fields around the extrusion thread and the surface structure have a certain influence on its fatigue life, especially when the edge distance ratio is less than 2. The tangential residual compressive stress of the cold extrusion internal thread decreases with the edge distance ratio being smaller, while the depth of the stress zone increases firstly and then decreases. Higher load will greatly reduce the fatigue life and dispersion of the thread; the fatigue life increases linearly with the increase of the edge distance ratio under lower stress level.