Fracture Failure Analysis Of WD618 Crankshaft In Truck

The crankshaft is the key component of the automobile engine, and also the core moving parts, which transforms the gas force of the piston and connecting rod into the torque output to drive the power device which is connected with the engine. In the operation of the force is very complex, mainly bear the load of bending and torsion, the load value is large and periodic variation, easy to cause the torsion and bending deformation of the crankshaft and even crack and fracture. In order to ensure the service life of the crankshaft, It must be strictly in accordance with the technical requirements, which includes structure design, material, processing technology and assembly and maintenance Against this background of the crankshaft fracture in a truck and conclusions are as follows:(1) Background investigation and actual condition investigation of failureThe service life of the WD618 crankshaft is 30 years, but the crankshaft was broken during the tenth year. Fracture failure is not overloaded, and there is no problem of maintenance. The reasons for the early failure of the use and maintenance are excluded. Many parameters are obtained, which includes the crankshaft structure, the use of materials and performance parameters of crankshaft machining technology, the shape and size of data query. In order to find out the real reason of crankshaft fracture failure, they provide technical basis for determining.(2) Macro analysis and micro analysis of the fracture of WD618 crankshaft are carried out.The fracture of the crankshaft was analyzed by means of stereo microscope, scanning electron microscope and X-ray energy spectrum analyzer. The fracture property of the crankshaft is fatigue fracture; The fatigue source is located on the surface of the crankshaft, which is near Spindle neck and crank arm corner R and the crack extends along the left and right sides until the final fracture. The area of the final fracture zone is less than 10% of the total area, which indicates that the alternating stress is small and the crankshaft is not directly broken, but the stress concentration in the presence of the source is the source of the source.(3) The physical and chemical analysis of the WD618 crankshaft is carried out.Macrostructure, chemical composition analysis, mechanical properties are test by the camera, hardness and fatigue machine of macrostructure. Macrostructure of main journal and connecting rod journal are not obvious defects. Crankshaft hardness is significantly less than that when the crankshaft is specified by the hardness. At the same time, tensile strength is lower than that of the manufacturer’s specified value. So they reduce the fatigue strength of WD618 crankshaft, prompting WD618 crankshaft fracture.(4) Metallographic examination and analysis of the fracture of WD618 crankshaft is carried out.Non metallic inclusion analysis, material microstructure analysis, main shaft nitriding layer analysis, heat treatment improvement measures, grain size analysis and verification of the brittleness of the nitriding layer are carried out. The distribution of inclusions is not uniform, and the types of the inclusions are mainly manganese sulfide; The crankshaft is not complete with the structure of the cord, with granular and needle like bainite structure and in the metallographic specimen of the fatigue source, the nitriding layer is found to be destroyed; The crankshaft is of fine grain size, and the material is of high strength. Non metallic inclusions are more and uneven distribution, easy to crack and there are cracks in the nitriding layer, which play a role in promoting the fracture of the crankshaft; Crankshaft forgings did not meet the contract requirements after heat treatment group, which cause the reduction of mechanical properties materials, so it is an important cause of the fracture.Then the improvement measures of heat treatment are put forward.(5) Finite element analysis of WD618 crankshaft is carried out.Through the Pro/E 3D solid model of crankshaft and ANSYS finite element analysis, there is a conclusion that crankshaft exists stress concentration phenomenon in the shaft neck and connecting rod shaft neck and crank arm handover R angle, but the largest part of the stress value is 181.26MPa, which is less than the material yield limit to meet the crankshaft of normal use requirement. The crankshaft deformation maximum is 0.72mm, which meet the design requirements of the rigidity of the crankshaft. For the rated speed of the diesel engine calibration is 2200r/min, the corresponding frequency 35.3Hz, when operating speed, the frequency value is smaller than the modal calculation of the first natural frequency of the crankshaft in the operation of the main axis of the neck, so the possibility of resonance is very small.The results show that the crankshaft fracture is fatigue fracture, and the crack caused by the grinding on the surface of the main shaft is the direct cause of the fracture of the crankshaft, and the failure of the structure after heat treatment decreases the fatigue strength of the crankshaft, which is the important reason for the crankshaft fracture.Non metallic inclusions can promote the fracture of the crankshaft. The main shaft neck and crank R angle attached to the existence of grinding cracks, providing a potential source of stress concentration. Grinding cracks should be eliminated, which will form a fault source and become a hidden danger in the use of the process.