Formation Mechanism And Mitigation Methods Of The Magnetic Particle Indication On Crankshaft With 120° Phase Angle

The crankshaft plays a key role in transmission in internal combustion engines and its mechanic heavily influences stability of internal combustion engines. Now it is prevailing that the casting crankshafts are replaced by forgings because the crankshafts forming by forging contains better microstructure than castings. During the hot forging process, the free defects in raw materials, irrational forging process design and operation deviation will result in lots of defections such as crack, coarse grains, mismatch, and fold and so on, so how to prevent and detect defects has become a very important facet in assuring the quality of crankshaft. Magnetic particle inspection technique is extensively applied in detecting defects distributing the surface layer of crankshafts. These defects which cannot be directly observed by eyes will show and are called Magnetic particle marks by Magnetic particle inspection and the location also can be obtained. The Magnetic particle marks are often found at present.Based on the experimental and numerical simulation, it was analyzed in the paper that the mechanism of Magnetic particle marks of forging crankshaft with a 120°phase angle. The results by physical and chemical Analysis show: the Magnetic particle marks reflect some micron dimension crack on surface of crankshaft; the micron dimension cracks were formed after disarticulation of inclusion which contained MnS and MoS and extended along with the forming direction; the microstructure of crankshaft contained severe banded structure which had something to do with the segregation of Mn, Mo and S. By means of numerical simulation for forging process of crankshaft, it was researched that Magnetic particle marks always appeared in the area of inner parting plan of NO.1, NO.3, NO.4, NO.6 Connecting Rod Journal and transition fillets in the view of the billet inner freedom defects'transition during the forging. The billet was divided into two layer, the outer higher quality layer and inner lower quality layer, and the corresponding finite element model of billet was built. The two layers metal's flow and distribution during forging were simulated by Deform 3D and the trend of accumulate of inner defects was achieved as well as the influence law of forging velocity on above two layers'flowing. The results show :after final-forging the inner low quality metal was distributed in the area of inner parting plan of NO.1, NO.3, NO.4, NO.6 Rod Journal and transition fillet with thin outer higher quality layer, and at these positions the inner low quality metal would exposed on forging surface after machine working, which are the dangerous areas ; the metal on the cross-section of rod journal flowing only at one direction intensified the trend of free defects accumulating in the dangerous area.According the analysis results of Magnetic particle marks above, the methods to mitigate Magnetic particle marks of forging crankshaft were brought up from the aspects of raw material and forging process. The structure of rib plate of pre-forging die was optimized. Comparing the original process and optimized process, the new pre-forging die realized the metal flow state along with the left side and right side at the same time at late final-forging process. The width of inner low quality metal was reduced by 28.9mm, which means better performance in mitigating Magnetic particle marks.