The crankshaft, whether it is the crankshaft of a car engine, the crankshaft of a marine engine or the crankshaft of an industrial pump, is subjected to the interaction of alternating bending and alternating torsional loads during the rotation process. The dangerous sections of the crankshaft, especially the transition between the journal and the crank are rounded. At this point, the crankshaft is often broken due to the high concentration of stress. Therefore, the service conditions require the crankshaft to have sufficient strength to ensure that the crankshaft does not have a break during operation. At present, the improvement of the fatigue resistance of the crankshaft by shot peening has been widely applied in a wide range, and the effect is quite satisfactory.
Compared with the defects of the traditional rolling process, that is, due to the limitation of the machining process of the crankshaft, it is difficult for the journal fillets to match the rollers, which often causes the rounding of the fillet and the deformation of the crankshaft after rolling is large. ,not effectively. The mechanism of shot peening is to use a pellet with strict control of diameter and a certain strength to form a projectile flow under high-speed airflow and continuously spray it onto the metal surface of the crankshaft, just like hammering with countless small hammers to make the crankshaft surface Extremely strong plastic deformation occurs, forming a cold hardened layer. In simple terms, because the crankshaft is subjected to various mechanical cutting forces during machining, the surface of the crankshaft, especially the cross-section of the crankshaft changes, is extremely uneven in stress distribution, and is affected by alternating stress during operation, so it is easy Stress corrosion occurs to reduce the fatigue life of the crankshaft. The shot peening process is to increase the fatigue resistance and safe service life of the workpiece by introducing a pre-compression stress to offset the tensile stress that the part will be subjected to in the later working cycle.
In addition, the forgings of crankshaft forgings are made of steel ingots directly or with hot-rolled steel. If the forging and rolling processes are improperly controlled, there will be component segregation in the embryos, coarse grain of the original microstructure, and unreasonable internal organization. Such as metallurgical and structural defects, so that the fatigue life of the crankshaft is reduced, the strengthening process can refine the structure of the structure and significantly improve its fatigue performance.
There are two most critical parameters for the shot peening process. One is the stress intensity, which is usually measured by “Almen test pieceâ€. A plurality of test pieces are fixed on different surfaces of the crankshaft, in particular, the cross-section of the crankshaft with the most concentrated stress is transferred to the rounded corner, and shot peening is performed together, and the compressive stress generated on the test piece causes the test piece to bow. The change in curvature is proportional to the energy of the shot impact. Another major parameter determining the quality of shot peening is coverage. The so-called coverage refers to the ratio of the area occupied by the surface crater after strengthening to the total strengthened surface. This parameter is defined by the design engineer of the crankshaft, usually between 100% and 200%. Some crankshaft applications may require coverage of more than 200%. .
Depending on the hardness of the crankshaft and the desired strength of the incoming compressive stress, the hardness of the shot peening is usually 50 - 55 HRC and the size is S 280 - S 330 (0.7 mm - 0.84 mm). This produces an intensity range on the "Almen test piece" of approximately 0.008 - 0.010 C (0.025 on the A scale).
Compared to shot blasting, the process parameters of shot peening are more stringent. For crankshaft reinforcement applications, the parameters that need to be monitored include:
- Shot peening speed
- Shot peening
- Pellet diameter
- Shot peening distance
- Enhanced time
- Coverage
The change of any of these parameters will affect the effect of the surface enhancement of the crankshaft to varying degrees.
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