In February 2021, an automobile company conducted a bench test and operated at full speed and full load for 65 h. the engine suddenly stopped. After disassembly, it was found that the rear end of the second connecting rod journal of the crankshaft was broken. The failed crankshaft is shown in Figure 1 and Figure 2.
The material of the failed crankshaft is qt800-3, and some processing processes are: rough turning connecting rod journal + fine turning countersunk groove + journal induction quenching + countersunk groove rolling + fine grinding journal. The strengthening method of the fillet of the journal is fillet countersunk groove rolling.
Failure analysis of qt800-3 nodular cast iron crankshaft:
- The effective coil width of the inductor in the quenching process is large, resulting in too long quenching area of the connecting rod journal and quenching of the rolling groove on one side;
- The graphite deformation on the surface of connecting rod journal is produced in the rolling process;
- The sharp angle effect formed by graphite deformation caused a large number of microcracks. During the bench test operation, the surface part of the connecting rod journal fell off with the journal metal matrix after being forced and extruded, resulting in Journal strain and poor lubrication, and then the pull pad locked;
- The pulling pad of crankshaft journal is locked, and the micro loose defect under the sub surface at the junction and transition between connecting rod journal and rolling groove becomes the fatigue source. When the stress exceeds the fatigue limit, the crankshaft breaks instantaneously.
Qt800-3 ductile iron crankshaft failure improvement scheme:
- Adjust the inductor coil width, which is taken as the center according to the technical requirements of quenching area
Lower limit, the clearance between the effective ring and the connecting rod journal shall be controlled at 1.5 mm.
- Expand the coverage angle of silicon steel sheet of inductor, optimize the distribution of silicon steel sheet and improve the electrical efficiency
Magnetic induction efficiency and permeability, improve the contour pattern of quenching zone.