Research on the generation mechanism of short rod-shaped magnetic traces in the new generation of aviation gear steel 15Cr14Co12Mo5Ni

ObjectiveThe 15Cr14Co12Mo5Ni alloy represents a new generation of low-carbon, high-alloy steels characterized by exceptional strength, elevated-temperature stability, and superior wear resistance, making it widely applicable in high-performance sectors such as aerospace engineering and automotive transmission systems. Magnetic particle inspection (MPI) serves as a critical non-destructive evaluation method for ensuring the structural integrity of components fabricated from this alloy. However, the presence of short rod-shaped magnetic indications frequently complicates defect assessment by introducing non-relevant signals. This study systematically examines the formation mechanism of such magnetic indications, with particular emphasis on their correlation with the alloy’s internal microstructural characteristics, carbide distribution patterns, and forging parameters. The findings provide a scientific foundation for optimizing forging practices, refining grain structure, mitigating excessive dislocation slip bands, and minimizing spurious magnetic indications in quality control processes.MethodsSamples were collected from both the magnetic-indication-displaying and non-displaying regions of the alloy gear shaft hole. By integrating magnetic particle inspection (MPI), scanning electron microscopy (SEM) observation, metallographic analysis, and energy-dispersive X-ray spectroscopy (EDS) analysis, comparative studies were conducted on the morphological characteristics, carbide compositions, and grain structure differences of magnetic indications at both macroscopic and microscopic scales.ResultsComprehensive macroscopic and microscopic investigations reveal that the formation of these magnetic indications is strongly associated with the chain-like clustering of Cr-Mo-rich carbides within the microstructure. These carbide aggregates induce local variations in magnetic permeability, thereby generating magnetic flux leakage fields that promote the accumulation of magnetic particles during inspection. Further analysis has elucidated a sequential microstructure evolution chain leading to the development of short rod-shaped magnetic traces: "coarse grains → dislocation slip bands → Cr/Mo segregation → interfacial directional precipitation → carbide banding → magnetic permeability gradient → magnetic particle accumulation."