Spline structures are essential load-transmission components in mechanical systems, and their fretting wear behaviour directly affects system performance, service life and safety. In this work, 16MnCrS5 steel in the uncarburised and carburised states was used as the self-mated friction pair. A combination of fretting tests and microstructural characterisation was applied to analyse the fretting-damage interface, evaluate energy dissipation and material damage, and clarify the fretting wear behaviour and damage mechanisms of 16MnCrS5 steel before and after carburisation. Running condition fretting maps (RCFMs) and material response fretting maps (MRFMs) were established from Ft-D curves, fretting wear volume and wear morphology. The results show that, with increasing displacement or decreasing load, the Ft-D curves of both uncarburised and carburised 16MnCrS5 steel change from closed linear responses to semi-open loops and finally to open parallelograms. The partial slip region (PSR), mixed fretting region (MFR) and gross slip region (GSR) were clearly identified. From the PSR to the GSR, both wear volume and coefficient of friction increased markedly for 16MnCrS5 steel before and after carburisation. The RCFMs and MRFMs of the two material states were similar. Compared with the uncarburised samples, the carburised samples exhibited lower wear volume, lower energy dissipation and a higher system deformation rate. These differences in fretting wear behaviour were closely associated with phase composition, thermomechanical properties and wear-debris evolution in the two materials. Moreover, operating the spline within the PSR can substantially reduce wear.

The main transformer control circuit is used to control and distribute electric power, and its reliability plays an important role in normal and stable operation of a nuclear power plant. In this paper, an unexpected fusing failure occurring on one power cable inside the cabinet of the main transformer control circuit of a nuclear power plant was addressed. By means of comprehensive investigation including morphologies observation of the fused area, materials characterization of the copper conductor and polymer insulation sheath, and compositions analysis of the corrosion products, it was identified that the failure was mainly resulted from the multiple factors in the manufacturing, installation and operation procedures. Under this condition, the root cause and contributing factors of the failure were determined, and a vicious circle that was originated from mechanical damage, accelerated by electrochemical corrosion, and functioned via heat accumulation was proposed to describe the failure process and relevant mechanisms. Based on the analysis results, the countermeasures were proposed, which would have reference values for failure prevention of power cables serving under similar environments.

Metallographic analysis techniques such as optical microscopy and scanning electron microscopy were used to observe the morphology of spline wear, confirming the presence of fretting wear characteristics. Microstructural analysis of spline cross sections using a metallographic microscope revealed surface carburization treatment on the splines. Hardness tests conducted from the surface to the core of the splines showed a gradual decrease in hardness, consistent with the observed microstructural features. The chemical composition of the spline was analyzed using a spectrometer, and the main component content was analyzed. By combining finite element analysis techniques, this study analyzed the effects of factors such as forces, eccentricity, and lubrication on spline wear. The results indicated that the varying degrees of spline wear in the hydraulic motors of charging pumps in the on-site conditions of the nuclear power plant were primarily attributed to differences in spline eccentricity after manufacture and assembly. Further investigation revealed that the fundamental cause of spline wear in hydraulic motors was the operation of the equipment under conditions deviating from the rated operating points, resulting in poor circulation of oil within the motors and insufficient spline lubrication. Specifically, operating the equipment at lower pressures and away from the rated operating points resulted in reduced oil flow velocity, increasing the friction between the spline mating surfaces. The poor lubrication of the spline and eccentricity are the primary reasons for spline wear, and the combination of these two factors exacerbates spline wear. To address the wear issue of the hydraulic motor, we replaced it with a linear-axis hydraulic motor and conducted a comprehensive analysis of the linear-axis hydraulic motor. The results indicate that the linear-axis hydraulic motor can effectively reduce wear, and after testing at the nuclear power plant, it meets the operational time requirements on-site. These findings offer valuable guidance for the maintenance and operation of charging pumps in nuclear power plants, facilitating the prevention of similar issues and enhancing equipment reliability and operational efficiency.

This paper presents an analysis of the fracture accident of a cylindrical roller bearing cage used in a charging pump in a nuclear power plant. The causes and mechanisms of bearing cage breakage were investigated by material failure analysis and simulation calculations. Macroscopic observation results confirmed that the cage fracture occurred at the stress concentration position. The microfracture morphology of the cage obtained from scanning electron microscopy showed a fatigue feature. The analysis of residual stress indicated large residual stress perpendicular to the fracture surface. The finite element calculation showed that when the bearing was moving in and out of the working area during operation, large working stress appeared at the stress concentration position. Working stress and residual stress acted together, approaching the fatigue limit of materials, and finally led to the cage fatigue fracture. The stress of the other two structural cages of the same type of bearing was also calculated, and no such large stress concentration was identified; thus, one plastic cage was temporarily used.