Metallographic examination as the feedback between product quality and manufacturing

Despite the increasing automation of the process of designing and manufacturing metal products, their failure remains a common phenomenon. The metallographic examination is appointed, which can only be carried out at a proper level by the specialized accredited organizations to identify the causes of such incidents. A metallographic examination is a tool that acts as feedback between the output quality of products and the entire chain of numerous operations during production. The purpose of this work is, using a practical example, to demonstrate the possibility and special significance of the conclusions of the metallographic examination for the development of the product manufacturing technology. Using the high-speed plasma spraying method, the authors applied the NiCrBSi coating to the surface of the locomotive wheel pair axle to increase its wear resistance. The life bench tests of the axle revealed the main fatigue crack, the tests were stopped, and the axle was artificially broken completely. The analysis of metal quality, including chemical composition, mechanical properties (strength, ductility, and impact hardness), microstructure, metal purity according to the non-metallic inclusions, and parameters of a surface layer hardened by rolling, showed its full compliance with the regulatory documentation. The thickness and hardness parameters of the NiCrBSi coating also corresponded to the declared ones. According to the fractographic analysis, the fatigue fracture was initiated at multiple points, which was a characteristic sign of a common objective reason for the insufficient strength of a product not associated with some random factor. The metallographic examination identified that the main reason for the failure of a wheelset axle is the coating’s insufficient fatigue strength. The numerous fatigue microcracks that originated in the coating grew into the base metal and led to the fatigue macrocracks formation at different height levels. The merging of these cracks led to widespread fatigue fracture surface formation.