Crack Initiation under Cyclic Loading in Thermally and Thermochemically Treated C60 Steel

The research context of this study focuses on optimizing the use of C60 steel in industrial applications where durability and fatigue resistance are critical. This material was selected due to its high mechanical properties, such as hardness and tensile strength, which make it suitable for demanding industrial applications. C60 steel is commonly used in components subjected to cyclic loading, such as shafts, gears, and springs, which makes it a relevant choice for studies on fatigue behavior.The relevance of the study is determined by the need to improve the operational reliability of structural steels widely used in mechanical engineering and power engineering. C60 steel is known for its combination of strength and hardness, but it is essential to evaluate how thermal and thermo-chemical treatments affect its performance under fatigue conditions. Investigating the crack formation process and the early stages of fatigue in C60 steel, comparing the effects of heat treatment (hardening and tempering) with those of thermo-chemical treatments (oxidation) on the steel's microstructure, and evaluating the performance of C60 steel under fatigue conditions based on the applied treatments provide insights that can help optimize the use of this steel in industries requiring high resistance and durability. In this research, a comparative analysis was conducted to evaluate the influence of thermal treatment (hardening and tempering) and thermochemical treatment (oxidation) on the fatigue behavior of C60 steel, with a particular focus on how these processes affect the initiation and propagation of fatigue microcracks within the material. The methodology of the study included fatigue tests performed on a four-point bending machine to determine the exact time of microcracking and fatigue life was defined as the number of cycles until the appearance of a technical crack, identified by a decrease in frequency of 0.02 Hz. The C60 steel samples were processed according to SR ISO 1099:2017 "Fatigue testing. Axial load method". To ensure consistency and comparability of results, the samples were made from the same material charge and machined under the same conditions. A total of 26 samples were used, 13 for each type of treatment: heat treatment by hardening and tempering and thermo-chemical treatment by oxidation. Due to the different mechanical properties obtained from the thermal and thermo-chemical treatment processes, the sets of samples were tested at varying forces. Frequency changes were monitored to evaluate the behavior of the materials under repeated stresses. Finally, the frequency changes were correlated with the number of cycles to identify when microcracks appeared and their evolution. The main results of this study show significant differences between the lifetime of thermally and thermo-chemically treated samples and the time of microcracks appearance in the material.