Why do steels with high carbon content break easily?

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Bars with a high carbon content have suffered from many fractures; for example, shafts made of 45# steel would break after being in use for only a short period of time. Samples are taken from the fractured components for metallographic analysis, but the cause of the failure is often not identified. Even if some reasons are found, they are not the actual causes. To ensure greater strength, carbon must also be added to steel, resulting in the precipitation of iron carbides. From an electrochemical perspective, the iron carbide acted as a cathode, accelerating the anodic dissolution reaction around the matrix. The increase in the volume fraction of iron carbides within the microstructure is also attributed to the low hydrogen overpotential properties of the carbides. Hydrogen easily forms on and adheres to the surface of steel. As hydrogen atoms penetrate into the interior of the steel, its volume fraction may increase, ultimately leading to a significant reduction in the material’s resistance to hydrogen embrittlement. The significant reduction in the corrosion resistance and hydrogen embrittlement resistance of high-strength steel not only harms the properties of such steel but also greatly limits its applications. If steel used in automobiles is exposed to various corrosive environments such as chlorides, stress corrosion cracking (SCC) may occur under stress, posing a serious threat to the safety of the vehicle body. The higher the carbon content, the lower the hydrogen diffusion coefficient and the higher the hydrogen solubility. Scholar Chan once proposed that various lattice defects such as precipitates (which act as trap sites for hydrogen atoms), potential differences, and vacancies are proportional to the carbon content; as the carbon content increases, hydrogen diffusion is suppressed, resulting in a lower hydrogen diffusion coefficient. Since the carbon content is proportional to the hydrogen solubility, the greater the volume fraction of carbides, which act as traps for hydrogen atoms, the lower the hydrogen diffusion coefficient within the steel, and the higher its hydrogen solubility. Hydrogen solubility also contains information regarding diffusible hydrogen; hence, the material exhibits the highest susceptibility to hydrogen embrittlement. As the carbon content increases, the diffusion coefficient of hydrogen atoms decreases while the surface hydrogen concentration increases, due to a decrease in the hydrogen overpotential at the steel surface. Based on the results of the passive voltage polarization tests, the higher the carbon content in the specimen, the more readily cathodic reduction reactions (hydrogen generation reaction) and anodic dissolution reactions occur in an acidic environment. Compared with the surrounding matrix with a low hydrogen overpotential, the carbides acted as a cathode, and their volume fraction increased. According to the results of electrochemical hydrogen permeation tests, the higher the carbon content and the volume fraction of carbides in the specimen, the lower the diffusion coefficient of hydrogen atoms and the greater its solubility. As the carbon content increases, resistance to hydrogen embrittlement also decreases. Slow strain rate tensile tests confirmed that the higher the carbon content, the lower the resistance to stress corrosion cracking. It is proportional to the volume fraction of carbides; as the hydrogen reduction reaction progresses and more hydrogen penetrates into the interior of the sample, anodic dissolution occurs, which also accelerates the formation of slip zones. As the carbon content increases, carbides precipitate within the steel. Under the action of electrochemical corrosion reactions, the likelihood of hydrogen embrittlement rises. To ensure that the steel possesses excellent corrosion resistance and resistance to hydrogen embrittlement, it is effective to control the precipitation of carbides as well as their volume fraction. The use of steel in automotive parts is limited, which is also attributed to its significant decrease in resistance to hydrogen embrittlement, a phenomenon caused by corrosion in aqueous solutions. In fact, this susceptibility to hydrogen embrittlement is closely related to the carbon content, with iron carbides (Fe2.4C/Fe3C) precipitating under low-hydrogen overvoltage conditions. Generally, to address surface local corrosion caused by stress corrosion cracking or hydrogen embrittlement, measures such as heat treatment to remove residual stresses and improving the efficiency of hydrogen traps are employed. It is naturally not easy to develop ultra-high-strength automotive steel that possesses both excellent corrosion resistance and hydrogen embrittlement resistance. As the carbon content increases, the hydrogen reduction rate increases, while the hydrogen diffusion rate decreases significantly. When using medium-carbon or high-carbon steel for components or drive shafts, the key to the technology lies in effectively controlling the carbide components in the microstructure.