Bars with high carbon content have undergone many breaks, such as the shaft made of 45# steel, which breaks after a short period of time. Sampling from the parts after the fracture and performing metallographic analysis often fails to find the cause, that is, it is not a practical reason to find some reasons.

In order to ensure higher strength, it is also necessary to add carbon to the steel, which in turn precipitates iron carbides. From an electrochemical point of view, iron carbide acts as a cathode, accelerating the anodic dissolution reaction around the substrate. The increase in the volume fraction of iron carbides within the microstructure is also attributed to the low hydrogen overvoltage characteristics of the carbides.

The surface of the steel is prone to generate and adsorb hydrogen. While the hydrogen atoms penetrate into the steel, the volume fraction of hydrogen may increase, and the hydrogen embrittlement resistance of the material is significantly reduced. The significant reduction in corrosion resistance and hydrogen embrittlement resistance of high-strength steels is not only detrimental to the properties of steel, but also greatly limits the application of steel.

If the automotive steel is exposed to various corrosive environments such as chloride, under the action of stress, the stress corrosion cracking (SCC) phenomenon may pose a serious threat to the safety of the vehicle body. (On January 17, 2019, "China Metallurgical News", Wang Yidong, deputy general manager of Anshan Iron and Steel Co., said: tackling high-strength and toughness-free carbide 1380 MPa bainite rail)

The higher the carbon content, the lower the hydrogen diffusion coefficient and the higher the hydrogen solubility. Chan has suggested that various lattice defects such as precipitates (as trap positions of hydrogen atoms), potentials, and pores are proportional to the carbon content. When the carbon content is increased, hydrogen diffusion is inhibited, so the hydrogen diffusion coefficient is also low. 

Since the carbon content is proportional to the solubility of hydrogen, as the carbide of the hydrogen atom trap, the larger the volume fraction, the smaller the hydrogen diffusion coefficient inside the steel, the higher the solubility of hydrogen, and the solubility of hydrogen also contains information about diffusible hydrogen. Hydrogen embrittlement is the most sensitive. As the carbon content increases, the diffusion coefficient of hydrogen atoms decreases, and the surface hydrogen concentration increases because of the decrease in hydrogen overvoltage on the surface of the steel.

According to the results of the driven voltage polarization test, the higher the carbon content of the sample, the easier the cathode reduction reaction (hydrogen generation reaction) and the anode dissolution reaction are in an acidic environment. In comparison with a peripheral matrix having a low hydrogen overvoltage, the carbide acts as a cathode and its volume fraction increases.

According to the results of the electrochemical hydrogen permeation test, the larger the carbon content and the volume fraction of the carbide in the sample, the smaller the diffusion coefficient of the hydrogen atom and the higher the solubility. As the carbon content increases, the hydrogen embrittlement resistance also decreases.

The slow strain rate tensile test confirmed that the higher the carbon content, the lower the resistance to stress corrosion cracking. In proportion to the volume fraction of carbides, as the hydrogen reduction reaction and the amount of hydrogen permeating into the interior of the sample increase, an anodic dissolution reaction occurs and the formation of a slip zone is accelerated.

When the carbon content increases, carbides are precipitated inside the steel. Under the action of electrochemical corrosion reaction, the possibility of hydrogen embrittlement increases. In order to ensure the steel has excellent corrosion resistance and hydrogen embrittlement resistance, the carbide is The control of precipitation and volume fraction is an effective control method.

The application of steel in automotive parts and components is subject to some limitations, due to the significant decrease in its resistance to hydrogen embrittlement, which is caused by corrosion of aqueous solutions. In fact, this hydrogen embrittlement sensitivity is closely related to the carbon content, and iron carbides (Fe2.4C / Fe3C) are precipitated under low hydrogen overvoltage conditions.

Generally, the surface local corrosion reaction caused by the stress corrosion cracking phenomenon or the hydrogen embrittlement phenomenon is removed, the residual stress is removed by heat treatment, and the hydrogen trap efficiency is increased. It is not easy to develop ultra-high-strength automotive steels that combine excellent corrosion resistance and hydrogen embrittlement resistance.

As the carbon content increases, the hydrogen reduction rate increases and the hydrogen diffusion rate decreases significantly. The key to the use of medium carbon or high carbon steel for components or drive shafts is the effective control of the carbide components in the microstructure.