Bars with high carbon content are prone to fracture. For example, shafts made of 45\. Sampling from the fractured parts for metallographic analysis often fails to find the cause. Even if it is far fetched, it is not the actual cause.

In order to ensure higher strength, carbon must also be added to the steel, followed by the precipitation of iron carbides. From the point of view of electrochemistry, iron carbides play a role of cathode and accelerate the anodic dissolution reaction around the matrix. The increase of the volume fraction of iron carbide in the microstructure is also attributed to the low hydrogen overvoltage characteristic of the carbide.

Hydrogen is easy to be produced and adsorbed on the steel surface. When hydrogen atoms penetrate into the steel, the volume fraction of hydrogen may increase, and finally the hydrogen embrittlement resistance of the material will be significantly reduced.

The significant reduction of corrosion resistance and hydrogen embrittlement resistance of high-strength steel is not only harmful to the performance of steel, but also greatly limits the application of steel.

If automotive steel is exposed to chloride and other corrosive environments, the possible stress corrosion cracking (SCC) phenomenon under stress will pose a serious threat to the safety of vehicle body.

The higher the carbon content, the lower the hydrogen diffusion coefficient and the higher the hydrogen solubility. The scholar Chan once proposed that various lattice defects such as precipitates (as the trap position of hydrogen atoms), potentials and empty holes are proportional to the carbon content. The increase of carbon content will inhibit hydrogen diffusion, so the hydrogen diffusion coefficient is also low.

Because the carbon content is proportional to the hydrogen solubility, the larger the volume fraction of carbides as hydrogen atom traps, the smaller the hydrogen diffusion coefficient inside the steel, and the hydrogen solubility increases. The hydrogen solubility also contains information about diffusible hydrogen, so the hydrogen embrittlement sensitivity is the highest. With the increase of carbon content, the diffusion coefficient of hydrogen atom decreases and the surface hydrogen concentration increases, which is caused by the decrease of hydrogen overvoltage on the steel surface.

According to the results of the driven voltage polarization test, the higher the carbon content of the sample, the cathodic reduction reaction (hydrogen generation reaction) and anodic dissolution reaction are prone to occur in the acidic environment. Compared with the surrounding matrix with low hydrogen overvoltage, the carbides play the role of cathode, and their volume fraction increases.

According to the results of electrochemical hydrogen permeation test, the larger the carbon content and the volume fraction of carbide in the sample, the smaller the diffusion coefficient of hydrogen atom and the greater the solubility. With the increase of carbon content, the hydrogen embrittlement resistance will decrease.

The slow strain rate tensile test shows that the higher the carbon content, the lower the stress corrosion cracking resistance. It is proportional to the volume fraction of carbide. With the increase of hydrogen reduction reaction and the amount of hydrogen injected into the sample, anodic dissolution reaction will occur and the formation of slip bands will be accelerated.

With the increase of carbon content, carbides will precipitate inside the steel, and the possibility of hydrogen embrittlement will increase under the action of electrochemical corrosion reaction. In order to ensure that the steel has excellent corrosion resistance and hydrogen embrittlement resistance, it is an effective control method to control the precipitation and volume fraction of carbides.

The application of steel in auto parts is limited, which is also due to the obvious decline of its resistance to hydrogen embrittlement, which is caused by aqueous corrosion. In fact, this hydrogen embrittlement sensitivity is closely related to carbon content. Iron carbides (fe2.4c/fe3c) are precipitated under low hydrogen overvoltage.

Generally, the local corrosion reaction on the surface caused by stress corrosion cracking or hydrogen embrittlement is carried out through heat treatment to remove the residual stress and increase the hydrogen trap efficiency. It is not easy to develop ultra-high strength automotive steel with excellent corrosion resistance and hydrogen embrittlement resistance.

With the increase of carbon content, the hydrogen reduction rate increased, while the hydrogen diffusion rate decreased significantly. Using medium carbon or high carbon steel as parts or transmission shafts, the key technology is to effectively control the carbide components in the microstructure.

Source: metal processing

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