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Why does steel with high carbon content break easily?

Sep 18, 2019

The bar with high carbon content has been broken many times, such as the shaft made of 4T steel, which has not been used for a long time. Sampling from the parts after fracture, metallographic analysis, often can not find the cause, that is, even if far-fetched will find some reasons, but also not the actual reasons.
In order to ensure higher strength, carbon must also be added to the steel, followed by the precipitation of iron carbides. From an electrochemical point of view, iron carbide plays a cathodic role and accelerates the anodic dissolution reaction around the matrix. The increase of the product fraction of iron carbides in the microstructure is also due to the low hydrogen overvoltage characteristics of the carbides.
The surface of steel is easy to produce and adsorb hydrogen. When hydrogen atoms infiltrate 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 brittleness resistance of high strength steel is not only harmful to the properties of steel, but also greatly limits the application of steel.
For example, automobile steel is exposed to chloride and other corrosion environment, under the action of stress, the possible stress corrosion cracking (SCC) phenomenon will pose a serious threat to the safety of the car body.
The higher the carbon content, the lower the hydrogen diffusion coefficient and the higher the hydrogen solubility. Scholar Chan has proposed that the precipitates (as the trap position of hydrogen atoms), potentials, voids and other lattice defects are proportional to the carbon content, and 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, as the carbide of the hydrogen atom trap, the larger the volume fraction is, the smaller the hydrogen diffusion coefficient is and the hydrogen solubility increases, and the hydrogen solubility also contains information about diffusive hydrogen. Therefore, the sensitivity of hydrogen embrittlement is the highest. With the increase of carbon content, the diffusion coefficient of hydrogen atom decreases and the surface hydrogen concentration increases, which is due to the decrease of hydrogen overvoltage on the surface of steel.
According to the results of dynamic voltage polarization test, the higher the carbon content of the sample is, the cathodic reduction reaction (hydrogen formation reaction) and anodic dissolution reaction are easy to occur in acidic environment. Compared with the peripheral matrix with low hydrogen overvoltage, the carbide plays the role of cathode and its volume fraction increases.
According to the results of electrochemical hydrogen penetration test, the larger the carbon content and the volume fraction of carbides in the sample, the smaller the diffusion coefficient of hydrogen atom and the higher the solubility. With the increase of carbon content, the hydrogen brittleness resistance will also decrease.
The slow strain rate tensile test shows that the higher the carbon content, the lower the stress corrosion cracking resistance. In direct proportion to the volume fraction of carbide, the anodic dissolution reaction will occur and the slip zone will be accelerated with the increase of hydrogen reduction reaction and hydrogen injection into the sample.
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 brittleness, It is an effective control method to control the precipitation and volume fraction of carbides.
The application of steel in automobile parts is limited by the obvious decline of its resistance to hydrogen embrittlement, which is caused by aqueous solution corrosion. In fact, this sensitivity to hydrogen embrittlement is closely related to carbon content, and iron carbide (Fe2.4C / Fe3C) is precipitated under the condition of low hydrogen overvoltage.
In general, the surface local corrosion reaction caused by stress corrosion cracking or hydrogen embrittlement is carried out by heat treatment to remove residual stress and increase the efficiency of hydrogen trap. It is not easy to develop ultra-high strength automobile steel with excellent corrosion resistance and hydrogen brittleness resistance.
With the increase of carbon content, the hydrogen reduction rate increases, while the hydrogen diffusion rate decreases significantly. The key to the use of medium carbon or high carbon steel as parts or transmission shafts is to effectively control the carbide components in the microstructure.