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Why do steels with high carbon content break easily? Part 2

2022-06-28

From the results of the dynamic voltage polarization test, the higher the carbon content of the sample, the more prone to cathodic reduction reaction (hydrogen generation reaction) and anodic dissolution reaction in acidic environment. Compared with the surrounding matrix with low hydrogen overvoltage, the carbide acts as a cathode with an increased volume fraction.

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

Slow strain rate tensile testing confirmed that the higher the carbon content, the lower the stress corrosion cracking resistance. Proportional to the volume fraction of carbides, as the hydrogen reduction reaction and the amount of hydrogen injected into the sample increase, the anodic dissolution reaction will occur, and the formation of the slip zone will also be accelerated.


When the carbon content increases, carbides will precipitate inside the steel. Under the action of electrochemical corrosion reaction, the possibility of hydrogen embrittlement will increase. In order to ensure that the steel has excellent corrosion resistance and hydrogen embrittlement resistance, the carbide Precipitation and volume fraction control are effective control methods.

The application of steel in auto parts is subject to some limitations, also due to its significant decrease in resistance to hydrogen embrittlement, which is caused by aqueous corrosion. In fact, this hydrogen embrittlement susceptibility is closely related to the carbon content, with the precipitation of iron carbides (Fe2.4C/Fe3C) under low hydrogen overvoltage conditions.

Generally, for the localized corrosion reaction on the surface caused by stress corrosion cracking phenomenon or hydrogen embrittlement phenomenon, 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 steel with 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. The key to using medium carbon or high carbon steel as parts or transmission shafts is to effectively control the carbide components in the microstructure..