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Why do steels with high carbon content break easily

Why is steel with high carbon content easy to break?

Bars with high carbon content have broken many times, such as 45# steel shafts, which break after a short period of use. Taking samples from the fractured parts for metallographic analysis, the causes are often not found, that is, it is far fetched to find some causes, which is not the actual cause.
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In order to ensure higher strength, carbon must also be added to the steel, followed by the precipitation of iron carbides. From the electrochemical point of view, iron carbides play a cathode role and accelerate the anodic dissolution reaction around the matrix. The increase of iron carbide volume fraction in microstructure is also attributed to the low hydrogen overvoltage characteristics of carbide.
Hydrogen is easily generated and adsorbed on the steel surface. While hydrogen atoms penetrate into the steel, the volume fraction of hydrogen may increase, and finally the hydrogen embrittlement resistance of the material is significantly reduced.
The significant reduction of corrosion resistance and hydrogen embrittlement resistance of high-strength steel is not only harmful to the properties of steel, but also greatly limits the application of steel.
If automotive steel is exposed to various corrosive environments such as chloride, the possible stress corrosion cracking (SCC) 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. Scholar Chan once proposed that various lattice defects such as precipitates (as the trap position of hydrogen atoms), potentials and empty holes are directly 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 directly 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 due to the decrease of hydrogen overvoltage on the steel surface.
According to the results of driven voltage polarization test, the higher the carbon content of the sample, the cathodic reduction reaction (hydrogen generation reaction) and anodic dissolution reaction are easy to occur in acidic environment. Compared with the surrounding matrix with low hydrogen overvoltage, the carbide plays the role of cathode and its 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 higher the solubility. With the increase of carbon content, the hydrogen embrittlement resistance will also decrease.
The slow strain rate tensile test shows that the greater the carbon content, the lower the stress corrosion cracking resistance. It is directly 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 band 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.

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The application of steel in auto parts is limited, which is also due to the obvious decline of its hydrogen embrittlement resistance, which is caused by aqueous solution 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, for the local corrosion reaction on the surface caused by stress corrosion cracking or hydrogen embrittlement, heat treatment is used 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.

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With the increase of carbon content, the hydrogen reduction rate increases, while the hydrogen diffusion rate decreases 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: China Flanges Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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