Why is high carbon steel easy to fracture?

The bars with high carbon content have been broken many times, such as 45 × steel shaft, which will break in a short time. It is often impossible to find out the cause of the fracture by sampling and metallographic analysis. That is to say, it is far fetched to find some reasons, not the actual reasons. In order to ensure higher strength, it is necessary to add carbon to the steel, which will precipitate iron carbide. From the point of view of electrochemistry, iron carbide plays the role of cathode and accelerates the anodic dissolution around the matrix. The increase of the volume fraction of iron carbide in the microstructure is also attributed to the low hydrogen overvoltage characteristics of the carbide. The surface of steel is easy to produce and absorb hydrogen. When the hydrogen atom penetrates 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 properties of steel, but also greatly limits the application of steel. For example, when the automobile steel is exposed to chloride and other corrosive environments, the SCC phenomenon may appear under the action of stress, which will pose a serious threat to the safety of the automobile body. The higher the carbon content, the smaller the hydrogen diffusion coefficient and the higher the hydrogen solubility. The scholar Chan once proposed that all kinds of lattice defects such as precipitate (as the trap position of hydrogen atom), potential and empty hole are in direct proportion to the carbon content. If the carbon content increases, the hydrogen diffusion will be restrained, so the hydrogen diffusion coefficient is also low. As 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 higher the hydrogen solubility. The hydrogen solubility also contains information about the 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 easier the cathodic reduction reaction (hydrogen generation reaction) and the anodic dissolution reaction will occur in the acid environment. Compared with the surrounding matrix with low hydrogen overvoltage, 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. 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 carbides, with the increase of hydrogen reduction reaction and the amount of hydrogen injected into the sample, the anodic dissolution reaction will occur and the slip band will be formed faster. With the increase of carbon content, carbides will precipitate in 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, the control of carbide precipitation and volume fraction is an effective control method. The application of steel in auto parts is limited by the obvious decrease of its anti hydrogen embrittlement, which is caused by the corrosion of aqueous solution. In fact, this hydrogen embrittlement sensitivity is closely related to the carbon content, and iron carbide (fe2.4c / Fe3C) is separated out under the condition of low hydrogen overvoltage. In general, for the local corrosion reaction caused by stress corrosion cracking or hydrogen embrittlement, heat treatment is used to remove the residual stress and increase the efficiency of hydrogen trap. It is not easy to develop super high strength automobile steel with excellent corrosion resistance and hydrogen embrittlement resistance. With the increase of carbon content, the hydrogen reduction rate increases, while the hydrogen diffusion rate decreases significantly. The key technology of using medium carbon or high carbon steel as parts or transmission shaft is to control the carbide component in microstructure effectively.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods