Why does the high carbon steel break easily?

In the process of processing, we will find that medium and high carbon steel such as spring steel, bearing steel, wheel and axle steel will be prone to multiple fractures, from the fracture sampling for metallographic analysis is often unable to find the reason. These steels generally required high fatigue resistance and the fracture toughness of steels is an important factor of fatigue performance.

From an electrochemical point of view, the addition of carbon to the steel to ensure higher strength leads to the precipitation of iron carbides, which acts as a cathode and accelerates 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. The surface of the steel is easy to generate and adsorb hydrogen, and the infiltration of hydrogen atoms into steel may increase its volume fraction, which ultimately reduces the material’s hydrogen embrittlement resistance significantly. If the automobile steel is exposed to various corrosive environments such as chloride, the possible stress corrosion cracking (SCC) will seriously affect the safety of the automobile.

The higher the carbon content, the lower the hydrogen diffusion coefficient and the higher the hydrogen solubility, which is caused by the decrease of the hydrogen overvoltage on the steel surface. The slow strain rate tensile test shows that the stress corrosion cracking resistance decreases with the increase of carbon content. Is directly proportional to the volume fraction of carbides. With the increase of hydrogen reduction reaction and hydrogen injection into the sample, the anodic dissolution reaction accelerates to form slip bands. The higher the carbon content is, the greater the possibility of hydrogen embrittlement of the carbides in the steel will be under the action of electrochemical corrosion reaction. In order to ensure that the steel has excellent corrosion resistance and hydrogen embrittlement resistance, the precipitation and control of carbides are an effective method.

The application of medium and high carbon steel in automotive parts is limited because of the obvious decrease of hydrogen embrittlement resistance. This hydrogen embrittlement sensitivity is closely related to carbon content, and iron carbide (Fe2.4C/Fe3C) precipitates under the condition of low hydrogen overvoltage. In general, heat treatment can be used to remove residual stress and increase the efficiency of hydrogen trap for the local surface corrosion reaction caused by stress corrosion cracking or hydrogen embrittlement. Carbide composition should be strictly controlled when using medium carbon or high carbon steel as parts or drive shafts.

At present, there are three methods to improve the fracture toughness of steel. The first method is to optimize the composition of steel, that is, to use the strengthening effect of Ni, V, N and other alloy elements. Second, the grain and structure of the steel are refined and homogenized during rolling and heat treatment. The third is to denature the hard phase inclusions in the steel. It must also be known that increasing the fracture toughness of the steel matrix itself does not solve the problem of a strong stress concentration in the steel matrix caused by hard inclusions, and therefore it increases the fracture toughness in a limited way.