Why The Higher Content Of Carbon, The More Brittle Steel Is?

As far as we all know, the higher the carbon content of steel has, the harder it is. When carbon is added to steel, iron carbide is precipitated. With the increase of carbon content, hydrogen reduction rate increases, while hydrogen diffusion rate decreases significantly. Effective control of carbides in microstructure is critical for using medium or high carbon steels as components and shafts. Medium and high carbon steels are widely used in many applications. For processing engineers, the higher the carbon content of the bars are prone to multiple fractures.

 

Electrochemical experiments showed that the anodic dissolution reaction around the matrix was accelerated by Fe - C compounds. The iron carbide volume fraction in the microstructure increases duo to the low hydrogen overvoltage property of carbides. The steel surface is easy to produce and adsorb hydrogen, the hydrogen atom into the steel internal infiltration, the volume fraction may increase, and finally the material's resistance to hydrogen brittleness can be significantly reduced. The significant reduction of corrosion resistance and hydrogen brittleness resistance of high strength steels is not only harmful to the properties of steels, but also greatly limits the applications of steel. For example, when automobile steel is exposed to various corrosive environments such as chloride, the stress corrosion cracking (SCC) phenomenon may occur under the action of stress, which will pose a serious threat to the safety of automobile body.

 

1045 steel bar fracture


With the increase of carbon content, hydrogen diffusion coefficient decreases and hydrogen solubility increases.Various lattice defects such as precipitates (trap locations of hydrogen atoms), potentials, and voids are proportional to the carbon content, which increases to inhibit hydrogen diffusion. As carbon content is proportional to hydrogen solubility, the larger the volume fraction, the smaller the hydrogen diffusion coefficient of 1045 steel rod core, and the higher the hydrogen solubility. 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 decreases and the surface hydrogen concentration increases, which is caused by the drop of hydrogen overvoltage on the steel surface. The results of the dynamic voltage polarization test show that the higher the carbon content of the sample, the more likely the cathode reduction reaction (hydrogen generation reaction) and anode dissolution reaction will occur in the acidic environment. Compared with the peripheral matrix with low hydrogen overvoltage, carbide acts as a 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 sample bar, the smaller the diffusion coefficient of hydrogen and the higher the solubility. As the carbon content increases, the resistance to hydrogen brittleness decreases. The slow strain rate tensile test confirmed that the higher the carbon content, the lower the resistance to stress corrosion cracking. As the hydrogen reduction reaction and the amount of hydrogen injected into the sample increase, the anodic dissolution reaction will occur, accelerating the formation of slip zone. With the increase of carbon content, carbides will be precipitated out in the steel. Under the action of electrochemical corrosion reaction, the possibility of hydrogen embrittlement will increase. In order to ensure excellent corrosion resistance and hydrogen brittleness resistance of the steel rod, controlling the precipitation and volume fraction of carbides is an effective method.

 

Medium carbon steel 1045 is limited in the application of automobile parts due to the decrease of its hydrogen brittleness resistance energy produced by aqueous solution corrosion. In fact, this hydrogen embrittlement sensitivity is closely related to carbon content, with iron carbide (Fe2.4C/Fe3C) precipitated at low hydrogen overvoltage conditions. Local surface corrosion reactions caused by stress corrosion cracking or hydrogen embrittlement can remove by heat treatment.