Why is high carbon steel prone to breaking?

Bars with a high carbon content have suffered numerous fractures.

For example, a shaft made of 45# steel may break if left unused for a long period of time.

Taking samples of the broken parts and performing metallographic analysis often fails to determine the cause of the fracture.

Even if a reason is identified, it may not be the real cause.

Why high carbon steel is easy to break

To increase the strength of steel, carbon must be added. This results in the precipitation of iron carbides. From an electrochemical point of view, iron carbides act as cathodes, accelerating the anodic dissolution reaction around the substrate. The increase in the volume fraction of iron carbides in the microstructure is also linked to the low hydrogen overvoltage characteristics of the carbides.

The surface of steel is prone to generating and absorbing hydrogen. As hydrogen atoms penetrate the steel, the volume fraction of hydrogen can increase, reducing the material's resistance to hydrogen embrittlement. This reduction in corrosion resistance and hydrogen embrittlement resistance significantly impacts the properties of the steel and restricts its applications.

For example, when automotive steel is exposed to corrosive environments such as chloride, stress corrosion cracking (SCC) can occur under stress, posing a threat to body safety.

The higher the carbon content, the lower the hydrogen diffusion coefficient and the higher the hydrogen solubility. Some researchers have suggested that various lattice defects, such as precipitates, potentials, and pores, increase proportionally with carbon content. As the carbon content increases, hydrogen diffusion is inhibited, leading to a decrease in the hydrogen diffusion coefficient.

The carbon content is proportional to the solubility of hydrogen, therefore, as carbides retain hydrogen atoms, the higher the volumetric fraction, the lower the hydrogen diffusion coefficient inside the steel. This results in an increase in hydrogen solubility and sensitivity to hydrogen embrittlement.

As the carbon content increases, the hydrogen diffusion coefficient decreases and the surface hydrogen concentration increases due to a decrease in the hydrogen overstress on the steel surface.

The results of the driven voltage polarization test show that the higher the carbon content of the sample, the easier it is for the cathode reduction reaction (hydrogen generation reaction) and the anode dissolution reaction to occur in an acidic environment.

Carbides act as cathodes and their volume fraction increases compared to the matrix with low hydrogen overvoltage. The results of the electrochemical hydrogen permeation test indicate that the higher the carbon content and the volume fraction of carbides in the sample, the lower the diffusion coefficient of hydrogen atoms and the greater their solubility. As the carbon content increases, the resistance to hydrogen embrittlement decreases.

The slow strain rate tensile test confirmed that the higher the carbon content, the lower the resistance to stress corrosion cracking. This is proportional to the volume fraction of carbides.

As the hydrogen reduction reaction and hydrogen permeation in the sample increase, the anodic dissolution reaction occurs, accelerating the formation of slip zones. As the carbon content increases, carbides precipitate inside the steel, increasing the possibility of hydrogen embrittlement under the action of electrochemical corrosion reactions.

To ensure excellent corrosion resistance and hydrogen embrittlement resistance of steel, controlling carbide precipitation and volume fraction is an effective method.

Why high carbon steel is easy to break

The use of steel in automotive parts and components is limited due to its lower resistance to hydrogen embrittlement.

This phenomenon is the result of corrosion caused by exposure to aqueous solutions.

Sensitivity to hydrogen embrittlement is directly linked to the carbon content of the steel.

Iron carbides (Fe2.4C/Fe3C) are formed under conditions of low hydrogen overvoltage.

To mitigate surface corrosion caused by stress corrosion cracking or hydrogen embrittlement, residual stress is typically removed through heat treatment, which also increases the efficiency of hydrogen retention.

It can be challenging to create ultra-high-strength automotive steels that offer exceptional corrosion resistance and hydrogen embrittlement resistance.

Why high carbon steel is easy to break

As the carbon content increases, the rate of hydrogen reduction also increases, while the rate of hydrogen diffusion decreases significantly.

The key to utilizing medium or high carbon steel for automobile components or drive shafts lies in effective control of the carbide components within the microstructure.

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