Views: 0 Author: Site Editor Publish Time: 2022-06-24 Origin: Site
1. Effect of carbon content on equilibrium microstructure of Fe-C alloy
The quantitative relationship between carbon content and microstructure and phase composition of FE-C alloy at room temperature can be concluded according to lever law
2. Influence of carbon content on mechanical properties
The more cementite content, the more uniform distribution, the higher the hardness and strength of the material, the lower the plasticity and toughness; However, when cementite exists at grain boundary or as matrix, the plasticity and toughness of the material decrease greatly, and the strength also decreases.
3. Effect of carbon content on process performance
For cutting machinability, it is generally considered that the plasticity of carbon steel is moderate, hardness is about HB200, and the cutting performance is the best. Too high or too low carbon content will reduce its machining performance.
Low carbon steel is better than high carbon steel for malleability. Because steel heating is single-phase austenite state, good plasticity, low strength, easy to plastic deformation, so the general forging is in the austenite state. When forging, the appropriate temperature must be determined according to the iron carbon phase diagram. The initial rolling and forging temperatures should not be too high to avoid overburning. Start rolling and temperature should not be too low, so as not to crack.
For casting, the fluidity of cast iron is better than steel, easy to cast, especially near the eutectic composition of cast iron, its crystallization temperature is low, the fluidity is good, and more has good casting performance. From the point of view of phase diagram, the larger the solidification temperature range, the easier to form dispersed shrinkage cavity and segregation, the worse the casting performance.
In general, the lower the carbon content, the better the welding performance of steel, so low carbon steel is easier to weld than high carbon steel.
4. Why does steel with high carbon content break easily?
Bars with high carbon content are prone to fracture, such as shafts made of 45# steel, which do not last long. Taking samples from the fractured parts and conducting metallographic analysis, the cause is often not found. Even if some causes are far-fetched, they are not the actual cause.
To ensure higher strength, carbon must also be added to the steel, which precipitates iron carbide. From an electrochemical point of view, iron carbide 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 characteristics of the carbide.
The surface of steel is easy to generate and adsorb hydrogen. As hydrogen infiltrates into the steel, the volume fraction of hydrogen may increase, resulting in a significant decrease in the hydrogen brittleness resistance of the material.
The significant reduction of corrosion resistance and hydrogen brittleness resistance of high strength steel will not only harm the properties of steel, but also greatly limit the application of steel.
If automotive steel is exposed to chloride and other corrosive environments, the possible stress corrosion cracking (SCC) phenomenon will pose a serious threat to the safety of automobile body under stress.
The higher the carbon content is, the hydrogen diffusion coefficient decreases and the hydrogen solubility increases. Scholar Chan once proposed that various lattice defects such as precipitates (as trap locations for hydrogen atoms), potential, and empty holes are proportional to carbon content, and the increase of carbon content will inhibit hydrogen diffusion, 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 the carbide as a hydrogen trap, 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 hydrogen diffusion coefficient decreases and the hydrogen concentration increases due to the decrease of hydrogen overvoltage on steel surface.
The results of the driven voltage polarization test show that the higher the carbon content of the sample is, the cathodic reduction reaction (hydrogen generation reaction) and anodic dissolution reaction tend 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 diffusion coefficient of hydrogen atom decreases and the solubility increases with the increase of carbon content and volume fraction of carbides in the sample. With the increase of carbon content, the hydrogen brittleness resistance decreases.
It is confirmed by slow strain rate tensile test that the stress corrosion cracking resistance decreases with increasing carbon content. It is directly proportional to the volume fraction of carbides. With the increase of hydrogen reduction reaction and hydrogen injection into the sample, anodic dissolution reaction will occur and the formation of slip band will accelerate.
With the increase of carbon content, carbide will precipitate in steel, and the possibility of hydrogen embrittlement will increase under the action of electrochemical corrosion reaction. In order to ensure that steel has excellent corrosion resistance and hydrogen embrittlement resistance, the control of carbide precipitation and volume fraction is an effective control method.
The limited use of steel in automotive parts is also due to a marked decline in its resistance to hydrogen embrittlement, which is caused by the corrosion of aqueous solutions. In fact, this hydrogen embrittlement sensitivity is closely related to carbon content, and iron carbides (Fe2.4C/Fe3C) are precipitated under low hydrogen overvoltage conditions.
In general, the partial surface corrosion reaction caused by stress corrosion cracking or hydrogen embrittlement is carried out by heat treatment to remove residual stress and increase hydrogen trap efficiency. It is not easy to develop ultra-high strength automotive steel with excellent corrosion resistance and hydrogen brittleness resistance.
With the increase of carbon content, hydrogen reduction rate increases, while hydrogen diffusion rate decreases significantly. Using medium carbon or high carbon steel as parts or drive shafts, the key technology is to effectively control the carbide components in the microstructure.