It can be seen from the iron-carbon equilibrium diagram reflecting the relationship between the steel structure and the carbon content of the steel and the temperature of the steel that when the carbon content is exactly 0.77%, it is equivalent to cementite (iron carbide) in the alloy When it accounts for about 12% and ferrite accounts for about 88%, the phase transformation of the alloy is realized at a constant temperature. That is to say, cementite and ferrite in this specific ratio will disappear at the same time if they disappear during a phase change (during heating), and if they appear, both will appear at the same time. At this point, this structure and The phase transition of pure metals is similar. For this reason, people treat this two-phase structure composed of a specific ratio as a kind of organization and named it pearlite. This kind of steel is called eutectoid steel. That is, steel with a carbon content of exactly 0.77% is called eutectoid steel, and its structure is pearlite.
The carbon content of commonly used structural steel is mostly below 0.5%. Because the carbon content is less than 0.77%, the amount of cementite in the structure is also less than 12%, so a part of the ferrite must be removed from the cementite to form a pearlite body. , There will be excess, so the structure of this steel is ferrite + pearlite. The smaller the carbon content, the smaller the ratio of pearlite in the steel structure, and the lower the strength of the steel, but the better the plasticity. This type of steel is collectively referred to as hypoeutectoid steel.
The carbon content of tool steel often exceeds 0.77%, and the proportion of cementite in this steel structure exceeds 12%. Therefore, in addition to the formation of pearlite with ferrite, there is excess cementite, so the structure of this type of steel It is pearlite + cementite. This type of steel is collectively referred to as hypereutectoid steel.