01 Equilibrium state diagram and metallography

In this equilibrium state diagram, carbon steels with carbon content less than 0.6% are carbon steels for machine structural use, and those with carbon content greater than 0.6% are carbon tool steels. The 0.765% carbon content point is called the eutectic point, and carbon steels with this carbon content are called eutectoid steels, those with less carbon content than this point are called sub-eutectoid steels, and those with more carbon content are called hyper-eutectoid steels.

02 Metallographic structure of sub-eutectoid steel

The metallurgical structure at higher temperatures _{than the} A3 line is austenite. As it cools, ferrite precipitates as it passes through the _{A3 line}. This is called Initially precipitated ferrite and is called Furthermore, ferrite and cementite ( _Fe3C ) precipitate simultaneously from austenite as it passes the _A1 line. This transformation from one phase to two phases simultaneously is called eutectic transformation, and the metallographic structure obtained by this eutectic transformation is called pearlite (ferrite and _Fe3C are arranged in layers).

The microscopic microstructures of various carbon steels with different carbon contents are shown above, which exhibit a mixed structure of pearlite and primary ferrite in equilibrium below the _{A1 transformation} point. Furthermore, the amount of pearlite increases with carbon content, and at 0.8% carbon content, which is the eutectic point, all of the carbon content is pearlite.

03 Metallographic structure of percolloidal steel

A characteristic feature of hypercoagulating steel is the presence of the Acm _line, at which temperature cementite ( _Fe3C ) precipitates from austenite, which is called primary cementite (Fe3C). The amount of cementite increases as one gets closer to the _{A1 line}, and since the _{A1 line} undergoes pearlitic transformation, the metallurgical structure at room temperature is a mixture of pearlite and precipitated _Fe3C.