When the steel is heated and held at a temperature above the Ac3 point, the structure is completely austenite, and then quenched
Since austenite does not have time to transform into ferrite and pearlite, it continues to cool in the state of supercooled austenite, and when it reaches a certain temperature, it transforms into an extremely hard structure called martensite. This temperature varies depending on the type of steel, but for structural steel it is usually 300-400°C.
It is called the Ms point (martensite start point).
The process of transforming the structure of steel from austenite to martensite is called quenching. Martensite, which is a quenched structure, is a bamboo leaf-like extremely hard structure, and because austenite is rapidly cooled, carbon becomes cementite (Fe3C) without time to precipitate, and is in a supersaturated solid solution state. is. The maximum hardness of steel obtained by quenching is related to the carbon content, and as shown in Fig. 1, the hardness increases as the carbon content increases up to about 0.60%, and after that it does not change much.
The heating temperature for quenching structural steel is the same as for full annealing, usually 30-50°C above Ac3 point. The temperature is maintained for about 30 minutes per 25 mm diameter of the treated material. After this, it is quenched in oil. (Depending on the type of steel, water cooling or air cooling may be used.) In this case, the heating temperature and holding temperature are particularly important factors.
The higher the heating temperature, the easier it is to quench, but the martensite structure after quenching becomes coarser, making the material brittle and, in extreme cases, quenching cracks.
On the other hand, when the heating temperature is lower than the Ac3 point, the steel does not become completely austenite, but has a mixed structure of austenite and ferrite. Therefore, when quenched from this temperature, austenite transforms to martensite, while ferrite undergoes no transformation and remains ferrite.
The structure after quenching becomes a mixed structure of hard martensite and soft ferrite. This means that the quenched product will have soft portions in some places, failing to achieve the original purpose of quenching. In addition, it is also necessary to set the optimal time to obtain uniform heating to the center of the steel with respect to the heat holding time.
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