As a coolant for quenching, structural steel usually uses oil, but other steels also use water or air. Salt baths are also used in special cases.
Because rapid cooling is important in quenching, it is necessary to put the material into the coolant immediately after heating and to stir it well after the addition. If the coolant is placed in the coolant and allowed to stand still, the heat of the steel warms the coolant that is in contact with the steel, causing it to evaporate locally and generate bubbles. There is a risk of unevenness in hardness after quenching.
Cooling should not be done until the steel is completely cooled. In other words, martensite is formed regardless of the cooling rate after passing the Ms point. On the contrary, rapid cooling causes rapid martensite transformation, and the greatly expanded martensite causes quench cracks and strain. It is for
Fig. 1 shows the basics of the quenching operation. In the case of oil quenching, the holding time in oil is said to be enough to produce white smoke when the steel is lifted up.
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