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QUICK FACT GUIDES FOR MACHINISTS
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GUIDES |
EFFECT OF RAPID COOLING OR QUENCHING CARBON STEEL Observations have shown that as the rate at which carbon steel is cooled from an austenitic state is increased, the temperature at which the austenite begins to change into pearlite drops more and more below the slow cooling transformation temperature of about 1300 degrees F. (For example, a 0.80 percent carbon steel that is cooled at such a rate that the temperature drops 500 degrees in one second will show transformation of austenite beginning at 930 degrees F.) As the cooling rate is increased, the laminations of pearlite formed by the transformation of the austenite become finer and finer up to the point where they cannot be det4cted under a high power microscope, while the steel itself increased in hardness and tensile strength. As the rate of cooling is still further increased, this transformation temperature suddenly drops down to around 500 degrees F. or lower, depending upon the carbon content of the steel. The cooling rate at which this sudden drop in transformation temperature takes place is called the critical cooling rate. When a piece of carbon steel is quenched at this rate or faster, a new structure is formed. The austenite is transformed into martensite which is characterized by an angular needlelike structure and a very high hardness. If carbon steel is subjected to a severe quench or to extremely rapid cooling, a small percentage of the austenite, instead of being transformed into martensite during the quenching operation, may be retained. Over a period of time, however, this remaining austenite tends to be gradually transformed into martensite even though the steel is not subjected to further heating or cooling. Since martensite has a lower density than austenite, such a change or "ageing" as it is called, often results in an appreciable increase in volume or "growth" and the setting up of new internal stresses in the steel.
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