Machinery's Handbook, 31st Edition
466 Heat Treatment of Steel bands or layers of ferrite and cementite that make up the pearlite begin to merge into each other. This process continues until the pearlite is thoroughly “dissolved,” forming what is known as austenite . If the temperature of the steel continues to rise and there is present, in addition to the pearlite, any excess ferrite or cementite, this also will begin to dissolve into the austenite until finally only austenite will be present. The temperature at which the excess ferrite or cementite is completely dissolved in the austenite is called the upper crit ical point . This temperature varies with the carbon content of the steel much more widely than the lower critical point (see Fig. 1). 2000
1900
1800
Austenite
1700
Austenite and Cementite
1600
1500
Austenite and Ferrite
Lower Critical Hardening Range
1400
1300
1200
Ferrite and Pearlite
Cementite and Pearlite
1100
1000
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6 1.7
Percent Carbon Content
Hypoeutectoid Steel
Hypereutectoid Steel
Fig. 1. Phase Diagram of Carbon Steel Effect of Slow Cooling on Carbon Steel.— If carbon steel that has been heated to the point where it consists entirely of austenite is slowly cooled, the process of transformation that took place during the heating will be reversed, but the upper and lower critical points will occur at somewhat lower temperatures than they do on heating. Assuming that the steel was originally fully annealed, its structure on returning to atmospheric temperature after slow cooling will be the same as before in terms of the proportions of ferrite or cementite and pearlite present. The austenite will have entirely disappeared. Effect of Rapid Cooling or Quenching on 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°F (704°C). For ex- ample, a 0.80 percent carbon steel that is cooled at such a rate that the temperature drops 500°F (278°C drop) in one second will show transformation of austenite beginning at 930°F (500°C). As the cooling rate is increased, the laminations of the pearlite formed by the transformation of the austenite become finer and finer up to the point where they cannot be detected under a high-power microscope, while the steel itself increases in hardness and tensile strength. As the rate of cooling is still further increased, this transfor- mation temperature suddenly drops to around 500°F (260°C) or lower, depending on the carbon content of the steel. The cooling rate at which this sudden drop in transformation
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