When steel is quenched, normalized and annealed using medium frequency induction heating equipment, the transformation of supercooled austenite follows the traditional basic rules. Understanding the decomposition products of supercooled austenite under different cooling rates provides a theoretical basis for formulating heat treatment processes. Today, the editor will briefly tell you about the supercooled austenite transformation of eutectoid carbon steel and low alloy steel.
(1) Undercooled austenite transformation of eutectoid carbon steel. After continuous cooling of supercooled austenite at different cooling rates, martensite (M), martensite + pearlite (P), pearlite and a small amount of residual austenite are obtained respectively. The following three heat treatment processes are introduced.
1. Completely quenched. The cooling rate during complete quenching is greater than 140°C.S, and 99% martensite (including a small amount of retained austenite) can be obtained after cooling. This cooling rate is also called the critical cooling rate and plays an important role in determining the quenching process.
2. Incomplete quenching. The cooling rate during incomplete quenching is less than 140-35℃.S-1, and the martensite and pearlite structure is obtained after cooling.
3. Normalizing and annealing. When the cooling rate is less than 35℃.S>-1, the pearlite structure will be obtained after cooling.
(2) Supercooled austenite transformation of low alloy steel. Because the addition of alloying elements to the steel makes the transformation of the supercooled austenite continuous structure more complicated, the analysis method is still the same. The transformation products after cooling are obtained according to different cooling curves. Analysis shows that the products transformed during the continuous cooling process of supercooled austenite include proeutectoid ferrite (a), bainite (B), pearlite, and martensite (M). The decomposition products of supercooled austenite under different cooling rates are analyzed as follows.
1. Completely quenched. The cooling rate during complete quenching is approximately greater than 30°C.S-1, and 100% martensite and a small amount of retained austenite are obtained after cooling. This cooling rate is also the critical cooling rate of 35CrMo steel, that is, the cooling rate through point A.
2. Incomplete quenching. The cooling rate during incomplete quenching is 30-0.5℃.S-1. After cooling, a mixed structure composed of martensite, bainite and a small amount of retained austenite can be obtained.
3. Normalizing and annealing. When the cooling rate is less than 0.5℃.S-1, a mixed structure composed of pearlite, proeutectoid ferrite and a small amount of retained austenite is obtained after cooling.
Today, this article briefly analyzes the supercooled austenite transformation of eutectoid carbon steel and low alloy steel. If you want to know more about the technical process of induction heat treatment of eutectoid carbon steel and low alloy steel, you can consult our technology personnel.
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