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Plain carbon steels and alloy steels differ in their content of additional alloying elements, which significantly affect the hardness and phase transformations that take place during the cooling process. In plain carbon steels, the only alloying element is carbon; hence, the CCT diagrams are relatively simpler. While for alloy steels, the presence of one or more additional alloying elements results in an alteration of the phase transformation temperatures and times, making the CCT diagrams more complex. In plain carbon steels, the phases present during cooling typically comprise of ferrite, pearlite, and cementite. In contrast, alloy steels exhibit a wider variety of phases, such as martensite, upper and lower bainite, and other secondary phases. The introduction of alloying elements delays the transformation from austenite to these phases. As a result, the cooling curves for alloy steels in the CCT diagrams are shifted to longer cooling times.

The presence of alloying elements in alloy steels significantly affects the rate at which transformations occur during cooling. Generally, alloying elements slow down the phase transformation rate compared to plain carbon steel. Alloying elements can be classified into two types depending on their effect on the transformation rates: 1. Elements that increase hardenability (e.g., chromium, nickel, and molybdenum): These elements retard the rate at which austenite transforms into ferrite and pearlite, thereby allowing the formation of martensite or lower bainite at slower cooling rates. 2. Elements that decrease hardenability (e.g., cobalt, silicon, and aluminum): These elements promote the formation of pearlite and ferrite, thereby hindering the formation of martensite or lower bainite at slower cooling rates. The CCT diagrams for alloy steels indicate these transformations and their dependence on the presence and concentration of the alloying elements. The cooling rates to achieve desired microstructures and properties in alloy steels are typically slower compared to those for plain carbon steels, due to the influence of the alloying elements.