The key difference between iron carbon diagram and TTT diagram is that iron carbon diagram uses equilibrium conditions, whereas TTT diagrams use non-equilibrium conditions.
What is Iron Carbon Diagram?
Iron carbon diagram is a phase diagram useful in understanding the different phases of steel and cast iron. Steel and cast iron are alloys of iron and carbon with other trace elements. Typically, an iron carbon diagram is drawn using the carbon concentration by weight on the X axis and the temperature scale on the Y axis.
In general, the x axis of the diagram shows the weight percentage of carbon ranging from 0% to 6.67%. This is because of the complexity, which limits the exploration of Fe3C to be focused only up to 6.67 weight percent of carbon. In this phase diagram, up to 0.008% of weight of carbon simply shows pure iron or iron. This metal exists in the alpha-ferrite form at room temperature.
In the second phase, from 0.008% to 2.14% carbon content by weight shows steel, an iron-carbon alloy. This range also shows different grades of steel named as low carbon steel or mild steel, medium carbon steel, and high carbon steel.
Upon the increment of the carbon content to above 2.14%, we can observe the stage of cast iron in the phase diagram. Cast iron is very hard. However, it has brittleness which severely limits the applications of this alloy and the methods for forming.
What is TTT Diagram?
TTT diagram stands for time-temperature-transformation diagram. This diagram is also called the isothermal transformation diagram. It gives the kinetics of isothermal transformations. It is usually used for steel, and it is important as a transformation diagram for non-equilibrium transformation. In addition to steel, there are martensite, bainite, etc., which can be shown in this diagram. These are called non-equilibrium metals because they cannot be formed by continuous cooling; thus, we cannot explain their properties using a phase transformation diagram.
The TTT diagram of steel has important applications such as austempering, martempering patenting, and isothermal annealing that are commonly useful in industry for the achievement of specific properties in steel.
However, the TTT diagram of steel is valid for one single composition, and if there is a varying composition, the plots, and curves also vary. Moreover, the diagram is understood only if steel immediately cools down from austenitizing temperature and is held constant during the completion of the transformation. In addition, the TTT diagram of steel is useful in explaining various concepts that are related to kinetic equilibrium and non-equilibrium changes in steel.
What is the Difference Between Iron Carbon Diagram and TTT Diagram?
Both, the iron-carbon diagram and TTT diagram, are important phase diagrams. The key difference between iron carbon diagram and TTT diagram is that the iron carbon diagram uses equilibrium conditions, whereas TTT diagrams use non-equilibrium conditions to draw the phase diagram. Moreover, in an iron carbon diagram, the temperature is plotted against the carbon composition, whereas in a TTT diagram, the temperature is plotted against time.
The below infographic presents the differences between iron carbon diagram and TTT diagram in tabular form for side-by-side comparison.
Summary – Iron Carbon Diagram vs TTT Diagram
The iron carbon diagram is a phase diagram useful in understanding the different phases of steel and cast iron. TTT diagram stands for time-temperature-transformation diagram. The key difference between iron carbon diagram and TTT diagram is that the iron carbon diagram uses equilibrium conditions, whereas TTT diagrams use non-equilibrium conditions.
1. “The Iron Carbon Phase Diagram.” Faculty of Engineering – Kiel – Germany.
1. “Iron carbon phase diagram” By AG Caesar – Own work Läpple, Volker – Wärmebehandlung des Stahls Grundlagen, Verfahren und Werkstoffe 8. Auflage, Seite 55ff.Weißbach, Wolfgang – Werkstofkunde Strukturen, Eigenschaften, Prüfung 17. Auflage, Seite 76ff. (CC BY-SA 4.0) via Commons Wikimedia
2. “T-T-T-diagram” By Metallos – Self-made with info from H. K. D. H. Bhadeshia, “Bainite in steels”. Cambridge University, Dept. of Materials Science and Metallurgy (1990). (CC BY-SA 4.0) via Commons Wikimedia