Phase Diagrams and Tie Lines

Metallurgy Background 2 Learning Activities Look up Keywords View Slides; Read Notes, Listen to lecture View Demo Do on-line workbook Do homework Lesson Objectives When you finish this lesson you will understand: Hypo and hyper euctectoid Steels How to calculate the percent of phases present in a Two phase region. Keywords: Hypo-eutectoid Steel, Hyper-eutectoid Steel, Tie line, Lever rule

Slow Cooled (Equilibrium) HYPOEUTECTOID STEEL Slow Cooled (Equilibrium) Phase Changes When Cooling From Austenite to Room Temperature Well, we have looked at the phase transformations that occur when a steel containing exactly 0.8% carbon is cooled; let’s now look at steels containing less than that eutectoid composition. These steels are called hypoeutectoid (meaning less than eutectoid composition). Starting at the high temperature austenite phase, we slowly cool into the two phase (austenite plus ferrite) region of the phase diagram. As we enter this region, ferrite begins to form along the austenite grain boundaries. The composition of the ferrite is something just slightly less than 0.02% carbon (see the right hand end of the tie line on the diagram). This means that the austenite is very slightly increased in carbon to make up for this material which is lower in carbon. Note that at this point the volume fraction of ferrite is small so the austenite is only slightly enriched in carbon. As cooling continues in this two phase region and we approach temperature closer to the eutectoid temperature, more and more low carbon ferrite is formed, and the austenite becomes enriched in carbon up to the 0.8% carbon eutectoid composition described previously. At this temperature just above the eutectoid temperature the structure consists of grains of ferrite at 0.02% carbon decorating the austenite boundaries with the remaining austenite having composition of 0.8% carbon. When we cool to just below the eutectoid temperature, the 0.8% carbon austenite transforms to pearlite laths just as described previously, making a final microstructure of ferrite at prior austentie grain boundaries surrounding a core of pearlite structure.

This just provides a closer look at the hypoeutectoid steel phase transformation upon equilibrium slow cooling.

And here is provided an actual photomicrograph of this hypoeutectoid steel slowly cooled microstructure. By looking at the volume fraction of ferrite and peralite, can you guess at what the original (nominal) composition of the austenite might be?

Another photomicrograph of a hypoeutectoid steel with significanly more ferrite (that is lower nominal carbon content in the austenite). Can you guess what the carbon content of this material might have been?

Tie Line to Calculate % Ferrite and % Austenite Austenitie Ferrite A tie line in the two phase region of a phase diagram is used to determine how much of each phase might be present and the composition of those phases. Lever Rule Tie Line Demo

Questions Turn to the person sitting next to you and discuss (1 min.): a hypereutectoid steel has carbon above 0.8%. What phase and percent phase do you expect at this tie line temperature?

Demonstration Homework A procedure called a “tie Line” can be used to determine the amount of each phase present in a Two Phase Compositional Region on a Phase Diagram. Please work the Demonstartion called “Phase Diagram Tie Line” found on the Demonstartion Page of the WE300 Website. Homework A tie line in the two phase region of a phase diagram is used to determine how much of each phase might be present and the composition of those phases. Doing this assignment will give you the concept of tie lines and practice doing tie line calculations. Please review this demonstration and do this homework as this is a most important concept. If you have difficulty with this, please ask for help in class or during office hours. Please do Assignment 6 “Metallurgy” found on the Assignment Page. Note, it will help if you do the above Demonstration first.