Eutectoid, Hypo-eutectoid & Hyper-eutectoid Alloys

Lab Session (4) Eutectoid, Hypo-eutectoid & Hyper-eutectoid Alloys

Objective

1. Get acquainted with Fe-Fe₃C phase diagram.
2. Study the effect of the eutectoid invariant reaction on steel microstructure.
3. Differentiate between eutectoid, hypo-eutectoid, and hyper-eutectoid microstructures.

Introduction

As discussed in the previous report, invariant reactions are those reactions accompanied by certain phase transformation and microstructural changes. These reactions happen at a given temperature and composition, giving them the name of a "zero degree of freedom'' reaction. Yet, with varying the content of the alloying element, it is possible to undergo invariant reactions, giving a certain amount of the "invariant" phase(s). In this report, the focus is upon the eutectoid reaction. In this reaction, a solid solution phase transforms into a 2-phase solid solution structure as indicated in the following equation and figure (1).

cooling f a ======> b + g

Figure (1) Eutectoid Reaction Same goes for the eutectoid reaction in terms of the production of a eutectoid phases at lower or higher contents of the alloying element, producing hypo-eutectoid and hyper-eutectoid alloys respectively. Definitely such microstructural changes influence the mechanical properties. That is why one of the important phase diagrams to investigate the effect of such changes would be the Fe-Fe₃C diagram (read as iron-iron carbide diagram) due to the branched and widespread applications of steel.

Figure (2) Fe-Fe3C phase diagram Courtesy of Georgia Tech Phase Diagrams

As shown in the Fe-Fe₃C diagram, the iron iron carbide system experiences the eutectoid reaction when the gamma (austenite) phase transforms into alpha (ferrite) and Fe₃C (cementite) phases. In fact, such transformation happens under certain conditions regarding the cooling rates, whether it is air or furnace cooled. This affects the microstructure of the produced eutectoid phases. In addition, increasing and decreasing the carbon content changes the amount of eutectoid phase

Procedures

Samples are made out of Fe-Fe₃C. Four specimens with different carbon content were investigated as follows:
-Specimen 1: 0.35% C
-Specimen 2,3 : 0.8% C with one furnace-cooled and another air-cooled
-Specimen 4: 1.3% C

Etching:
-Specimen 1: etching for 15 seconds in 2% Nitric acid in alcohol (nital) solution.
-Specimen 2 &3: grinding and polishing with diamond paste and finishing with a 1 micron grade, followed by etching with the 2% nitric acid in alcohol solution.
-Specimen 4: Similar to 1

Testing Devices

Optical Microscope

Results/Discussion

Answer the following questions in the body of your discussion Refer to your textbook pages 485-500 1. Describe the phase transformation for each specimen from the liquid state to the solid state using the phase diagram.
2. Discuss the effect of the cooling rate on the microstructure.
3. Compare the expected microstructure to the actual one for all specimens.
4. Sketch all the microstructures under results.
**Attach the answers to the handout questions to the report and consider them within your discussion too.

Further References/Images

--The following sites and books contain useful information and images for several articles that we have encountered in this experiment. Make sure to check them.

Kalpakjian, Serope. Manufacturing Process for Engineering Materials. Addison Wesley, 3^rd Ed., 1997.

Smith, William F. . Principles of Materials Science and Engineering. McGraw Hill, 3^rd Ed., 1996. (p. 128-132)

Academic Page for a similar experiment¶¶¶ http://compfab.me.lsu.edu/me3701/experiments/metallography/expt6.htm

Academic Page for a similar experiment¶¶¶ http://compfab.me.lsu.edu/me3701/experiments/metallography/expt7.htm

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