Heat Treatment of Plain Carbon Steels

Lab Session (5) Heat Treatment of Plain Carbon Steels

Objective

1. Get more acquainted with Fe-Fe₃C phase diagram.
2. Focusing on the plain carbon steel zone in the Fe-Fe₃C system.
3. Studying the effect of the various cooling methods on steel microstructure.
4. Studying the Austenite-to-Martensite transformation and its impact on microstructure.

Introduction

The iron-iron carbide system (Fe-Fe₃C) is one of the most importance phase diagrams for engineering applications. Steel alloys are perhaps the most widely used alloys due to the possibility of "tailoring" a steel alloy of certain desirable properties for a given application. Variations in the carbon content or the heat treatment processes have a decisive effect on the alloy strength, ductility, machineability, grain size, residual stresses, hardness and toughness. As shown on figure 1, the carbon content has a decisive effect on the yield strength, tensile strength, and hardness.

Figure 1 Effect of carbon content on steel alloys properties After L.H. Van Vlack 1989 A basic step is to shed more light on the Fe-Fe₃C system. As shown in figure (2), the Fe-Fe₃C system is composed of different ranges of phases that exist under different carbon contents and temperature conditions.

Figure 2 Fe-Fe3C system Courtesy of Georgia Tech Phase Diagrams

The following table introduces the different phases and their properties:

Phase	Properties
Cementite (Fe₃C) Iron Carbide	It has 6.67% Carbon content by weight. It is a typical hard and brittle interstitial compound of low tensile strength but high compressive strength. Its crystal structure is orthorhombic.
Austenite g	An FCC interstitial solid solution of carbon dissolved in iron, with maximum solubility of 2.08% C at 1148C and decreases to 0.8 at 723 C. Austenite is normally unstable at room temperature. Yet, using heat treatment, it is possible to obtain austenite at room temperature.
Ferrite a	A BCC interstitial solid solution of a small amount of carbon dissolved in iron, with a maximum solubility of 0.02% at 723 C, and decreases to 0.005% at 0 C. It is the softest structure on the iron-iron carbide diagram.
Pearlite (a+ Fe₃C )	(Eutectoid invariant reaction) At 0.83% C, austenite undergoes phase transformation in an invariant reaction known as the eutectoid reaction transforming to ferrite and cementite. This is done by heating plain carbon steels to 750 C and keeping it held for sufficient time to fully austenize it. Then equilibrium cooling leads to the formation of a lamellar structure of alternate plates of ferrite and cementite. Within a range between 0.83 and 2.0%C, hyper-eutectoid steel is formed and at %C less than 0.83 hypo-eutectoid is formed (which was discussed in the previous report).
Ledeburite (a+ Fe₃C)	(Eutectic invariant reaction) At 4.3% C, mixture of austenite and cementite transforms into ferrite and cementite. Within the range from 2.08 to 4.3%C, a hypo-eutectic alloy is formed, and beyond 4.3% hyper-eutectic is formed (discussed in report 3). Such a region is not for plain carbon steels, but for cast irons (higher carbon content).
Ferrite d	A BCC interstitial solid solution but with greater lattice constant. It has a maximum carbon solubility of 0.09% at 1465 C.

Figure 3 Different Fe-C system microstructures Source: Van Vlack 1989

Heat Treatment of Plain Carbon Steels:

Plain carbon steels are iron alloys of carbon content less than 2.08%. Heat treatment is changing the microstructure of steels alloys to change the strength, ductility, hardness, machinability, grain size and wear resistance. There are general reasons for heat treatment:
1. Hardening: a heat treatment to increase the hardness of the steel alloys.
2. Tempering: a heat treatment that reduces the brittleness of a steel without significantly lowering its hardness and strength. All hardened steels must be tempered before use.
3. Softening: a heat treatment to increase the machinability of steels.
4. Re-crystallization: transformation of cold-worked grains to an undistorted shape.
5. Stress relief: removing internal stress from a metal that has been subjected to cold working or welding.
6. Hot working operations
7. Diffusion of alloying elements: remove internal stress from a metal that has been subjected to cold working or welding.

Heat treatment by itself is a process of varying the heating and cooling rates of plain carbon steels, accordingly different combinations of mechanical properties can be obtained.

One of the commonest heat treatment processes is the formation of Martensite. The process of formation of Martensite can be pinpointed as follows:
1. Heating a plain carbon steel alloy (from the eutectoid region, either hypo or hyper) to the austenite region
2. Having it held there for a while of time to fully austenize its structure.
3. Exposing it to a non-equilibrium cooling rate, the austenite will transform into Martensite.

Figure 4 Martensite Van Vlack 1989 There are several ways of quenching plain carbon steels, such as using air, oil and water. It also varies whether agitated or non-agitated stream is used. As the heat transfer rate increases, the formation of Martensite gets closer. Martensite is a meta-stable phase consisting of a supersaturated interstitial solid solution of carbon in BCC iron (or in body-centered tetragonal iron, which is caused by a slight distortion of the BCC iron unit cell). Martensite is the hardest and most brittle steel phase. That is why it usually undergoes further heat treatment to enhance its ductility (martempering).

Procedures

Five specimens were used:
1. X17: 0.8%C, heated for 1 hour at 800 C, then furnace cooled.
2. X18: Same as X17, but cooled in still air instead (normalized).
3. OQ: 0.33% C with 3.6% alloying elements, oil quenched and no tempering.
4. OQ: tempered for 2 hours at 200 C.
5. OQ: tempered for 2 hours at 600 C.

Common preparation procedures such as grinding and polishing were taken, followed by etching using 3ml of HNO₃ + 100 ml of CH₃OH (methyl alcohol), etched by immersing for 15 seconds.

Testing Devices

Optical Microscope

Results/Discussion

Answer the following questions in the body of your discussion Refer to your textbook (p.496-523) Investigate the following factors:
1. The use of different types of cooling media (air and oil) to investigate the effect of the cooling fluid on the formed structure.
2. The variation of carbon content and the impact on the microstructure.
3. The effect of tempering temperature on the formed structure.
4. The effect of the tempering time on the microstructure.
5. The Martensitic structure and its types.
Sketch all the microstructures and comment under results.
**Attach the answers to the handout questions to the report and consider them within your discussion too.
BONUS: Define TTT Diagrams

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.

Georgia Tech Phase Diagrams Page

Useful page 1

Useful page 2

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)

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