M.E.T.U.

Department of Materials and Metallurgical Engineering

Met.E 206

Materials Laboratory

 

EXPERIMENT II

 

By Prof. Dr. Haluk Atala

 

HEAT TREATMENT AND MICROSTRUCTURE I

 

THEORY

 

Heat treatment of metals and alloys involve heating and cooling operations in the solid state. The aim is to obtain the desired mechanical properties, such as ductility, toughness, strength or hardness. The required properties depend on the application. For example, for a cutting tool, hardness is important, but for a wire drawing high ductility is more usefull.

 

The mectanical properties of metals and alloys are directly related to their microstructure. For example, martensite gives the hardness in ferrous alloys. Therefore, during a heat treatment process, the phase transformations must be controlled very well. Very slow cooling rates yieldsstable phases at room temperature. In the case equilibrium phase diagrams are used. However, fast cooling rates may form metastable phases which are not present in the phase diagrams.

 

In all type of steels, austenite is the stable phase at high temperatures. Therefore, it can be stated that, heat treatment of steels is aimed to control the transformation of the austenite to another phase or phase mixtures.

 

Equilibrium Phase Diagrams

 

Figure 1 shows the Iron-Carbon equilibrium phase diagram. Ferrous alloys hving less than 2 %C are called steels but more steels contain less than 0.8 %C. The stable phases present in the diagram are:

γ :

Austenite (FCC),

Not stable at room temperature

a :

Ferrite (BCC),

Very soft and ductile

Fe3C :

Cementite (iron carbide),

Very hard and brittle

At room temperature, when a steel having a content of 0.5 % is observed under microscope, a two-phase microstructure is seen: Proeutectoid Ferrite + Pearlite (Figure 1).

 

            Proetuctoid Ferrite: It starts to precipitate within a + γ phase region. It is a soft phase. (schematically shown in Figure 1).

 

            Pearlite: The austenite retained without transforming to ferrite within a + γ will go into eutectoid reaction and transform to pearlite. Pearlite is a lamellar mixture of ferrite and cementite (Figure 1). As ferrite is soft, but cementite is very hard and brittle, aptimum combination of combination of mechanical properties are obtained from this phase mixture.

 

When the carbon content of steels are increased from 0.0 % to 0.8 %, the pearlite content incrases from 0 % to 100 %. For this reason, low carbon steels are ductile but high carbon steels have higher strength. When a steel cooled very slowly, the phases present are dictated by the equilibrium phase diagram. Therefore, mechanical properties are only a function of carbon content.

 

Figure 1. The equilibrium phase diagram of Iron – Carbon. The microstructure of a slowly cooled plain carbon steel having a composition Co is also incleded.

 

Non- Equilibrium Cooling

 

When an alloy is cooled fast, several changes in microstructure can be seen:

 

  1. The equilibrium phases can be seen at room temperature, but their morphology (shape) can be different.
  2. The microstructure at high temperatures can be frozen, and they can be seen at room temperature.
  3. A completely new phase can appear, which is not present in the equilibrium phase diagram.

 

For the fast cooling rates, phase diagrams can not be used and TTT diagrams (time- temperature-transformation) are needed. Figure 2 shows a hypothetical diagram. On the diagram, several cooling rates are superimposed. The slowest cooling rate is furnace cooling, in which the specimen is left in the furnace and the power is shut down. So, specimen is cooled to room temperature in furnace. Air cooling is somewhat faster than furnace cooling. When the specimen is taken out of the furnace and cooled in air down to room temperature, we call this air cooling. The faster cooling rate is obtained by water quenching. In quenching, the specimen is taken out of the furnace and immediately dropped in water. Quenching medium can be oil or water, but fastest cooling rate is obtained with water.

 

Figure 2. The cooling curves superimposed on a hypothetical TTT (time-temperature-transformation) diagram of a plain carbon steel.

 

In furnace cooling, the cooling rate is very slow and can be assumed that equilibrium conditions are satisfied. Therefore, the specimen follows the phase boundaries in the equilibrium phase diagram.

 

In air cooling, the cooling rate is faster. For a plain carbon steel, the phase diagram is still valid, and you can see the phases dictated by the phase diagram (compare Figures 1 and 2). Only the morphology and relative amount of the phases are changed. The reason for this is the larger ∆T (T-TE), the amount of undercooling. As ∆T is more effective on nucleation rate in comparison to growth rate, much finer grains are expected for faster cooling rates.

In water quenching, a completely new phase can appear. For example in ferrous alloys, austenite transforms to martensite, if cooled fast enough. Martensite has a needle-like structure and it is very hard.

 

Effect of Alloying Elements

 

Figure 3.a shows a typical TTT diagram for a plain carbon steel. The addition of carbon and alloying elements effect several parameters in TTT diagrams, such as the shape, martensite start temperature and relative position of the curve. In Figure 3.b, the effect of a small amount of Cr is seen. The TTT curve is shifted to right. The higher the percentage of Cr, the larger becomes this shift. Therefore, transformation of austenite to martensite becomes easier in alloy steels.

 

a

b

 

Figure 3. The schematic representation of the effect of alloying elements on the TTT curve of a eutectoid steel.

 

OBJECTIVE

 

To apply different cooling rates to various type of steels and to see the effect of cooling rate and steel composition on final hardness values.

 

APPARATUS

 

  1. A muffle Furnace.
  2. AISI 1060 steel bar.

(The chemical composition: 0.5 %C)

  1. AISI 1020 steel bar.

(The chemical composition: 0.5 %C)

  1. AISI D1 high alloy steel bar.

(The chemical composition: 2 %C-12 %Cr)

  1. A hardness indenter.
  2. A water tank.

 

PROCEDURE

 

  1. Cut 3 pieces of AISI 1060, 1 piece of AISI 1020 and 1 piece of AISI D1 steel. Therefore, you have 5 specimens alogether.
  2. Heat the muffle furnace to 950°C.
  3. Put the specimens in the muffle furnace and wait for 20 minutes.
  4. After 20 minutes, take the AISI D1 specimen and one of the AISI 1060 pecimens out of the furnace and cool in air.
  5. Similarly, take one of the AISI 1060 specimens and drop in water tank.
  6. Leave the AISI 1020 specimen and 3rd AISI 1060 specimen in furnace. Shut down the furnace and let it cool down slowly. When the furnace temperature reahes approximately 500°C, you can take out the specimen. The steps 4 to 6 are summarized in Table 1.
  7. Gring the cross-sections of all the specimens and take the hardness values.

 

 

 

SPECIMEN NO.

STEEL GRADE

COOLýING RATE

1

AISI 1060

Furnace Cooling

2

AISI 1060

Air Cooling

3

AISI 1060

Water Quenching

 

AISI 1060

Oil Quenching

4

AISI 1020

Furnace Cooling

5

AISI D1

Air Cooling

 

IMPORTANT: Take all precautions not to burn your hands and any other part of your body, when you take the specimens out of the furnace. Do not touch the specimens cooled in air at least for 10 minutes.

 

QUESTIONS

 

  1. Tabulate the hardness values af all the five specimens.
  2. What are the phase(s) present in AISI 1060 steel at 950°C?
  3. What are the phase(s) present in AISI 1020 steel at 950°C?
  4. Compare the hardness values of AISI 1060 specimens.(Specimens 1, 2, 3, and 4). Which one has the highest and which one has the lowest hardness values? Why?
  5. Compare the hardness values of furnace cooled AISI 1060 and AISI 1020 (Specimens 1 & 4). Do you see a difference in hardness values? Discuss.
  6.  Compare the hardness values of normalized AISI 1060 and AISI D1 steels (Specimens 2 & 5). Discuss the difference in hardness values in terms of composition and microstructure.
  7. Schematically draw the TTT curves of AISI 1020, 1060 and D1 steels.

 

SUGGESTED READINGS

 

1.       W. D. Callister, “Materials Science and Engineering: An Introduction”, John Willey and Sons Inc. (1990), pp. 305-320

  1. S. H. Avner, “Introduction to Physical Metallurgy”, McGraw-Hill Company, (1974), pp. 224-283

3.      C. R. Barret et. Al, “The principles of Engineering Materials”, Prentice Hall Inc. (1973), pp. 161-170, pp. 304-314