E45 Lab 6 Heat Treatment of Steel

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Engineering 45 Properties of Materials

Laboratory

© Copyright 2001 Professor Ronald Gronsky the Arthur C. and Phyllis G. Oppenheimer Chair in Advanced Materials Analysis Department of Materials Science & Engineering University of California Berkeley, California 94720-1760

E 45

Lab 4

Heat Treatment of Steel

Objective
To understand how the processing (heat treatment) of steel affects both microstructure and properties (hardness)

Overview
This lab is designed to illustrate the versatility of steel as a structural material. One sample of a single grade of steel will be subjected to heat treatments similar to those employed in industry. Students will monitor the changes resulting from these simple inexpensive treatments, noting the dramatic changes in both microstructure and hardness that result.

References
   G. Krauss, Principles of Heat Treatment of Steel, American Society for Metals, Metals Park, OH, (1980). W.T. Lankford, et.al., Eds., The Making, Shaping, and Treating of Steel, 10th edition, Association of Iron and Steel Engineers, Pittsburgh, PA, (1985). Metals Handbook, 9th edition, Vol. 4, Heat Treating, American Society for Metals, Metals Park, OH, (1981).

Background
Steel is an alloy of iron (Fe) and carbon (C), with concentrations of carbon from 0.1 to 2.0 weight percent. Carbon is soluble in Fe because the C atoms are small enough to fit into interstitial locations between Fe atoms without too much distortion of the lattice. Carbon is soluble in the fcc phase of Fe (called austenite or gamma-Fe, or ) up to approximately 2%. However, in the bcc phase of Fe (called ferrite or alpha-Fe or ), the maximum solubility is only about 0.02%. When austenite is cooled below a critical temperature called the eutectoid temperature (727°C), it becomes unstable. Most of the Fe tends to precipitate as nearly pure ferrite and most of the C tends to come out as the intermetallic compound Fe3C (cementite). The transformation of austenite requires redistribution of C atoms from a random solid solution to one in which nearly all of the C is contained in the Fe3C precipitates. At temperatures just below the eutectoid decomposition temperature the driving force for the transformation is low, and nucleation of the two new phases, ferrite and cementite, is slow. It is therefore necessary to hold the specimen at temperature for a considerable time to allow the transformation to take place in those regions of the specimen that are just

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below 727°C. The lower the temperature, the greater the driving force for the reaction to occur, causing a higher nucleation rate. At temperatures below about 540°C, however, the rate of transformation again becomes slower. This is due to the decreasing mobility of C atoms in the austenite. In order for the transformation to proceed, C atoms must continually diffuse away from the growing ferrite regions to the growing Fe3C regions. If the austenite is cooled so rapidly that there is not enough time for nucleation and growth of ferrite and Fe3C, an entirely different kind of transformation takes place called the martensitic transformation. In this reaction the fcc austenite changes instantly by a shear mechanism to a body-centered tetragonal structure, trapping C in the new phase, called martensite. These reactions are best illustrated in a figure known as a time-temperaturetransformation (TTT) diagram. It portrays the decomposition kinetics of austenite, obtained by isothermally holding a sample for different times until the reaction is complete. An example is shown here.

804  727

 T (°C)

  Fe3C

Ms Mf 0.1 1 10 time (seconds) 100

Fig. 4-1 Schematic TTT curve for a hypoeutectoid plain carbon steel. The poreutectoid ferrite constituent forms below 804°C, and the eutectoid constituent below 727°C. Both the start and the finish of the athermal martensitic transformation are seen at lower temperatures.

The mechanical properties of steel depend upon the dispersion of C atoms or of cementite particles in the Fe matrix. Since redistribution of C atoms requires diffusion and since the rate of diffusion decreases exponentially with decreasing temperature, the coarsest dispersions of Fe3C and of ferrite result from the transformation that is allowed to take place just below the critical temperature. The resulting eutectoid product is known as coarse pearlite. If the transformation is forced to take place at lower and lower temperatures by increasing the rate of cooling, then finer and finer dispersions of the hard brittle carbon-rich phase (Fe3C) and the relatively soft ductile ferrite are produced, known as fine pearlite. If

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the transformation takes place at temperatures below about 400°C, the ferrite and cementite form as extremely fine needles. This microstructure is known as bainite. The hardness of steel generally increases with the fineness of the dispersion of the phases in the microstructure. If the rate of cooling is increased still further, the precipitation of carbon as Fe3C can be suppressed altogether, forming martensite, in which the C atoms are still distributed nearly at random. This is the hardest structure possible for any given steel. However, since martensite has low ductility, many machine parts are first rapidly cooled to produce martensite, then "tempered" by holding at slightly elevated temperatures to redistribute the carbon as fine precipitates. The resulting microstructure is known as tempered martensite. Part I. Heat Treatment
Equipment

   

ASTM 1040 cold-rolled steel specimen Furnace and wire for suspending specimen in hot zone Chromel™ + alumel™ thermocouple Computer data acquisition system

Procedures

Each group will be given a sample of 1040 cold rolled steel. The sample will first be heated uniformly in the austenitic region of the phase diagram. The sample will then be heated under conditions resulting in a temperature gradient along its length, using the same procedure as that in Lab 4, and illustrated in the sketch below.

Wire support

Thermocoupl e Insulator top

900°C Furnace Sample

1/8 inch gap

100°C Water

1/8 inch immersion

Fig. 4-2 Schematic experimental set-up. It may be necessary to replenish the water supply due to evaporative losses.

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Connect the thermocouple to the data acquisition interface box. Start the data acquisition program and begin to record data. Center the sample in the furnace and heat to 1050°C. Hold at this temperature for at least 5 minutes. Then quickly drop the sample so that it is in the position shown in the figure. Set up the apparatus ahead of time so that the specimen can be dropped into position as quickly as possible. Adjust the power input so that the temperature at the top of the sample is 900°C. If the water level decreases, add more water with the squirt bottle to avoid wetting the furnace. Hold the temperature at 900°C for 15 minutes. This can be accomplished by adjusting the power source. After 15 minutes quench the sample by dropping it into the water and vigorously agitating it until cool. Turn off the power, and when the furnace has cooled to about 50°C remove the sample. Stop recording data.

Part 2. Metallographic Polishing
Equipment

  

Polishing clamp Wet belt grinder (in the grinding rooms) Polishing paper (nos. 240, 320, 400, 600)

Procedures

Mount the sample in the clamp and remove the scale by pressing the sample lightly on the grinder to obtain a flat surface on one side. Do this for both sides of the sample. Have the T.A. check the surface. Then grind by hand on successively finer paper down to 600 grit. The amount of grinding necessary on each paper can be determined by changing the direction of the grinding by 90° each time a change to finer paper is made. Continue grinding until the scratches of the previous grit have been removed. Remember to wash the sample between paper changes.

Part 3. Metallographic Etching
Equipment

 

Electropolisher Polishing solution 60ml distilled water 50ml butyl cellulose 350ml ethyl alcohol, 95%, (and just before using, add) 40ml perchloric acid, 60%

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Procedures

Your GSI will do the electropolishing. Afterwards, quickly wash the sample in running water, rinse with alcohol and dry under the hot air blower. Be careful not to touch the polished surface at any time, or else the fine grinding and polishing steps will need to be repeated.

Part 4. Hardness Indentation Test
Equipment

 

Rockwell hardness tester Measurement Ruler

Procedures

Take a series of Rockwell “C” hardness measurements at 1/8" intervals along the edge of the sample as shown in the plan schematic to the right. Record the results on the data sheet.

1 2 3 4 5 6 7 8

Part 5. Metallography
Equipment

  

Glass slide Specimen press Microscope (100x, 500x)

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Procedures

Mount the sample on a glass slide with plasticene or clay using the specimen press to level the surface. This will keep the sample in focus as it is moved with the microscope stage. Choose five areas along the edges where the hardness readings are different and/or where the microstructure looks different. Sketch the microstructures at each of the five areas, using your own data sheet. Notice the difference in grain sizes and the density of dislocations, and record these on your data sheet. Also, look for the answers to the questions in the following section when examining the sample.

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Lab Report
Precise data presentation is expected, accompanied by relevant references (the phase diagram for the Fe-Fe3C system can be found in many publications). Answers to all of the following questions are also required.
Question 1

For each of the five microstructures shown in your results, draw a representative cooling curve on a TTT diagram corresponding to the heat treatment received in that area of the sample. Use one small TTT diagram for each of the five cooling curves, or one large (full page size) TTT diagram for all five cooling curves, clearly coded and labeled. Verify the phases present in each microstructure using the cooling curve. Specify the order in which the phases appeared. Use the phase diagram as necessary.
Question 2

Make a plot of the hardness vs. the distance from the 900°C end of the sample. Use your hardness data to rank martensite, ferrite, and pearlite from the hardest to the softest. Support your results by discussing the nature of the crystal structure and bonding of the phases (martensite, ferrite, and carbide).
Question 3

Does the transformation of austenite into ferrite begin at the austenite grain boundaries and propagate inward, or begin inside the grains and propagate toward the boundaries? Include a sketch of the microstructure from the appropriate part of the specimen illustrating the answer to this question. Explain.
Question 4

How could a structure containing only ferrite and martensite be produced in a 1040 steel specimen? Illustrate on a TTT diagram.
Question 5

Could you heat treat a 1040 steel specimen so that it contains only ferrite? Describe how this can be achieved, or explain why it cannot be done.
Question 6

Why isn't martensite on the Fe-Fe3C phase diagram? Why doesn't its absence from the phase diagram prevent it from being an important engineering material?

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DataSheet: Hardness Measurements

Position 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Minor Load

Major Load

Hardness

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