TAM 224/CEE 210 11–1

11. Heat Treatment of Plain Carbon Steels 11.1. Objective The objective of this experiment is to demonstrate some important heat-treatment processes—including annealing, normalizing, quenching and tempering—and the effects that these processes have on the

microstructure and mechanical properties of a com-mon structural steel. The microstructure is examined under a microscope, and mechanical properties are determined from tensile and hardness tests.

Also, the Jominy hardenability test is conducted on a steel sample to illustrate how cooling rate affects the hardness and microstructure of the steel.

11.2. Apparatus Two tabletop Vulcan model 3-550 electric furnaces with a maximum operating temperature of 1100°C are used—one to austenitize all specimens (Fig. 1), and the other to temper some of the specimens. A water bath is used for quenching. Hardness is determined by using a Rockwell hardness machine. Tension tests to failure are conducted in an Instron model 4400 testing machine.

Jominy heat treatment is conducted using a water jet in a recirculating water tank (Fig. 2).

A Nikon microscope with 5X to 100X objectives and a TV camera and monitor are used to examine the

microstructure of previously prepared heat-treatment samples.

11.3. Materials Eight identical carbon-steel round tensile speci-mens with the same nominal dimensions as those in the previous tension-test lab will be used. Four of these eight specimens will have been resting in one of the electric ovens at 850°C (1550°F) for at least an hour before the beginning of the lab session, leaving them in the austenitic condition. The remaining four

Fig. 1. Removal of specimens from austenitizing oven

prior to quenching in water.

Fig. 2. Hot Jominy specimen being cooled from below

by a water jet.

11–2 Behavior of Engineering Materials TAM 224/CEE 210

specimens are prepared ahead of time by the laboratory technician—two annealed (A) specimens,1 and two normalized (N) specimens;2 these specimens will be at room temperature, ready for testing.

One Jominy sample of 1045, 4340, or 6150 steel will be available for hardenability testing. This sample will also be in an austenitizing oven at a temperature between 850°C and 900°C at the beginning of the laboratory session.

11.4. Experimental Procedure The class will be broken into four groups—A, B, C, D. All groups will participate in the initial quenching of the four tensile specimens that are in the austenitizing oven, and in the water-jet cooling of the hot Jominy specimen. Thereafter, groups A and B may collaborate in part, as may groups C and D, as suggested in the accompanying table.

Heat treatment 1. Using gloves and tongs, remove one of the

specimens from the 850°C furnace (Fig. 1) and

1An annealed specimen is one that is heated to the austenitizing temperature, held at that temperature for at least one hour, then allowed to oven-cool with the oven door closed. The cooling time is approximately one day. Annealed specimens are identified by white paint on one end.

2A normalized specimen is one that is heated to the austenitizing temperature, held at that temperature for at least one hour, then removed from the oven and allowed to air-cool. The cooling time is approximately one hour. Normalized specimens are identified by blue paint on one end.

quench as quickly as possible in the water bath provided. Stir the specimen continuously and vigorously, without dropping it, for about 30 seconds. If the quenching process is successful, it will produce a hard martensitic microstructure. Put this quenched (Q) specimen aside for the moment. Quench the remaining three tensile specimens in a similar manner.

As suggested by the time-temperature-transition (TTT) plot in Fig. 3, the desired martensite will be produced if the specimen can be quenched from above 800°C to below 240°C in less than about one second. Otherwise, it is likely that only the softer components of ferrite and pearlite (ferrite + cementite) will be produced instead.3

2. Place two of the quenched specimens in the smaller furnace, which will have been set at a temperature in the range of 400°C (750°F) to 600°C (1100°F). After about 30 minutes, remove these specimens from the furnace and allow them to air-cool on one of the ceramic tiles. This heat treatment results in a tempering (T) of the hard martensitic structure obtained when the material was quenched; the unstable martensite decom-poses into a fine-structured, stable solution of α (ferrite) and Fe3C (cementite).

3Constant-cooling curves should be used in this discussion, not TTT diagrams; however, the constant-cooling curves for the iron–carbon system are very similar to TTT curves above the bainite nose. See, for example, Callister (2000).

Suggested group tasks

A B C D

Quench 4 specimens; put 2 in tempering oven; prepare 1 Jominy specimen; take notes on rest of lab

Observe microstructure of A, N, Q, T

Rockwell test 1 Jominy specimen; remove

tempered specimens from oven

Perform Rockwell tests on 1 Jominy specimen

Observe microstructure of A, N, Q, T

Rockwell-B 1 A, test in

tension

Rockwell-B 1 N, test in

tension

Rockwell-B 1 A, test in

tension

Rockwell-B 1 N, test in

tension

Rockwell-C 1 Q, test in

tension

Rockwell-C 1 T, test in

tension

Rockwell-C 1 Q, test in

tension

Rockwell-C 1 T, test in

tension

0

100

200

300

400

500

600

700

800

900

0.1 1 10 102 103 104 105

Time (sec)

Tem

pera

ture

(C

)

Roc

kwel

l-C

har

dnes

s

α + γ

A1

A3γ (austenite)

martensite

martensiteγ +

bainitebainite

γ +

pearliteα +

pearliteγ + α +γ

Ms

Mf

23

30

39

49

62

Fig. 3. Time-temperature-transition curves for 1050 steel.

(Adapted from Askeland (1989), Fig. 12-8.)

TAM 224/CEE 210 Heat Treatment of Plain Carbon Steels 11–3

Tempering reduces the strength and hardness of the steel, but increases its ductility and impact properties.

3. Using a wire wheel, remove the scale from the two specimens that your group will test. Determine the Rockwell-B hardness of each of your specimens by taking the average of three readings on the grip end of the specimen that was not held with the tongs during quenching. A self-centering support platen should be used for this test. Record the data in Table 1.

4. With each of your two specimens, perform an instrumented tension test in the Instron testing machine, taking the material to failure. Make a detailed sketch of the fracture surfaces and final shape of the specimen, and measure the final diameter at the point of failure. Continue filling out Table 1. Exchange necessary data with another group—A with C, B with D, for example, to complete the required descriptive information.

5. Using the Nikon microscope, observe the micro-structure of previously prepared annealed, normalized, quenched, and quenched-and-tempered samples mounted in epoxy. Sketch the structure of each type in Table 1.

Jominy hardenability 6. Using a dummy Jominy specimen at room

temperature, practice placing the specimen in the end-quenching jig and turning on the recirculating water pump. Turn off the pump and remove the dummy specimen.

The actual Jominy specimen to be end-quenched will have been placed in one of the electric furnaces prior to the laboratory session at approxi-mately 845°C (for 1045 or 4140 steel) or 870°C (for 6150 steel).

7. Under close supervision, remove the heated Jominy specimen with tongs and gloves from the electric furnace, place it in the jig, and turn on the water pump. Observe the rate at which various parts of the specimen are cooled (Fig. 2). Allow the specimen to cool completely before removing it from the jig.

8. Using a wire brush, wire wheel, or file, remove the scale from the specimen. Then use a belt sander to sand a narrow flat surface along the length of the specimen for Rockwell hardness tests. Note.—The laboratory technician may have already used this procedure to prepare a Jominy specimen for you to test.

9. Perform Rockwell hardness tests at several points along the narrow sanded portion, using LabView software to record and display the data. Concen-trate most of the data near the cooled end. Observe the range of hardness values obtained and the positions along the length at which the measurements are made.

10. (Optional) Using the Nikon microscope, observe the differences in microstructure at various points in the sectioned and etched Jominy samples provided.

11.5. Analysis of Results

Heat treatment 1. Calculate the averages of the three Rockwell hard-

ness values for each sample. If a mixture of Rockwell-B and Rockwell-C numbers is used, then convert average Rockwell hardness numbers to Brinell hardness numbers using the table in the Compression and Hardness Lab or the conversion chart reprinted below.

0 100 200 300 400 500 6000

20

40

60

80

100

120

0

100

200

300

400

500

600

Brinell hardness number, HB

Roc

kwel

l B-

& C

-sca

le h

ardn

ess

Vic

kers

har

dnes

s nu

mbe

r, H

V

Rockwell B

Vickers

Rockwell C

2. Plot nominal stress–strain curves for all four specimens on the same graph. Label the curves.

3. Calculate the Young’s modulus E, the yield strength σy , the ultimate strength σu , and the percent reduction of area %RA for each sample tested.

4. Complete Table 1 by filling in results from the Tension Test lab.

Jominy hardenability 5. Prepare a graph showing the spatial dependence

of surface hardness along the axis of the Jominy specimen.

11–4 Behavior of Engineering Materials TAM 224/CEE 210

11.6. Points for Discussion Note.—Your lab instructor will indicate which of the

following questions are to be addressed in your report.

Heat treatment 1. Discuss your sketches of the microstructures,

pointing out relevant differences between them.

2. Compare hardness values with the values of yield strength and ultimate strength for each specimen. Are these relations what you would expect?

3. Compare hardness values with your observations of the microstructure. Explain in some detail.

4. Compare ultimate strengths, amount of plastic deformation prior to failure, and apparent modes of failure of all tensile specimens. Comment on differences in load–deflection behavior and give reasons for these differences.

5. Discuss the applicability of equilibrium phase diagrams, TTT diagrams, and constant-cooling curves for the heat treatments considered.

6. State which heat treatment(s), if any, could give rise to ultimate strength values that are strongly affected by any surface flaws that may be present. Justify your predictions.

7. Discuss any inconsistencies that exist in the observed relation between hardness values and ultimate strength, for the various heat treatments considered in this laboratory session.

8. Determine the effect of the various heat treatments on the value of Young’s modulus E. Be sure to include data for the as-received material (from the Tension Test lab).

Jominy hardenability 9. Describe the practical purpose of Jominy

hardenability testing. Why is the test devised so that only one end of the rod is cooled?

10. Discuss the spatial dependence of hardness observed in your sample. Rate your material with regard to ease of hardenability.

11. Discuss any observed relations between micro-structure in the Jominy specimen and your hardness measurements.

11.7. References Askeland, D. R. 1989. The Science and Engineering of

Materials, 2nd ed. Boston: PWS-Kent, Chapter 12.

Callister Jr., W. D. 2003. Materials Science and Engineering—An Introduction, 6th ed. New York: Wiley, Chapters 9–11, esp. Sections 9.17–9.19, 10.5–10.9, 11.7–11.8.

Flinn, R. A., and P. K. Trojan. 1990. Engineering Materials and Their Applications, 4th ed. Boston: Houghton Mifflin. See Chapter 5, esp. Sections 5.5-5.6, for heat treatment, and Section 8.17 for the Jominy hardenability test.

Van Vlack, L. H. 1980. Elements of Materials Science and Engineering, 4th ed. New York: Addison-Wesley, 364.

Young, J. F., S. Mindess, R. J. Gray, and A. Bentur. 1998. The Science and Technology of Civil Engineer-ing Materials. Upper Saddle River, N.J.: Prentice Hall, Section 13.3.

Notes

TAM 224/CEE 210 Heat Treatment of Plain Carbon Steels 11–5

Table 1—Microstructure, tensile properties and hardness data

Measurement or property Material: _____ Steel

Quantity Symbol Units Annealed Normalized Quenched Tempered As Rec’d*

Processing

Microstructure produced (description)

Microstructure (sketch)

Initial data

Diameter d mm

Cross-sectional area A0 mm2

Gage length l0 mm

Modulus and strength

Yield load Py kN

Maximum load Pmax kN

Yield strength σy MPa

Ultimate strength σu MPa

Young’s modulus E GPa

Type of behavior

Detail of fracture surface (sketch)

Ductility—Percent reduction of area

Final diameter df mm

Final area A f mm2

Percent red. of area %RA —

Hardness tests

Rockwell hardness HR B or C ___, ___, ___ HR__

___, ___, ___ HR__

___, ___, ___ HR__

___, ___, ___ HR__

Average Rockwell HR B or C ______ HR__

______ HR__

______ HR__

______ HR__

______ HR__

Equiv. Brinell hardness HB —

Test date: Group: Student’s name:

*Specimen used in the Tension Test lab Printed 7/17/03