Laboratory

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THERMAL TREATMENT PROCESSES

ME 311: Manufacturing Processes Course

Objectives:

Learn to use Rockwell hardness machine to measure hardness of different specimens.

Understand the processes and effects of annealing and quenching on different specimens.

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Section I: Introduction to Thermal Treatment Processes A metallic material can be altered via mechanical and/or thermal means to change the mechanical

properties of the material. Such treatments are most often used to alter strength, hardness, and

ductility of a material.

1. Heat Treatment: Tempering of Hardened Steel The heat treatment process for steel and steel alloys is composed of three steps:

a) Heating to a temperature at which austenite is formed (austenitizing)

b) Rapid cooling (quenching)

c) Reheating to stabilize structure (tempering) Austenite is a solid solution of carbon in iron in a face centered cubic (FCC) crystalline structure

which is stable at elevated temperature. The temperature at which austenite formation occurs

depends primarily on carbon content of the steel. This temperature can be determined from the

phase equilibrium diagram for the particular steel composition. The material must be held at the

austenitizing temperature for a period of time to ensure completeness of the phase transformation

and homogeneous structure. The amount of time required is dependent on the size and shape of the

work piece as well as its composition.

After the time required for austenite formation, the material is rapidly cooled by quenching. Most

often, quenching is accomplished by immersing the material in oil or water although air quenching is

also used. Under conditions of rapid cooling, austenite transforms into an unstable (non

equilibrium) phase known as martensite. This phase is a supersaturated solution of carbon in iron in a

body centered tetragonal structure. Martensite is very hard, relatively brittle phase which provides

the ability of strengthening steel to very high levels. Quenching usually results in a structure

composed of martensite plus ferrite (solid solution of carbon in iron in body centered cubic

structure) and iron carbide (cementite). The proportions present after quenching depend on carbon

content and cooling rate. Higher carbon content and rapid cooling tend to produce larger

proportions of martensite.

Martensite is hard and brittle. In order to reduce brittleness, increase ductility, and relieve internal

stresses from rapid cooling step, the material is subjected to a second heating operation known as

tempering. The temperature for the tempering step must be below the austenite transformation

temperature and is usually between 400F and 800F. The resultant properties of the steel after

tempering depend on the time allowed for tempering as well as the temperature. In this lab, a

constant temperature will be used and the effect of tempering time on hardness will be investigated.

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Figure 1: Iron-Carbon Phase Equilibrium diagram. (Source: Material Science and Metallurgy, 4th

edition, Pollack, Prentice-Hall 1988)

2. Hardenability: The Jominy Test

The Jominy end quench test is the most commonly used method for determining the hardenability

of steel. Hardenability is the relative capacity of steel to be hardened by transformation to

martensite. In this test, a standard specimen (octagonal cross-section bar stock) is heated to the

austenite range and quenched by impinging a stream of cold water on one end of the specimen while

the specimen is held in a vertical orientation. This procedure results in cooling rates ranging from

very rapid at the end impinged by the water to very slow at the opposite end. With the variation in

cooling rate along the length, a single specimen will be composed of material ranging from hard

martensite to soft pearlite.

The effect of cooling rate on the resultant structure of steel can be observed on a time-temperature

transformation diagram. A sample diagram is provided (Figure 2) that shows three cooling rates

resulting in cooling curves A, B, and C. Curve A represents conditions at the end of the Jominy

specimen nearest the water stream. The material at this end will be composed primarily of

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martensite and should be relatively hard. Curve C is indicative of conditions farther along the bar

and indicates a softer composition which includes ferrite and pearlite.

The effect of cooling rate is evidenced by obtaining hardness measurements along the length of the

bar. A plot of hardness versus distance (Figure 3) is the standard method for presenting

hardenability data.

Figure 2: Effect of cooling rate on hardness

Figure 3: Effect of distance on hardness

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Name: Section:_ Group:_

Section II: Heat Treatment Exercise

Due Date:

The exercises of laboratory 2 investigate heat treatment techniques and measurement of hardness of

different specimen.

The instructor/TA will demonstrate heat treatment process and use of Rockwell hardness machine.

A. Steel Heat Treatment

The instructor/TA will provide samples of AISI 1080 steel. These samples were first heated to a

temperature of 1700F (Austenizing temperature for steel). One sample was quenched in water

and another in oil. For each sample, remove the scale formed during heat treatment using sand

paper or emery cloth. Using Rockwell tester, obtain and record five Rockwell C hardness

measurements on the polished and unpolished surface of each sample. Record these readings in

Table 1.

Table 1: Hardness Rockwell C for polished and scaled surfaces for heat treated AISI 1080 Steel

Measurement Water quenched Oil quenched Number Polished Scaled Polished Scaled 1 2 3 4 5

The instructor will provide different samples from the above experiment that were placed in the

furnace preheated to 650F for 5 min, 10 min, 20 min and 30 min. These samples were then

allowed to cool in air. Measure the Rockwell C hardness on the cut surface of each sample and

record in Table 2.

Table 2: Hardness vs Tempering Time for AISI 1080 Steel austenized at 1700F and then tempered at 650F

Measurement number

Water Quenched Oil Quenched

Initial 5 mins 10 mins 20 mins 30 mins 30 mins 1 2 3 4 5

B. Jominy Test

An AISI 1040 or 4140 alloy specimen will be given with indentations along the length. Use the

Rockwell hardness machine to measure Rockwell hardness values. Each measurement should be

made in the center of the flat area. Record the data in Table 3. Note: One person should

perform the hardness measurement while another person holds the specimen steady in position

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on the machine. This will avoid erroneous reading caused by the overhanging weight of the

specimen.

Table 3: Hardness vs distance, Jominy test

Distance from end

Hardness Distance from end

Hardness

1040 Steel 4140 Steel 1040 Steel 4140 Steel 1/16 57 58 1-1/8 38 40 1/8 55 57 1-1/4 36 39 3/16 55 56 1-3/8 36 37 1/4 56 54 1-1/2 33 36 5/16 55 57 1-5/8 33 34 3/8 52 56 1-3/4 33 36 7/16 52 54 1-7/8 32 34 1/2 48 52 2 31 33 9/16 49 53 2-1/4 34 33 5/8 50 53 2-1/2 32 33 11/16 48 49 2-3/4 32 32 3/4 45 47 3 31 34 13/16 43 46 3-1/4 31 32 7/8 43 43 3-1/2 31 31 15/16 41 42 3-3/4 30 31 1 39 40 4 27 30

Using the data obtained and recorded in the tables above, prepare a report which answers the

following questions.

1. What effect does surface scale have on hardness measurements ?

2. How does tempering time affect the hardness of AISI 1080 steel ?

3. What is the effect of cooling rate on hardness of 1040 and 4140 steel per the Jominy test ?

In support of your answers to these questions, include graphs as follows:

a) Mean hardness vs tempering time for water quenched 1080 steel

b) Hardness vs distance for both Jominy tests

c) Bar graph of Table 1 data showing 95% confidence intervals for each condition. Compute

mean hardness, sample standard deviation for each condition. Use small sample statistical

methods (n=5) and present in tabular for such as shown below. Show your calculations.

Treatment Mean Hardness Standard Deviation 95% confidence limits Water-polished Water-scale Oil-polished Oil-scaled

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