Lab 1: Heat Treatment and Indentation Hardness
Fu, Amy
Scott, Shin
Shan, SiSi
Son, Jennifer
Wen, Alexander
Date Submitted: February 10, 2011
Date Performed: February 3, 2011
Lab Section: 104 (Group 20)
Lab GSI: Hua Xiang
Abstract
The objective of this experiment is to understand how the microstructures of AISI 1018 and AISI
1045 carbon steel can be changed by heat treatment. A test is conducted with a Rockwell hardness tester before and after the heat treatment in order to determine the hardness caused by the treatment. Testing is done with two different tips: a diamond indenter and a steel ball indenter, which are pressed into the specimen by the tester. The hardness can then be read from the gauge. The Rockwell tester calculates the hardness by using the depth, force, and indentation of the tips. The most important result that this lab will show is the percent increase in hardness of the two varieties of steel after heat treatment. This leads to the conclusion that that heat treatment (as well as higher carbon content) increases the hardness of steel.

1. Introduction
The purpose of this lab is to understand how the microstructure of low-carbon (AISI 1018) and medium-carbon (AISI 1045) steel can be altered by heat treating and then quenching to increase the mechanical strength (hardness) of steel. This is done by using the TTT diagrams and knowledge of phase transformation in order to heat the steel samples to a specific temperature, and then rapidly cooling in a specific time to get the desired hardness. Then, a test of the heat-treated samples is conducted to ascertain their actual hardness and whether they confirm with the theory.
The theory behind the heat-treating of this lab has to do with the microstructure of the steel.
When the steel is made by a hot-rolling process, it is allowed to cool slowly, which gives it time to settle to its lowest energy state. This can be seen in an equilibrium phase diagram of carbon steel. Now, if steel is cooled rapidly (that is, within five seconds) from above 730°C, the material does not have enough time to go to its lowest energy state; instead, the austenite phase transforms to a metastable martensite phase
[1]. This is called a martensitic phase transformation and can be seen on a Time-Temperature-
Transformation phase diagram (TTT diagram).
The theory behind the indentation hardness testing in this lab is based on hardness, which is the ability of a material to resist plastic deformation due to indentation by a rigid body [1]. The sample is tested for hardness by using the Rockwell hardness tester, which presses two different rigid points (of known hardness and size) into the specimen, first with a minor load and then with a major load, then reading the hardness from the gauge. The minor load is only applied to establish a reference position for depth measurement and to penetrate through any surface scale of foreign particles [2]. The major load establishes the reading. The numbers read from the gage are suffixed by the letter H, followed by R, then
C or B. H stands for hardness, R for Rockwell, and B and C for the scale that was read from the gauge corresponding to the different points used in the test.
The importance of this lab is that it allows one to understand the results and the process to get to these results so that they can be used in practice. The different heat-treating and tempering processes create different characteristics that have their pros and cons. This lab will give one the understanding to be able to choose and optimize metals for the purpose intended.

2. Theory
The Phase Diagram
Materials of different compositions exhibit different microstructures (and thus different material properties) at different temperatures. A phase diagram is a map of these equilibrium microstructures over composition and temperature. Figure 1.1 in Mechanical Testing of Engineering Materials [1] shows the
Iron-Carbon phase diagram, and both materials used in this lab (AISI 1018 and AISI 1045 carbon steel) can be found on this diagram, thus allowing us to identify the microstructures of the two materials when they are at equilibrium state. However, heat-treated materials are not in equilibrium, so a TTT diagram
(explained below) is needed to determine the microstructure.
The Time-Temperature-Transformation (TTT) Diagram
Materials with the same chemical composition can exhibit entirely different physical properties because of differing microstructures. This can be caused by altering the temperature of the material rapidly, without allowing the microstructure to reach its quasi-static state. A map of such transformations is called a Time-Temperature-Transformation (TTT), or an Isothermal Transformation diagram. This diagram has temperature on the Y-axis and time on the X-axis. A different TTT diagram exists for each composition: Figure 1.6 (a) in Mechanical Testing of Engineering Materials [2] shows the TTT diagram for eutectic steel, and Heat Treater’s Guide [3] includes TTT diagrams for AISI 1019 steel and AISI 1045 steel, which are both very different. One may graph the temperature inside a material versus time by starting the “stopwatch” when temperature transformations begin after the equilibrium phase, and tracing the changing microstructure in the material at each time using the TTT diagram.
The Rockwell Hardness Test
Hardness is a measure of a material’s resistance to deformation when a load is applied. The
Rockwell Hardness Scales are standardized ways of quantifying hardness. The Rockwell Hardness Test applies a minor load followed by a major load to the material using a standardized indenter and compares the depths of penetration caused by the loads. Different Rockwell scales exist for different indenter types and loads, and the scale used is chosen depending on the hardness of the material to be tested.
The Rockwell test is commonly used because of its convenience and because it inflicts minimum damage to the material, but other hardness tests, such as the Brinell and the Vickers test, exist. Rockwell hardness numbers can be converted to Brinell or Vickers numbers using a table. Yield strength can be approximated as one third of the Vickers hardness number. The approximate ultimate tensile strength can also be estimated from the hardness number using tabulated values.

3. Experimental Procedure
Heat Treatment
The materials used were AISI 1018 carbon steel and AISI 1045 carbon steel. The equipment consisted of an environmental-control furnace, tongs, and a container filled with water at room temperature (approximately 25°C). The samples were heated to 850°C in the furnace, then held at that temperature for one hour. The samples were retrieved with the tongs and, within three seconds, were plunged into the water for quenching. Afterwards, the samples were beadblasted to remove the carbon buildup, then tempered for one hour in an oven at 400°C. When the tempering was completed, the samples were air-cooled to room temperature.
Indentation Hardness
The procedure used was the Rockwell hardness test (ASTM E 18). The materials used were heattreated and non-heat-treated AISI 1018, and heat-treated and non-heat-treated AISI 1045. 46.5-B, 93.8-B,
26.4-C, and 62.4-C calibration test blocks were also used. The equipment used was a Rockwell hardness tester. For the first set of tests, the tester’s B-scale was used, which required a 1/16 in. (1.588 mm) ball indenter tip and two weights that resulted in a load of 100 kg f
. The second set of tests used the C-scale, with a diamond tip and three weights that yielded a load of 150 kg f
.
To set up the tester, the appropriate indenter tip was inserted and screwed into place. Weights were placed on the lever mechanism to provide the desired loading. To begin testing with the B-scale, the tester was calibrated using two different test blocks (46.5 and 93.8). Each block was placed on the anvil, then raised to just below the indenter’s level by turning the capstan hand-wheel in the counter-clockwise direction. The specimen’s position was adjusted so that previous indentations in the material were at least three indentation diameters away from the point to be tested. The hand-wheel was turned until the tester’s small gauge reached the red dot that indicated the presence of a minor load (10 kg f
), then the bezel of the large gauge was rotated to set the dial to zero. The crank handle was pushed slightly so that it moved on its own; this motion released the weights that applied the major load to the specimen. When the handle stopped rotating, it was pulled forward again to its starting position (at which there is no major load). The
Rockwell number corresponding to the B-scale was then read and recorded. The hand-wheel was turned in the clockwise direction to remove the minor loading.
This procedure was done three times for each calibration block, then the average of the three trials was computed. When calibration was completed, the same three-trial procedure was repeated for each of the four steel samples. The entire process (involving both the calibration and the actual tests) was then repeated in the C-scale, after the indenter tip and weights were appropriately adjusted.

4. Results
Rockwell Hardness Test (B-Scale)
Sample Test 1 Test 2 Test 3
Average of
3 Tests
Standard
Deviation
Deviation from
Calibration
Standard
Calibration
Test Block
46.5
Calibration
Test Block
93.8
AISI 1018
NHT
AISI 1018 HT
AISI 1045
NHT
AISI 1045 HT
53.0
95.0
83.0
110.0
92.0
111.0
52.0
95.0
82.0
108.0
91.5
111.0
52.5
95.0
85.0
107.0
92.0
110.0
52.50
95.00
83.33
108.33
91.83
110.76
0.5
0.0
1.2
1.2
0.2
0.5
-6.0
-1.2
N/A
N/A
N/A
N/A
AISI 1018 showed a 30.0% increase in hardness after heat treatment, while AISI 1045 showed a
20.6% increase in hardness after heat treatment. The formula for calculating increase in hardness is:
%
 (
 1)  100% where HT stands for “heat-treated” and NHT stands for “non-heat-treated.”


Rockwell Hardness Test (C-Scale)
Sample Test 1 Test 2 Test 3
Average of
3 Tests
Standard
Deviation
Deviation from
Calibration
Standard
Calibration
Test Block
26.4
Calibration
Test Block
62.4
AISI 1018
NHT
AISI 1018 HT
AISI 1045
NHT
AISI 1045 HT
26.5
62.5
-5.0
31.0
7.0
30.0
26.5
63.0
-3.0
29.0
12.0
33.0
26.0
63.0
0.0
28.0
10.0
34.0
26.33
62.83
-2.67
29.33
9.67
32.33
0.2
0.2
2.1
1.2
2.1
1.7
0.1
-0.4
N/A
N/A
N/A
N/A
AISI 1018 showed a 1198.5% increase in hardness after heat treatment, while AISI 1045 showed a 234.33% increase in hardness after heat treatment.
The formula to calculate the percent increase in AISI 1045 is the same as the formula previously used for the B-scale tests. However, to accommodate for the negative starting value of AISI 1018, this formula was used instead:
Sample
AISI 1045 NHT B-scale
AISI 1045 HT B-scale
AISI 1045 NHT C-scale
AISI 1045 HT C-scale
AISI 1018 NHT B-scale
AISI 1018 HT B-scale
AISI 1018 NHT C-scale
AISI 1018 HT C-scale
%


100 %
Average Rockwell
Hardness
91.83
110.67
9.67
32.33
83.33
108.33
-2.67
29.33
Approximate
Tensile
Strength
(MPa)
635
1010
550
950
H v
(MPa)
1912.95
3119.58
1559.79
2962.62
Approximate
Yield Strength in Tension
(MPa)
637.65
1039.86
519.93
987.54
* Empty entries are a result of inappropriately converted hardness values.
Yield strength formula:

5. Discussion
In this experiment, we observed that steel with higher carbon content has a correspondingly higher hardness value. Our experiment also demonstrated that heat treatment of steel resulted in a marked increase in hardness.
The results of our experiment agree with the theoretical model of steel’s atomic behavior. The introduction of carbon changes the crystal structure of iron. One result is that higher carbon content introduces more dislocations, preventing atoms from sliding past one another, increasing iron’s resistance to deformation. Using the Equilibrium Phase Diagram (EPD), the crystal structure can be inferred based on the carbon composition. While both samples are hypoeutectoid, the diagram shows the difference in composition is significant. Thus, AISI 1018 is expected to be composed of ferrite and austenite, while AISI 1045 is composed of primarily austenite near the eutectoid temperature of 727 o C.
With increasing composition of carbon, the prevalence of cementite, a comparatively harder alloy of steel, increases. The additional cementite distributed between grains of ferrite, austenite, and pearlite increases the alloys’ resistance to material deformation. Thus, we expect AISI 1018 to have lower hardness readings than AISI 1045, which our data substantiates. Carbon content also affects the crystal structure of heat-treated iron. When steel is cooled quasi-statically, carbon atoms diffuse, resulting in softer, ductile alloys (such as austenite and ferrite), as is displayed in the Fe-C EPD. However, quenching austenite, during which carbon does not have time to diffuse, allows the formation of additional harder alloys. With the quasi-static constraint lost, the EPD becomes invalid, and the Time-Temperature-
Transformation diagram (TTT) is used instead. Quenching gives rise to two alloys of carbon that cannot be created quasi-statically: bainite and martensite. Due to the order of magnitude of time on the TTT diagram, we expect to find martensite in higher concentration; martensite is the hardest phase of iron, due to its body-centered tetragonal (BCT) crystal structure. Because martensite requires carbon to form, we expect both samples to form martensite, thus increasing in hardness. The data showed that heat-treated
1045 experienced a 234.33% increase in hardness, while 1018 experienced a 1198.5% increase, although it was still softer than 1045, which shows that harder steel can be obtained by increasing carbon content.
Because both tensile strength and hardness are indicators of a material’s resistance to plastic deformation, hardness values can approximate tensile strength and yield strength in tension. Using the original Rockwell hardness scale as a metric, the relative magnitudes of tensile and yield strength is consistent with our expectations.
Understanding the causes and effects of changes in material properties is a topic of particular importance in today’s society. Due to the ubiquity of steel as construction material in bridges, automobiles, and medical tools, the material properties of steel need to be well-understood. However, this distinction extends not only to steel, but also to all materials that society trusts to behave as designed.

6. Conclusions
The purpose of this experiment was to investigate the effect of microstructure changes on the indentation hardness of low-carbon and medium-carbon steel. The data obtained in this experiment succeeded in demonstrating that higher carbon content in steel results in more hardness. Furthermore, the data shows that heat-treated steel has a significant increase in hardness over its non-heat-treated counterpart, which verifies that heat treatment of steel also has a significant effect on its material properties. In this study, only one material property (hardness) was examined for one alteration of the sample (heat treatment). In future experiments, a greater understanding of how processes change the microstructure of material, thus altering the material properties, can be ascertained by conducting tests on a more comprehensive list of processes and properties.
7. References
1.
K. Komvopoulos, Mechanical Testing of Engineering Materials , 2 nd edn (San Diego, CA:
University Readers, 2011).
2.
N. E. Dowling, Mechanical Behavior of Materials , 3rd edn (Upper Saddle River, NJ: Prentice
Hall, 2007).
3.
H. Chandler, Heat Treater’s Guide , 2 nd edn (Metals Park, OH: ASM International, 1995).

Item
Abstract and
Introduction
Theory
Experimental
Procedure
Results
Discussion and
Conclusions
Fu,
Amy x
Scott,
Shin x
Shan,
SiSi
Son,
Jennifer
Wen,
Alexander x x x
Item
Abstract and
Introduction
Theory
Experimental
Procedure
Results
Discussion and
Conclusions
Spelling, etc.
Appearance and formatting
TOTAL
Students
Fu, Amy Scott, Shin Shan, SiSi Son, Jennifer Wen, Alexander
% with 3 or 4