ENGINEERING MATERIALS LAB REPORT EXPERIMENT 1 Report Title
:
Annealing Test
Subject
:
UEME 2133 Engineering Materials
Course
:
Bachelor of Engineering (Hons) Chemical Engineering
Name of lecturer
:
Ms. Ng Tan Ching
Date of Experiment :
16/06/2016
Name of Student
Student ID No
Year and Semester
Goh Xin Ying
1407419
Y2S3
Lim Lee Shyan
1302230
Y2S3
Ng Sin Long
1406379
Y2S3
Toh Ler Kang
1500127
Y2S2
Wong Yee Foong
1304402
Y2S3
ANNEALING TEST OBJECTIVE
To determine the hardness of a carbon steel and an alloy steel per annealing time.
INTRODUCTION
Hardenability is a capacity ability of alloy to change its microstructure to martesite as a consequence of given heat treatment. It is also a qualitative steel property which indicates the depth of steel at a certain hardness can be achieved during quenching. With the annealing test, a disc specimen is heated up to 900°C which is austenite range at different times in furnace (10 minutes and 20 minutes) to undergo austenitization. Austenitization is the process of heat iron based metal from pearlite or ferrite to austenite. The austenitized specimen is then dipped into quenching medium such as water at room temperature to allow rapid cooling. During quenching process, the cooling rate on the surface of specimen is maximum leads to formation of martensite. However, this is impossible to cool the specimen at uniform rate throughout the quenching process. Surface of specimen is cooled more rapidly than the interior of specimen. Therefore, austenite will transform over a range of temperature. This might form variation of microstructure and properties on the position or depth within the specimen (Lpsindia.com, n.d.). The hardness is increased with the amount of martensite and austenite of microstructure in steel. Martensite microstructure is the hardest and strongest structure but it is also the most brittle.
The presence of alloying elements (nickel, chromium and molybdenum) delays the pearlite to austenite reaction therefore increases the ha rdenability. The grain size of austenite and carbon content will also affect the hardenability (Mehran Maalekian, 2007). Besides, delay of alloying materials leads to longer formation time for austenite. (Rose-hulman.edu, n. d.).
EQUIPMENT AND MATERIALS
1. Furnace 2. Rockwell Hardness Tester 3. Grinder/Polisher Machine 4. Metallurgical Microscope 5. Sand Paper (4 Grade) 6. Carbon steels AISI 1191 7. Alloy steels AISI 7225 8. Etchant solution (Ethanol 97 mL + Nitric acid 3 m L)
PROCEDURE
1. The furnace temperature is heated up to 900°C. 2. Once the furnace temperature is stabilized at 900°C, all 4 steel specimens are put into the furnace. 3. A specimen for zero hour is spared as a comparison. 4. The furnace door has to be properly closed.
5. The first sample is taken out for both compositions after 10 minutes heating. The samples are then quenching into water. 6. Once the samples are cold, the samples are grinded and polished until it is clean and flat. 7. Rockwell hardness test is performed on the polished surface randoml y. 8. The samples are then dipped into the etchant solution for 10 to 15 minutes before micrographic is taken. 9. Steps 1 to 8 are repeated for annealing time for 20 minutes.
RESULTS Table 1: Rockwell Hardness (HRC) Reading at Different Time.
Time
Rockwell Hardness (HRC)
(min)
Alloy Steel
Carbon Steel
1
21.21
1
8.91
2
20.09
2
8.63
3
26.19
3
9.07
1
58.69
1
60.58
2
55.93
2
60.16
3
58.44
3
58.44
1
53.22
1
52.15
2
53.28
2
51.71
3
52.94
3
52.11
0
10
20
22.50
57.69
53.15
8.81
60.03
51.99
Graph 1: The Hardness reading of Alloy Steel and Carbon Steel at Different Time.
A) Micrograph for 0 Minutes Annealing Time Carbon Steel AISI 1191
Figure 1
Alloy Steel AISI 7225
Figure 2
B) Micrograph for 10 Minutes Annealing Time Carbon Steel AISI 1191
Figure 3
Alloy Steel AISI 7225
Figure 4
C) Micrograph for 20 Minutes Annealing Time Carbon Steel AISI 1191
Figure 5
Alloy Steel AISI 7225
Figure 6
DISCUSSIONS
In this experiment, two types of materials which is carbon steels (AISI 1191) and alloy steels (AISI 7225) were used to determine the hardness. Each type of material has a total of 3 sets in which the furnace was heated up to 900°C at 0 minute, 10 minutes and 20 minutes. The 3 sets of different annealing time for carbon and alloy were used to make comparison based on the hardness of each material. The comparison methods were done by Rockwell hardness test and by analysing the microstructure through the micrograph taken. After annealing test, the heated materials was quenched immediately in the water after taking out from the furnace in order to ensure the maximum rapid cooling rate. By quenching process, martensite arrangement was formed because the carbon atoms unable to form cementite or iron carbide and the atoms were trapped within a “frozen” austenite structure. Due to difficult movement of dislocation, the quenched material became extremely hard and brittle (Annealing, hot working and quenching, 2016). Carbon steel and alloy steel consist of different atom arrangement forms. Hardenability is commonly measured as the distance below a quenched surface in which the metal exhibits specific percentage of martensite in the microstructure (Bocchini, G, 2004). The level of hardenability of the element depends on the different arrangements in atom structure. Alloy steel capable of forming martensite when quenched. Alloy steel has the higher hardenability because it consists of manganese, nickel and other elements which can increase the hardenability of the material. Based on the results in Table 1, both alloy and carbon steel have the highest Rockwell hardness readings at 10 minutes of heating. Regardless of the time used, the Rockwell hardness
readings for alloy steel are always higher than carbon steel. Carbon steel is a plain steel that contains only carbon and iron, while alloy steel contains elements other than carbon and iron. For example, manganese, silicon, boron, chromium, vanadium and nickel (Olivia, 2011). These impurities in alloy steel will undergo dislocation, thus distort the arrangement of atoms. The lattice strain field interaction between dislocation and impurity atoms is to restrict the dislocation motion, increasing its hardness (Callister, W. D. & Rethwisch, D. G., 2014). Refer to Graph 1, 0 minute annealing showed that alloy steel has a higher HRC reading than carbon steel. The purpose of doing annealing test in 0 minutes is to act as a control. As we can observe from the micrograph obtained (Figure 1 &2), there is a lot of scratching line shape on both material surface but no nuclei formed. The dark pattern area indicates that the microstructure is inhomogeneous with accumulating dislocation density concentrated at grain boundaries. This proved that there are no dislocations to be found since both materials did the least cold work. Therefore, the hardness readings of both materials at 0 minute are the lowest in the graph. Refer to Graph 1, both materials (carbon and alloy) have the highest hardness reading (HRC) at 10 minutes of annealing test. It is because the process of recrystallization is taking place in both sample at that moments. Therefore, it allows the grain size to grow and relieves most of the residual stress in the materials. Indeed, the hardness and toughness of both of the materials are improved. In contrast with the result for 0 minutes annealing, carbon steel has a higher Rockwell hardness than alloy steel in 10 minutes. This is because recrystallization always occurs faster in pure metal than alloys. The motion of grain boundary occurs as there is growth of new nuclei
during recrystallization. (Annealing-Recrystallization, 2016). Thus, the impurity atom more likely to segregate and interact with recrystallized grain boundaries to reduce their mobility and effectively cancel out most of the strain which surrounds a dislocation (PLAYING WITH PROPERTIES, 2016). This is due to the formation of a new set of strain-free and equiaxed grains that have low dislocation densities and the occurrence of precold-worked condition (Annealing-Recrystallization, 2016).
Based on carbon steel in Figure 3, there is annihilation of dislocation with most of the dark area clearing off. Grain elongation is observed indicating the recovery of deformed grain. Besides, the microstructure in Figure 3 is having a ferrite-austenite duplex phase. The annealing process affects the spatial distribution of ferrite at grain boundaries due to oxidation at metal surface. Normalizing also produces a uniform fined grain structure of ferrite and pearlite with large grain size. (Industrial Engineering Letters, 2014). In figure 4, there is some formation of fine grains as compared to Figure 2. This showed that the nuclei is formed during recrystallization. For 20 minutes of annealing process, the hardness readings for both steel are dropped, which are lower than 10 minutes. In this stage, both of the materials undergo further recrystallization and grain growth. The grain growth might decrease the strength and hardness of materials. The hardness of the materials reduces due to the longer heating time. Based on Figure 5 and 6 , the microstructure becomes courser in this stage. The oxidation of the materials resulting the surface of both materials becomes rough, thus we can conclude that the longer the time for heating, the more the oxidation possibility of the material. (Annealing-Recrystallization, 2016).
The
formation of new grains keep increasing while the new grains that are formed from small nuclei grow and restore its mechanical properties (Callister, W. D. & Rethwisch, D. G., 2014).
The purpose of dipping the samples into the etchant solution (97ml of ethanol and 3ml of nitric acid) is to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal (Black Church Print Studio, 2016). After dipping the etchant solution, the surface of samples becomes muddy, therefore a clearer view of the micro-structures under microscope. There are some possible error sources occurred in this experiment. First, the temperature of furnace may decrease to the surrounding when we insert or take out the specimens. This will cause the heating temperature to change in order to reach the original temperature which is in the furnace. Second, the cooling rate for the specimens during quenching process is different because outer surface of specimen which exposed to water undergoes direct heat transfer (convection) while inner part of specimen undergoes indirect heat transfer by conduction from outer surface with higher temperature. Third, the water supply pipe was malfunction during the grinding process, thus we poured water on the sandpaper by hands. This may affect the performance of polishing whereby leading to overheating of specimens due to friction. Fourth, when tested with Rockwell hardness test, the specimen surface which is uneven and attached with foreign materials may affect the result. Fifth, the micrograph can be affected when the specimen is not handling carefully. Based on the possible errors that might occur in our experiment, we took few precaution steps to minimize the errors. The furnace door should be closed properly and quickly after inserting or taking out the specimens. Small round specimens is used in this experiment to ensure constant cooling rate throughout the specimens. Make sure the water is poured in the constant rate during the grinding process in order to polish the surface of specimens. The specimens must be grinded until it is a flat surface before testing in Rockwell hardness test. The specimens have
to handle with care after dipping in the etchant solution so that a better micrograph image can be obtained. Apart from that, extra care must be given when using the Rockwell Hardness Tester. Make sure the specimen surfaces are maintain in a clean state (Iowa State University of Science and Technology, 2016). The thickness of specimens should be at least 8 times larger than the indenter penetration depth (Iowa State University of Science and Technology, 2016). The scale is turned down slowly until it reaches a desired position. The elevation handle must be turned into anti-clockwise direction after the hardness reading is taken to prevent the indenter from damage.
CONCLUSION
Before annealing, the hardness of alloy steel and carbon steel are 22.50HRC and 8.81HRC respectively. After annealing for 10 minutes, the hardness for alloy steel and carbon steel has increased to 57.69 HRC and 60.03 HRC respectively. The hardness for alloy steel and carbon steel has decreased to 53.15 HRC and 51.99 HRC respectively after annealing for 20 minutes. The results show that annealing heat treatment has changed the microstructure of samples and affect their hardness.
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