ENG 364 Materials Engineering Laboratory
PRAC 2 – Hardening & Impact Testing.
Aim
1. The purpose of this practical is to determine the effect of various hardening treatments and crystal structure on the material properties on toughness.
2. The metals considered will be steel (BCC) and aluminium (FCC).
3. The material properties considered will be the Vickers hardness and the impact energy values.
Specifications:
You must read the following Australian Standard before coming to the class. The standard can be accessed by going to the CDU li
ary link and using your student credentials for login. Only two students/staff can be logged on at the same time. Logout as soon as you have finished.
AS 1544.1 – 2003: Methods for impact testing on metals, part 2: Charpy.
AS 1544.4 – 2003: Methods for impact testing on metals, part 4: Cali
ation of the testing machine.
AS 1544.5 – 2003: Methods for impact testing on metals, part 5: Assessment of fracture surface appearance of steel.
AS 1817.1 – 2003: Metallic materials - Vickers hardness test - Test methods (ISO 6507-1:1997, MOD)
Introduction
Medium and high ca
on steels can be hardened by heating the steel to form austenite and then quenched in oil or water to form martensite. This martensitic steel can be very
ittle so it is often tempered to increase the toughness.
Solution treatment about 540C for 1 hour and followed by quenching is used on aluminium. The solution treated aluminium is then age hardened at about 175C for 8 hours to harden the aluminium.
Impact testing
A typical impact testing machine (for charpy and izod)is shown over the page. The pendulum has a striking velocity of 3 to 4 m/s and the standard Izod test piece is 10 mm x 10 mm square with a machined notch. The notch has a 45 degree included angle, is 2mm deep with 0.25mm root radius. The standard test set up is shown over the page too.
The energy abso
ed on impact by the specimen is an indication of the toughness of the material. You will have to cali
ate the impact pendulum before the test.
Callister W.D. Jr., Materials Science and Engineering : An Introduction , John Wiley and Sons, 5th ed., 2000
Vickers hardness testing
The Vickers hardness number HV is given by:
2
854
1
d
P
.
HV
=
Where
1. P = Load (kg)
2. d = arithmetic mean of the two diagonals of the indentation (mm)
The diamond pyramid hardness test has the advantage over the Brinell test (ball indenter) in that the hardness numbers are independent of the load used because all impressions made with the pyramidal diamond are geometrically similar. Select the appropriate force range on the testing machine.
The hardness testing machine is likely to provide a digital readout of the hardness values.
http:
en.wikipedia.org/wiki/Vickers_hardness_test
Procedure:
We will test different specimens: some heat treated and others with a different crystal structure. These samples will vary depending on availability. So please listen to your lecturer carefully.
We will determine the Charpy impact and Vickers hardness VHN values for each of these.
Charpy
Safety
WARNING - EXTREME CAUTION IS TO BE OBSERVED AT ALL TIMES. No student is to stand in front of the Pendulum at any time. All students must wear safety glasses.
Cali
ation
a. Energy Losses
In the impact tests, allowance must be made for energy losses due to bearing friction in the pointer (indicator), air drag on the pendulum, and bearing friction of the pendulum. Hence it is necessary to determine these energy losses so that co
ections can be applied to the impact test results.
. Pointer Bearing Friction
Set the pointer at 150 [J] and then release the pendulum as under normal test conditions but without any specimen in the anvil. Record the indicator reading. Leaving the pointer in this recorded position (should be near 0), again release the pendulum. The distance by which the pointer is displaced from the recorded position is a measure of the energy loss (joules) due to bearing friction in the pointer.
c. Pendulum Air Drag and Bearing Friction
Leave the pointer in the last recorded position. Again release the pendulum from the suspended position and allow it to make five to-and-fro oscillations. Then move the pointer so that on the eleventh swing the pendulum moves it forward not more than one degree (so that the energy loss from the Pointer bearing friction can be minimized to be disregarded).
The amount of displacement from the previous recorded position divided by eleven is a measure -of the energy loss (joules) due to air drag and bearing friction.
Determination of Charpy Impact Values
Ca
y out the impact test for each of the four specimens and record:
(a) energy abso
ed in
eaking the specimens (J)
(b) type of fracture (you may wish to use the stereomicroscope)
Vickers Hardness
Before ca
ying out the hardness tests the samples should be sanded with 400 grit silicon ca
ide paper. Best practice would then be to etch the parts to remove any deformation layer, however, we won’t do this today.
1. Place your first specimen in the Vickers hardness testing machine. Determine the hardness with a suitable load, at three different locations, measuring the diameter of both diagonals each time.
2. Repeat for other samples.
Report: Not more than 4 pages
The report should contain:
· Results, charts, discussion including comparison to theory, predictions and e
o
· Conclusions and suggestions for improvement
· Answer to the following questions
You must answer the following questions.
1. Can the toughness be changed by heat treatment? Why?
2. Can temperature of use affect the material toughness? Why?
3. How is toughness important to improve safety in structures and components?
4. Discuss the relationship and possible compromises between hardness, strength and toughness in the applications of materials.
Marking Scheme
Item
Marks allocated
Results, charts
25
Conclusions and suggestions for improvement
15
Answers to questions
60
Total
100
d
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Material
Hardness
Test 1
Test 2
Test 3
Average
Steel
(As-Received)
232
226
227
228
Steel
(As-Quenched)
615
616
628
620
Aluminium
77.8
78.3
79.9
78.7
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