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MMY820S - Mechanical Metallurgy - 1st OPP - JUN 2023 |
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nAmlBIA UnlVERSITY
OF SCIEnCE Ano TECHn OLOGY
FACULTY OF ENGINEERING AND SPATIAL SCIENCES
DEPARTMENTOF MECHANICAL,MINING AND PROCESSENGINEERING
QUALIFICATION:BACHELOROF ENGINEERINGIN METALLURGY
QUALIFICATIONCODE: 0BBMET
LEVEL:8
COURSECODE: MMY820S
COURSENAME: MECHANICALMETALLURGY
SESSION:June 2023
DURATION: 3 HOURS
PAPER:THEORY
MARKS: 100
EXAMINER(S)
FIRST OPPORTUNITY QUESTION PAPER
Prof. Sofya Mitropolskaya
MODERATOR:
Prof JosiasVan der Merwe
INSTRUCTIONS
1. Answer all questions.
2. Read all the questions carefully before answering.
3. Marks for each questions are indicated at the end of each question.
4. Please ensure that your writing is legible, neat and presentable.
PERMISSIBLE MATERIALS
1. Examination paper.
THIS QUESTIONPAPERCONSISTSOF 6 PAGES(Including this front page)
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Question1 [25 marks]
(a) Table Ql shows typical data for strength of copper alloys: pure copper, bronze (a solid
solution of tin or beryllium in copper) and brass (a solid solution of zinc in copper). Rank
the strengthening mechanisms (as indicated in the table) in order of effectiveness.
[2]
Table Ql. Strengthof a selectionof copperalloys.
Alloy
Process route Main strengthening mechanism
Yield
strength
(MPa)
Pure Cu
Cast
None
35
Bronze: Cu + 10% Sn Cast
Solid solution strengthening
200
Brass: Cu + 30% Zn Cast
Solid solution strengthening
90
Brass: Cu + 30% Zn Cold-rolled
Solid solution strengthening+
400
dislocation strengthening
Bronze: Cu + 2% Be Heat treated Precipitation strengthening
1000
(b) The yield strength oy of plain carbon steel is dependent on the grain sized, and the relation
can be described by the equation:
+ k~ Uy= <J0
where Oo and k are material constants. The yield strength of a plain carbon steel is 622 MPa
for a grain size of 180 µm and 663 MPa for a grain size of 22 µm.
(i) Calculate the yield strength of the steel for a grain size of llµm.
[10]
(ii) Explain briefly the physical significance of the Oa constant.
[1]
(iii) What methods can you recommend to ensure grain refinement of a plain carbon steel?
[2]
(c) The critical strength 6c of a plain carbon steel equals 900 MPa. Ultrasonic non-
destructive inspection of an axle made of this steel has revealed a microcrack
S0µm long (2a = S0µm). Is it safe to operate such an axel? Estimate with the aid
of Griffith's criterion:
where o-cis the critical stress required for propagation of the brittle crack;
ys is the energy of the new surface area per unit of area;
Eis Young's modulus
a is a half-length of a critical crack that will propagate spontaneously;
1t = 3,14;
if the following parameters apply:
I Iron
Oc, MPa
I 900
Ys, J/m 2
1,2
E, GPa
205
[10]
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Question2 [25 marks]
(a) Figure Q2-1 reveals stress-strain curves for a selection of engineering alloys. Use the
diagrams to find:
(i) The metal with the lowest yield strength. [1]
(ii) The metal with the highest tensile strength [1]
(iii) The metal with the lowest ductility [1]
800
,.......,_700
ro
Q..
600
........500
C
t) 400
(/)
-(/)
(.I.) ..
300
Cl) 200
100
Drawn
-brass
Annealed
brass
/
Annealed/
copper
00
10 20 30 40
50 60
Strainen (%)
Figure Q2-1. Stress-strain curves for a selection of engineering alloys.
(b) Figure Q2-2 shows an S-N curve for AISI 4340 steel after heat treatment.
(i) What is the endurance limit for this steel? [3]
(ii) If cycled for 100 cycles at an amplitude of 1200 MPa, will it fail? [2]
(iii) If cycled for 100 000 cycles at the amplitude of 1000 MPa, will it fail?[2]
1500
1400
e,a"'.. 1300
1200
c" 1100
oi
"O
1000
a-~. 900
E
<II 800
(/)
(/)
700
in 600
500
400
102
4340 low-alloy steel,
quenched and tempered 425 •c
Stress ratio R "' -1
1Q3
1Q4
1Q5
106
107
1Q8
Number of cycles to failure, N1
Figure Q2-2. Fatigue behaviour of low-alloy steel 4340.
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(c) A newly developed vaccine against COVID-19 is to be transported and stored under
extremely low temperature (minus 70 °q. The vaccine will be administered with the aid
of refrigerating containers. Fig. Q2-3 features the Charpy impact energy of three
candidate steel grades (A, B, and C) in a wide temperature range, from +80°C to -80°C.
(i) Which steel grade should be selected for the refrigerating containers manufacturing (the
operating temperature is as low as minus 70 °C): steel A or steel B or steel C? Briefly
provide reasons.
(10]
(ii) Speculate upon the type of steel suitable to meet the target (lattice type, alloying
system).
[5]
-.
-o'
.QcJ
0
JS 50
">-'
""'
C
<10
QJ
t,
"a.' 20
/
/
/
/
.§
/
>a.
__./
"''-
.ur:;
0
-80
-40
o
Temperature,
•40
0c
.A
B
C
..-ao
Figure Q2-3. Charpy impact energy as a function of
temperature for candidate steel grades A, B, and C
Question3 [SOmarks)
In April 1912, the Royal Mail Ship (RMS) Titanic struck an iceberg and sank off Newfoundland
in less than 3 hours with a loss of over 1,500 people. It was only in 2015 when a piece of the
Titanic hull steel was recovered and Charpy impact tests as well as chemical analysis, fracture
surface and microstructure examination were carried out on test specimens machined from
the sample. A modern hull steel of A!SI1018 grade was also examined for comparison. It is
obvious that Titanic was seriously damaged due to collision with the iceberg. However, why
did the ship sink so quickly after the collision? Being a failure investigator, you should consider
all the data provided and answer the following questions:
(i) Did the Titanic hull steel become brittle at the ice water temperature? What evidence can
you provide with the aid of Figure Q3-1 and Figure Q3-2? Point out and label the regions
of your concern.
[15]
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AISI 1018
163
a
C
C
136
"
",£5 . 108
;,:._
l:!l
u-iai:i:.. 81
0
0
54
t Tr:rnsition
tcmper:1turc
27
0
··100
C
C
0
0
D <c <O
0
Titanic long
0
0
00
0
t I•
0
0
Titanic trans
0
0
0
00
100
200
T<!mpcrature. ~c
Figure Q3-l. Charpy impact energy versus temperature for specimens taken longitudinal and
transverse to the rolling direction of the Titanic's hull steel. Results for a modern steel AISI
1018 are shown for comparison.
(ii) Compare and contrast the chemical composition of the Titanic's hull steel with that of a
modern steel AISl1018 (Table Q3). What is wrong with the chemistry ofTitanic's hull steel?
Discuss possible disadvantages of the steelmaking processes responsible for the poor
chemical analysis of the Titanic's hull steel.
[15]
Table Q3. The compositionof the Titanic's hull steel in weight% and the compositionof a
comparablemodernsteel. Resultsfor a modernsteel,AISI1018, are shownfor comparison.
Element
TitaHic
AISI 10l8
Carbon
Sulfur
Manganese
Phosphorus
Nitrogen
Oxygen
0.21
0.065
0.48
0.027
0.004
0.013
0. 18-0.23
0.05 max
0.6-1.0
0.04 max
0.0026
{iii) Label the regions of your concern in the Titanic's hull steel micrograph (Figure Q3-3 a).
What is wrong with this microstructure? How does it compare and contrast with the
microstructure of modern 1018 steel {Figure Q3-3 b)? Note the magnification. Based upon
the microstructural features, discuss the metallurgical defects leading to failure.
[20]
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Figure Q3-2. The fracture surface (a) of the Titanic's hull steel and (b)the fracture surface of modern
1018 steel after Charpy impact test at o0c.
1------l
20"'" b
1--------<
SOµm
Figure Q3-3. The microstructure (a) of a longitudinal section of the Titanic's hull steel in comparison
with that of modern 1018 steel (b). Polished sections, nital etched .
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