MAT4443


CANDIDATES ARE ADVISED TO READ THROUGH THE
INSTRUCTIONS GIVEN ON THIS PAGE CAREFULLY BEFORE
ATTEMPTING ANY QUESTIONS






Spring Semester Exam 2018-2019

METALLURGICAL PROCESSING

Length of Exam: 3 Hours


There are SIX Questions
Attempt FOUR questions ONLY


PLEASE WRITE YOUR ANSWERS IN THE ANSWER BOOK
PROVIDED NOT ON THE EXAM PAPER.

ANYTHING WRITTEN ON THIS PAPER WILL NOT BE CONSIDERED.






Department Of
Materials
Science &
Engineering.

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Attempt FOUR questions ONLY

1. Methods of reducing the inclusion content of steel are of great interest to
steelmakers, because some advanced applications require super clean steel.

(a) Explain the main differences between exogenous and indigenous inclusions
and indicate the possible origin of each type of inclusion. [4 marks]

(b) Why do we need to remove the oxygen from liquid steel prior to solidification?
[2 marks]

(c) Why must aluminium not be used as a deoxidizer in the production of tyre wire
steel? [2 marks]

(d) Why is sulphur generally detrimental to the mechanical properties of steel?
[4 marks]

(e) Describe in detail, using relevant figures and diagrams if necessary, the
vacuum arc remelting (VAR) and electroslag refining (ESR) processes that
could be used to produce clean steel. [6 marks]

(f) In continuous casting, describe how the tundish plays a key role in reducing
inclusion content? [2 marks]

2. The main processing routes for steel production are the basic oxygen steelmaking
(BOS) and electric arc furnace (EAF) processes. Approximately 35% of the world’s
steel is produced using the electric arc furnace route, with oxygen steelmaking
accounting for the rest.

(a) Explain the differences in the two processes with regards to the use of oxygen.
[2 marks]

(b) Phosphorous reversion is a potential problem that can occur in BOS
steelmaking. Describe what the problem is and suggest 2 methods of reducing
the possibility of it happening. [4 marks]

(c) The BOS process is not well suited for removing sulphur from liquid steel.
Explain why this is so and describe two methods of removing Sulphur that are
more effective. [6 marks]

(d) Explain why excess carbon is sometimes added during the EAF process?
[2 marks]

(f) Steel is sometimes described as an infinitely recyclable material, but this can
present some problems, particularly with respect to residual elements. Give 2
examples of a residual element and explain why are they of importance in
electric steelmaking practices. [6 marks]
TURN OVER


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3. This question is about the extrusion process

(a) On the same axes, draw typical pressure displacement curves for direct and
indirect extrusion and explain why the pressure curves are different.
[4 marks]

(b) Sometimes the surface layers of the billet to be extruded contain oxides and
other impurities. Explain why the presence of these oxides in the extruded
product can be undesirable and suggest two methods of avoiding the problem.
[4 marks]

(c) Why is a higher strength metal such as titanium usually extruded at higher
extrusion speeds than lower strength metals such as aluminium?
[2 marks]

(d) It is proposed to use a temperature of 450°C to extrude an aluminium billet of
75mm diameter into a circular rod of 15mm diameter. Calculate the extrusion
pressure required. [6 marks]

data:

flow stress of material at 450°C = 90 MPa
extrusion constant A = 0.8
extrusion constant B = 1.7
indirect extrusion press used

(e) If the capacity of the extrusion press is 2.5 MN, calculate whether the proposed
extrusion temperature in part d) is sensible. Show your working.
[4 marks]



4. Continuous casting is used to produce 95% of the worlds cast steel.

(a) Why does the mould oscillate during continuous casting? [2 marks]

(b) What are the 4 main functions of the mould flux? [4 marks]

(c) Correct choice of mould powder can minimise the formation of defects such
as longitudinal cracks in the cast product. Explain how this defect can originate
and how we can use mould powders to stop their formation.
[4 marks]

Question 4 continued over page - CONTINUED

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(d) Please write the most appropriate answer to each of the following 5
statements. More than one of the options may be true, but may not be the
most appropriate answer.

A. Models utilising cellular automata fall into which general modelling group:

i. Black box
ii. Grey box
iii. White box
iv. Nearly-white box
v. Hybrid
[2 marks]

B. The mode of heating that is most dominant during a high temperature
metal forming operation is:

i. Conduction
ii. Convection
iii. Radiation
iv. Deformational
v. Interfacial
[2 marks]
C. During dynamic recovery:

i. The hardness of the metal increases
ii. The resistivity of the metal increases
iii. There is a release in deformational energy
iv. The density of the metal decreases
v. None of the above
[2 marks]
D. An example of a black box model would be:

i. Artificial neural network
ii. Constitutive equation
iii. Monte-Carlo-Potts
iv. Cellular automata
v. None of the above
[2 marks]
E. Crystallographic texture influences:

i. Recrystallisation
ii. Normal anisotropy
iii. Planar anisotropy
iv. Grain growth
v. All of the above
[2 marks]

TURN OVER


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5. The diagram below illustrates the macroscopic plastic flow behaviour of austenite
under two separate, and dramatically different conditions, designated as either
“High Z” or “Low Z” conditions.



(a) Define the term, ‘Z’. [1 mark]

(b) Define the functional relationship that Z has with processing parameters such
as deformation temperature and strain rate. [3 marks]

(c) Describe how the microstructure at points A, B & C would evolve for the
austenite under “High Z” conditions, and briefly explain why. [6 marks]

(d) Describe how the microstructure at points A, D & E would evolve for the
austenite under “Low Z” conditions, and briefly explain why. [6 marks]

(e) How would the flow behaviour of the “High Z” conditions change, if instead of
austenite, the metal being deformed was aluminium under similar homologous
temperatures? Briefly describe why. [4 marks]




CONTINUED





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6. The vertical forming pressure for plane strain compression can described by:

=
2
√3
[

ℎ
(1 −
2

)]

(a) Describe the significance of the terms in the above expression. [3 marks]

(b) Describe under which conditions the above expression will be at a maximum
and minimum, and illustrate using an appropriate diagram. [5 marks]

(c) Assuming that plane strain compression is conducted on a slab of metal
whose yield strength is 180 MPa, calculate the maximum pressure at the onset
of plastic flow. Assume that the width of the slab is 20.3 cm, the height is 2.5
cm and the average coefficient of friction is 0.10. [6 marks]

(d) Determine the expression for the horizontal forming pressure (q), and
calculate its minimum value. [6 marks]


END OF QUESTION PAPER


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