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EXAMINATION PAPER: ACADEMIC SESSION 2018/2019
Campus Medway
Faculty Engineering & Science
Level 5
Exam Session April/May 2019
MODULE CODE MECH 1069
MODULE TITLE Materials 2
Duration of examination 2 HOURS
Q1
Figure Q1 shows the iron-carbon phase diagram. The following phases are indicated: austenite (γ), ferrite (α), cementite (Fe3C).
(a) What phase is present in a Fe-0.4%C steel at 1000 oC?
[1 Mark]
(b) What phases are present in the same alloy at room temperature?
What is the carbon content of the two phases?
What is the proportion of the two phases?
[6 Marks]
(c) A 0.8% C steel is fully pearlitic at room temperature. What does pearlite consist of? Describe its appearance.
[3 Marks]
(d) Describe the patenting and drawing process by which 0.8%C steel is formed into high strength steel wire. What gives the wire its exceptionally high strength?
[5 Marks]
(e) How does the strength and ductility of normalised steel depend on the carbon content? Explain these relationships
(f) What feature of a phase diagram indicates a good casting alloy and why? What, therefore, is the (approximate) carbon content of Cast Iron?
Figure Q1
ANSWERS(purchase full paper to get all the solutions)
1a)
Austenite phase
1b)
ferrite (α) and cementite (Fe3C) phases are present in the same alloy.
The carbon content of ferrite (α) is 0.022% and the carbon content of cementite (Fe3C) is 0.738%. the proportion of the two phase is 0.76%
1c)
Pearlite is a two phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt.%) and cementite(12.5 wt.%) that occurs in some steels and cast irons.
1d)
0.8%C steel is formed into high strength steel wire by subjected he steel with 0.8%C to patenting treatment before final wire drawing, wherein the patenting treatment is performed by the steps of heating at a temperature in the austenite range above point, cooling of a temperature in the range of below the point and above . At such a cooling rate that does not cross the pearlite transformation starting line in the isothermal transformation diagram, applying plastic deformation in that temperature range with a reduction rate of at least 20% and causing pearlite transformation without heating to the austenite range.
1e)
The ductility of steels varies depending on the types and levels of alloying elements present. An increase in carbon content will increase the strength but decrease the ductility, and vice- versa.
1f)
The features of phase diagram that indicate a good casting alloy is the low melting point, good fluidity , castability, excellent machinability, resistance to deformation and wear resistance properties of a cast iron.
The approximate carbon content of a cast iron is 2.5-4.0%
Q2
(a) A structure is to be fabricated from a steel alloy that has a fracture toughness (KIC) value of 100 MNm-3/2 and a yield strength of 860 MN/m2. The crack size resolution limit of the crack detection apparatus is 3 mm. Assume that the applied (design) stress is one half of the yield stress.
Determine whether a critical crack size to cause fracture for this plate may be detected.
Assume that the crack is an edge (surface) crack
(b) An oil drilling platform is made from structural low carbon steel. The steel oil drilling platform would be highly susceptible to corrosion in a marine environment without protection. The steel gains its protection from a large block of magnesium tethered by electrically conducting cables to the legs of the platform. What is this form of protection called? Explain how this gives corrosion protection to the steel.
[4 Marks]
(c) Explain why the addition of around 13% chromium to steel helps protect it from corrosion. What is the protection provided by chromium?
Why does mild steel not benefit from any similar protection?
(d) What is the role of nickel in stainless steels?
[1 mark]
(e) What is the role of Molybdenum in stainless steels?
(f) Name three advantages and on disadvantage of austenite stainless steels compared to ferritic and martensitic stainless steels.
(g) A 304 stainless steel has the following composition (weight%)
Carbon (C): 0.05
Silicon (Si): 0.9
Manganese (Mn): 1.8
Sulphur (S): 0.015
Phosphorus (P): 0.018
Chromium (Cr): 19.4
Molybdenum (Mo): -
Nickel (Ni): 8.4
Niobium (Nb): -
Nitrogen (N): -
(i) Estimate the Chromium Equivalent (CrE) and Nickel Equivalent (NiE) from the following equations
CrE = Cr + Mo + 1.5 Si + 0.5 Nb
NiE = Ni + 30 C + 0.5 Mn + 25 N
(ii) Then use the Schaeffler diagram (figure Q2) to estimate the percentage of austenite and ferrite in the alloy
Continued on Next Page
Figure Q2
Q3
(a) The 3xxx (Aluminium–Manganese) and 5xxx series (Aluminium– Magnesium) wrought aluminium alloys cannot be strengthened by heat treatment processes. Explain how they get their strength.
Name one application for each of these alloys
(b) Figure Q3(b) shows the Aluminium – Copper phase diagram
Aluminium - 4% Copper (indicated on the diagram) is an alloy that may be strengthened by heat treatment.
Describe in detail the heat treatment process that results in precipitation (age) hardening of the alloy. Explain the changes that occur in the material, and to its microstructure, at each stage of the process.
Figure Q3(b)
Continued Next Page
(c) A typical ageing (hardness) versus time curve for an alloy aged at 130 oC is shown in figure Q3(c).
(i) Explain the shape of the curve by consideration of the changes in the microstructure during the ageing process and the effect these changes have on the motion of dislocations.
(iii) In your answer book reproduce the shape of this curve, and on the same axes sketch the shape of the ageing curve for the same material aged at a higher temperature. Make it very clear on the sketch what the effect of the higher ageing temperature on the ageing response is.
[2 Marks]
Figure Q3 (c)
(d) What is the problem with using aluminium alloys for the external structure of high-speed military aircraft?
(e) Cast aluminium alloys are replacing cast iron for many automotive engine parts. What are the advantages and disadvantages of aluminium alloys compared to cast iron for use as automotive pistons?
Q4
(a) Why is the tensile strength of conventional concrete so low (about 4 MN/m2)?
(b) Describe and explain how high tensile strength cement can be produced by improvements in processing?
(c) Why is it necessary to cool large volumes of poured concrete when it is setting and hardening?
How might you do this in practice?
What would happen if you did not cool the concrete?
(d) Concrete consists of cement and aggregate. Aggregate is cheaper than cement and it therefore makes sense to include as much as possible in the concrete. How is this achieved in practice?
(e) Cement and concrete are generally used under compressive loading conditions. Describe the stages in the compressive failure of these two materials.
Sketch a stress-strain curve for cement and concrete in compression. Indicate on the stress strain curve the regions and points corresponding to the failure stages that you have described.
(f) Why are concrete bridge beams reinforced with pre-stressed tendons?
Describe the method of pre-tensioning tendons.
Q5
(a) What are the five main generic classes of ceramics?
For each generic class give one example of a specific component made from that class
[10 Marks]
(b) Why are ceramics inherently brittle materials?
(c) Why are ceramics stronger in compression than tension?
(d) Why is the three-point bend test preferred over the simple tensile test for measuring the strength of ceramics in tension? The modulus of rupture is determined from the three-point bend test. Why is this different from the tensile strength of the material?
(e) Explain what causes thermal shock in materials and why ceramics are particularly susceptible?
(f) A piece of alumina ceramic (whose properties are given in Table Q5 is heated to progressively higher temperatures (T), and is then dropped into cold water at 20 oC.
Determine the maximum temperature (T) that it can withstand before fracture will occur.
Thermal expansion (ΔL) is given by:
ΔL = LO.α.ΔT
Where:
LO = original length of sample
α = coefficient of thermal expansion
ΔT = change in temperature
Last updated: Jun 23, 2021 11:09 AM
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