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Instructions to Candidates

 

Answer 4 Questions

Q1

(a)       Dust and dirt collected in structural sections may lead to water entrapment and corrosion

Consider the examples given in Figure Q1(i) and provide simple solutions to this issue which would help prevent the dirt entrapment

 

 

Dirt and dust collection

 

 

Figure Q1(i)

[2 Marks}

 

(b)       Sketch design changes that might be made to the tank indicated in figure Q1 (ii) to help prevent corrosion?

Figure Q1(ii)

[2 Marks]

 

(c)        Table Q1 shows a list of standard electrode potentials for some common metals when immersed in aerated water.

(i)         If nickel (Ni) and copper (Cu) were immersed in water and electrically connected what would occur?

[1 Mark]

(ii)        Why in general is it bad engineering practice to connect two dissimilar metals

[2 Marks]

(iii)       The list of standard electrode potentials shows that aluminium (Al) is below iron (Fe), which indicates that it is more reactive.  Common experience tells us that iron corrodes more severely than aluminium.  Explain this anomaly.

[2 Marks]

(iv)       How does anodising increase aluminium`s corrosion protection in outdoor applications?

[2 Marks]

(v)        The list of electrode potentials indicates that wet corrosion is not possible for gold (Au), platinum (Pt), mercury (Hg) and silver (Ag).  Why is wet corrosion not possible?

[1 Mark]

(vi)       What is immersed current cathodic protection?  Explain it and sketch a diagram to show how a steel pipeline may be protected by such a system.

[4 Marks]

Table Q1

(d)       Stainless iron consists of iron and chromium.  What makes it “stainless” and protects it from corrosion.  How much chromium is needed to achieve this effect?

[3 Marks]

(e)       Figure Q1(iii) shows an iron chromium phase diagram for a 0.6 wt% carbon steel.  With reference to the phase diagram, indicate how you might produce a hardened steel. What phase is present in the hardened steel?

Give an example of a use of an hardenable stainless steel

[3 Marks]

Figure Q1(iii)

(f)        AISI 304 and AISI 306 stainless steels contain nickel in addition to chromium

(i)         What is the purpose of the nickel in these steels ?

(ii)        Outline THREE key properties of austenitic stainless steels

[3 Marks]

SOLUTION

1a)

The simple solution is to avoid entrapped dust and water.

1b)

1ci)

There is a potential difference between them equal to their seperation on the standard reduction potential chart.

 has potential =

Copper has  a potential =

Potential difference between them =

1cii)

It result in bimetallic galvanic corrosion.

1ciii)

 Aluminium forms aluminium oxide which dulls the surfaces of the aluminium but protects it against further oxidation.

When iron forms oxides, the oxides does not form a shield against further oxidation, so the rusting continues, albeit slowly.

1civ)

If aluminum is intended for outdoor use – the soluiton is to artificially thicken the protective oxide film.

1cv)

Wet corrosion is not possible because they are more resistant to interaction with other chemicals, therefore corrosion resistant.

1cvi)

It is a corrosion protection system consisting of sacrificial anodes connected to an external power source. The external power source often a DC power supply, provides the current necessary to drive the electrochemical reaction required for cathodic protection to occur.

1d)

What makes stainless iron stainless is the presence of chromiumin the alloy. And at least 11-13% of chromium is needed.

1e)

Martensite phase is present

Uses

1. Non-rusting ball bearing
2. Surgical scalpels and domestics knives.

1fi)

Nickel enchances its important properties such as formability weldability and ductility.it also increases corrosion resistance in certain application.

Properties

1. It is unstable at room temperature.
2. It has composition of fe – 0.1%C. 1%Mn 18%Cr 8% Ni
3. It has been transformed to ferrite.

Q2

(a)       Figure Q2(i) shows the iron (Fe) rich end of the iron (Fe) – titanium (Ti) phase diagram

Figure Q2(i)

Answer the following questions

 

(i)         What phase (or phases) is present in a Fe – 3 wt% Ti alloy at 1200 ^oC ?

[1 Mark]

(ii)        What is the composition (alloy content) of any phases that are present in a Fe – 3 wt% Ti alloy at 1200 ^oC ?

[1 Mark]

(iii)       If a Fe – 3 wt% Ti alloy is slowly cooled from 1200 ^oC, at what temperature does a transformation occur?  What is the transformation that takes place?

[3 Marks]

(iv)       What phase (or phases) is present in a Fe – 10 wt% Ti alloy at 800 ^oC ?

[2 Marks]

(v)        What are the compositions (alloy content) of any phases that are present in a Fe – 10 wt% Ti alloy at 800 ^oC ?

[3 Marks]

(vi)       Estimate the proportions of any phases that are present in a Fe – 10 wt% Ti alloy at 800 ^oC.

[4 Marks]

(vii)      A phase Fe₂Ti is indicated on the phase diagram

What type of phase is this an example of?

[1 Mark]

            (viii)     Between what two temperatures is austenite present

[2 Marks]

            (ix)       At what temperature does a eutectic point exist ?

[1 Mark]

 

(b)       Figure Q2(ii) indicates the copper rich end of the copper (Cu) - beryllium (Be) phase diagram.

 

1. What is the maximum amount of beryllium that is soluble in the α – Cu phase?

[1 Mark]

2. At what temperature is this achieved?

[1 Mark]

3. What is the maximum solubility of Be in α – Cu at 300 ^oC?

[1 Mark]

4. Copper beryllium alloys are the strongest of all copper alloys (with yield strengths up to 1400 MPa) and are used as springs, for example.

By consideration of the phase diagram (and after choosing a suitable alloy content) suggest how you might heat-treat the alloy to achieve such high strengths.

[4 Marks]

Figure Q2(ii)

Q3

(a)       What loading conditions are necessary for fatigue failures to occur?

[1 Mark]

(b)       Give ONE example, using either a Mechanical Engineering application or a Civil Engineering application, where fatigue might occur, explaining briefly why.

[2 Marks]

(c)        A S-N plot is shown in Fig Q3(i) and indicates typical curves for a number of alloys.

            (i)         What is the fatigue limit of the 1045 steel?

            (ii)        If steel is loaded below its fatigue limit what will happen?

(iii)       What is the fatigue strength of 2014-T6 aluminium at 10⁸ cycles to failure?

(iv)       What is the fatigue lifetime of red brass at a stress amplitude of 150 MPa ?

[4 Marks]

Figure Q3(i)

 

(d)       The fatigue lives from the S-N curve of a certain steel are:

 

Stress (MN/m2)	Fatigue life (cycles)
380	3 x 107
420	5 x 106
460	1 x 105

 

If a component manufactured from this steel is subjected to cyclic loading and undergoes 12 000 000 cycles at 380 MN/m² and 800 000 cycles at 420 MN/m², how many cycles can the material be expected to withstand at 460 MN/m² before fatigue failure occurs?

[5 Marks]

Assume that Miners Law applies, and that failure occurs when:

            nN =  1 

           

(e)       Why is brittle fracture initiated at pre-existing cracks?  What is dangerous about cracks?

 

[2 Mark]

 

(f)        A structural plate (Fig Q3 (ii)) of cross-sectional area 1200 mm² is found to contain an edge crack a of length 24 mm.  The width (W) of the plate is 80 mm.  If the plate was an aluminium alloy of fracture toughness 30 MN/m^3/2 would the design load (P) of 100 kN cause failure.

 

[6 Marks]

            Assume that stress intensity K is given by:

 

                                   

 

            Where t is the thickness of the plate

 

            Y values are given graphically in Figure Q3 (iii)

           

Figure Q3 (ii)

 

Figure Q3 (iii)

 

(g)       Describe the essential features of a Charpy impact test and how the test can be used to indicate whether a material is suitable for service at a particular temperature.

 

What does the term Impact Transition Temperature mean?

Explain why steels have an impact transition temperature and aluminium alloys do not.

[5 Marks]

Q4

(a)       Why is the tensile strength of conventional concrete so low ?

[2 Marks]

(b)       in producing cement and concrete why is it important to add the correct amount of water to the cement mix?

            What is the ideal cement-water ratio?

[4 Marks]

(c)        Why might it take up to 100 days for concrete to achieve its highest strength?

[2 Marks]

(d)       Name one advantage and one disadvantage of heat evolution during concrete setting

[2 Marks]

(e)       Why is the use of Portland cement restricted to developed countries

[3 Marks]

(f)        Why is Portland Cement stronger than Pozzolana Cement

[3 Marks]

(g)       What are the advantages and disadvantages of high alumina cement

[4 Marks]

(h)       Cement and concrete are generally used under compressive loading conditions.  Describe the stages in the compressive failure of these materials.

[5 Marks}

Q5

 

(a)       From an environmental viewpoint, what are the advantages of reducing the weight of cars?

 

[2 Marks]

 

(b)       Table Q5 shows the properties of three candidate materials for car body applications.

 

Table Q5

 

	Density (ρ) Mg/m3	Elastic Modulus (E) GN/m2	Yield Strength (σ) MN/m2
Mild Steel	7.8	210	220
High Strength Low C steel	7.8	210	500
Aluminium Alloy	2.7	70	200

 

The lightest car body panels for a given function (bending stiffness or flexural strength level) is the material with the highest value of (E^1/3/ρ) or (σ^1/2 /ρ) as appropriate.

Rank the materials given in Table Q5 in terms of these stiffness and strength criteria.

 

[5 Marks]

 

(c)        Conventional (ferrite-pearlite) mild steel gets its strength from the amount of pearlite in the microstructure. 

 

1. What is pearlite and what does it consist of?

 

2. Explain why car body panels cannot be produced with higher pearlite contents to increase their strength.

 

[4 Marks]

 

(d)       New weldable low carbon steels have been developed for use as car body panels and other structural applications in mechanical and civil engineering.  High strength low alloy (HSLA) steels are now commonly used. Discuss the hardening/strengthening mechanisms that operate within these steels to give them their high strength.

 

[4 Marks]