Reference no: EM131022172

QUESTION 1:

a) Distinguish between rimmed, capped, semi-killed, and killed steel in terms oftheir deoxidisation treatment and quality.

b) What are the major differences between killed and rimmed steels?

c) Explain the effect of ingot segregation on the properties of rolled plate andsections.

d) Why are as-cast structures anisotropic? What is the effect of anisotropicstructures on the properties of materials? How can the effects of anisotropybe reduced or removed from as-cast components?

e) Briefly explain the AISI-SAE system of alloy steel designation. In youranswer, also explain what the numbers 30 in 1030 signify about that particularsteel?

f) Why do alloy steels have greater hardenability than plain carbon steels?

QUESTION 2:

a) Explain the following strengthening mechanisms - work (or strain) hardening,solid solution strengthening, and precipitation hardening.

b) Outline the advantages associated with using conventional quenching andtempering of steels as a mechanism to strengthen steels.

c) Explain what is meant by the term ‘secondary hardening' in tool steels.

d) Indicate three (3) properties of tool steels that are important in the majority ofapplications in which these steels are used.

e) Making reference to an appropriate TTT diagram, what is meant by the terms‘ausforming', austempering and martempering?

f) Draw a creep curve (strain vs. time) and clearly identify Stages 1, 2 and 3 ofcreep behaviour on the creep curve. (ii) Explain the importance of the three(3) stages to engineering practice.

g) List four (4) factors that are important in selecting creep resistant alloys.

QUESTION 3:

a) Indicate, and explain the effects of, two (2) important characteristics of oxidefilms if the oxide is to form an effective barrier to further oxidation of a basemetal.

b) Name, and indicate, the principal characteristics of the three (3) main classesof stainless steel. In your answer, indicate where each type of stainless steelcan be used.

c) Briefly describe the process of erosion corrosion and indicate severalmethods of preventing erosion corrosion.

d) Using a neat sketch to illustrate your answer, describe the type of corrosionknown as galvanic corrosion. In your answer, clearly show anode and cathoderegions and reactions, and the flow of electrons (or current).

e) Briefly explain the Galvanic series (of alloys) and how engineers use thisseries in practical situations.

QUESTION 4:

a) Briefly describe the surface hardening treatment of carburising. Indicate thetype of metals most suitable for surface hardening in this manner.

b) Describe the surface hardening treatment of nitriding. Indicate the type ofmetals most suitable for surface hardening in this manner.

c) Compare the relative merits of pack carburising with nitriding as surfacehardening treatments.

d) Some steels are surface hardened by quench and tempering the wholecomponent, then re-heating and quenching just the surface layers of thatcomponent. Why do some steels need to carburised while others can besurface hardened by re-heating and quenching the surface layers?

e) How can the impact strength of a material be determined and why is itimportant?

QUESTION 5:

a) Briefly explain the three Stages - Stage 1, Stage 2 and Stage 3 - involved in atypical fatigue failure.

b) Describe the appearance of a typical fatigue failure.

c) (i) Sketch an S-N curve for a metal that exhibits an endurance (or fatigue)limit. Clearly indicate what is measured on each axis of the graph. (ii) Making reference to the S-N curve, indicate what is meant by endurance(or fatigue) limit and fatigue life.

d) (i) Sketch a log da/dN vs. log ?K curve for a metal. Clearly indicate what ismeasured on each axis of the graph. (ii) Making reference to the log/log curve, explain the significance of the slopeof the curve to crack growth rates.

e) In relation to the fracture toughness of a material, define and explain theconditions: plane stress and plane strain.

f) What is the effect of the following variables on the fracture toughness andfracture appearance of a material: (i) temperature, (ii) severity of notch, (iii)thickness, and (iv) strain rate? (Each section is worth 10 Marks. Total = 40 Marks.)

QUESTION 6:

a) Outline and explain the principal differences between dispersionstrengthening and fibre strengthening of composite materials.

b) Briefly outline the advantages of fibre-reinforced composite materials overother engineering materials.

c) Outline the conditions necessary for underbead cracking to occur in weldedsteels, and indicate the type of material in which this type of cracking is mostlikely to occur.

d) Outline three (3) possible reasons for pre-heating or post-heating a weldedsteel component or section.

e) With the help of a suitable sketch, indicate what microstructural changes mayoccur in the HAZ of cold worked, commercially pure aluminium plate when it iswelded.

QUESTION 7.

a) Three major wear processes are termed: adhesive, abrasive and erosivewear. Briefly explain each of these wear processes.

b) The abrasive wear process includes the mechanisms: micro-ploughing, micro-cutting, micro-fatigue and micro-cracking. Clearly explain these mechanisms.

c) During the process of adhesive wear, a transfer layer is formed. Explain (i)how the transfer layer is formed, and (ii) the significance of its formation to theprocess of wear.

d) In relation to the adhesive wear process, what is surface delamination?

e) Briefly describe the two wear situations termed: two-body and three-bodywear. Indicate what the effect of chemical segregation can be on theproperties of cast materials.