Question 1 (40 marks) (a) Describe the Hall-Petch relationship (2 marks) State t
ID: 2086646 • Letter: Q
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Question 1 (40 marks) (a) Describe the Hall-Petch relationship (2 marks) State the THREE basic requirements that need to be fulfilled to achieve martensite in (3 marks) (b) plain carbon steels (c) Differentiate between the terms hardness and hardenability (2 marks) (d) Provide an advantage and a disadvantage of hot working. (2 marks) (e) For steels, describe briefly the heat treatment methods of full annealing, normalising and (6 marks) (f) Differentiate between stress relaxation and creep in terms of their loading conditions and (4 marks) (g) Describe how the modulus of elasticity varies with temperature for a crystalline, semi- (3 marks) (h) Show how the density of a binary composite may be related to the densities of the (3 marks) (i) Describe briefly how the deformation behaviour of thermoplastics, thermosets and (6 marks) (G) Describe why high strength low alloy (HSLA) steels are intended primarily for structural (3 marks) tempering. the mechanical response of the material crystalline and amorphous thermoplastic polymer individual constituents through the rule of mixture elastomers are related to their microstructures members (k) Describe the phase transformations in a eutectic and a eutectoid reaction (2 marks) Describe why a low carbon steel cannot be flame or induction hardened. Briefly describe one method through which the low carbon steel can be case hardened. (1) (4 marks)Explanation / Answer
Answer:
(a) Hall - Petch Relationship: Yield pressure and Grain size is related by this equation.
sigma {y} = sigma {0} + k_{y}/sqrt{d}
where, sigma {y} = yield stress , sigma {0} = matter stable for the starting stress for dislocation movement (Resistance of the lattice to dislocation motion) , K_{y} = strengthening coefficient ( a constant unique to each material) d = grain size ( diameter) Higher the grain size, more dislocations can pile up, thus with less force dislocations can be moved. This implies large grain size leads to less yield stress and vice versa.
(b) 1. The eutectoid temperature for plain carbon steel is 720 Celsius. To get Martensite structure, the steel should be heated beyond this temperature. 2. In Plain Carbon steels, on rapid cooling (quenching) of the Austenitic form of Iron, the carbon atoms at such high cooling rate do not have enough time to diffuse out of the crystal structure in large quantities to form cementite. As a result, Martensite is formed. 3. The presence of alloying elements reduces the critical cooling rate complete martensite formation. If this critical cooling rate is not achieved then steel will be martensitic in the outer regions since they cool faster. In the inner core the slower cooling rate will give rise to bainite, ferrite, and pearlite.
(c) Hardness: The resistance of a material to plastic deformation by indentation. It also refers to stiffness, or resistance to scratching, abrasion, or cutting. It is a measure of how resistant a material is to various kinds of permanent shape change when a compressive force is applied. Hardenability: is a way to indicate a material's potential to be hardened by heat treatment. Usually for steels, how deep the steel is hardened due to quenching (say) from high temperature. A steel with high hardenability has a low critical cooling rate. This means continuing from the above question's answer, Martensite can be formed at a much lower cooling rate if that steel has higher hardenability.
(d) Hot working - Advantage: During this process, recrystallization of the material is possible, which prevents strain hardening of the material. Thus it provides high ductility and easy workability(less force needed). Disadvantage: Due to thermal effects, oxidation of surface is possible. Thermal contraction and warping can occur due to uneven cooling.
(e): Annealing: Annealing is a process of heating steel (most common) slightly above its critical temperature (723 Celsius). It is then cooled very slowly (furnace cooling). Full Annealing involves heating it 50 Celsius above the critical temperature and maintaining at that temperature for some time. Then it is cooled very slowly inside the furnace itself. Normalizing: It is a heat treatment process for making a material softer. It does not produce uniform material properties of annealing. A material can be normalized by heating it to a specific temperature and then letting the material cool to room temperature outside the furnace. Tempering: Tempering involves preheating previously quenched or normalized steel to a temperature below its critical range, holding it for some specified time. Then cooling it to obtain desired mechanical properties. Tempering is used to reduce the brittleness of quenched steel.
(f) Stress relaxation is the time-dependent decrease of stress in a material under constant strain. Creep is an increase in plastic strain under constant stress. Stress relaxation is a decrease in stress under constant strain.
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