Question

reasons. a) Why might HSLA steel makers am to have dynamic recrystallisation dung the high temperature rolling period and then to avoid static recrystallisation during the hold period? (1 mark) (1) Why are interstitial free steek relatively sof? (1 mark) b) Why might the folowing products have higher cold fomability than Dual Phase HSLA steek? Give your TRIP assisted steel (1 mark) Ferrite Pearlite HSLA steek. (1 mark) c) What is meant by the Dead Metal Zone in a flow pattem? (1 mark) d) Explain how you might improve tensile strength and/or facture toughness of an alminium alloy by using a technique of severe plastic deformation processing (1 mark) e) Why does the tensile strength of metal increase with increasing strain rate | (2 imiks) f) Explain what work hardening is, in terms of dislocation glide along slip systems? (2 marks)

Answer 1

a (i) Recrystallisation is the collective term applied to those stages in which the crystal orientation is altered through the passage of high angle grain boundaries through the material. During recrystallisation, new grains are nucleated and then grow at the expense of other deformed grains.

At higher strain rates, dynamic recrystallisation is initiated by the growth of the high angle cell boundaries formed by dislocation accumulation. In the industrial hot rolling (high temperature) processes the strain rates are relatively high such that only single peak dynamic recrystallisation is likely to occur, if any.

Static recrystallization is usually described as taking place in two stages: nucleation of new grains, and the growth of these grains at the expense of deformed ones. Some feature of static recrystallisation are:

* A minimum amount of deformation (critical strain) is necessary before static recrystallisation can take place.

* The lower the degree of deformation, the higher the temperature required to initiate static recrystallisation.

a (ii) The term ‘Interstitial Free steel or IF steel’ refers to the fact, that there are no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. IF steels have interstitial free body centered cubic (bcc) ferrite matrix. These steels normally have low yield strength, high plastic strain ratio, high strain rate sensitivity and good formability.

b (i) The Transformation-Induced-Plasticity (TRIP) effect is used for enhancing the formability of cold formable sheet steels.

b (ii) High Strength Low Alloy (HSLA) steels, or more precisely microalloyed low carbon steels, are the classic solution for the automotive industry when high strength and good cold formability is required. By adding small amounts of niobium, titanium, vanadium strength is increased by grain refinement, precipitation, and solid solution hardening without significantly impairing formability. These steels are supplied in different strength levels as hot or cold rolled grades; the annealing after cold rolling might be of the batch annealing or the continuous annealing type.

c With increase in friction, friction on the container wall cause the grid lines to curve. In seeking minimum energy flow movement at the transition to the die is arrested and a dead metal zone is formed.

d There are two major procedures for SPD processing. First, equal-channel angular pressing (ECAP) refers to the repetitive pressing of a metal bar or rod through a die where the sample is constrained within a channel bent through an abrupt angle at, or close to, 90 degrees. Second, high-pressure torsion (HPT) refers to the procedure in which the sample, generally in the form of a thin disk, is subjected to a very high pressure and concurrent torsional straining.

e If you use a low strain rate, the material has time to stretch and will usually neck down(the cross sectional area will reduce) just before breaking. This reduction in cross sectional area creates a weak point that limits the maximum load.

Alternatively, by using a high strain rate, test piece has less time to deform and neck down so you should get a higher measured load.

f The increased dislocation density associated with the bulk deformation of a material is responsible for work hardening; that is, dislocation themselves are obstacles to dislocation motion. Depending on the degree of interaction experienced between moving dislouitions, dislocations can be either soft or hard obstacles.

This does not apply to plastics. In this case, a low strain rate allowing the sample to stretch, causes the polymer strands to align along the load axis and actually increases the strength of the material during the test.