21-Mat-A5 Phase Transformations and Thermal Treatment · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2014 — 10-Met-A5, Mechanical Behaviour and Fracture of Materials. Three hours, closed book, any non-communicating calculator permitted. Eight questions of 20 marks each; the rubric states that five questions constitute a complete paper and that only the first five appearing in the answer book are marked. All eight are answered here, because this set is a study resource rather than an exam script. Several questions ask explicitly for essay-format answers, and the marking scheme rewards clarity and organisation, so the discursive answers below are written as structured prose rather than as note form.
The printed exam header reads 10-Met-A5, Mechanical Behaviour and Fracture of Materials. The paper examines strengthening and deformation, creep and fatigue testing, fracture mechanics, toughening of engineering materials, deformation processing selection, and environmental degradation; it has no classical phase-transformation or heat-treatment (TTT/CCT diagram, hardenability, tempering-curve) questions. The answers below are written to the printed subject.
Reference texts. The answers below are keyed to the works normally recommended for this syllabus code:
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Process: hot rolling of a cast billet/bloom through a sequence of universal (H-beam) mill stands into the finished I-profile, typically followed by controlled (accelerated) cooling and, for HSLA structural grades, thermomechanical controlled processing (TMCP) rather than a separate normalising heat treatment. Hot working is the only practical choice for a section this large: the flow stress at rolling temperature (roughly 1100–1250 °C for structural steel) is a small fraction of the room-temperature value, so the very large cross-section reduction from billet to I-section is achievable with equipment of reasonable size and force, and continuous dynamic recrystallisation keeps refining the grain structure and relieving strain energy pass-to-pass rather than allowing strain hardening (and the associated residual stress and reduced weldability) to build up as it would in cold working. Because the beam will be welded into a bridge structure, the finished microstructure must also be weld-friendly: a fine-grained, low-carbon-equivalent, homogeneous ferrite–pearlite (or ferrite–bainite for TMCP grades) structure free of the large residual stresses a cold-formed section would carry, giving good heat-affected-zone toughness and resistance to hydrogen cracking in the weld.
Process: hot (or warm) closed-die forging, typically followed by a light cold coining/sizing operation on the striking face and finished with a quench-and-temper heat treatment. Forging is chosen over machining the head from bar stock because forging aligns the internal grain flow with the head's contour — the metal is pushed to follow the claw and poll geometry rather than being cut across, so the fibre structure runs continuously around the most highly loaded features (the claw roots, the eye) instead of being severed there as it would be if the shape were simply machined from solid, which is exactly what gives a forged head its superior impact toughness and resistance to chipping/cracking in service. The bulk shaping needs hot working because the deformation (round rod to a complex 3-D head shape) is far too severe for cold forming without cracking; the optional cold coining step work-hardens and improves the surface finish and dimensional precision of the striking face specifically, and the subsequent quench-and-temper sets the final hardness gradient — hard, wear-resistant striking face with a tougher core and claw.
Process: cold wire drawing (reducing hot-rolled rod through a series of dies to the final fine wire diameter) followed by cold bending into the clip shape, with at most a very light, low-temperature stress-relief (not a full anneal) afterward. This is one of the few cases where the strain hardening that cold working produces is exactly the desired outcome rather than a side effect to be removed: a paper clip needs to be an elastic spring, recovering its shape after being flexed to insert or remove papers, which requires as high a yield strength relative to modulus as the low-carbon spring-wire steel can be given, and cold drawing is precisely the process that raises yield strength substantially through dislocation multiplication while the wire is reduced to its final gauge. No annealing (or only a very light stress-relief anneal well below the recrystallisation temperature, to stabilise residual stresses and dimensions without softening the wire) is applied, because a full anneal would recrystallise the wire, remove the strain hardening, and destroy the spring action the product depends on.
Process: deep drawing and wall ironing (D&I) of a flat, cold-rolled aluminium-alloy sheet blank (a work-hardenable, non-heat-treatable alloy such as AA3104 for the can body, AA5182 for the easy-open end), entirely a cold-working sequence: the blank is first cup-drawn, then redrawn to a smaller diameter, then pushed through a series of ironing rings that simultaneously thin and elongate the cup wall to its final gauge while the base is left thick to form the can bottom. Cold working is used throughout because the process is a room-temperature, extremely high-speed, thin-sheet forming operation, and the strain hardening it produces is again beneficial rather than incidental: it raises the thin can wall's strength and dent/buckle resistance without adding weight, which is essential for a product sold on minimising material cost per unit. A stabilising bake (a low-temperature anneal well short of recrystallisation, applied during the interior/exterior coating and curing cycle) relieves some residual stress and sets a controlled final temper without fully softening the work-hardened wall, after which the neck and flange are formed by further cold spinning/necking operations to receive the separately formed end.
