21-Mat-A5 Phase Transformations and Thermal Treatment · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2013 — 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. Several questions ask explicitly for essay-format answers, and the marking scheme rewards clarity and organisation, so the 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 has no phase-transformation or heat-treatment question in the classical (TTT/CCT diagram, hardenability, tempering-curve) sense; the syllabus actually examined is deformation, strengthening, creep, fatigue, fracture, toughening, deformation processing and environmental degradation. 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.
The decision in each case turns on the same three questions, and it is worth setting them out before the four answers. How much deformation is required, and can the material supply it? Hot working — above the recrystallisation temperature, conventionally above about $0.5\,T_m$ — offers a flow stress an order of magnitude lower and unlimited ductility, because recovery and recrystallisation restore the structure as fast as the deformation damages it. What tolerance and surface finish are needed? Hot working leaves scale, and thermal contraction on cooling limits dimensional accuracy, so any close-tolerance or bright-finish feature must be produced cold. What final properties are wanted? Cold work leaves the material strain-hardened and anisotropic, which may be exactly what is desired or may have to be removed by annealing.
Process: hot rolling in a universal beam mill. A continuously cast beam blank or bloom is reheated to 1200–1250 °C and passed through a reversing breakdown stand and then through a universal roughing and finishing group, in which horizontal rolls reduce the flanges and vertical rolls reduce the web in the same pass, so that the section is progressively developed to shape. Very large sections are instead fabricated by cutting plate and welding, but the rolled beam is the standard product.
Hot working, decisively. The total reduction from bloom to beam is enormous and could not be achieved cold at any credible force; at 1200 °C the flow stress of a structural steel is perhaps a tenth of its room-temperature value and its ductility is effectively unlimited. Hot working also does metallurgical work that the finished beam needs: it closes the centreline porosity of the cast bloom, breaks up and redistributes the coarse as-cast dendritic structure and its segregation, and — through repeated dynamic and static recrystallisation — replaces a coarse cast grain size with a fine wrought one. Since the beam is to be welded, toughness in and near the heat-affected zone is the governing property, and fine grain size is the only strengthening mechanism that raises strength and lowers the transition temperature at the same time.
No cold working and no separate anneal. Surface finish and tolerance requirements are loose (the beam is a structural section, not a machine part), so there is nothing to gain from a cold pass, and any cold work would be undone by welding anyway. Modern practice replaces the old normalising anneal with thermomechanical controlled processing: the finishing passes are made at a controlled low temperature in the non-recrystallisation range to pancake the austenite grains, followed by accelerated cooling on the run-out table, which produces a fine ferrite–bainite structure directly off the mill. The result is a weldable, low-carbon-equivalent, high-toughness steel to CSA G40.21 350W/350WT or ASTM A709 that needs no further heat treatment. The only post-process thermal operation is a local stress-relief or post-weld heat treatment on the fabricated girder, if the plate thickness and restraint call for it.
Process: hot closed-die (impression-die) forging, followed by trimming, piercing of the eye, and heat treatment. A length of medium-carbon steel bar (about AISI 1045–1080) is induction-heated to roughly 1100–1200 °C, pre-formed by fullering or rolling to distribute the volume between the head and the claw, then struck in a blocker die and a finisher die on a drop hammer or a mechanical press. The flash is trimmed, the eye is pierced and finished on a mandrel, and the part is shot-blasted.
Hot working, again for two independent reasons. First, the shape change is severe and three-dimensional — a round bar must become a head, a neck, an eye and a bifurcated claw — and cold forging that shape would require forces beyond any practical press and would crack the bar long before the die filled. Second, and specific to this product, hot forging produces a continuous grain flow that follows the contour of the part, wrapping around the eye and running out into the claw. Because a hammer is loaded impulsively and repeatedly at exactly those locations, the fracture path is forced to run across the flow lines rather than along them, and impact toughness and fatigue life are markedly better than for the same shape cut from bar (where the flow lines are simply severed by the cutter).
Then heat treatment, which is where the properties actually come from. The forging is normalised to refine the grain size left by the finishing temperature, then austenitised, quenched and tempered to a hardness of roughly 50–55 HRC in the striking face and 40–47 HRC in the claw. In practice the striking face and the claw are hardened locally by induction and the eye region is left soft and tough, so that the head cannot spall a chip (a well-known and serious injury mechanism) but also cannot fracture through the eye. There is no cold-working step; the only cold operation is straightening or coining for dimensional touch-up, and a full anneal would defeat the purpose.
Process: melt extrusion of the profile, or pultrusion if the beam is fibre-reinforced. Pellets are melted and homogenised in a single- or twin-screw extruder, pumped through a profile die that forms the I-, box- or channel section, then pulled through a vacuum calibrator and a cooling bath by a haul-off, and cut to length. For a genuine structural beam the material is normally a fibre-reinforced thermoset instead: in pultrusion, continuous glass rovings and mat are drawn through a resin bath and then through a heated die in which the polyester or vinyl-ester matrix cures, emerging as a fully cured profile of constant section.
The hot/cold vocabulary does not transfer directly, and saying so is part of the answer. Polymers have no dislocations and do not recrystallise, so “hot working” and “cold working” in the metallurgical sense do not apply. The equivalent classification is by temperature relative to the transitions: extrusion is carried out above $T_m$ for a semicrystalline polymer or well above $T_g$ for an amorphous one, which is the analogue of hot working — the material flows viscously at low stress and retains no memory of the deformation once relaxed. Cold forming of polymers (cold drawing, as in Question 6) does exist and does orient the chains, and it is used deliberately in fibres, films and strapping, but it is not how a construction beam is made.
What replaces annealing. Rapid cooling in the calibrator freezes in molecular orientation and thermal stresses that will later cause warping, creep and stress cracking, so extruded profiles are commonly given a controlled slow cool or a post-extrusion annealing soak just below $T_g$ (or below $T_m$ for a semicrystalline grade) to relax orientation and, in a crystalline polymer, to complete and stabilise crystallisation. That is a genuine anneal in the same spirit as the metallic one — a thermal treatment to remove the stored energy of processing — even though the mechanism is chain relaxation rather than recrystallisation. The design caution to state is that polymers creep under sustained load at ambient temperature, so a polymer beam is designed on a long-term (apparent) modulus, not the short-term value.
Process: cold drawing and wall ironing (DWI). Coil of AA3104-H19 aluminium about 0.28 mm thick is lubricated and fed to a cupping press, which blanks discs and deep-draws them into shallow cups in one stroke. Each cup then enters a bodymaker, where a punch drives it at high speed through a redraw die and then through two or three ironing rings of decreasing clearance. Ironing does what deep drawing cannot: it deliberately thins the wall, from about 0.28 mm to roughly 0.10 mm in the sidewall while leaving the base thick, and lengthens the can in proportion. The dome is then formed against a doming punch in the same stroke, the ragged top is trimmed, the can is washed, decorated, necked in a series of dies and flanged; the end is separately stamped from AA5182-H19 and scored.
Cold working throughout, and the choice is deliberate on three counts. Dimensional tolerance and surface finish are extreme — wall thickness must be held to a few micrometres, and the decorated surface must be bright — and neither is achievable with hot metal. Production rate is measured in hundreds of cans per minute per line, which rules out heating. Most importantly, the strain hardening is the product: the can wall is so thin that only heavily cold-worked metal can carry the internal pressure (typically 620 kPa) and the axial column load of stacking. The coil arrives in the H19 (fully strain-hardened) temper, and the ironing adds further work, so the finished wall is stronger than the incoming sheet.
Annealing is conspicuously absent, and the reason is the same. There is no interstage anneal, because annealing would destroy exactly the property the process is generating; the alloy is chosen (a 3xxx manganese-bearing alloy, work-hardening rather than heat-treatable) so that it can accept that much cold work without an intermediate softening step. The nearest thing to a thermal treatment is the decorating and inside-lacquer bake at roughly 200 °C for a few minutes, which is a paint-curing operation but which does recover a small part of the cold work; can designers account for the resulting few per cent loss of strength in the wall-thickness calculation. Earing caused by the crystallographic texture of the rolled sheet is controlled by balancing the rolling and annealing schedule at the mill, before the coil ever reaches the cupping press.
