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.
The material chosen for both tests is a quenched-and-tempered low-alloy steel (AISI 4340) turbine-fastener bolt, a typical component that must be qualified against both damage modes.
Creep test procedure (ASTM E139). A cylindrical specimen with a reduced gauge section is loaded in axial tension inside a furnace held at a fixed elevated temperature (for 4340, a service-representative test might run at 450–550 °C), under a constant dead-weight (lever-arm) load that produces a fixed nominal stress well below the room-temperature yield strength. An extensometer records gauge elongation continuously over the test duration, which can run from hours to many thousands of hours. The output is a plot of strain against time at fixed stress and temperature; the test either runs to rupture (creep-rupture test) or is stopped once the secondary (minimum) creep rate has been established.
Fatigue test procedure (ASTM E466). A smooth, polished hour-glass or dog-bone specimen is subjected to a fully-reversed or pulsating cyclic axial (or rotating-bending) stress at a fixed amplitude and mean stress, typically at room temperature and at a frequency of tens of hertz, and the number of cycles to failure $N_f$ is recorded. The test is repeated at several stress amplitudes on nominally identical specimens (a full test programme needs 8–12 specimens to define a scatter band) to build up a stress–life relationship.
The two curves plot fundamentally different variables against different axes because the damage clocks are different: creep accumulates continuously with time under a fixed load, so strain vs. time at constant stress is the natural representation, with the three classical stages — decelerating primary creep as dislocation density and substructure build up, a roughly constant-rate secondary (minimum-creep-rate) stage that dominates design life, and an accelerating tertiary stage as necking or internal cavitation reduces the load-bearing area, ending in rupture. Fatigue accumulates with cycles, so stress amplitude vs. $\log N_f$ (the S–N or Wöhler curve) is the natural representation; ferrous alloys typically show a fatigue (endurance) limit below which the curve flattens and life becomes effectively infinite, while most non-ferrous alloys (aluminium, for example) continue to decline slowly and are instead characterised by an endurance strength at a stated number of cycles (commonly $10^7$–$10^8$).
The machined-from-solid part is expected to have the higher toughness, essentially always, for the same steel composition and comparable heat treatment.
The reason is porosity. Conventional press-and-sinter powder metallurgy (PM) parts retain residual porosity — typically several percent by volume even after sintering, and more if the part is used in the as-pressed or lightly sintered condition rather than fully densified by hot isostatic pressing (HIP) or powder forging. Each pore acts as a pre-existing internal notch: it locally raises the stress by a stress-concentration factor, provides a ready nucleation site for a void under triaxial tension ahead of any crack tip, and reduces the net load-bearing cross-section. Under monotonic loading the pores lower the tensile ductility and impact energy sharply relative to a fully dense wrought or cast-and-machined part of nominally the same alloy; under cyclic loading they are equally effective fatigue-crack initiation sites, so PM parts also show lower fatigue strength unless post-densified. A machined part, cut from solid wrought or forged bar stock, starts from a material that has already been hot- or cold-worked to full density (and, if forged, has favourable grain flow as well — see Question 7), so it carries none of this population of internal notches.
The comparison is not unconditional. If the PM route is followed by a densification step — hot isostatic pressing, powder forging, or metal-injection-moulding with a full sinter-HIP cycle — residual porosity can be driven down to a fraction of a percent, closing most of the toughness gap and in some cases (very fine, homogeneous PM microstructures free of the segregation and large inclusions that a cast-and-wrought route can carry) even favouring the PM part. PM's real advantage is net-shape economy and the ability to blend elemental powders (or retain fine, uniform carbide distributions) that would be impractical from an ingot; toughness is traded away unless that extra densification step is taken. Absent such a step — the case as posed — the machined part wins on toughness.
