21-Mat-A5 Phase Transformations and Thermal Treatment · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2015 — 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 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.
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 here is a 2.25Cr–1Mo low-alloy steel, used for elevated-temperature pressure parts, which is subject to both modes in service.
Creep testing (ASTM E139). A round tensile specimen with threaded or shouldered ends is placed in a lever-arm frame and dead-weight loaded, so that the load stays constant while the specimen extends. It sits inside a three-zone split furnace controlled to within about $\pm 2$ °C, with thermocouples in contact with the gauge length, and is soaked at temperature for long enough to stabilise before loading. Extension is measured on the gauge length itself by a high-temperature extensometer with rods passing out of the furnace to a linear transducer, and strain is logged continuously for the duration of the test — hours for a screening test, tens of thousands of hours for design data. Two variants matter: a true creep test records strain against time and is usually stopped at a defined strain, while a stress-rupture test is run to failure and reports only the rupture life and the elongation. Because the load is constant, the true stress rises as the section necks; a servo-controlled constant-stress test removes that artefact where the mechanism is under study.
Fatigue testing (ASTM E466). A polished specimen — surface finish matters enormously, since fatigue starts at the surface — is cycled at constant amplitude either in rotating bending (the classical R. R. Moore machine, giving fully reversed loading at $R=-1$) or, more usefully for design, in axial load control on a servo-hydraulic frame where the mean stress and the ratio $R = \sigma_{\min}/\sigma_{\max}$ can be set independently. The specimen is cycled at 10–100 Hz until it separates, and the number of cycles is recorded. Since a single test yields one data point, a whole population of specimens is tested at a series of stress amplitudes, with run-outs (typically $10^{7}$ cycles) recorded as arrows. Crack-growth data, by contrast, come from a different test — a pre-cracked compact-tension specimen cycled under load control with the crack length followed by compliance or potential drop (ASTM E647) — and yield the Paris-law constants used in Question 3.
How the two representations differ. The contrast is instructive and is the point of the comparison. One creep test produces an entire curve — strain against time on linear axes for one combination of stress and temperature — and a family of such curves is needed to cover the design space; the number pulled out for design use is the minimum (secondary) creep rate $\dot{\varepsilon}_{\min}$ or the time to 1 per cent strain or to rupture, and those are then cross-plotted against stress on log–log axes or collapsed onto a Larson–Miller parameter $P = T(C + \log t_r)$. One fatigue test, in contrast, produces a single point, and the curve emerges only from a population; the axes are stress amplitude against $\log N$, the scatter is wide (a factor of five in life at a given stress is unremarkable), so the curve is drawn as a mean with a survival probability attached. Creep data are a continuous time-based record from each specimen, with comparatively modest scatter; fatigue data are statistical and cycle-based.
The part machined from a solid wrought block is expected to have the higher toughness, and by a wide margin for a conventionally pressed-and-sintered powder part. The reasons are microstructural and mostly reduce to one word: porosity.
Residual porosity dominates. A pressed-and-sintered ferrous part typically reaches 85–93 per cent of theoretical density, so 7–15 per cent of its volume is void. Those pores do three things at once. They reduce the load-bearing section, so the true stress is higher than the nominal stress everywhere. They are irregular, angular and interconnected, so each is a stress concentrator with a small root radius — a population of ready-made cracks distributed uniformly through the part. And, most damagingly for toughness, they remove the ligaments over which plastic work would otherwise be done: ductile fracture proceeds by void nucleation, growth and coalescence, and in a porous body the nucleation step is already complete, so the fracture energy collapses. Impact energy and $K_{Ic}$ fall far faster than strength does — a part at 90 per cent density may retain 75 per cent of the tensile strength but only 20–30 per cent of the impact energy. Fatigue endurance falls similarly, because the pores are also crack initiation sites.
Prior-particle boundaries and oxides. Even at full density, a powder part can retain a network of oxide films and inclusions decorating the original particle surfaces. These prior-particle boundaries are a continuous brittle path and give intergranular fracture at low energy. Cleanliness of the powder, and the atmosphere used during sintering, are what determine whether this second defect population is present.
What the wrought block brings. The machined part starts from material that has been hot-worked from an ingot or continuous cast bloom. Working closes the casting porosity, breaks up and disperses the inclusions, destroys the as-cast dendritic segregation, and recrystallises the structure to a fine equiaxed grain size. The result is fully dense, with a fine grain size (Hall–Petch benefit and cleavage-path benefit) and small, well-distributed inclusions. Machining then removes material without changing that structure, apart from a thin work-hardened surface layer.
The qualifications an examiner will look for. Three are worth stating, because the blanket answer is not universally true. First, full-density powder routes change the verdict: hot isostatic pressing, powder forging (used for automotive connecting rods precisely because toughness and fatigue strength matter), and metal injection moulding to above 98 per cent density recover most of the toughness, and HIP'd powder-metallurgy superalloy and tool-steel products are actually tougher than their wrought equivalents because they avoid the coarse segregated carbide network that ingot solidification produces. Second, the wrought part is anisotropic: rolling and forging align the inclusions, so the short-transverse toughness of a thick plate can be half the longitudinal value, and if the part is machined so that the crack plane lies parallel to the rolling plane the advantage narrows. Third, machining can introduce its own damage — tensile residual stresses, surface tearing, grinding burn, hydrogen from a subsequent plating operation — whereas a net-shape powder part avoids cutting the grain flow at all, which is why a forged (rather than machined) part is the toughest option of the three.
Summarising: for the same steel composition and heat treatment, the ranking in toughness is forged > machined from wrought bar > full-density powder route (HIP or powder forging) > conventional pressed-and-sintered powder part, and the gap between the last two is far larger than any of the others.
