04-BS-11 · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-11, Properties of Materials — May 2017. 3 hours, closed-book examination (approved Casio or Sharp calculator only). Any five of the seven questions constitute a complete paper (only the first five in the answer book are marked); all questions are of equal value. All seven questions are solved below for completeness.
Reference texts: Callister & Rethwisch, Materials Science and Engineering: An Introduction, 9th ed. (dislocations and slip / Schmid’s law, mechanical behaviour, ionic crystal geometry and the radius-ratio rule, X-ray diffraction, diffusion and Fick’s laws, polymer molecular weight, ASTM grain size, strengthening and annealing, steel heat treatment and hardenability, casting defects, ceramics, glasses and composites).
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.
All four structures start from the same fully austenitized 0.45% C steel; what differs is the cooling path. Ferrite and pearlite are produced by slow (furnace) cooling: proeutectoid ferrite first rejects from austenite along the grain boundaries, and the remaining austenite, reaching the eutectoid composition, transforms to lamellar pearlite. All martensite requires quenching fast enough (typically water) to miss the nose of the transformation curve entirely and carry the austenite below the $M_s$–$M_f$ range, where it shears diffusionlessly to martensite. All bainite is obtained by austempering: quench rapidly to an intermediate temperature (about 250–550°C, between the pearlite nose and $M_s$) and hold isothermally until transformation is complete, then cool. All spheroidite forms by holding the steel just below the $A_1$ line (≈700°C) for a long time, so the cementite coalesces into rounded globules in a ferrite matrix — the softest, most machinable condition.
In the Jominy test a standard bar (25 mm diameter × 100 mm long) is austenitized and then quenched at one end only by a controlled water jet, so the cooling rate falls off smoothly with distance from that end. After quenching, a flat is ground along the bar and Rockwell C hardness is measured at fixed intervals from the quenched end. Plotting hardness versus Jominy distance gives the hardenability curve. This measures hardenability — the depth to which a steel will harden (how far martensite forms before the slower interior cooling produces softer bainite/pearlite) — which is distinct from maximum hardness (set mainly by carbon content). It lets engineers compare steels and predict the through-hardness of real parts of different section sizes.
Causes. Gas porosity arises chiefly from dissolved gases coming out of solution as the metal freezes: the solubility of hydrogen (and nitrogen/oxygen) drops sharply on solidification, so gas is rejected and trapped as rounded pores. Common sources are hydrogen picked up from moisture — damp moulds or cores, wet tools, humid air, or oxide/oil on the charge — air entrained by turbulent pouring, and gases generated by mould–metal reactions. (Shrinkage cavities are a separate, more angular defect from liquid-to-solid volume contraction.) Detection. Castings are checked non-destructively by radiography (X-ray or gamma-ray, which shows internal voids), ultrasonic inspection, and — for surface-breaking pores — dye-penetrant testing; leak/pressure testing and density (Archimedes) measurements or destructive sectioning confirm internal soundness.
