21-Mat-B6 Ceramic Materials · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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.
[Figure not reproduced: Left: polygonal ferrite grains with dark pearlite islands, scale 40 micron. Right: fine acicular martensite, scale 20 micron. See the official exam paper or the cited reference text.]
Left micrograph (ferrite + pearlite). The pearlite area fraction of the printed micrograph is estimated at roughly 0.41–0.52, centred near 0.45–0.48. Treating area fraction as equal to weight fraction and applying the hypoeutectoid lever rule between the ferrite solubility limit ($C_\alpha\approx0.022$ wt% C) and the eutectoid composition (0.77 wt% C): $$C_0=C_\alpha+W_{pearlite}(C_{eutectoid}-C_\alpha)\approx0.022+(0.41\text{–}0.52)(0.748)\approx0.32\text{–}0.41\text{ wt\% C}$$ i.e. approximately 0.35–0.40 wt% C — a mid-low-carbon hypoeutectoid steel, comparable to a normalized AISI 1035–1040-class plain-carbon steel. The coarse, well-formed polygonal ferrite grains (no acicularity) confirm slow, near-equilibrium cooling rather than any accelerated transformation.
Right micrograph (fully acicular martensite). There is no resolvable ferrite or pearlite at all — the entire field is the fine, needle-like (acicular) plate structure characteristic of as-quenched martensite. Achieving a FULLY martensitic structure in a plain-carbon steel (no alloying to boost hardenability) by a practical quench requires both a fast quench AND enough carbon to depress the pearlite-nose transformation-start time comfortably past the achievable cooling rate; this combination is reliably seen only in near-eutectoid to hypereutectoid compositions. Estimated carbon content: approximately 0.8–1.0 wt% C (near-eutectoid to hypereutectoid).
The left specimen's coarse, well-developed polygonal proeutectoid ferrite grains together with clearly resolved lamellar pearlite islands are the signature of slow, near-equilibrium cooling from the austenite phase field — i.e. the steel was austenitized above $A_3$ and then either furnace-cooled (full anneal) or air-cooled (normalized) at a rate slow enough for both proeutectoid ferrite to nucleate and grow at prior-austenite grain boundaries and for the remaining austenite to transform diffusionally, lamella by lamella, into pearlite. Neither phase shows any acicular or plate-like character, ruling out any appreciable undercooling below the pearlite transformation range.
The right specimen's fine, acicular, plate/lath morphology with no visible prior-phase boundaries is diagnostic of martensitic transformation: the steel was austenitized (heated above $A_3$/$A_{cm}$ and held to fully dissolve carbon into solution) and then rapidly quenched — typically in water or brine for a plain-carbon steel of this hardenability — fast enough to bypass the pearlite "nose" of its TTT/CCT diagram entirely, carrying the austenite diffusionlessly down through $M_s$ to below $M_f$ so essentially the whole volume shears to martensite before any diffusional product can form.
