21-Mat-B6 Ceramic Materials · December 2013
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.
Austenite nucleation on reheating (austenitization) is heterogeneous in every practical steel, and it is controlled by the same two factors regardless of starting microstructure: (1) the availability of α/Fe3C interphase boundary area, which simultaneously supplies a locally carbon-rich environment and lowers the nucleation activation energy, and (2) prior high-angle grain boundaries, wherever no better (carbide-adjacent) site exists. The three starting microstructures below differ enormously in how much α/Fe3C interfacial area they already contain and how that area is distributed — and that distribution is what controls both where austenite nucleates and how fine the resulting initial austenite grain size will be.
In an annealed hypoeutectoid low-carbon steel the microstructure is predominantly large, soft proeutectoid ferrite grains, with only a small volume fraction of pearlite located mainly at ferrite grain boundaries and triple points (it formed there last during the original cooling). On reheating, the α/Fe3C lamellar interfaces WITHIN this residual pearlite are the most carbon-rich, highest-interfacial-energy sites available, so austenite nucleates there FIRST and grows rapidly as it consumes the pearlite pocket. Nucleation also eventually occurs at plain ferrite/ferrite grain boundaries once the temperature is high enough, but these sites are carbon-starved (no adjacent cementite) and are both slower to activate and less numerous.
A spheroidized structure consists of a continuous ferrite matrix containing many small, discrete, globular (spheroidal) Fe3C particles, produced by a prolonged sub-critical anneal that breaks up the original lamellar or network cementite into low-surface-energy spheres. Every one of these numerous spheroids presents its own α/Fe3C interphase boundary, so on reheating austenite nucleates essentially AT THE SURFACE OF EACH CARBIDE SPHEROID throughout the matrix. Because the sphere population is fine and numerous, this produces a large number of independent, well-dispersed nucleation events and, once transformation is complete, a FINE and uniform initial austenite grain size — one practical reason components are spheroidize-annealed before a final hardening treatment.
A fully pearlitic (eutectoid) steel presents an enormous amount of α/Fe3C interfacial area intrinsically, since the ENTIRE microstructure is made of fine, alternating lamellae. On reheating, austenite nucleates profusely along these lamellar interfaces throughout every pearlite colony — not only at colony (former austenite grain) boundaries — because every lamella boundary is simultaneously carbon-rich and a high-energy interface. The nucleation rate here is the HIGHEST of the three starting microstructures, because the interfacial area per unit volume is highest (a finer lamellar spacing exposes even more boundary length), which is why pearlite-to-austenite reversion is characteristically fast and yields the finest initial austenite grain size of the three cases, for comparable heating rates.
