Question 2 of 7: Thermomechanical Treatment and Austempering
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Reference texts: Krauss, Steels: Processing, Structure, and Performance, 2nd ed.; Reed-Hill & Abbaschian, Physical Metallurgy Principles, 4th ed.; Callister & Rethwisch, Materials Science and Engineering: An Introduction, 10th ed.; ASM Handbook, Vol. 4, Heat Treating; Porter, Easterling & Sherif, Phase Transformations in Metals and Alloys, 3rd ed.
Check: this paper's printed header reads "Met-B6, Physical Metallurgy of Iron and Steel" and every question is ferrous physical metallurgy (equilibrium microstructures, thermomechanical treatment and austempering, CCT/TTT curve theory, martensite thermodynamics and crystallography, high-speed tool-steel heat treatment, cast-iron carbon morphology, and surface hardening) with no ceramics content anywhere.
Question II: Thermomechanical Treatment and Austempering (10 marks)
Austenitize the steel in the normal way, above A3/Acm.
Quench RAPIDLY — fast enough to miss the pearlite/ferrite nose entirely — down into the temperature bay that separates the diffusional (pearlite) C-curve from the displacive (bainite) C-curve of a suitably alloyed (typically Mo/Cr-bearing) steel's double-nosed TTT diagram.
Hold the steel isothermally in that bay, where the austenite is metastable but transformation is very slow on either side, and plastically deform it there (hot- or warm-work: forging, rolling) while it is still 100% austenite.
Immediately after deforming, continue quenching on down through Ms to form martensite from the now heavily deformed (dislocation-dense, fine-substructured) austenite.
Temper as normal.
Fig. 2.1 — double-nosed TTT diagram: quench into the bay between the pearlite and bainite noses, hold + deform there, then quench through Ms and temper.
This is worth doing because deforming the austenite BEFORE it transforms leaves the martensite that eventually forms inside a much finer, higher-dislocation-density parent substructure than an equivalent conventionally quenched-and-tempered steel: the prior-austenite grain size is refined, the martensite lath/packet size is refined, and the resulting tempered martensite combines HIGHER strength with BETTER toughness than the same composition heat-treated conventionally (ausformed high-strength steels and gun-barrel/spring-wire practice are the classic applications) — a combination ordinary strengthening cannot achieve, since most strengthening mechanisms trade toughness away as strength rises.
2.2 — (ii) Austempering
Austenitize the steel above A3/Acm, as usual.
Quench RAPIDLY (into a molten salt bath) to miss the bainite nose entirely, arriving at an isothermal hold temperature set just ABOVE Ms.
Hold isothermally at that temperature until the bainite reaction runs to completion — i.e. until the isothermal transformation crosses the ENTIRE bainite C-curve, start to finish.
Air-cool to room temperature: since the austenite has already been fully consumed by bainite during the hold, this final cool causes no further transformation (no martensite forms).
Fig. 2.2 — single-nosed (bainite) TTT diagram: quench past the nose to just above Ms, hold isothermally through the whole bainite C-curve, then air-cool with no further transformation.
The resulting microstructure is 100% bainite (typically lower bainite, given the low hold temperature) — ferrite laths with fine, dispersed carbides, rather than the plate martensite a conventional quench would give. Because the transformation happens ISOTHERMALLY, the whole cross-section transforms uniformly and at essentially the same time, so austempering avoids the large thermal gradients (and the associated distortion and quench-cracking risk) of a conventional interrupted quench-and-temper, while bainite's own microstructure gives a strength/toughness/ductility combination that is often superior to tempered martensite of the same hardness — and no separate tempering step is needed afterward.
