21-Mat-A6 Materials Selection and Design for Materials Processing · May 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.
For a eutectoid composition the TTT diagram is a single C-shaped curve (no proeutectoid phase field), bounded above by the eutectoid temperature $A_1=727\,{}^{\circ}\text{C}$, above which austenite is stable indefinitely, and below by the $M_s$ line, below which martensite forms athermally on cooling through it. Between the printed start and finish curves, isothermal holding at a given temperature transforms untransformed austenite into pearlite (upper shelf, coarse near $A_1$ and finer as the nose is approached), or into bainite (lower shelf, upper bainite just below the nose, lower bainite closer to $M_s$).
The main function of thermomechanical processing during controlled rolling of a eutectoid steel is austenite grain and substructure refinement prior to transformation. Rolling below the austenite no-recrystallization temperature $T_{nr}$ ("pancaking") deforms the austenite grains without letting them recrystallize, building up a high density of deformation bands, sub-boundaries and elongated grain-boundary area — all additional heterogeneous nucleation sites for the transformation product. On the TTT diagram this shows up as the start (and finish) curve shifting to the LEFT (shorter incubation and completion times) and, for the pearlite shelf, sometimes rising slightly, because the extra nucleation sites let transformation begin sooner at a given undercooling. The practical consequence is a finer, more uniform pearlite (or acicular ferrite) colony size and interlamellar spacing than the same steel transformed from an unrefined, undeformed austenite grain structure, at the cost of reduced hardenability (it becomes harder to bypass the nose and reach a fully martensitic structure at a given cooling rate, precisely because the curve has moved left).
(i) Conventional (quench-and-temper). The steel is quenched fast enough to miss the nose of the curve entirely (the red path in the figure), crossing directly from the austenite field to below $M_s$ in one continuous cool, forming untempered (hard, brittle) martensite. It is then reheated to a sub-$A_1$ tempering temperature to precipitate fine transition/equilibrium carbides and relieve quench stresses, trading some hardness for toughness. Advantage: the highest attainable hardness/strength combination for a given steel, with toughness independently tunable via the temper temperature. Disadvantage: the fast, uninterrupted quench produces the largest thermal and transformation gradients through the section, and hence the highest residual stress and greatest risk of quench cracking and distortion of the three routes.
(ii) Austempering. The steel is quenched rapidly enough to miss the pearlite nose but is then held ISOTHERMALLY in the bainite bay, above $M_s$ (the blue path), until the bainite transformation runs to completion (a horizontal segment crossing both the start and finish curves in the bainite field), and is only then air-cooled to room temperature. Because the isothermal hold is above $M_s$, no martensite forms and no separate temper step is needed; bainite is produced directly. Advantage: substantially lower residual stress, distortion and cracking risk than direct quenching (the section equalizes in temperature during the isothermal hold, rather than transforming under a steep gradient), while bainite gives a favourable combination of strength and toughness/ductility for a given hardness — often superior to tempered martensite at the same hardness.
(iii) Martempering (marquenching). The steel is quenched rapidly to a temperature just ABOVE $M_s$ (the purple path) and held only briefly — long enough to equalize the temperature across the section, but NOT long enough to cross the bainite start curve (the hold sits to the left of the nose/bainite-start line) — then cooled slowly (e.g. air) through $M_s$ so the whole section transforms to martensite more uniformly and simultaneously than a direct quench would allow; the resulting (still untempered) martensite is subsequently tempered conventionally. Advantage: minimizes the thermal-gradient-driven stresses, distortion and cracking associated with an uncontrolled martensitic transformation front sweeping through the section, while still delivering the same final tempered-martensite properties as route (i).
