Question 6 of 8: Reading the TTT Diagram of a Eutectoid Steel — Four Isothermal/Interrupted-Quench Heat-Treatment Paths

Question 6: Reading the TTT Diagram of a Eutectoid Steel — Four Isothermal/Interrupted-Quench Heat-Treatment Paths (20 marks: 5 each)

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.

The printed TTT diagram (a classic eutectoid-steel C-curve, austenitized at 750 °C) carries the following fixed features used throughout: the eutectoid temperature at 727 °C; a pearlite field above ∼550 °C; a pearlite+bainite field between ∼550 and ∼400 °C; a bainite field below ∼400 °C down to $M_s$; $M_s\approx220\,{}^{\circ}$C and $M_{90}\approx120\,{}^{\circ}$C (90% martensite line), with a martensite+retained-austenite field between them. All four paths are drawn fast enough (<1 s to their hold temperature) to miss the transformation-start C-curve entirely on the way down — the transformation, if any, therefore happens ONLY during the isothermal hold, or (for paths a/b) not at all above $M_s$.

[Figure not reproduced: Schematic TTT diagram for a eutectoid steel (austenitized 750 °C), redrawn from the paper's own Fig. 8.11, with the four cooling/holding paths (a)–(d) superimposed as printed. See the official exam paper.]

6.1 — (a) Rapid quench to room temperature in <1 s

The quench is fast enough to miss the C-curve nose entirely, so the austenite survives, undecomposed, all the way down to $M_s\approx220\,{}^{\circ}$C. Below $M_s$ the diffusionless, athermal martensitic transformation begins — a temperature-triggered (not time-triggered) shear transformation that proceeds instantly at whatever temperature is reached, converting a fraction of the remaining austenite to martensite for every degree of further cooling. Because the quench continues past $M_{90}\approx120\,{}^{\circ}$C down to room temperature, essentially ALL of the austenite transforms: the resulting microstructure is ∼100% (untempered) martensite — a hard, brittle, supersaturated body-centred-tetragonal phase, with the high hardness/low toughness typical of an as-quenched eutectoid steel.

6.2 — (b) Quench to 160 °C in <1 s, hold for years

The quench again misses the C-curve, so no diffusional transformation occurs on the way down. However, 160 °C lies BETWEEN $M_s$ (220 °C) and $M_{90}$ (120 °C): as the specimen cools THROUGH $M_s$ to 160 °C, the athermal martensite reaction is triggered and a PARTIAL fraction of the austenite transforms instantly — interpolating between 0% at $M_s$ and 90% at $M_{90}$, roughly half of the austenite converts to martensite by the time 160 °C is reached. Because martensite formation is athermal (temperature-dependent only, not time-dependent), holding at a CONSTANT 160 °C for even "several years" produces NO further martensite — there is no additional undercooling to drive more transformation. The remaining, untransformed austenite is also outside the diagram's plotted bainite field at this temperature (the bainite curves stop at $M_s$), so on the diagram it does not decompose isothermally either, no matter how long the hold. The resulting microstructure is therefore a mixture of martensite (formed instantly, partial fraction) and retained austenite, with no further transformation shown on the diagram. Practical caveat beyond the diagram: a TTT diagram only tracks austenite decomposition, so it cannot show two slow changes a real multi-year hold at 160 °C would bring — the fresh martensite would undergo low-temperature (first-stage) tempering with transition-carbide precipitation, and part of the retained austenite could slowly decompose to lower bainite. Read strictly off the printed diagram, the expected answer is martensite + retained austenite.

6.3 — (c) Quench to 650 °C, hold 1 day, then quench to room temperature

650 °C sits in the pearlite field, but close to the eutectoid temperature (727 °C) where nucleation is SLOW (low undercooling below $A_1$ means a low nucleation rate for pearlite colonies, even though growth itself is diffusion-controlled and not otherwise unusual). One full day (∼$8.6\times10^4$ s) is far longer than the time needed for transformation to go to completion at 650 °C even with this slow nucleation, so by the end of the hold the austenite has fully transformed. Because nucleation was slow, relatively few pearlite colonies formed, each growing to a comparatively large size with widely spaced ferrite/cementite lamellae — i.e. coarse pearlite. Since no austenite remains, the final quench to room temperature produces no further transformation at all: the microstructure is 100% coarse pearlite, unaffected by the final quench.

6.4 — (d) Quench to 550 °C, hold 1 day, then quench to room temperature

550 °C sits right at the nose of the C-curve (the fastest-transforming temperature on the whole diagram, at the boundary between the pearlite and pearlite+bainite fields), where BOTH nucleation and growth rates are maximized, so transformation to completion here takes only seconds to minutes — trivially finished within the one-day hold. The high nucleation rate at the nose produces many closely spaced pearlite colonies with very fine, closely spaced ferrite/cementite lamellae — fine pearlite (right at the boundary with the bainite field, this is the finest pearlite the diagram produces; nucleation this vigorous is also why the nose sets the shortest time-to-transform anywhere on the C-curve). Because 550 °C is also the top edge of the diagram's “pearlite plus bainite” field, a trace of upper bainite is possible, but the product is essentially all fine pearlite. As in part (c), the austenite is fully consumed during the isothermal hold, so the final quench to room temperature changes nothing further: the microstructure is 100% fine pearlite.