21-Mat-A6 Materials Selection and Design for Materials Processing · May 2015
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.
A TTT diagram is constructed from a set of ISOTHERMAL experiments: austenitize, then quench INSTANTLY to a fixed holding temperature and record the time to start and finish transformation at that one temperature, repeated across many temperatures. It answers "how long does transformation take if I hold at exactly this temperature?" and is the appropriate map for interrupted-quench (hold-and-transform) treatments.
A CCT diagram for the SAME steel is instead constructed from CONTINUOUSLY cooling experiments at a series of fixed cooling rates from the austenitizing temperature, recording the temperature/time at which transformation starts and finishes along each continuous cooling curve. Because the steel spends time transforming while still cooling through a RANGE of temperatures (never held isothermally), the CCT start/finish curves are always shifted to LONGER times and (for the pearlite/bainite noses) somewhat LOWER temperatures than the TTT curves for the identical alloy — continuous cooling effectively "samples" a moving target rather than one fixed isothermal hold, so slightly more undercooling/time is needed before the diffusional reaction can nucleate and grow. A further structural difference for a eutectoid steel: on a CCT diagram the bainite portion of the curve is frequently truncated or absent altogether, because by the time a continuously cooling eutectoid steel has cooled enough to reach the bainite temperature range, the pearlite reaction (encountered first, at higher temperature, while cooling through it) has usually already consumed the transformable austenite along realistic cooling paths — bainite is far more readily reached on a TTT diagram (direct isothermal hold in the bay) than by continuous cooling of this composition.
The primary objective of TMP is to combine controlled DEFORMATION with controlled TRANSFORMATION so as to refine the final microstructure — principally by refining the austenite grain/substructure (through pancake rolling below the no-recrystallization temperature $T_{nr}$) before it transforms, which multiplies the number of nucleation sites available to the diffusional transformation and yields a much finer ferrite/pearlite (or acicular ferrite/bainite) grain size than conventional (deform-then-heat-treat-separately) processing could achieve for the same alloy composition. This lets the designer obtain a superior combination of strength AND toughness (both improve with finer grain size, per Hall–Petch and the ductile-to-brittle transition temperature shifting down) without relying on additional alloying or a separate heat-treatment step, which is also the basis of low-carbon HSLA microalloyed steel design.
All three routes are variants of an INTERRUPTED quench that holds the steel isothermally in the metastable austenite region above the transformation curves, but they differ in WHERE that hold sits and WHETHER plastic deformation is applied during it:
(i) Ausforming. The steel is quenched rapidly enough to miss the pearlite nose and is held ISOTHERMALLY in the metastable-austenite "bay" between the pearlite and bainite noses (a region where austenite is supercooled but transformation is still slow) — and, uniquely among the three, is PLASTICALLY DEFORMED (heavily worked) while held there, before finally being quenched on through $M_s$ to martensite (and subsequently tempered). The deformation refines and adds substructure to the metastable austenite itself, and that refined substructure is inherited by the martensite that eventually forms from it, giving simultaneously higher strength AND better toughness than conventionally quenched-and-tempered martensite of the same composition — the TMP benefit of part (b) applied directly to a hardenable, quench-and-temper steel.
(ii) Martempering (marquenching). The steel is quenched rapidly to a temperature just ABOVE $M_s$ and held only briefly, long enough to equalize temperature across the section but not long enough to cross the bainite-start curve, then slow-cooled (e.g. air) through $M_s$ so the whole cross-section transforms to martensite more simultaneously and with less thermal gradient than a direct quench — reducing residual stress, distortion and cracking risk relative to conventional quench-and-temper, for the same final tempered-martensite properties. No deformation is applied during the hold.
(iii) Marstraining. This is martempering's hold PLUS a deliberate plastic-deformation step applied to the austenite WHILE it sits in the brief isothermal hold just above $M_s$ (before the final quench through $M_s$ to martensite). It therefore combines martempering's stress/distortion benefit (uniform, low-gradient transformation through $M_s$) with an ausforming-like substructure-refinement benefit, but applies the working step at a much lower temperature (just above $M_s$, rather than in the pearlite/bainite bay), which limits how much recovery of the deformation substructure can occur before the final quench and further raises strength.
