21-Mat-A6 Materials Selection and Design for Materials Processing · Dec-12-Mtl-A6 2018
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: the TTT diagram of this alloy steel shows two SEPARATE C-curves — an upper ferrite–pearlite curve and a lower bainite curve — divided by a “bay” of sluggish, metastable austenite (Ni, Cr and Mo retard the diffusional reactions and split the noses). The CCT diagram is read along cooling curves instead, so it is used to find the critical cooling rate for full martensite and the slower rates that give bainite + martensite or ferrite + pearlite; it cannot show the bay as an isothermal holding window. (In a plain-carbon eutectoid steel such as 1080 the two noses merge into a single C-curve and the bainite region is largely suppressed on the CCT diagram, which is one reason the processes in part (c) are applied to alloy grades.)
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.
The three routes are read off the schematic TTT diagram below. Ausforming and martempering are both INTERRUPTED quenches that hold metastable austenite without letting it cross a transformation-start curve, but they differ in WHERE the hold sits and WHETHER the austenite is deformed; marstraining is different in kind, because its deformation is applied to the martensite AFTER transformation:
(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 hold and the deformation must both finish before the bainite-start curve is reached, which is why the process needs an alloy steel with a wide, deep bay. 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, and the martensite is tempered afterwards as usual.
(iii) Marstraining. The steel is first fully hardened: quenched (directly, or by a martempering route) through $M_s$ and $M_f$ so the austenite transforms to martensite. The MARTENSITE is then plastically strained by a small amount, typically about 1–5%, at or near room temperature, and finally given a low-temperature temper. On the TTT diagram the path therefore runs below $M_s$ before any deformation, which is the opposite of ausforming, where the austenite is worked before it transforms. The strain multiplies dislocations in the already very strong martensite, and during the subsequent low-temperature temper carbon atoms and fine transition carbides pin those new dislocations (a strain-ageing effect), so the yield strength and elastic limit rise well above those of conventionally quenched-and-tempered martensite, with some loss of ductility. Some texts extend the term to straining carried out during the martensite transformation itself, between $M_s$ and $M_f$; in either version, the deformation acts on martensite rather than on metastable austenite.
