21-Mat-B6 Ceramic Materials · December 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.
| Grade | C | Cr | Mo | W | V | Mn | Co | Si |
|---|---|---|---|---|---|---|---|---|
| T1 | 0.65–0.80 | 3.75–4.00 | 0.2–0.5 | 17.25–18.75 | 0.9–1.3 | 0.1–0.4 | 0.1–0.15 | 0.2–0.4 |
T1 is a heavily alloyed high-speed steel, carrying roughly 18 wt% W, ≈4 wt% Cr and ≈1 wt% V, all strong carbide-forming elements. In the as-annealed (spheroidize-annealed, machinable) starting condition, most of this alloy content sits in coarse, thermally stable primary carbides ($M_6C$-type tungsten-rich carbide and $MC$-type vanadium carbide/carbonitride) that are far more refractory and slower to dissolve than ordinary cementite. Tool-steel heat treatment DELIBERATELY relies on getting a substantial fraction of these alloying elements (especially W, plus some Cr and V) into solution in the austenite before quenching, because it is exactly that dissolved alloy content that (a) confers deep hardenability (air-hardening capability, see 4.2) and (b) later precipitates as ultra-fine secondary hardening carbides during tempering (see 4.3), which is what gives high-speed steel its hot hardness and cutting-edge wear resistance. Dissolving these stable primary carbides requires heating close to their own solvus/eutectic temperature — for W-rich $M_6C$-type carbides in this system that is near 1200–1280 °C — which is why T1's austenitizing temperature (1250–1280 °C) sits so much higher than a plain-carbon or low-alloy steel's (typically 750–900 °C).
Once the heavy alloy content (W, Cr, Mo, V) is taken into solution by the high-temperature austenitizing soak, that SAME dissolved alloy content dramatically depresses the pearlite/bainite "nose" of the steel's TTT/CCT diagram to very long times, i.e. it gives the steel extremely high hardenability. With the nose pushed out that far, even the comparatively slow, uniform cooling rate available from still air (or a gentle forced-air fan, which mainly improves temperature UNIFORMITY through the section rather than raw cooling speed) is already faster than the now-very-long critical cooling time needed to avoid the nose — so the steel can be taken essentially straight to martensite without ever needing a severe oil or water quench. This is exactly why high-speed and other highly-alloyed tool steels are classed as "air-hardening": a slow, gentle air cool is not merely tolerated, it is the RECOMMENDED practice, because it also minimizes the thermal-gradient-driven distortion and quench-cracking risk that a fast liquid quench would otherwise impose on the complex, often thin-sectioned tool geometries these steels are used for.
As-quenched high-speed steel martensite contains a substantial fraction of RETAINED austenite (not fully martensite), because the heavy alloying that depresses the pearlite nose (needed for 6.2) also depresses $M_s$/$M_f$ well below room temperature, so the quench never fully completes the austenite-to-martensite transformation. Each temper cycle serves two compounding purposes at once: (1) it precipitates a dense dispersion of very fine, coherent secondary alloy carbides (the "secondary hardening" reaction, from the same dissolved W/V/Cr/Mo/Co that was put into solution during austenitizing) from the primary martensite, which is what raises — rather than simply softens — the hardness on tempering (unlike a plain-carbon steel, whose hardness only falls with tempering); and (2) the resulting compositional and volume change, together with the temper's own heating/cooling cycle, converts a substantial fraction of the previously RETAINED austenite into fresh, untempered (and therefore brittle) martensite on cooling back to room temperature after each cycle. A single temper therefore leaves behind a mixture of properly tempered martensite AND freshly formed, still-untempered (brittle, and itself still containing some retained austenite) martensite from the just-converted retained austenite. A SECOND temper is needed to temper THAT freshly formed martensite; and because even the second temper still converts a further (smaller) increment of any remaining retained austenite into new untempered martensite, a THIRD temper is the standard practice to ensure the retained-austenite-to-martensite conversion is driven essentially to completion and every generation of martensite formed along the way is properly tempered, leaving a stable, fully tempered, minimal-retained-austenite final microstructure.
