Question 6 of 7: High-Speed Tool Steel (T1) — Austenitizing Temperature, Air Quench, and Multiple Tempering

Question VI: High-Speed Tool Steel (T1) — Austenitizing Temperature, Air Quench, and Multiple Tempering (15 marks)

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.

6.1 — (i) Why austenitizing must go as high as 1250 °C

T1 carries very large amounts of strong carbide-forming elements — tungsten and vanadium especially, with chromium contributing as well — which form coarse, extremely thermally stable alloy carbides (M₆C-type W-rich carbide, MC-type VC) during solidification and subsequent processing. These carbides have very LOW solubility in austenite at the moderate austenitizing temperatures ($800$–$900^{\circ}\text{C}$) used for plain-carbon or low-alloy steels, and do not dissolve appreciably at such temperatures. To get enough W, Cr, V and C into solid solution in the austenite — essential so that on quenching and tempering they can re-precipitate as the FINE secondary carbides responsible for this steel's characteristic secondary hardening and hot hardness (red hardness) — the steel must be heated close to its incipient-melting temperature, roughly $1250^{\circ}\text{C}$ for T1, where the coarse carbides finally dissolve substantially. Austenitizing too low leaves the alloy content locked up in undissolved primary carbides, starving the matrix of the alloying needed for both adequate hardenability and the secondary-hardening response on tempering.

6.2 — (ii) Why still-air or slow fan cooling is a workable quench

Once the heavy alloy content (W, Cr, 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 — exactly the hardenability-shifting mechanism developed in Question IV(ii), here at an extreme level. 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.

6.3 — (iii) Why T1 (and similarly D2) require a minimum of three tempers

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 (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 — or, for a high-Cr cold-work grade such as D2, predominantly Cr — put into solution during austenitizing) from the primary martensite; 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. A single temper therefore leaves behind a mixture of properly tempered martensite AND freshly formed, still-untempered 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 standard practice to drive the retained-austenite-to-martensite conversion essentially to completion, leaving a stable, fully tempered, minimal-retained-austenite final microstructure. D2, despite deriving its deep hardenability mainly from Cr-rich carbides rather than T1's W/V-rich carbides, shares the identical retained-austenite mechanism (its own heavy alloying likewise depresses $M_s$), which is why the same "minimum of three tempers" rule is quoted for both grades.