21-Mat-B6 Ceramic Materials · May 2013
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.
Given. AISI T1 ("18-4-1") tungsten high-speed steel, nominal composition:
| Element | C | W | Cr | V | Mn | Si |
|---|---|---|---|---|---|---|
| Range (wt%) | 0.65–0.80 | 17.25–18.75 | 3.75–4.50 | 0.90–1.30 | ≤0.40 | ≤0.40 |
T1 carries very large amounts of strong carbide-forming elements — tungsten and vanadium especially — 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 °C) used for plain-carbon or low-alloy steels, and they do not dissolve appreciably at such temperatures. To get enough W, Cr, V and C into solid solution in the austenite — which is essential so that on quenching and tempering they can re-precipitate as the FINE secondary (M2C/MC) 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 °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 for the secondary-hardening response on tempering.
The very high total alloy content of T1 (W, Cr, V, C all in solution after the 1250 °C austenitizing treatment) gives it exceptionally high hardenability — the alloying pushes the pearlite/bainite "nose" of its TTT/CCT curve far out to long times, so even the comparatively SLOW cooling rate achieved by still air, or a gentle fan-assisted air flow, is more than sufficient to bypass that nose everywhere in the tool section and reach $M_s$ without forming any pearlite or bainite. A fast liquid quench (oil or water) is therefore neither necessary (hardenability is not the limiting factor) nor desirable: HSS tools are often geometrically complex (drills, end mills, form cutters) and a severe quench would impose large thermal gradients, promoting distortion and, worse, quench cracking in a material that is already hard and only modestly tough as-quenched. Air hardening achieves full martensitic hardening with minimal thermal shock and minimal dimensional distortion.
The very high alloy content that depresses the pearlite nose (part ii) also depresses $M_s$/$M_f$ substantially, so T1 as-quenched retains a SIGNIFICANT fraction of untransformed (retained) austenite between the martensite laths — commonly 20–30% by volume, far more than in a plain-carbon steel. The first temper (typically ∼550–570 °C, chosen to coincide with the secondary-hardening peak) precipitates fine, coherent alloy carbides (chiefly M2C/MC) from the supersaturated martensite. That precipitation DEPLETES the surrounding, still-untransformed retained austenite of carbon and alloying, which RAISES its $M_s$ back above room temperature — so on cooling down from that first temper, the retained austenite (now effectively a lower-alloy austenite) transforms to a FRESH, untempered, brittle martensite. A second temper is then required to temper THIS newly formed martensite and to continue the secondary-hardening carbide precipitation; a third temper ensures any still-remaining retained austenite has likewise transformed and been tempered, and that residual stresses are fully relieved, leaving a uniform tempered-martensite-plus-secondary-carbide structure with no untempered (crack-prone) martensite anywhere in the tool. Skipping the later tempers would leave pockets of fresh, brittle, high-stress martensite in service, a common cause of in-service HSS tool cracking.
