21-Mat-B6 Ceramic Materials · December 2016
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.
D2 is a heavily alloyed, high-carbon tool steel (roughly 12 wt% Cr plus Mo and V). This heavy substitutional alloying gives it exceptionally high hardenability — per Question III(ii), the alloying content pushes the TTT/CCT nose so far to the right (to long times) that the critical cooling rate needed to bypass pearlite/bainite formation and reach martensite is very LOW, low enough that even still air or a gentle fan-forced air stream cools the piece through the nose fast enough to still produce a fully martensitic structure. Because such a mild quench already achieves the required transformation, it is strongly preferred over a severe liquid quench (oil or water): a slow, uniform air cool minimizes the thermal gradients through the section, which in turn minimizes the residual (quench) stresses, distortion, and risk of quench cracking that a fast liquid quench would otherwise impose — a critical concern for D2 tooling, which is typically a complex, high-value, precision-machined die or punch shape that cannot tolerate warping.
As-quenched martensite in a highly alloyed, high-carbon steel like D2 retains a significant fraction of untransformed RETAINED austenite, because the heavy alloying and high carbon content depress $M_s$ and especially $M_f$ (often below room temperature), so the quench never fully completes the martensitic transformation. Tempering D2 is also typically done at a secondary-hardening temperature (around 500 °C) where fine alloy carbides precipitate; at this temperature a portion of the retained austenite can itself transform — on cooling back to room temperature after the FIRST temper, this retained austenite converts to FRESH, untempered (hard and brittle) martensite, effectively reintroducing the very problem the temper was meant to remove. A SECOND (and often third) temper cycle is therefore needed to temper this newly formed martensite in turn and to further condition/decompose any remaining retained austenite, until the microstructure stabilizes with little or no untempered martensite or retained austenite left. Multiple tempers also allow more complete stress relief and a more uniform distribution of the fine secondary-hardening carbides, improving toughness and dimensional stability while preserving the high working hardness.
