= Solution
In <tempered martensite>, the metastable carbon-supersaturated martensitic matrix can evolve toward a lower-carbon ferritic matrix containing fine carbides. <Carbon> segregation and precipitation reduce <supersaturation> and tetragonal lattice distortion; recovery can reduce internal <strain> while the original plate or lath pattern remains recognizable. This is tempering, rather than a fresh austenite-to-pearlite transformation.<Tempered Martensite by H. K. D. H. Bhadeshia>[Introduction and Virgin Microstructure, carbon redistribution and retained plate structure]{ref}
A generic sketch consequently replaces a uniform highly strained plate interior by a lower-carbon matrix with dispersed carbide particles, as in the third panel above. This provides a qualitative materials explanation for a changed fracture response. It does not specify a manufacturing optimization, and tempering is not a guarantee of monotonic toughness improvement in every <steel>: carbide morphology, impurities and retained <austenite> matter. The source reports the observed outcome, so the explanation must not be turned into a universal process recommendation.
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