In this lecture on Heat Affected Zone in Transformation-Hardening Ferrous Metals, we explore how the weld thermal cycle influences microstructural and property changes in steels and cast irons during welding. The session explains the formation of coarse- and fine-grained heat affected zones, the role of peak temperature, soaking time, and cooling rate, and how these factors determine grain size and phase transformations. We examine the effects of partial and heterogeneous austenite formation, and how different zones evolve into martensite, pearlite, bainite, or ferrite based on cooling rates.