In this lecture on HAZ in Work and Precipitation Hardening Metals, we discuss how welding heat causes softening by reducing dislocation density in work-hardened metals and dissolving or coarsening precipitates in precipitation-hardened alloys. Peak temperature and soaking time drive recovery, recrystallization, grain growth, and reversion, leading to hardness loss, especially near the fusion boundary. Differences between fusion and solid-state joining are also highlighted.