Metallurgy and heat treating are a vital part of metal fusing. It is not simply a good bead or welding technique that assures a safe, quality weld. Filler material must match the base metal. Heat treatment is often involved after the machining and finish of alloy metals. Chemistry is a critical part of metal selection and the choice of welding filler materials used with specific alloys.
Metallurgy and heat treatment are not abstract processes for manufacturing only. I have repaired obsolete gears with chipped teeth, where the 8620 base metal has been case hardened to 0.035" or so depth at