Heat-affected zone (HAZ)
The portion of the base material adjacent to the weld seam that has not melted, but whose microstructure and mechanical properties were altered by intense heat exposure.
The heat from welding doesn't just go into the weld, but also into the strip next to it. That strip gets its own unwanted heat treatment. Right next to the weld, the grain grows; next to that, the steel becomes fine-grained or soft, depending on the material. If something breaks, it usually happens in this zone and not in the weld itself. In hard steels, the zone next to the weld can become hard and brittle during cooling – that's hardening. In quenched and tempered parts, it tends to soften. With stainless steel, you have to be careful not to stay too long in the critical temperature range, otherwise it becomes susceptible to corrosion. You can control this via the cooling rate: preheating, maintaining interpass temperature, and adjusting the heat input. When measuring hardness, you don't test in the weld center, but right next to it – that's where the peak is.