Why the heat affected zone is where things break
The region beside a weld, and why it is weaker than either side.
The heat-affected zone is where things break because it is the band of parent metal beside a weld that gets hot enough to have its internal structure altered without ever melting. That alteration very often leaves it weaker than both the unaffected parent metal further away and the weld itself, making it the most likely place for a joint to fail even though nobody deliberately worked that region at all.
Heated by accident, never remade
A weld's molten pool cools and solidifies in a way the designer and welder can control, choosing filler composition, cooling rate and process to end up with a structure that performs well. The metal immediately beside the pool receives none of that control. It gets whatever heat conducts outward from the weld as a side effect, rising high enough to change its internal grain structure without reaching the melting point that would let it be refined the way the weld metal was.
Heat has to travel outward through the surrounding solid by ordinary thermal conductivity for the weld to form and cool at all, so there is no way to melt one region of a continuous piece of metal without altering the metal next to it. That uncontrolled heating can coarsen the grain structure, soften a region previously hardened by earlier processing, or, in certain alloys, leave it harder and more brittle. Either direction of change can turn this narrow band into the weakest link of a carefully designed joint. The effect depends so heavily on the alloy, the heat input and the cooling rate that the same process can produce very different zones on two metals that look alike on a datasheet, which is why a welding procedure proven on one alloy has to be checked again before it is trusted on a superficially similar one.
The middle band of a seared steak
Searing a steak in a very hot pan leaves a browned crust on the surface, a distinct separately cooked band just beneath it, and a barely warmed pink centre. The same cut of meat and the same pan produced all three, and each owes its texture entirely to how far it sat from the heat. Nobody set out to cook that middle band. It exists because heat had to pass through it on the way inward, and its texture, neither raw nor properly seared, is a consequence of that transit.
The heat-affected zone is the metallurgical version of that middle band: enough heat to change, not enough to be remade on purpose. And just as the middle of the steak tends to have the least predictable texture of the three, the heat-affected zone tends to have the least predictable properties of any region in a welded joint.
Controlling the zone through heat input
Because the zone is a side effect of heat, the reliable ways to control it all work through how much heat goes in and how it is allowed to cool: the choice of process, the heat input, and where relevant a controlled pre-heat or post-weld heat treatment. Welding procedures for demanding applications specify the permitted heat input and cooling behaviour for this reason, since a sound weld surrounded by a badly disturbed zone can still leave the joint weaker than the design intended.
Brazing stays well below the parent metal's melting point and generally puts in less heat for a shorter time, so it produces a far narrower and milder heat-affected zone. That is part of why brazing is preferred for thin sections and precision assemblies. The zone shrinks toward irrelevance as heat input drops and grows toward dominance as it rises, which makes it a physical record of how much heat a joining process demanded. Reading that record on a finished joint is the habit the next article carries forward, since the region beside a weld can end up stronger than the weld metal just as easily as weaker, depending on choices made before the torch was lit.