Why do some welded components need heat treatment after welding?

The primary purpose of post-weld heat treatment for certain welded components is to eliminate residual weld stresses and improve the microstructure and properties of the weld joint.

Reducing Residual Stress
Localized high temperatures during welding can cause uneven metal expansion, generating residual stresses upon cooling. These stresses can reduce the component's load-bearing capacity and even cause cracks. Heat treatment (such as stress relief annealing) can induce plastic flow in the metal, relieve stresses, and prevent deformation and cracking.

Improving Microstructure and Properties
Rapid cooling may produce a hard and brittle structure (such as martensite) in the weld zone. Heat treatment (such as normalizing and tempering) can achieve grain refinement and adjust the balance between hardness and toughness. For example, carbide precipitation during tempering can reduce hardness and increase toughness, preventing hydrogen-induced cracking (particularly important for low-alloy, high-strength steels).

Specific Mechanism of Action
Diffusible Hydrogen Emission: Post-weld heat treatment prolongs the cooling time between 800°C and 500°C, promoting the diffusion and escape of hydrogen from the weld metal and preventing hydrogen-induced cracking. ‌

‌Temperature Gradient Control‌: Local or global preheating can reduce temperature differences (temperature gradients), lower welding strain rates, and prevent cracking. ‌
‌Microstructure Control‌: Heating re-austenitizes the metal, and subsequent cooling produces a uniform, fine-grained structure, improving mechanical properties.