Definition of heat affected zone

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Photonics Dictionary

heat affected zone: The heat-affected zone (HAZ) refers to the portion of a material, typically metal, that experiences changes in its microstructure and mechanical properties due to the heat generated during welding, cutting, or other thermal processes. The extent and characteristics of the HAZ depend on factors such as the heat input, the material's composition, and the cooling rate.

Formation: During welding or similar processes, intense heat is applied to the base metal and adjacent areas. This heat causes the material within the HAZ to undergo thermal cycles, resulting in localized changes in microstructure and properties.

The HAZ may experience various microstructural changes, including:

Grain growth: Increase in the size of grains due to recrystallization and grain boundary migration.

Phase transformation: Changes in the crystalline phases or transformation to different microstructural phases depending on the material and alloy composition.

Precipitation and hardening: Precipitation of secondary phases and changes in material hardness or strength.

Mechanical properties: The mechanical properties of the material within the HAZ can differ significantly from those of the base metal, potentially leading to variations in tensile strength, ductility, toughness, and corrosion resistance.

Classification: The HAZ is often classified into different zones based on the extent of thermal exposure and resulting microstructural changes. These zones typically include the:

Fully austenitic zone: Closest to the fusion line where the material has been heated above its austenite transformation temperature.

Partially transformed zone: Where partial transformation of the microstructure occurs, often characterized by a mixture of phases.

Subcritical heat-affected zone: Farthest from the fusion line where the material remains below its critical transformation temperature but experiences thermal cycling.

Understanding and controlling the HAZ is critical in welding and thermal processing operations to ensure the integrity and performance of welded joints and processed materials. Techniques such as preheating, post-weld heat treatment, and selecting appropriate welding parameters are used to minimize adverse effects on the HAZ and optimize material properties.

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