What is the Heat-Affected Zone (HAZ) in welding?

What is the Heat-Affected Zone (HAZ) in welding?

• A. The base metal portion that remains unaffected by heat
• B. The base metal portion that has melted
• C. The portion of base metal whose properties have been altered by the heat of welding but not melted
• D. The area around the joint where coating is damaged

Answer: (C)

During welding, the heat travels into the base metal near the joint. The heat-affected zone is that portion of the base metal whose properties have been altered by that heat but which has not melted. The weld metal itself melts and forms the joint, while the HAZ sits adjacent to it and experiences thermal exposure without melting. In this zone, microstructure and mechanical properties can change—grain growth, phase changes, and residual stresses can occur depending on heat input and cooling rate. This contrast in behavior (altered properties without melting) is what distinguishes the HAZ from both the untouched base metal and the melted weld metal.

During welding, the heat travels into the base metal near the joint. The heat-affected zone is that portion of the base metal whose properties have been altered by that heat but which has not melted. The weld metal itself melts and forms the joint, while the HAZ sits adjacent to it and experiences thermal exposure without melting. In this zone, microstructure and mechanical properties can change—grain growth, phase changes, and residual stresses can occur depending on heat input and cooling rate. This contrast in behavior (altered properties without melting) is what distinguishes the HAZ from both the untouched base metal and the melted weld metal.