What is heat-affected zone (HAZ)?

Short Answer:

The heat-affected zone (HAZ) is the portion of the base metal that does not melt during welding but experiences high temperatures due to the welding heat. These high temperatures change the metal’s microstructure and mechanical properties. The HAZ lies between the weld metal and the unaffected base metal.

Changes in hardness, strength, and grain size occur in the HAZ, which can make it weaker or more brittle than the rest of the material. Controlling the welding heat input and cooling rate helps minimize the size and negative effects of the heat-affected zone.

Detailed Explanation :

Heat-affected Zone (HAZ)

The heat-affected zone (HAZ) is a critical part of any welded joint. It refers to the area of the base metal that has been thermally influenced by the welding heat but has not melted. When welding is performed, the temperature distribution across the joint is not uniform. The metal near the weld pool reaches high temperatures and undergoes structural changes, while areas farther away remain unaffected.

The HAZ is located between the fused weld metal (which melts and solidifies) and the unaffected base metal (which remains at room temperature). Although it does not melt, the heat-affected zone experiences thermal cycles—heating and cooling—that can alter its physical and mechanical properties. These changes may affect the overall strength, hardness, and toughness of the welded joint.

Formation of the Heat-affected Zone

During welding, a high-temperature heat source such as an electric arc, flame, or laser melts the filler and part of the base metal to form the weld bead. The heat then spreads outward from the molten zone to the surrounding metal. As the temperature decreases with distance from the weld, different areas of the base metal experience different degrees of heating.

This temperature gradient results in different metallurgical transformations within the base metal. Some areas close to the fusion line may reach near-melting temperatures, while regions farther away may reach only moderate temperatures. The area that experiences temperature high enough to cause microstructural changes but below the melting point is the heat-affected zone.

The HAZ does not have a uniform structure; instead, it is divided into sub-zones based on temperature ranges and resulting transformations.

Sub-zones of the Heat-affected Zone

  1. Fusion Boundary Zone:
    This region lies closest to the weld metal. The temperature here is almost at the melting point of the base metal. Grains become coarse due to excessive heat, which can reduce toughness.
  2. Coarse-Grained Zone:
    Slightly away from the fusion line, this area experiences high heat but not enough to melt. The grains grow larger as the temperature remains high for a longer period. This zone is generally the weakest part of the HAZ because of its brittleness.
  3. Fine-Grained Zone:
    This region is heated to a temperature where grain refinement occurs. It usually possesses better strength and toughness compared to the coarse-grained area.
  4. Intercritical Zone:
    Here, the temperature is high enough to partially transform the structure but not sufficient for full recrystallization. Some phases transform, while others remain unchanged, leading to non-uniform properties.
  5. Subcritical Zone:
    The outermost part of the HAZ where the temperature is lower. The metal here undergoes recovery or stress relief but retains most of its original structure.

Each of these zones affects the final performance of the weld joint differently, depending on the type of material and welding parameters used.

Effects of the Heat-affected Zone

The HAZ significantly influences the mechanical and metallurgical properties of the welded structure. The main effects include:

  1. Change in Hardness:
    Depending on the metal and cooling rate, hardness can increase or decrease. For example, in steels, rapid cooling may form hard but brittle martensite.
  2. Change in Strength and Toughness:
    The altered microstructure can make the HAZ either stronger or weaker than the base metal. Usually, the coarse-grained region becomes brittle.
  3. Residual Stresses and Distortion:
    Uneven heating and cooling create internal stresses, which may lead to warping or cracking.
  4. Change in Grain Size:
    Grain growth due to prolonged heating reduces mechanical strength and impact resistance.
  5. Susceptibility to Cracking:
    The HAZ is often the most vulnerable area for hydrogen cracking or fatigue failure, especially in high-strength steels.

Factors Affecting the Heat-affected Zone

  1. Welding Process Used:
    Different welding methods produce different heat inputs. Processes like MIG or TIG have smaller HAZs compared to oxy-acetylene or submerged arc welding.
  2. Heat Input:
    Higher heat input enlarges the HAZ, while controlled heat input keeps it narrow and minimizes property changes.
  3. Cooling Rate:
    Fast cooling can cause brittleness, while slow cooling allows for better grain refinement and ductility.
  4. Base Metal Composition:
    Metals like carbon steel, alloy steel, and aluminum respond differently to heat. Steels with high carbon content are more sensitive to HAZ changes.
  5. Preheating and Post-heat Treatment:
    Preheating reduces thermal gradients and stress, while post-heat treatment restores ductility and toughness.

Minimizing the Heat-affected Zone

To reduce the negative effects of the HAZ, several precautions can be taken:

  • Use low heat input and control welding speed.
  • Apply preheating to reduce temperature differences.
  • Use proper welding technique to distribute heat evenly.
  • Perform post-weld heat treatment to relieve residual stresses.
  • Select appropriate filler materials compatible with the base metal.

By managing these factors, the size of the heat-affected zone can be minimized, ensuring better weld quality and durability.

Conclusion

The heat-affected zone (HAZ) is an important part of a welded joint that determines the strength and performance of the weld. Though it does not melt, the HAZ undergoes thermal and structural changes due to the heat of welding. These changes can make the metal brittle or weak if not controlled. Proper selection of welding parameters, preheating, and post-weld heat treatments are essential to reduce HAZ problems. Maintaining a narrow, well-controlled HAZ ensures a strong, reliable, and long-lasting welded structure.