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Glossary

Heat-Affected Zone (HAZ)

The region of base metal adjacent to a weld that is altered in microstructure and properties by the heat of welding without melting.

Definition

What is Heat-Affected Zone (HAZ)?

The Heat-Affected Zone (HAZ) is the band of base metal surrounding a weld that was exposed to sufficient heat to alter its microstructure — changing grain size, precipitate distribution, and mechanical properties — without reaching melting temperature. The extent and severity of the HAZ depends on material type, heat input (voltage × amperage × time), and preheat/interpass temperature management. In structural steel, the HAZ may become harder and more susceptible to hydrogen cracking; in aluminum alloys, the HAZ softens as heat-treatment temper is reversed; in stainless steel, excessive HAZ temperatures cause sensitization — chromium carbide precipitation that reduces corrosion resistance. Minimizing heat input through process selection (TIG over MIG) and technique controls HAZ size and its effects.

Common Applications

  • • Quality control inspection of welds
  • • Weld procedure qualification
  • • Post-weld heat treatment planning
  • • Material selection for weldable designs

FAQ

Frequently Asked Questions

How does the heat-affected zone affect weld strength?
In most structural steels, the HAZ can be harder than the base metal (martensite formation), but also more susceptible to brittle fracture. In heat-treatable aluminum alloys (6061-T6), the HAZ softens significantly — losing up to 50% of temper strength. In stainless steels, sensitization in the HAZ can cause intergranular corrosion in service environments.
How do you minimize the heat-affected zone?
Minimizing HAZ requires low heat input — achieved by reducing amperage and voltage, increasing travel speed, and using processes with concentrated arc energy (TIG vs. MIG). Interpass temperature limits, backing bars (heat sinks), and strategic weld sequencing all reduce cumulative heat input in multi-pass welds.
What is sensitization in stainless steel welds?
Sensitization occurs when stainless steel is held in the 425–870°C temperature range long enough for chromium carbides to precipitate at grain boundaries — depleting the adjacent area of chromium below the threshold needed for the passive oxide layer. Sensitized stainless corrodes rapidly in service environments containing chlorides or acids. Low-carbon grades (304L, 316L) and stabilized grades (321, 347) resist sensitization.

Related Terms

See Also

TIG Welding (GTAW) MIG Welding (GMAW) CWB Certification Stainless Steel Fabrication Aluminum Fabrication

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