Heat Affected Zone (HAZ) Definition
The heat affected zone (HAZ) is the part of the base metal beside a weld or thermal cut that was heated enough to change its microstructure and properties, but not melted. Hardness, strength and toughness can all shift in this band. It sits between the fusion zone (the melted weld metal) and the unaffected base metal.
"HAZ," "welding HAZ" and "heat-affected zone" all mean the same thing, and the term also applies to plasma, laser and oxy-fuel cutting. It forms because metal near the weld reaches different peak temperatures and cools at different rates, so its structure changes without melting (TWI, Wikipedia). The HAZ is often the weakest area of a welded joint, which is why welding codes and procedures control it (CWB Group). For a deeper look, see our article on the heat affected zone in welding.
What Is the Heat-Affected Zone in Welding? How It Forms and Why It Matters
Welding heat conducts from the molten pool into the surrounding metal. The peak temperature and cooling rate change with distance from the weld, and those two factors decide the final structure (ScienceDirect). Metal close to the weld gets hottest, while metal farther away barely changes. If you want background on how heating and cooling change steel microstructure, that guide covers the basics.
HAZ sub-zones in carbon steel
In carbon steel, the HAZ breaks into four regions. This table lists them in order from the weld, with approximate temperatures that depend on the steel.
| Sub-zone | Approx. peak temperature | Structure | Key effect |
|---|---|---|---|
| Coarse-grain | About 1100°C and above | Large grains, high hardness | Lower toughness, highest hydrogen cracking risk |
| Fine-grain | Just above about 900°C | Refined grains | Good toughness |
| Intercritical | About 727°C to 900°C | Mixed structure | Possible local brittle zones |
| Subcritical | Below 727°C | Tempered or softened | Little visible change |
Hardness rises in the coarse-grain region when the steel's carbon equivalent is high and cooling is fast. That raises the risk of cold cracking (TWI).
Heat input and HAZ width
Heat input is the main thing you can control. The formula is:
heat input (kJ/mm) = (volts × amps × 60) / (travel speed in mm/min × 1000), multiplied by process efficiency.
Higher heat input widens the HAZ and slows cooling. HAZ width depends mainly on heat input, process, material and thickness: laser cutting produces the narrowest, and stick welding and oxy-fuel cutting typically the widest. These indicative ranges shift with plate thickness and travel speed:
- Laser cutting: about 0.1–0.5 mm
- Plasma cutting: about 0.5–3 mm, depending on thickness and amperage
- Oxy-fuel cutting: about 2–6 mm or more on thick plate
- TIG welding: about 1–5 mm
- MIG welding: about 2–6 mm
- Stick welding: about 3–8 mm
For more on how TIG and MIG heat input differences affect the zone, see our process comparison. Stud welding uses very short, localized heat, so it produces a small HAZ.
How material changes the HAZ
Different metals react differently to the same heat (Inspectioneering):
- Carbon steel can harden near the weld. Our guide on how stainless and carbon steel respond differently to welding heat goes further.
- Austenitic stainless steel risks sensitization, where chromium carbides precipitate in roughly the 450–850°C range and reduce corrosion resistance.
- Heat-treatable aluminum alloys such as 6061 can lose significant strength in the HAZ, sometimes about 30–40% or more. See our notes on aluminum welding.
HAZ in plasma and laser cutting
Thermal cutting creates a HAZ too. Plasma leaves a thin hardened layer on the cut edge, which can affect machining or later weld quality. Laser has a narrower HAZ because the beam is tightly focused (Wikipedia). To limit the effect, use the correct amperage and speed, keep consumables in good condition, and grind or remove the cut edge before welding. Our high-tolerance laser cutting services page covers laser work in more detail.
How the HAZ is controlled
Fabricators manage the HAZ with a few proven methods (CWB Group):
- Preheat and interpass temperature limits
- Lower-heat-input processes or faster travel speed
- Controlled weld sequencing
- Post-weld heat treatment or stress relief
These controls are written into qualified welding procedure specifications under AWS D1.1 and CSA W59. At BSG, we work under CWB and AWS certification with 30+ qualified welding procedures across structural steel, mining and automotive work, and HAZ control is built into each procedure. You can read more about CWB and AWS qualified procedures and welding procedures by project type. It matters most on load-bearing work such as structural steel.
Heat Affected Zone FAQ
How to fix a heat affected zone?
You can't undo a HAZ, but you can restore its properties. Options include post-weld heat treatment or stress relief to soften hard zones and reduce residual stress, grinding out any cracks and rewelding under a qualified procedure with proper preheat, or replacing the part if damage is severe. The right fix depends on the material, code requirements and service conditions, so involve a qualified welding engineer or certified fabricator. Our steel welding guide gives wider context on how heat affects base metal.
How big is the heat affected zone of a weld?
It's typically a few millimetres wide. Indicative ranges are about 1–5 mm for TIG, 2–6 mm for MIG and 3–8 mm for stick, and the width grows with heat input, plate thickness and slower travel speed. Aluminum and thick sections can show wider effects. Welding procedure specifications and testing confirm the actual size for a given job (TWI).
How to reduce heat affected zone in welding?
Lower the heat input by using a lower-heat process or settings, increasing travel speed and using stringer beads. Control preheat and interpass temperatures rather than overheating, sequence welds to balance heat (such as backstepping), and use clamps or heat sinks where appropriate. Follow the qualified welding procedure to keep the HAZ within code limits. Our MIG vs TIG process comparison shows how process choice plays in, and CWB certified structural steel welding explains how certified procedures keep it in check.
What is the heat affected zone (HAZ) in laser cutting?
In laser cutting, the HAZ is the thin band of metal along the cut edge that is heated enough to change its microstructure without melting. Because the beam is tightly focused, it's typically very narrow, often a fraction of a millimetre, and narrower than in plasma or oxy-fuel cutting. Speed, power and assist gas all affect its width (CWB Group).
If your project needs HAZ-aware fabrication, our team offers certified welding services from our Kitchener facility.