What Is a Heat-Affected Zone (HAZ)? Definition, Causes & Control
What Is a Heat-Affected Zone (HAZ)?
The heat-affected zone (HAZ) is the portion of base metal adjacent to a weld or thermal cut that has not melted but has undergone permanent microstructural changes due to heat exposure. When you weld or thermally cut metal, the intense heat doesn't stay contained to the area that actually melts. It radiates outward, and the surrounding solid metal absorbs enough thermal energy to alter its internal grain structure, hardness, and mechanical behaviour, without ever becoming liquid itself.
According to TWI Global, the HAZ sits immediately adjacent to the fusion zone, which is the region where base metal actually melted and fused with filler material. Temperatures within the HAZ typically range from around 200 °C up to just below the metal's melting point. That range is wide enough to trigger phase transformations, grain growth, and significant changes in hardness, all without the material ever liquefying.
As described by Wikipedia's overview of the heat-affected zone, this makes the HAZ metallurgically distinct from both the weld metal (fusion zone) and the unaffected base metal further away. Because it has been structurally changed without being fully remelted and homogenised, it's frequently the weakest or most brittle region in a finished joint, a fact that the CWB Group highlights as central to understanding weld quality.
For anyone specifying or reviewing welded assemblies, understanding the HAZ is foundational. Our Welding Services page covers the processes we apply, and our Steel Welding: A Comprehensive Welder's Guide goes further into how these thermal effects play out across different steel grades.
HAZ in Depth: Zones, Influencing Factors, Mechanical Effects & How to Control It
The Structure of a Thermally Processed Joint
A welded joint isn't simply "weld metal" and "base metal." Moving outward from the weld centreline, you'll find at least four distinct sub-regions. The fusion zone is where full melting occurred. Just beyond it is the partially melted zone (PMZ), where temperatures reached the solidus boundary. Next is a narrow unmixed zone at the weld boundary. Then comes the true HAZ, where solid-state metallurgical transformations happened without any melting at all, as documented in Wikipedia's technical breakdown.
Within the true HAZ itself, there are further gradations. Closest to the fusion line, temperatures approach the solidus, producing coarse austenite grains that cool into brittle martensite or bainite in carbon and alloy steels. This coarse-grain sub-zone is typically where the worst mechanical degradation occurs. Further out, a fine-grain recrystallised sub-zone and a partially transformed sub-zone exist, each with progressively milder property changes.
How Heat Input Controls HAZ Width
Heat input is the single most controllable variable a fabricator has. It's calculated using the formula: (Voltage × Amperage × 60) ÷ Travel Speed in mm/min. Doubling heat input roughly doubles HAZ width in carbon steel, which is why travel speed and parameter selection matter so much in practice. TWI Global's FAQ on HAZ causes and Inspectioneering's HAZ reference library both identify heat input as the dominant variable fabricators can directly control.
Material-Specific Behaviour
Different metals respond to HAZ heat in very different ways, and that's where process knowledge becomes essential.
Carbon and alloy steels are prone to martensitic hardening in the HAZ, which raises hardness but reduces toughness. A hardness value exceeding 350 HV in the HAZ of carbon steel is widely used under standards such as AWS D1.1 and CSA W59 as a threshold indicating unacceptable brittleness risk. In structural steel applications, this is a direct structural integrity concern, and it's why steel plate fabrication specifications always address preheat and interpass temperature requirements.
Aluminium alloys don't harden in the HAZ the way steel does. Instead, they suffer softening due to dissolution of strengthening precipitates. This is a key challenge our team addresses when providing aluminum welding services in Ontario, and it's particularly critical when you're welding steel to aluminum, where two very different HAZ responses occur side by side.
Stainless steel faces a different problem called sensitisation, where chromium carbide precipitates at grain boundaries in the HAZ, reducing corrosion resistance in those zones. We cover this in detail in our article on whether you can fabricate stainless steel. Controlling interpass temperature and using low-carbon or stabilised grades (304L, 316L, or titanium-stabilised alloys) significantly reduces sensitisation risk.
Process Selection and HAZ Width
Welding process choice directly governs how much heat enters the base metal. Laser welding produces the narrowest HAZ of any common fusion process, often under 1 mm. TIG/GTAW follows, then MIG/GMAW, with submerged arc welding (SAW) generating the widest thermal influence. The same principle applies to cutting: as we explain in Demystifying How Laser Cutting Works, laser cutting produces a HAZ of only 0.1–0.5 mm along the cut edge, compared to several millimetres for plasma cutting or oxyfuel processes. That's one of the primary reasons engineers choose laser cutting for parts requiring tight tolerances or further welding at the cut edge, a comparison we make directly in Laser Cutting vs. Waterjet Cutting.
For processes like stud welding, the HAZ is typically small due to the short arc duration, though it's still present and must be accounted for in load-bearing applications. By contrast, high heat-input joining methods can produce HAZ widths that compromise performance under stress, which is part of why spot welding falls short in high-stress applications.
Practical Controls: Preheat, Interpass Temperature, and PWHT
Preheat slows the post-weld cooling rate, reducing the risk of martensitic transformation and hydrogen-induced cold cracking (HIC) in medium- and high-carbon steels. In multi-pass welds, enforcing a maximum interpass temperature (commonly 250 °C for carbon steel) prevents cumulative heat build-up that would progressively widen and degrade the HAZ.
Post-weld heat treatment (PWHT), typically stress-relief annealing or tempering, is the standard corrective method for reducing HAZ hardness and residual stress after welding is complete. It restores toughness and dimensional stability without any further fusion.
On the shop floor, low heat-input techniques such as pulsed MIG, cold metal transfer (CMT), and stringer beads rather than weave passes all reduce HAZ width without sacrificing fusion quality. Proper fixturing and clamping also matter: controlling distortion caused by differential thermal expansion across the HAZ and base metal prevents secondary residual stress concentrations from forming.
HAZ in Industrial Applications
In automotive fabrication, HAZ softening in advanced high-strength steels (AHSS) used for body-in-white structures can reduce joint strength by 20–40% relative to the base metal. In mining and heavy equipment work, HAZ embrittlement in wear-resistant plate steels such as AR400 or AR500 is a primary failure risk.
This is exactly where certified welding procedures make a measurable difference. As we explain in Why Partnering with a CWB and AWS Certified Manufacturer Matters, BSG Inc. holds CWB and AWS certification across 30+ welding procedures. That means the amperage, voltage, travel speed, preheat, and interpass limits for every procedure are formally validated to keep HAZ properties within specification for each base material, not estimated on the fly. The CWB Group's own guidance on HAZ emphasises that certified procedures are the most reliable mechanism for HAZ control in production welding environments.
Heat-Affected Zone: Frequently Asked Questions
Q: How do you fix a heat-affected zone?
The microstructural changes in a HAZ can't be reversed by rewelding, but their effects can be corrected. Post-weld heat treatment (PWHT), such as stress-relief annealing or tempering, is the most common corrective method. It reduces HAZ hardness and residual tensile stress, restoring toughness. In thermal cutting applications, grinding or machining the cut edge removes the narrow HAZ band entirely. Shot peening or laser peening can introduce compressive residual stresses that counteract HAZ-induced brittleness. In all cases, prevention through controlled heat input and proper preheat is more cost-effective than remediation after the fact.
Q: How big is the heat-affected zone of a weld?
HAZ width depends on the welding process, heat input, and base material. In precision laser welding, it can be less than 0.5 mm wide. TIG (GTAW) welding on thin material typically produces a HAZ of 1–3 mm. MIG (GMAW) welding on structural steel commonly yields 3–6 mm. High heat-input processes such as submerged arc welding on thick plate can produce a HAZ exceeding 10 mm. Fabricators measure HAZ width using Vickers microhardness traverses across a weld cross-section or by metallographic examination of etched samples.
Q: How do you reduce the heat-affected zone in welding?
Reducing HAZ width means lowering the total heat delivered to the base metal. Practical methods include reducing amperage and voltage while maintaining adequate fusion, increasing travel speed, using pulsed MIG or cold metal transfer (CMT) processes, running stringer beads rather than wide weave passes, and enforcing a maximum interpass temperature. Selecting an inherently lower heat-input process, such as TIG or laser welding, for critical joints is the most effective single change you can make. CWB/AWS-certified welding procedures specify these parameters formally for each material and application.
Q: What is the heat-affected zone (HAZ) in laser cutting?
In laser cutting, the HAZ is the thin band of material along the cut edge that has been heated above its transformation temperature but not removed by the laser beam. Because laser cutting concentrates energy into an extremely small spot at high travel speed, the HAZ is typically only 0.1–0.5 mm wide, far narrower than the HAZ from plasma cutting (up to several millimetres) or oxyfuel flame cutting. This narrow HAZ is a key advantage for parts that will be subsequently welded or that require tight dimensional tolerances. For applications where even a minimal HAZ is unacceptable, waterjet cutting is a non-thermal alternative, as we explain in our laser cutting vs. waterjet cutting comparison.