Understanding the Heat-Affected Zone (HAZ) in Welding

Every arc weld leaves two obvious things behind: a bead of solidified filler and, right next to it, a narrow band of parent metal that never melted but did not escape unchanged. That band is the heat-affected zone, and in practice it is where many welded joints either prove themselves or fail. Good heat affected zone welding practice starts with understanding this band, because it explains why a joint can pass visual inspection and still crack, soften, rust or fracture in service.

This article explains what the HAZ is, how it forms, what it looks like in carbon steel, stainless steel and aluminium, and which practical levers a welder or engineer has to control it. It builds on the fundamentals covered in welding metallurgy basics, but goes deeper into the thermal cycle and the decisions that follow from it.

What the Heat-Affected Zone Actually Is

The HAZ is the part of the base metal whose microstructure and properties have been altered by the welding heat, without the metal having been melted. It sits between the fusion zone (the weld metal, which was liquid) and the unaffected base metal, which stayed too cool to change.

Three points are worth fixing in mind from the start:

• The HAZ is not filler metal and was never molten.
• It is made of the same material as the parent plate, so its chemistry is essentially unchanged; what changes is the structure.
• Its width is usually a few millimetres, but it varies with process, heat input, plate thickness and material.

The boundary between the fusion zone and the HAZ is often called the fusion line or fusion boundary. Because this narrow region saw the highest temperatures without melting, it is frequently the most problematic part of the joint.

How the HAZ Forms: The Weld Thermal Cycle

Welding is a moving heat source. Each point in the base metal experiences a rapid rise to a peak temperature, a brief time at or near that temperature, and then cooling at a rate set by the heat input, plate thickness and starting temperature of the plate.

Peak temperature falls steeply with distance from the weld. A point next to the fusion line may approach the melting range, while a point a few millimetres away might only reach a few hundred degrees Celsius. This gradient is why the HAZ is not one uniform region but a series of sub-zones, each with its own thermal history.

Two parameters of the cycle matter most:

1. Peak temperature, which decides what transformations or reactions can occur.
2. Cooling rate (often described as the time to cool through a critical temperature range), which decides what structure forms on the way back down.

Faster cooling generally favours harder, more brittle structures in hardenable steels. Slower cooling generally gives coarser, softer structures. Neither extreme is automatically good, which is the heart of HAZ control.

HAZ Sub-Zones in Carbon and Low-Alloy Steel

Carbon and low-alloy steels are the most studied case, and the textbook picture is a sequence of sub-zones moving outward from the weld.

Coarse-grained HAZ

Closest to the fusion line, the steel is heated well into the austenite range. Grains grow quickly at these temperatures, and on cooling they transform into a coarse structure. In hardenable steels this region can contain hard martensite or bainite, and the large grain size reduces toughness. This coarse-grained HAZ is usually the first suspect for brittle fracture and for hydrogen-related cracking.

Fine-grained HAZ

Further out, the peak temperature is high enough to transform the steel to austenite but not high enough for significant grain growth. On cooling it forms a fine, uniform structure. This is often the most favourable part of the HAZ, with good strength and toughness.

Intercritical HAZ

Still further out, the peak temperature falls between the lower and upper transformation temperatures, so only part of the structure transforms. The result is a mixed structure that can contain small islands of hard constituents. In multi-pass welds, a region that was already coarse-grained can be reheated into this range, creating the localised brittle zones that toughness testing is designed to catch.

Subcritical HAZ

At the outer edge, the metal never reaches the transformation range. Changes here are subtle: tempering of an existing structure, or softening in steels that were strengthened by cold work or heat treatment. In quenched-and-tempered or thermomechanically processed steels, this region can be the softest part of the joint.

Note: The names and boundaries of these sub-zones vary between textbooks and standards, and real HAZs blur into each other. Treat the list as a map of tendencies, not as sharp lines you could measure with a ruler.

What Controls the Size and Properties of the HAZ

The HAZ you get is the product of several interacting variables. Understanding them lets you change the outcome on purpose rather than by accident.

Heat input

Heat input is the energy delivered per unit length of weld. The common expression is:

Heat input (kJ/mm) = (Voltage × Current × 60) / (Travel speed in mm/min × 1000) × thermal efficiency factor

The thermal efficiency factor accounts for the fact that not all arc energy reaches the workpiece. European practice, as described in EN 1011-1 and summarised in AWS guidance, uses roughly 0.8 for shielded metal arc, flux-cored and gas metal arc welding, 0.6 for gas tungsten arc and plasma arc welding, and 1.0 for submerged arc welding. Always confirm the factor your governing code or procedure specifies.

Higher heat input generally widens the HAZ and slows cooling. Lower heat input narrows the HAZ and speeds cooling. That single trade-off drives many of the decisions below.

Process and technique

Different processes deposit heat differently. A concentrated, low-energy process such as TIG at modest current gives a narrow HAZ but a slow travel speed can still push total heat input up. High-deposition processes and thick-section submerged arc welding run much higher heat input. Weave width, travel speed and bead sequence all change the thermal cycle, even with the same machine settings on the dial.

Plate thickness and joint design

Thick plate conducts heat away in three dimensions, so it cools faster than thin sheet at the same heat input. A thick joint with a low-energy bead can therefore cool dangerously fast, while a thin sheet with high heat input can overheat and distort. This is why the same procedure cannot simply be copied across thicknesses.

Starting temperature

Preheat and interpass temperature change the starting point of the cycle and so slow the cooling rate. This is the main practical tool for managing hardening in susceptible steels.

Material composition

Carbon content and alloying elements determine how hardenable a steel is and therefore how severe the HAZ response will be. The same weld on two different steels can produce very different HAZs.

HAZ Problems in Carbon and Low-Alloy Steels

Hydrogen-assisted cold cracking

In ferritic steels, the classic HAZ failure mode is hydrogen-assisted cold cracking, also called hydrogen cracking or delayed cracking. It needs three ingredients at the same time:

• a susceptible (hard) microstructure,
• diffusible hydrogen in the weld area,
• tensile stress from restraint and residual stress.

In carbon-manganese steels such cracks normally start in the HAZ and can extend into the weld metal, as technical guidance from TWI describes. Because the cracking can appear hours or days after welding, inspection delay schedules matter for susceptible materials.

The control strategy attacks each ingredient: use low-hydrogen consumables and keep them dry, preheat to slow cooling, and reduce restraint where the design allows it.

Preheat and interpass control

Preheat requirements are normally taken from the welding procedure specification, which in turn follows a code or standard. For structural steel, AWS D1.1 includes prequalified minimum preheat and interpass temperature tables. It also provides an Annex B with alternative methods based on steel composition, thickness, diffusible hydrogen level and assumed restraint, and AWS D14.8M (equivalent to ISO/TR 17844) describes four methods to avoid cold cracks. The alternative methods can give lower, more tailored preheat than the conservative tables, but they require good knowledge of composition and hydrogen level.

The practical lesson is that more preheat is not automatically better. Excessive preheat or interpass temperature can soften the HAZ or degrade toughness in some steels, and it makes working conditions harder. The goal is the right amount, set by the procedure.

Hardness and sour service

Hardness is a convenient proxy for susceptibility to some forms of cracking. In sour (hydrogen sulphide) service, specifications place upper limits on hardness in the parent metal, weld metal and HAZ, because the HAZ is often the hardest region. This is why hardness traverses across the weld, through the HAZ and into the parent metal, are a routine part of procedure qualification for demanding applications.

Toughness loss and brittle zones

The coarse-grained HAZ can show reduced impact toughness. In multi-pass welds, local brittle zones can form where one pass reheats the coarse-grained HAZ of an earlier pass. Impact testing with notches placed in the HAZ, as specified by the procedure qualification requirements, is how this risk is checked.

Softening in high-strength steels

Quenched-and-tempered and thermomechanically controlled steels owe their strength to a carefully produced microstructure. The welding cycle can partly undo it in the outer HAZ, creating a soft band. Designers of high-strength structures account for this by limiting heat input and by using joint strength rules that recognise a weaker HAZ.

HAZ in Stainless Steel

Sensitization in austenitic grades

In austenitic stainless steels, the main HAZ concern is sensitization. When the steel dwells in roughly the 950 to 1450 °F range (about 510 to 790 °C), chromium carbides can precipitate at grain boundaries. This depletes the adjacent metal of chromium and can leave it vulnerable to intergranular corrosion. Welding, stress relief and hot forming can all pass through this range.

Three established approaches reduce the risk:

1. Solution annealing after welding, followed by rapid cooling, which dissolves the carbides.
2. Low-carbon grades such as 304L, 316L and 317L, with carbon below about 0.03%, which are designed to resist sensitization during typical welding. They do not protect against long service exposure in the critical range.
3. Stabilised grades such as 321 and 347, in which titanium or niobium ties up carbon.

The narrow band where this happens in the HAZ sits some distance from the fusion line, in the region that spends time in the sensitizing range, which is why corrosion attack often appears as a line parallel to the weld.

Other stainless concerns

Heat tint (oxide) in the HAZ also reduces corrosion resistance if it is not removed, and duplex stainless steels need controlled heat input to keep a balanced ferrite-austenite structure. TWI notes that reduced corrosion resistance is a common side effect of the high temperatures experienced by the HAZ in stainless steels.

HAZ in Aluminium Alloys

Aluminium behaves very differently from steel. There is no martensite to worry about; instead the HAZ response depends on how the alloy was strengthened.

• Non-heat-treatable alloys (for example 5xxx series) that gain strength from work hardening can lose some of it in the HAZ as the cold-worked structure recovers and softens.
• Heat-treatable alloys (for example 6xxx and 7xxx series) gain strength from fine precipitates. The HAZ can dissolve or coarsen these precipitates, producing a softened zone whose strength is lower than the parent metal.

Because the softening is a thermal effect, it cannot be fully avoided in fusion welding of strengthened alloys. Designers typically treat the welded joint as weaker than the parent material and rely on design values for welded condition. Lower heat input, appropriate filler metal choice and, where possible, post-weld heat treatment (not always practical) all influence the result. Always use the alloy-specific guidance in the relevant design standard.

Common Misconceptions

"A narrow HAZ is always better." A narrow HAZ comes from low heat input and fast cooling. In hardenable steels, that is exactly the condition that promotes hard microstructures. Narrow is better for distortion and softening, not necessarily for cracking.

"The weld metal is the weak point." In many structures the filler is designed to match or overmatch the parent metal, and the failure initiates in the HAZ instead.

"If it looks clean, it is sound." HAZ problems such as hydrogen cracks, sensitization or softening are not visible on the bead surface.

"HAZ means the same thing in every material." The mechanisms in steel, stainless and aluminium are different. Rules of thumb from one do not transfer to another.

Practical Ways to Control the HAZ

The following checklist summarises the levers discussed above. Use it as a conversation starter with your welding engineer, not as a substitute for a qualified procedure.

• Follow the welding procedure specification for heat input, preheat and interpass limits.
• Measure and record voltage, current and travel speed so heat input can be calculated, not guessed.
• Keep low-hydrogen consumables dry and stored as the manufacturer and code require.
• Clean joints of oil, moisture and coatings that can introduce hydrogen.
• Check preheat with a calibrated temperature indicator over the full joint, at a defined distance from the weld.
• Plan the bead sequence so that later passes temper, rather than harden, earlier ones where the procedure permits it.
• For stainless steel, select low-carbon or stabilised grades when welding in the critical thickness range, and clean heat tint.
• For aluminium, design for the strength of the welded condition rather than the parent condition.

A hypothetical example

Imagine two welders joining the same 25 mm structural steel plate. The first runs stringer beads at high travel speed on a cold plate in a draughty workshop. The second preheats to the procedure value, keeps the interpass temperature in range and uses dry low-hydrogen electrodes. This is a hypothetical scenario for illustration, but it shows the pattern: the first joint has a much faster cooling rate and more hydrogen available, so its HAZ is the more likely site of delayed cracking, even though the two beads may look equally good.

Testing and Inspecting the HAZ

Because many HAZ effects are invisible on the surface, the evidence comes from testing:

• Macro-etch examination reveals the HAZ width and shape on a cross-section.
• Hardness traverses (for example Vickers) measure peaks and dips across the weld, HAZ and parent metal.
• Charpy impact tests with the notch located in the HAZ check toughness.
• Microstructural examination identifies grain size and phases.
• Corrosion tests for stainless steels, such as the ASTM A262 practices referenced for intergranular attack, detect sensitization.
• Non-destructive testing such as magnetic particle or ultrasonic inspection, timed after any required delay, checks for cracks.

Acceptance limits come from the applicable code, specification or customer requirements, so there is no universal pass number.

When the HAZ Matters Most

Not every project needs the same level of HAZ attention. A light-duty bracket in mild steel is forgiving. Concern increases with:

• higher-strength or higher-carbon steels,
• thick sections and high restraint,
• low service temperatures where toughness is critical,
• corrosive environments, especially for stainless steel,
• fatigue-loaded structures where a local soft or hard zone becomes a crack initiation site,
• safety-critical equipment such as pressure vessels, pipelines and structural steelwork.

In those cases, qualified procedures, trained welders and documented inspection are the sensible route.

Conclusion

The heat-affected zone is the part of the joint that never melted but never stayed the same. Its structure is written by the thermal cycle, and its behaviour depends on the material: hardening and hydrogen cracking in steel, sensitization in austenitic stainless steel, and softening in strengthened aluminium.

The central message is practical. You cannot eliminate the HAZ, but you can shape it through heat input, preheat, interpass temperature, consumable choice and technique, all anchored in a qualified procedure. Treat the HAZ as part of the design, not as a by-product, and welded joints become far more predictable.