What is the ‘heat affected zone’ in welding?
The heat affected zone (HAZ) is that area of metal that has not been melted and has undergone changes in properties as a result of being exposed to relatively high temperatures during welding. The HAZ is the area between the weld or cut and the base (unaffected), parent metal.
The HAZ area can vary in severity and size depending on the properties of the materials, the concentration and intensity of the heat, and the welding or cutting process used.
The width of a heat-affected zone (HAZ) is influenced by the amount of heat going into the material which is related to the heat input of the welding process. The size of the HAZ is also influenced by the thermal diffusivity.
Those materials with a high level of thermal diffusivity are able to transfer the h eat faster, which means for a certain level of heat input, they cool quicker and, as a result, their HAZ width is reduced.
The thermal diffusivity of copper is significantly greater than that of steel and so, for the same heat input, the HAZ of the copper would be narrower than that of a steel.
What Does the Colour Tell?
As a result of using different temperatures during manufacturing processes, a variety of tints are present in the HAZ. These tints range from light yellow to dark blue in increasing order of the temperature.
The band colours in order of temperature progression are:
| Colour | Cutting temperature |
| Light yellow | 290º C |
| Straw yellow | 340º C |
| Yellow | 370º C |
| Brown | 390º C |
| Purple brown | 420º C |
| Dark purple | 450º C |
| Blue | 540º C |
| Dark blue | 600º C |
actors that further affect the formation of these heat tints are:
- Surface condition – Rougher surfaces oxidise faster producing more pronounced colouration.
- Surface contamination – Impurities like rust, paint and oil also affect the tint. The contamination may change the heat tint but the extension of the HAZ is unaffected.
- Oxygen availability – As limiting access to oxygen reduces oxidation, using an electrode coating or a protective gas for welding can affect the heat tint.
- Chromium content – Chromium increases oxidation resistance. Therefore, higher chromium content reduces the intensity of the heat tint.
What are the Causes of Heat-Affected Zones?
A zone is formed between the melted metal and the unaffected base metal called the heat affected zone (HAZ).
In this zone, the metal is not melted but the heat has led to changes in the metal’s micro-structure. These changes in structure can reduce the metal’s strength.
The cause of the forming of the HAZ is clearly heat. The width of the zone still depends on several factors, like thermal diffusivity and choice of cutting methods.
Thermal Diffusivity
Thermal diffusivity of metal plays the primary role in determining how HAZ will affect the metal. It is the ratio of the thermal conductivity of the metal divided by its density and specific heat capacity at constant pressure.
In simple words, the thermal diffusivity of a metal is the measure of how fast heat will be transmitted through its body. If the thermal diffusivity is high, the metal will be able to transmit heat sooner.
This leads to faster cooling and the HAZ will be narrower. On the other hand, low thermal diffusivity will keep heat in the metal for a longer duration and create a wider HAZ.
The thermal diffusivity of stainless steel AISI 304 is 4.2 mm2/s, whereas for structural steel it is 11.72 mm2/s. This means that structural steel, when subjected to heat, will create a smaller HAZ as it will cool down quicker.
The creation of HAZ depends upon various other factors too. The zone width depends on the amount of heat generated, the duration of the exposure to heat and material thickness.
Thin sheet metal heats up more quickly and therefore creates a larger heat affected zone.
The Choice of Cutting Method
Every thermal cutting method is a little different. Thus, the resulting heat affected zone also varies.
Flame cutting and arc welding produce the maximum amount of heat and have the widest HAZ of all.
Fast and steady welds keep the exposure to heat to a minimum. Thus, having an experienced welder can reduce the HAZ size and consequently lead to a stronger joint.
The size of the HAZ in plasma cutting is relatively thinner as cutting speeds can be manipulated to give a thin HAZ.
Laser cutting will create an even smaller HAZ because it has a narrow cutting kerf and the heat is applied to a small area.
Processes such as water jet cutting and shearing do not create a HAZ as they do not involve overheating the material. This is something to consider when designing parts that need extra reliability.
What are the Effects of Heat-Affected Zones?
Due to the heating experienced within the HAZ, the microstructure and properties in this region change so that they differ to that of the base material.
These changes are usually undesirable depending on the material, the changes may give – higher or lower strength, susceptibility to cracking, reduced corrosion resistance or lower toughness. As a result of this, the HAZ is often an area where failures can occur.
Reduced corrosion resistance is a common side-effect of the high temperatures experienced by the HAZ in stainless steels.
The heat produced in the weld bead area causes chromium carbides to precipitate around the grain boundaries in the HAZ, causing the local chromium content to drop below 10.5%, at which point the steel loses the ability to form a passive film and is no longer stainless. This results in intergranular corrosion, also known as sensitisation or weld decay.
In conventional steels, hydrogen embrittlement is another undesirable side-effect of the high temperatures, whereby atomic hydrogen dissolved in the weld metal is trapped in the cooling weld, and is rejected into the transforming HAZ.
The hydrogen diffuses to the region of greatest strain (usually the weld toes or HAZ), creating additional pressure within the lattice and potentially causing cracking.
Hydrogen can be removed by the correct selection of welding parameters, and preheating or postheating as appropriate.
In some cases, the HAZ is harder and stronger than the parent material, which can cause problems, but in others, such as aluminium, the HAZ is softer and weaker than the parent material.
These can both be challenges for design and use of components.
How to Reduce the Heat Affected Zone
It is impossible to completely eliminate the Heat Affected Zone but it can be reduced. The key is your speed, whether you are welding or cutting metal meaning shorter exposure to heat leaves a lesser HAZ.
Being able to reduce your speed does depend on the equipment you are using and the operator. Having knowledge of setting up machines to the best performance leads to the best results. Annealing is also another method which helps reduce the HAZ.
This includes heating the metal holding it a a specific temperature to fortify the elemental bonds. The most effective way to get rid of the entire heat-affected zone is to machine it away but, this does lead to a loss of material.
Which Types Of Welding Processes Produce Less HAZ?
With regard to welding processes, assuming the same material, thickness and joint being welded, then those processes that give lower heat inputs will cool faster.
This will lead to a smaller HAZ. Conversely, the higher heat input processes will have a slower rate of cooling, thus leading to a larger HAZ.
For the purposes of discussion, we can rank the heat input of the common welding processes as:
- Low: Gas Tungsten Arc Welding (GTAW)
- Medium: Shielded Metal Arc (SMAW), Gas Metal Arc (GMAW, Flux Cored Arc (FCAW) and Metal Cored Arc (MCAW)
- High: Submerged Arc Welding (SAW)
- Very High: Electro Slag Welding (ESW). The Electroslag Welding Process cannot be regarded as a common welding process but, it is used for fabricating thick materials in one pass and is included here for comparison purposes.
By reference to Table 1 below, three processes with typical heat inputs (typical welding parameters) have been selected in the medium to very high range for the welding of steel.
The difference in HAZ size is immediately evident. The SMAW process, with a heat input of 1.4 KJ/mm has a 2 mm wide HAZ while the very high heat input ESW, at a heat input of 88 KJ/mm, has a 17.80 mm wide HAZ
| Process | Current | Voltage | Travel Speed | Heat Input | Width of HAZ |
| Amps | Volts | mm/sec | KJ/mm | mm | |
| Electroslag (ESW) | 800 | 34 | 0.32 | 88 | 17.80 |
| Sub Arc (SAW) | 600 | 28 | 5.1 | 3.3 | 3.10 |
| Shielded Metal Arc (SMAW) | 200 | 23 | 3.4 | 1.4 | 2.00 |