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What is the Heat Affected Zone (HAZ)?

   

The heat affected zone (HAZ) is a non-melted area of metal that has undergone changes in material properties as a result of being exposed to high temperatures. These changes in material property are usually as a result of welding or high-heat cutting. The HAZ is the area between the weld or cut and the base, parent metal.

Heat Affected Zone (HAZ) DiagramThe 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.

What are the Causes of Heat-Affected Zones?

The heat affected zone is influenced by the level of thermal diffusivity, which is dependent on the thermal conductivity, density and specific heat of a substance. Those materials with a high level of thermal diffusivity are able to transfer variations of heat faster, meaning they cool quicker and, as a result, the HAZ is reduced. On the other hand, those materials with a lower coefficient retain the heat, meaning that that the HAZ is wider. Generally speaking, the extension of the HAZ is dependent on the amount of heat applied, the duration of exposure to heat and the properties of the material itself. When a material is exposed to greater amounts of energy for longer periods the HAZ is larger.

With regard to welding procedures, those processes with high rates of heat input will cool faster, leading to a smaller HAZ, whereas slower rates of heat input will have a slower rate of cooling, leading to a larger HAZ. In addition, the size of the HAZ also grows as the speed of the welding process decreases. Weld geometry is another factor that plays a role in the HAZ size. 

High temperature cutting operations can also cause a HAZ and, similarly to welding procedures, those processes that operate at higher temperatures and slow speeds tend to create a larger HAZ, while lower temperature or higher speed cutting processes tend to reduce the HAZ size. The depth of the HAZ is determined by the cutting process, cutting speed, and the material properties and thickness. 

Different cutting processes have differing effects on the HAZ, regardless of the material being cut. For example, shearing and waterjet cutting do not create a HAZ as they do not overheat the material, while laser cutting creates a small HAZ due to the heat only being applied to a small area. Meanwhile, plasma cutting leads to an intermediate HAZ, with the higher currents allowing for an increased cutting speed and thereby a narrower HAZ, while oxyacetylene cutting creates the widest HAZ due to the high heat, slow speed and flame width. Arc welding falls between the two extremes, with individual processes varying in heat input.

HAZ Areas

While the HAZ occurs between the weld or cutting zone and the unaffected parent metal, the HAZ itself has different areas according to how close they are to where the cutting or welding heat was applied to the material.

The cutting or weld zone is the liquid region where the process itself takes place and is adjoined by the fusion boundary. The fusion boundary is the border of the fusion zone where the liquid and solid phases of the metal coexist. Further from the weld or cutting zone is the HAZ itself, which is where the non-melted parent metal has experienced changes to the microstructure. The HAZ can be broken down into the grain growth zone (closest to the heated area), the recrystallized zone, the partially transformed zone and the tempered zone. Outside of these HAZ areas is the unaffected base material.

The various HAZ areas are formed by differing temperatures in the base metal further from the weld or cut itself. This can, for example, be clearly seen by a series of visible coloured bands, caused by surface oxidation, near a weld in stainless steel. These different colours, also known as heat tint, offer an approximate indication of the temperature reached by the metal. In the case of stainless steel type 1.4301 (AISI 304) heated in open air, the band colours and associated temperatures are as follows:

Light yellow 290 °C / 550 °F

Straw yellow 340 °C / 640 °F

Yellow 370 °C / 700 °F

Brown 390 °C / 735 °F

Purple brown 420 °C / 790 °F

Dark purple 450 °C / 840 °F

Blue 540 °C / 1000 °F

Dark blue 600 °C / 1110 °F

The heat tint colours depend on the material’s resistance to oxidation, with those metals with a higher steel chromium content showing less intense colouration as they are more resistant to oxidation. The use of protective gas and electrode coatings can also reduce heat tint as they partially shield the metal from oxidation. Conversely, rougher surfaces oxidise faster, causing darker colours. In addition, paint, oil, rust and even fingerprints can alter the heat tint, although they do not impact the extent of the HAZ itself.

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 and lead to this area becoming weaker than the base metal. This can, in turn, lead to residual stress, welds cracks (both hot cracking and cold cracking), increased brittleness, lowered material strength and a lessened resistance to corrosion. As a result of this, the HAZ is often the area where failures occur.

Furthermore, in stainless steels, surface nitriding can lead to increased hardness and a decreased weldability.  Corrosion is another 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, causing the 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.

Hydrogen embrittlement is another undesirable side-effect of the high temperatures, whereby gas diffuses through the weld metal, creating pressure within the lattice and reducing the tensile strength and toughness of the metal. Unless the hydrogen gas is removed it can cause a spontaneous type of cracking in just 24 hours after heating.

In some cases, the heating can generate localised hardening and austenitic stainless steels can become martensitic, increasing both hardness and brittleness. In other cases, heated metal can become weaker.

How to Reduce the Heat Affected Zone

Reducing the heat affected zone can help alleviate the related problems of HAZ cracks, corrosion, embrittlement, and so forth. This can be achieved by heat treatment following the normalisation or annealing of the material. However, this process can be costly and time-consuming and may not offer a complete solution. Heat treating is also limited by the comparative size of the part and the torch or furnace used for the treatment.

Cutting can also be used an alternative to heat treatment to reduce the impact of the HAZ. This can be achieved through either grinding or cutting, although care needs to be taken not to introduce more heat to the part.

Machining is the most effective way to remove the entire HAZ, but this can reduce the material yield as well as increasing labour and machine time.

Finally, heat tint from oxidation can be ground away or removed with fine sandpaper. This will expose the underlying layer and activate self-passivation of the chromium – although this can also weaken the part.

Ideally, the heat affected zone should be minimised through the selection of the correct welding or cutting process for the material. TWI can help with this type of selection as well as being able to advise on the best materials for your job.

For assistance with welding, cutting, materials selection and the heat affected zone, please email:


contactus@twi.co.uk