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 (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.
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What are the Causes of Heat-Affected Zones?
The heating associated with welding and/or cutting generally uses temperatures up to and often exceeding the temperature of melting of the material in question, depending on the welding process used. However, the heating and cooling thermal cycle associated with these processes is different to whatever processing has occurred with the parent material previously. This leads to a change in microstructure associated with the heating and cooling process.
The size of a 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 as well as the amount of heat going in to the material. 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 width 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 low heat input will cool faster, leading to a smaller HAZ, whereas high heat input will have a slower rate of cooling, leading to a larger HAZ in the same material. 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, as it affects the heat sink, and a larger heat sink generally leads to faster cooling.
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 width of the HAZ from the cut edge 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 heat the material, whilst 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.