What is the 'Heat Affected Zone' in welding and which types of Welding Processes produce less HAZ?

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 located between the weld and the unaffected base metal and is illustrated graphically in Figure 1.

Figure 1 Graphical Representation of the Heat Affected Zone Location

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 heat 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.

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

Table 1. Comparison of Welding Process and HAZ Width

So, from this representative table, it can be seen that the lower heat input welding processes produce the smallest heat affected zones, all other things being equal. Figure 2 illustrates a macro-section from a portion of a 75mm, multipass SMAW weld at a magnification of less than x 5. The darker etching heat affected zone is seen located between the weld metal and base (parent) metal. The heat input was controlled at 1.5KJ/mm max and the material, due to thickness, was also preheated to 150 deg C. The resulting HAZ is 2.00 mm in width.

Figure 2 Macro of a SMAW Weld showing the three zones. Base Metal, Weld metal and the Heat Affected Zone (HAZ)

In terms of the metal properties, there are other things to consider when evaluating those of the HAZ itself. Due to the variations in temperature across the HAZ, the individual portions will have differing properties as they see different temperatures for different amounts of time. This fact, and its importance or otherwise, is beyond the scope of this article but may be covered in a sister article at a future date.

Other than welding, most cutting processes also produce a HAZ and precautions may have to be taken after cutting depending on the metal and the cutting process used. Conversely, the waterjet cutting process does not create a HAZ as it does not heat the material.