Arc Welding Heat-Affected Zone

Welding obviously involves quite a lot of heat, and for every stick, flux-core, MIG, or TIG, there is a corresponding arc welding heat-affected zone on either side of the metal. This is an area where the metal has experienced a change in its material properties due to the high temperatures associated with arc welding. It is a transition zone between the welded metal and the unaffected metal of the workpiece.

An arc welding heat-affected zone can vary in size depending on the welding process used, the metal’s size and properties, the heat’s intensity, and the length of time arc welding was used. How big of an issue a heat-affected zone depends on the welding process. In some low-precision processes like automotive welding, a large heat-affected zone is considered a plus for cosmetic reasons. The pressures involved will never pose a problem for the weld or the base metal. In high-pressure or high-purity applications, an excessively large arc welding heat-affected zone can severely compromise the suitability of the workpiece.

What Is an Arc Welding Heat Affected Zone?

Arc welding heats a metal well beyond its melting point, albeit in a very small area relative to the overall size of the workpiece. The areas surrounding the weld are subjected to heating and cooling that are substantially different compared to those processes that created the base metal in the first place. This leads to changes in the microstructure of the material. The type and extent of these changes largely depend on:

• Metal Dimensions: More material equals more mass to absorb and dissipate heat across. Thicker materials will generally be able to handle more heat for longer, minimizing the heat-affected zone compared to thinner metals.
• Metal Type: The type of metal also plays a role. Copper and aluminum are thermal conductors par excellence, allowing heat to dissipate quickly through their mass. Steel and stainless steel are far less conductive, meaning they are far more prone to heat-affected zones. Other metals like titanium are not thermally conductive in any meaningful sense but are so resistant to heat that heat alone will not meaningfully impact their strength. Instead, the issue is that heat makes the metal reactive, allowing brittle compounds to form if the hot titanium isn’t properly shielded.
• Weld Parameters: Generally, flux-based welding processes like shielded metal arc welding (SMAW) and flux corded arc welding (FCAW) will result in greater heat input and larger arc welding heat-affected zones than gas-shielded processes like gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW). Factors such as travel speed and oscillation can also substantially alter the properties of an arc welding heat-affected zone with slower movement increasing heat input and the extent of the heat-affected zone. 
• Heat Treatment: Although it sounds nonintuitive, pre and post-weld heat treatments can actually mitigate the microstructural effects of an arc welding heat-affected zone. By evenly heating a piece before welding starts, the differences between the heat-affected area and the rest of the workpiece are reduced. Likewise, post-heat treatments can reduce and remove the internal stresses that come from different portions of the workpiece cooling at different rates.

Microstructural alterations in the heat-affected zone are ultimately due to metal expanding when heated and contracting when cooled. As one area of metal is heated, it expands compared to parts of the metal that are not, leading to microstructural alterations. These can be further compounded if cooling is uneven when the heat is removed. The effect leads to distortions in the metal and internal stresses as distorted metal presses and pulls against itself. Thermally conductive metals like copper and aluminum are less prone to this sort of distortion because their conductivity helps dissipate heat throughout the entire workpiece and distributing the heat helps keep expansion more equal between heat affected and cool zones in the workpiece. These are essentially metallurgical changes in the metal due to heat, and there is also a range of chemical changes involved with arc welding heat-affected zones.

Chemical Reactions In Heat-Affected Zones

Elemental materials, or materials that are overwhelmingly composed of single types of atoms, are by their nature highly reactive with other elements. Metals are some of the most atomically pure materials that people encounter in their daily lives. Their reactivity only increases when energy is introduced to these materials, and arc welding is very energy-intensive. One of the hallmarks of arc welding heat-affected zones is the chemical bonding of the base metal with atmospheric gases. This bonding is usually indicated by surface-level changes in the coloration of the metal. Chemical bonding types include:

• Nitriding: Nitrogen is the most abundant element in the atmosphere, and under high heat conditions, this gas can bond to metal forming nitrides. These nitrides are very hard (cemented nitrides are used in tooling bits) but brittle and can reduce the metal’s weldability.
• Oxidation: The most reactive gas in the atmosphere is oxygen, and it takes relatively little heating for most metals to react with oxygen-forming oxides. These oxides are what are responsible for the coloration changes that indicate heat-affected zones.
• Hydrogen: Hydrogen has an unusual reaction with metals. Atmospheric hydrogen can diffuse into molten metal during welding. When the molten metal cools, this hydrogen can be expelled into the grain boundary between the weld and the heat-affected zone leading to embrittlement and cracking.

These chemical reactions under heating are so common that the resulting colorations in the heat-affected zone can be used to identify heating in some metals. The colors form a basis for accepting or rejecting the resulting weld. The common coloration coding for stainless steel welding, for example, can be seen in the chart below.

Stainless Steel Coloration Guide
Color	Temperature Celsius	Temperature Farenheit
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 tints that result in an arc welding heat-affected zone are minimized in stainless steels with high chromium contents resulting in diminished coloration. Shielding gas can also prevent heated metal from coming into contact with atmospheric oxygen leading to a distinctive line of uncolored metal at the border of the weld and the heat-affected regions. In spite of its lack of color, this area is a part of the heat-affected zone and can potentially have metallurgical changes.

How Do Heat-Affected Zones Affect Metal?

The changes in a heat-affected zone can be both metallurgical and chemical. They can have a significant impact on the overall strength of the final assembly. Heat-altered metals are likely to be harder than the original base material but trade away ductility and lower the overall strength of the assembly as the hardened areas create a point around which stresses can accumulate instead of being evenly distributed. Hydrogen diffusion in the weld or an arc welding heat-affected zone can cause cracking without introducing additional external stressors to the welded assembly. Chemical changes can vastly reduce the material’s corrosion resistance in both the near and long term. 

Whether or not these are undesirable changes depends on the role the assemblies are used in. In certain automotive roles like air or exhaust systems, it is unlikely for the contained gases to ever hit the thresholds needed to strain even heat-altered high-strength metals like titanium or even basic steels. In aviation or aerospace, the strains of daily operation can be such that even minute alterations of a base metal from the calculated norm can cause a critical failure of a weld or the surrounding arc welded heat-affected zone. In pharmaceutical welding, even slight surface alterations can have significant implications for product hygiene and corrosion resistance.

Avoiding the negative impacts of arc welding heat-affected zones is a matter of choosing the right welding process. Gas-shielded arc welding methods like GTAW or GMAW are highly recommended for high-purity applications like aviation welding and biopharmaceutical welding, as well as in high-heat and high-stress applications like power generation and petrochemical welding.