What is the ‘Heat Affected Zone’ in Welding?
The Heat Affected Zone (HAZ) refers to the region of base metal that has not melted during welding but has been subjected to elevated temperatures significant enough to alter its microstructure and mechanical properties. This zone exists between the actual weld metal and the unaffected parent material. The extent and characteristics of the HAZ are influenced by various factors, including the material properties, the heat input during welding, and the specific process employed.
The size of the HAZ can vary considerably depending on the thermal properties of the material. For instance, metals with high thermal diffusivity, such as copper, transfer heat more rapidly, resulting in a narrower HAZ. Conversely, materials with lower thermal diffusivity, like steel, tend to develop a broader HAZ under the same welding conditions. The heat input, which depends on the energy supplied during welding, directly impacts the width and severity of the HAZ.
What Does the Colour Indicate in the Heat Affected Zone?
The variation in colour within the HAZ is a visual indicator of the temperature history experienced by the material. Different tints correspond to specific temperature ranges during heating, providing insight into the thermal cycle. These colours range from light yellow to dark blue, with each hue associated with a particular temperature threshold. Understanding these colours helps in assessing the extent of thermal influence and potential microstructural changes.
| Colour | Approximate Temperature |
| Light yellow | 290° C |
| Straw yellow | 340° C |
| Yellow | 370° C |
| Brown | 390° C |
| Purple brown | 420° C |
| Dark purple | 450° C |
| Blue | 540° C |
| Dark blue | 600° C |
Several factors influence the development of these heat colours, including:
- Surface condition: Rough, oxidized surfaces tend to produce more pronounced colouration due to faster oxidation rates.
- Surface contamination: Impurities such as rust, paint residues, or oil can alter the heat tint, although they generally do not affect the HAZ size.
- Oxygen availability: Limiting oxygen access, often through protective gases or coatings, can reduce oxidation and influence colour development.
- Chromium content: Higher chromium levels improve oxidation resistance, thus diminishing the intensity of heat tint coloration.
Causes and Formation of the Heat Affected Zone
The HAZ forms primarily due to localized heating during welding or cutting. When metal is heated but not melted, its microstructure undergoes transformations that can weaken the material or alter its properties. The width and characteristics of the HAZ depend on the heat input, thermal properties, and process parameters.
Role of Thermal Diffusivity
Thermal diffusivity is a key factor in determining how the HAZ develops. It is defined as the ratio of thermal conductivity to the product of density and specific heat capacity. Metals with high thermal diffusivity, such as copper, transfer heat quickly and cool down faster, resulting in a narrower HAZ. In contrast, materials like steel with lower thermal diffusivity tend to retain heat longer, creating a wider HAZ.
For example, the thermal diffusivity of stainless steel AISI 304 is approximately 4.2 mm2/s, whereas structural steel exhibits a higher value of around 11.72 mm2/s. Consequently, under identical welding conditions, structural steel cools more rapidly, producing a narrower HAZ.
Influence of Cutting and Welding Methods
The method employed during cutting or welding significantly affects the size and severity of the HAZ. Processes like flame cutting and arc welding generate substantial heat, resulting in a wider HAZ. In contrast, techniques such as plasma cutting allow for better control of heat input, leading to a thinner HAZ.
Advanced methods like laser cutting utilize a concentrated light beam, delivering localized heat that minimizes the HAZ. Water jet cutting and mechanical shearing do not involve heat application, hence they produce no heat-affected zone, making them preferable for applications requiring high structural integrity.
Impacts and Consequences of the Heat Affected Zone
The alterations in microstructure within the HAZ can lead to various mechanical and chemical property changes, often adversely affecting the material’s performance. Common effects include reduced toughness, increased susceptibility to cracking, and diminished corrosion resistance.
For stainless steels, high temperatures during welding cause chromium carbides to precipitate at grain boundaries, lowering local chromium content below the critical 10.5%. This phenomenon, known as weld decay or sensitization, results in intergranular corrosion, compromising the material’s corrosion resistance.
In conventional steels, hydrogen embrittlement may occur due to atomic hydrogen trapped during cooling, leading to potential cracking at stress concentration points like weld toes or within the HAZ. Proper welding techniques, preheating, and post-weld heat treatments can mitigate these issues.
Interestingly, in some materials such as aluminum alloys, the HAZ can become softer and weaker than the base metal, whereas in others, it may become harder and more brittle, posing challenges in design and service life predictions.
Strategies to Minimize the Heat Affected Zone
While it is impossible to eliminate the HAZ entirely, its size can be significantly reduced through various approaches. The most effective method involves increasing the welding or cutting speed, thereby decreasing heat input and exposure time.
Proper equipment setup and skilled operation are essential. Additionally, techniques like annealing — heating the metal to a specific temperature and then controlled cooling — can relieve residual stresses and reduce microstructural alterations within the HAZ.
Another way to eliminate the HAZ is through machining or grinding, removing affected material. However, this involves material loss and may not always be practical for all applications.
Which Welding Techniques Result in Smaller HAZ?
Welding processes with lower heat inputs generally produce smaller HAZs. The ranking of common welding methods based on typical heat input and their impact on HAZ size is as follows:
- Low heat input: Gas Tungsten Arc Welding (GTAW) or TIG welding
- Moderate heat input: Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Flux Cored Arc Welding (FCAW), and Metal Cored Arc Welding (MCAW)
- High heat input: Submerged Arc Welding (SAW)
- Very high heat input: Electro Slag Welding (ESW)
For instance, when welding steel with similar materials and joint configurations, the heat input directly influences the HAZ width. As an example, using typical parameters:
| Process | Current | Voltage | Travel Speed | Heat Input | Width of HAZ |
| Electro Slag Welding (ESW) | 800 Amps | 34 Volts | 0.32 mm/sec | 88 KJ/mm | 17.80 mm |
| Submerged Arc Welding (SAW) | 600 Amps | 28 Volts | 5.1 mm/sec | 3.3 KJ/mm | 3.10 mm |
| Shielded Metal Arc Welding (SMAW) | 200 Amps | 23 Volts | 3.4 mm/sec | 1.4 KJ/mm | 2.00 mm |
This comparison clearly demonstrates that processes with higher heat inputs produce significantly wider HAZs, potentially affecting the overall integrity and properties of the welded joint.