What is Heat Affected Zone (HAZ) πŸ‘¨‍🏭 2022


Definition

It is the region of the base metal that was not fused during welding, but whose microstructure and mechanical properties have changed due to the heat of the weld.

Many people confuse the HAZ with the fused zone. 

Although the molten zone has many chemical and metallurgical transformations during its melting and solidification, and is thus affected by heat or temperature, it is not what we call HAZ. 

To better understand the distinction of these definitions, please look at the picture below:

The weakness of a HAZ

HAZ is a region that can become the weak point in a weld joint that, under normal conditions, would be strong enough. 

This is common in carbon steels, where in addition to the HAZ being less resistant than the weld metal (WM), it can be a source of defects and the source of a catastrophic failure.

Even though it is less resistant than WM, the qualification of a welding procedure must ensure that HAZ is stronger or at least preserves the characteristics of the original base metal.

Some of the reasons for the fragility of the HAZ of a carbon steel can be seen below:

  • The grains in the HAZ grow due to heat and peak temperature. Coarse-grained microstructures have lower toughness at low temperatures and, in general, lower corrosion resistance.
  • Heat also favors the coalescence and spheronization of steel components, such as cementite, reducing mechanical strength.
  • The typical thermal cycling of welding and its rapid cooling of the HAZ can aid and enhance the formation of a hard, brittle crystalline structure known as martensite.
  • The generated martensitic structure is fundamental for hydrogen absorption and cracking (cold cracking). This crack is generally not visible to the human eye but can severely compromise performance or reduce equipment life.
  • The increase in hardness caused by the new martensitic microstructure decreases corrosion and impact resistance.

One observation is that other types of materials may have a stronger HAZ, contrary to our traditional paradigm. As a result, other considerations may apply.

Regions of a HAZ

The embrittlement concepts above are generic. In addition to varying from material to material, they have variations within the HAZ itself. 

According to GRONG & AKSELSEN, a single pass weld can be divided into 5 very characteristic regions, depending on the peak temperature:

  1. Partially molten region: Temperature near melting point.
  2. Coarse grained region: Peak temperature above 1100ΒΊC.
  3. Fine-grained region: Peak temperature just above critical transformation temperature.
  4. Intercritical region: Peak temperature slightly below critical transformation temperature.
  5. Subcritical region: Peak temperature slightly below AC1 or AR1 temperature.

The representation below provides a perfect analogy between the thermal distribution, the peak temperatures reached and the division into regions.

A multipass welding naturally presents a greater complexity of analysis, as the following passes will temper the previous ones. 

It is important to note that the generalization of the above topic is only to facilitate understanding (didactics) and to direct quality control efforts.

The extension of a HAZ

It is worth noting that the HAZ is the zone affected by heat (temperature) in the base metal, and that it is not synonymous with a heated zone. 

The region that has its increased temperature is much larger, but we only consider the area “affected” or “damaged” by heat as a HAZ.

See below two macrographs with an indication of the HAZ. The macro on the right is much more evident given the much darker color of the HAZ.

Material property's change will depend mainly on:

  • Base material.
  • Thermal conductivity coefficient.
  • Preheat temperature.
  • Heat input.

Base material

HAZ characteristics fundamentally depend on the the thermal cycle and thermal distribution. And that depends on the type of base metal and welding procedure.

Depending on the type of metal being welded, the effects of the thermal cycle can vary a lot.

In the case of non-transformable metals (eg aluminium), the most marked structural change will be grain growth (or annealing in the case of work-hardened alloys).

Thermal conductivity coefficient

The thermal conductivity of the base material plays a big role. 

If the coefficient is high, the material has a high cooling rate and a small thermally affected zone. 

A low coefficient results in a higher heat concentration, a slower cooling rate and, consequently, a higher HAZ.

Heat input (by welding process)

The amount of heat introduced by the welding process plays an important role, for example:

  • Gas welding has a high heat input making the HAZ increase in size.
  • Laser welding, on the other hand, gives a very concentrated and limited amount of heat, resulting in a small HAZ.
  • Arc welding is between these two extremes, with great variation between each process according to the heat input.

Learn Welding

Would you like to learn about welding? Check my Quick Welding Course.

Citation

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COMMENTS

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Hi, I'm Gelson Luz.

I'm a Mechanical Engineer, expert in Welding and passionate about:

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Materials: What is Heat Affected Zone (HAZ) πŸ‘¨‍🏭 2022
What is Heat Affected Zone (HAZ) πŸ‘¨‍🏭 2022
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