What Does Heat-Affected Zone (HAZ) Mean?
A heat-affected zone (HAZ) is the portion of the base metal that was not melted during brazing and cutting/welding, but whose microstructure and mechanical properties were altered by the heat. This alteration can be detrimental, causing stresses that reduce the strength of the base material, leading to catastrophic failures.
The HAZ occurs inside the metal and cannot be seen. The HAZ may need to be partially or completely removed (by grinding or some other process) before the metal part can be used.
Corrosionpedia Explains Heat-Affected Zone (HAZ)
Preferential corrosion of a heat-affected zone is a widely known phenomenon. High-temperature exposure and welding can significantly affect the microstructure and properties of the heat-affected zone that makes them more susceptible to corrosion. The microstructural changes that occur in heat-affected zones include carbide precipitation and intermetallic phase formation.
Heat affected zones can be of varying size and strength. The extent and magnitude of the HAZ is inversely proportional to the thermal diffusivity and cooling rates of the material:
- Where thermal diffusivity is high, the material cooling rate is high and the HAZ is small.
- Where thermal diffusivity is low, the cooling rate is slower and the HAZ is larger.
The extent and magnitude of property change depends primarily on:
- Base material
- Weld filler metal
- Amount and concentration of heat input by the welding process
The width of the HAZ is influenced by:
- Cut speed - In general, faster speeds result in a smaller HAZ.
- Amperage (when using plasma) - For a given thickness of metal, a higher amperage (and consequently a faster cut speed) results in a smaller HAZ.
- Type of metal being cut - Increased temperatures and longer cutting times will result in a wider HAZ.
All thermal cutting processes create a heat-affected zone in the cut metal. The amount of heat inputted by the welding process plays an important role:
- Oxyfuel welding uses high heat input and increases the size of the HAZ.
- Laser and electron-beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ.
- Arc welding falls between these two extremes.
The heat from the welding process and subsequent re-cooling causes change from the weld interface to the termination of the sensitizing temperature in the base metal. These changes can be minimized by following proper welding procedures and using low-carbon stainless steel alloys.