Heat affected zone

from Wikipedia, the free encyclopedia
Annealing colors in the HAZ through TIG welding.

In the heat affected zone (HAZ), a structural change takes place during production without melting the material . This can result in unwanted changes in the material properties , or even warpage and cracks due to thermal expansion . Especially in processes with high energy requirements, the spread of heat in the material is clearly recognizable by the tarnish color .

The changed properties depend heavily on the material used, but often form a weak point in the component.

Size of the zone

The extent of the heat affected zone depends on the geometry and thermal conductivity of the workpiece, the temperature of the process used and its duration / feed rate . If the energy can be concentrated on a small area, i.e. the material can be heated faster, a higher feed speed can be used and the time can be reduced, which results in a smaller HAZ. If the thermal energy is distributed more quickly (higher heat conduction), the HAZ is also larger. Examples:

  • A steel with high thermal conductivity can be quenched by its own heat conduction and converted into a martensite structure ( hardening ).
  • If the heat is removed very slowly, the surface layer can burn faster if the energy input is too high.

In general, it can be summarized as follows: workpieces with low thermal conductivity and processes with high energy density have a smaller heat-affected zone.

Manufacturing examples

Cutting kerfs in plasma / laser and oxy-fuel cutting. WEZ in orange.

A typical example for comparing manufacturing processes are laser / plasma and oxy-fuel cutting . The plasma arc creates a curved cutting surface, the laser beam creates a very narrow and straight kerf due to the high concentration of energy and the flame in oxy-fuel cutting heats the material more intensely on the processing side.

In the case of thermoplastics , the long polymer chains can break (thermally induced chain cleavage and recrosslinking), which results in greater brittleness . The temperature required for this depends heavily on the chemical composition of the plastic . Increased plasticizer losses can also be seen in this area.

In machining production, enough frictional heat can arise to negatively influence the surface layer or to fall outside the tolerance due to thermal expansion . In most cases, it is sufficient to use cooling lubricant or to reduce the speed.

In the welding process, the differences in the generation of heat lead to different results. The HAZ can be divided into four zones for transformable steels:

  1. Overheating zone
  2. Normalization zone
  3. Zone of incomplete austenitization
  4. Recrystallization and tempering zone

Zones 2 and 3 do not apply to non-transformable steels (without α-γ-α transformation).


Friction stir welding : HAZ in light red, mechanical and thermal influence in dark red.

The consequences can be minimized by conventional post- treatment methods , or even completely eliminated by heat treatment . The preheating of the workpieces and the resulting reduced temperature difference can also be very successful.

Alternatively, processes with lower heat generation can also be used, e.g. E.g .: water jet cutting instead of laser cutting. When friction stir no molten bath is generated, but the material in the pasty state (attained by frictional heat ) to each other "kneaded". The lower temperature results in a better transition to the cold material.

Possible consequences


Web links

Individual evidence

  1. ↑ Heat affected zone. Retrieved May 28, 2020 .