what is Heat affected zone

what is Heat Affected Zone – HAZ in welding?

Base metal vs HAZ

Metals in welding results can be categorized into 3 parts, namely: base metal (initial workpiece), heat affected zone (heat affected area) and weld metal (weld metal). The base metal is a base metal part where heat and welding temperature do not cause changes in the structure or properties of the metal. The heat affected zone (HAZ) is a base metal that is adjacent to the weld metal which undergoes a thermal cycle of heating and cooling during the welding process so that this area is the most critical of the welded joint. Weld metal is a part of metal which at the time of welding melts and then freezes, the composition of the weld metal consists of the parent metal component and the added material from the electrode.

The heat affected zone is formed during welding and includes the area of the material next to the weld, in which metallographic examinations can still detect structural changes through the welding process. These microstructural changes also change the properties of the material, so that the smallest possible HAZ is sought. This is achieved with low energy range, ie as concentrated as possible heat input at high welding speed, eg.  laser welding.

the heat-affected zone (HAZ) is divided into four zones

The heat-affected zone adjoins the weld on both sides. It extends to the base material, which was not influenced by the heat introduced during welding. For convertible steels, the heat-affected zone is divided into four zones:

  1. coarse grain or overheating zone,
  2. fine grain or normalization zone,
  3. zone of incomplete austenitization,
  4. Recrystallization and tempering zone. The heat affected zone of non-transforming steels shows only a coarse grain and a recrystallization and tempering zone.

Since crystallization often begins simultaneously at many points in the melt, a piece of metal is composed of a multiplicity of individual crystals grown together (also referred to as crystallites). This is the reason why metals often have no geometric shape typical for crystals. Crystallites are usually too small to be detected by the naked eye.

The association of crystallites or grains in metal is called a structure. Microstructure features include grain size, grain shape, and different phases or precipitations. The formation of the structure is determined by many factors such as the content of alloying elements or the manufacturing or processing conditions. The structure of a metallic material is usually visible only after a metallographic preparation.

heat affected zone
austenitic zone in Fe-Fe3c phase diagram

Coarse grains in Heat Affected Zone – HAZ

Coarse grains that occur in the HAZ region will cause the material to become very brittle. Apart from being caused by coarse grains, agitation can also be caused due to stress concentration due to weld defects. In connection with this, the reduction in embrittlement on the weld boundary is a very important effort in ensuring the toughness of the welded joints.

The best toughness will be obtained if a double structure of martensite and bainite is formed. Whereas when upper bainite and coarse ferrite are formed, the weld toughness becomes very low. The change in structure is caused by differences in chemical composition and differences in cooling speed due to welding heat, preheating.

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