weld heat affected Zone
The welded joint can be divided into three main zones having different micro structures:
- base metal zone;
- heat affected zone;
- and a weld seam weld zone.
In gas welding, due to slow heating , the heat affected zone (near- heat zone) is larger than in the case of arc welding . The thermal influence zone has several structural sections caused by the heating temperature in the range of 450-1500 ° C and differing in the shape and structure of the grain.
The thermal influence zone consists of the following sections:
- incomplete fusion
- incomplete re-crystallization
- Cinematic area
Incomplete Melting Area
is transitional from deposited metal to the base. It is a region of a base metal heated slightly above the melting temperature, and is in a solid-liquid state. The nature of this section determines the quality of the welded joint, since it fuses the crystals of the weld metal with the grains of the base metal.
the region of the main, highly heated (from 1100 to 1500 ° C) metal with a coarse-grained structure and reduced mechanical properties. The metal in this zone has the structure of large pearlite grains with a ferrite network. In steels with a high carbon content, hardening structures are possible at the overheating site.
the region of the base metal heated in the range from 930 to 1100 ° C. The metal at these temperatures is relatively short and during cooling during subsequent re-crystallization acquires a fine-grained structure with the highest mechanical properties.
Incomplete Re-crystallization Site
the region of the base metal heated in the range of 720-930 ° C. This area is characterized by the fact that around large grains of ferite that have not undergone re-crystallization, there are small grains of ferrite and pearlite formed as a result of re-crystallization.
the region of the base metal heated in the range from 450 to 720 ° C. The site is characterized by the restoration of the shape and size of the broken metal grains, previously subjected to practice or pressure treatment.
the region of the base metal heated within 200 to 450 ° С does not receive visible structural changes. However, it is characterized by a decrease in plastic properties.
The total length of the weld heat affected zone in gas welding, depending on the thickness of the metal, is about 8 to 28 mm. To improve the structure and properties of the weld metal and the heat affected zone performed by gas welding, hot forging of the weld metal, heat treatment by heating with a welding torch and general heat treatment of the welded product by heating in furnaces and slow cooling are used.