In Multi-pass Welds, the nomenclature of the weld, the zones affected by the welding heat when a butt weld is made by more than one pass or layer, and the nomenclature applying to the grooves used in butt welding are shown in figure 6-14. Figure 6-15 is based on weld type and position.
The primary heat affected zone is the area fused or affected by heat in the first pass or application of weld metal. The secondary heat zone is the area affected in the second pass and overlaps the primary heat zone. The portion of base metal that hardens or changes its properties as a result of the welding heat in the primary zone is partly annealed or softened by the welding heat in the secondary zone. The weld metal in the first layer is also refined in structure by the welding heat of the second layer. The two heating conditions are important in determining the order or sequence in depositing weld metal in a particular joint design.
Multipass welds enable a variety of weld joints and plate thicknesses to be welded with the same procedures and materials. In certain base materials, the multiple pass welding technique must be used to maintain adequate properties in the base HAZ (heat affected zone).