The material used for the large-diameter head and tube sheet of the pressure vessel is thick plate low-alloy steel. The welding process is usually a submerged arc automatic welding and pre-weld preheating welding method. However, in actual production, separation deformation occurs at both ends of the joint during preheating. When the deformation is severe, the positioning weld is pulled off, so that the welding work cannot be performed, and the root bead of the submerged arc automatic welding is prone to crystal cracking. In order to solve the above problems, based on the many practices, a non-preheating welding method for thick plate heads is summarized.

1. The effect of preheating

When preheating, the heating expands the base metal. Since the ends of the joint are free ends, the restraint is small, and the separation deformation occurs under the action of the internal expansion stress, and the ends tend to open. If the size of the positioning weld is increased, the amount of misalignment in the direction of the thickness during preheating will increase, resulting in an outfit.

2. Influence of welding method on rotary deformation

The rotational deformation typically occurs when the unwelded portion of the joint moves (closes or separates) when welded from one end of the plate to the other. The rotational deformation is affected by the line energy and the welding speed. With manual arc welding with small line energy, the unwelded portion of the joint tends to close; when the large-line energy submerged arc is used to automatically weld the steel sheet, the unwelded portion of the joint tends to open.

The submerged arc automatic welding welding head joint seam, the separation force generated during the preheating and the separation force generated during the welding may be large enough to break the positioning weld seam.

3. The influence of the groove form

Thick plate joints usually open X-shaped groove. When using submerged arc automatic welding, the size of the blunt edge should not be too small, otherwise it will burn through. Thus, due to the large dilution ratio of the root bead, the weld beading coefficient is seriously deteriorated, and crystal cracks are inevitably caused, which is the main cause of cracks in the root bead.