Welding process of 304 lined stainless steel composite pipe

(304+Q235B) is a new type of composite pipe, which has the characteristics of corrosion resistance, high temperature resistance, high elongation, high strength, etc. It is widely used in the process pipelines of new and expanded petrochemical plants. The biggest use problem of this kind of pipe is: the problem of complete fusion welding of the base layer and the composite layer. In order to achieve a good welding effect, the correct welding method is:

1. Preparation before welding

Before welding, clean up the paint, dirt and oxide layer of the contact layer, so as to prevent the impurities from being decomposed into H2O and CO2 when heated during the welding process.

The base layer and composite layer should be preheated before welding. The preheating adopts electric heating method, based on the center line of the counterpart, the two sides are not less than 3 times the wall thickness, and not less than 50mm, and prevent overheating.

2. Welding process

The composite layer is welded first, and then the base layer is welded. The process is: sealing layer – bottom welding – transition layer – filling layer – cover layer.

1) When the pipe is butt welded, the inner wall should be flush, and the misalignment of the inner wall should not exceed 10% of the thickness of the pipe wall, and should be ≤ 2mm.

2) When welding, multi-layer welding should be used as far as possible, the joints of each welding layer should be staggered as much as possible, and the welding bead should not be too wide or too thick.

3) The tube must be filled with argon during spot welding to ensure the quality of the spot welding seam. When backing welding, the overhead welding position adopts the inner wire filling, and the vertical welding and horizontal welding positions adopt the outer filling wire method for welding.

4) The arc should be struck in the groove, and it is forbidden to strike the arc at the non-welding part. The arc connection should ensure penetration and fusion. The arc crater should be filled when the arc is extinguished. When the welding is about to end, the argon gas flow should be reduced to prevent air pressure. Too much can cause the weld to sink.

5) The welding wire should not be in contact with the tungsten grade or directly penetrate into the arc column area of ​​the arc to prevent the stability of the arc from being destroyed and tungsten inclusion defects.

6) The temperature between tracks should be controlled below 150℃. During the construction, the cooling method of water cooling on both sides of the welding seam is adopted, that is, the wet towel is wrapped on the steel pipes on both sides of the welding seam 80mm away from the welding seam, so that the welding seam can be reduced to below 150℃ as soon as possible.

3. After welding

After welding, the welding slag, weld bead spatter and other contaminants on the surface of the weldment should be carefully cleaned, and the welding seam should be partially trimmed if necessary. After welding, post-heat treatment should be carried out to eliminate residual stress. Post-weld treatment should be carried out according to design requirements. Electric heating method should be used for local heat treatment. The post-weld treatment of the base layer should select the heat treatment temperature according to the material requirements of the base layer, and other parameters should be calculated according to the total thickness of the stainless steel lined pipe (7±1.5) mm. Welding process of 304 lined stainless steel composite pipe