Welding process of marine steel plate

Due to the relatively high alloy content of marine steel plates, the hardening tendency of steel is relatively large. During welding, the hardness near the fusion line is the largest, which reduces the ductility and toughness of the area. Due to the heat of welding, a hardened structure is generated. At the hardened structure, the stress or strain exceeds a certain limit, and the hydrogen exceeds a certain limit, and welding is prone to occur. Cold crack. Therefore, certain welding process measures must be adopted to ensure welding quality. 
　　
Selection of welding wire 
According to China Classification Society CCS , the welding wire must match the base metal.
　　
Preparation before welding 
Now take the 50mm  board butt weld as an example to introduce the welding process 
1. Base metal groove processing 
Groove processing generally adopts CNC cutting machine to automatically process, in order to reduce welding deformation and improve welding quality. The processing of the groove surface requires smoothness and no sawtooth. After processing, clean the oxide scale and rust in time, and then perform welding. Because cold cracks all occur in the "hydrogen-rich zone", a large amount of hydrogen is dissolved in the molten pool under the action of high temperature welding. During the cooling and solidification process, the transformation of austenite into ferrite causes a sharp decrease in the solubility of hydrogen. At this time, hydrogen is trying to escape, but due to the rapid cooling rate, hydrogen is too late to escape and remains in the weld metal, so that the hydrogen is in a supersaturated state. When the hydrogen concentration is high enough, root cracks or weld toe cracks are likely to occur. Moisture in the welding material, rust, oil stains and environmental humidity at the groove of the weldment are all causes of hydrogen enrichment in the weld. Therefore, the edge of the groove must be cleaned before welding. 
2. Assembly 
Assemble according to the drawing requirements, the gap between the workpieces during assembly should not exceed 2mm, and the misalignment should not exceed 2mm. The gap between the grooves should be consistent. After the plates are spliced, tack welding is carried out. The tack welding uses the same wire as the formal welding, and the height of the tack welding should not exceed 2/3 of the weld height. The length of tack welds is generally 5mm~10mm, and the length of weld spacing is 400mm. Tack welds should be set as far as possible on the root-cleaning side, and spot welding should be avoided at the welding start side to reduce welding distortion. After tack welding, check the weld for defects, and clean up the weld in time. 2.3 Preheat before welding 
　　
Considering the chemical composition and thickness of the plate, the hardening tendency of steel is relatively large, and welding cracks are prone to appear. Therefore, the plate must be preheated before welding. The function of preheating is to slow down the cooling rate of welded joints, and appropriately extend the cooling time of welded joints at 800℃ below AC3  line to 500℃ below AC1  line and 500℃~1000℃ on AC1  line, thereby reducing the quenched structure and conducive to hydrogen escape. To avoid cold cracks. For marine D36  high-strength steel, the preheating temperature should be controlled at 100℃~150℃. The range of local preheating is generally 100mm~200mm on both sides of the weld, and the temperature is strictly controlled.