Please help me to analyze the cause of cracking.

First of all, it should be determined that it is an equipment manufacturing problem. Manufacturing problems include equipment material and manufacturing process. Judging from the picture, this is a pressure vessel, not an on-site installation part. Therefore, this problem needs to be solved by the equipment manufacturer, and the person who unties the bell must also tie the bell.

What is the base material? Although welding will produce stress, even if the welding process such as specifications and improper welding sequence will produce more stress, it will not be enough to cause cracking. The main consideration is the base metal. If the base metal needs to be preheated before welding, but is not preheated or the preheating temperature is not enough, cracking and expansion will occur under the influence of heat. The other is whether there are non-metallic inclusion defects in the base material.

The opening position did not meet the requirements, the welds overlapped, and the overall preheating was insufficient, resulting in excessive stress.

The material 16Mn has very good weldability, so the material reason should be ruled out. However, the picture shows that this structure has disadvantages such as stress concentration and overlapping heat-affected zones. The welding process should appropriately adjust parameters, number of weld bead layers, current size, etc. to minimize the level of stress concentration. At the same time, the heat treatment process must keep up, and preheating and post-weld heat treatment must be in place in a timely manner. I don’t know if it’s due to limited space or other reasons. The tube section of the pipe box is designed to be so short, and the nozzle is so large and so close to the flange. If reinforcement is needed, priority should be given to overall reinforcement, such as increasing the thickness of the section or forging the pipe. As shown in the picture, the reinforcement ring is welded to the flange cone section. The design should be economical under the premise of safety.

The other party's technology said that the welding has been evaluated and there is no problem. The temperature on the day of welding is above 0°C and no preheating is required.

It seems to be stress cracking, caused by improper welding process or improper selection of base material.

The picture shows a crack in the board. Although the welds have overlapping factors and should not be welded like this, can the stress be so great that it tears the plate? Have you considered the strength of the board? Is this a crack discovered after lifting or a problem discovered after welding? It should be analyzed comprehensively from all aspects. personal opinion

Mainly considering the material defects of the base metal, there is no problem with the welding process. Because these cracks occur in local areas of the forging flange, the overlap of the welds must have an impact on them. From a design perspective, it is necessary to change the reinforcing structure type of the nozzle. The base metal is more likely to have non-metallic inclusion defects.