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Analysis and treatment of welding crack of 304 stainless steel flange

In our power generation enterprises, stainless steel pipes and flanges are often used for equipment pipeline connection. In some water systems or acid-base pipelines, stainless steel is often used to replace carbon steel. In the welding process, the welding joints or flanges are often cracked. Therefore, it is an important content to analyze and discuss the cracks and welding work at this stage. This article will discuss the related problems of 304 stainless steel flange cracks, analyze the real causes, and provide some reasonable suggestions for the specific treatment process.

Generally speaking, 304 stainless steel has excellent plasticity and corrosion resistance, but its yield strength is relatively poor. Since the internal carbon content is very scarce, it has relatively good weldability and is suitable for the manufacture of installation parts. However, its own thermal coefficient is relatively small, so it has a relatively high resistivity. The actual welding work must strictly abide by the relevant regulations to prevent burning problems.

Brief description of the problem

In this experiment, the internal cooling water pipe was selected, and the connection method was flange connection. The internal materials of the inner cold water pipe and the flange are all 304 stainless steel, and a K-shaped groove is set at the specific weld position.
During the overhaul, an obvious crack was found in the internal weld of the inner cold water pipe. After polishing it, a large amount of welding slag was found inside. Specifically, the part that has not been welded is about 16 mm, the welding depth of the inner cold water pipe is about 12 mm, and the outer welding part of the draft tube is 16 mm.
After discovering the problem, the crew expanded the scope of inspection, isolated the internal cooling water system, and inspected the internal welds, and found that the cracks were very similar to the previous cracks and had certain commonalities. At the same time, the most obvious flange itself has very obvious cracks.
The initial treatment method selected is to replace the flange, polish all welds, and re-weld. The specific welding method is argon arc welding. In order to ensure that the progress of maintenance will not be delayed, the temperature between the layers is not controlled. After completing the welding work, it was found that the cracks in the 304 stainless steel flange body gradually increased.

Cause analysis

(1) Analysis of weld cracks. The 304 stainless steel flange and draft tube were inspected by means of spectral inspection, and it was found that the internal chemical composition was completely different from that of 304 stainless steel.
The main cause of weld cracks is that there is insufficient penetration in the actual welding process, and the welding slag between the layers is not cleaned. From this, it can be judged that there are certain defects in the weld itself, which leads to the existence of cracks on the surface of the weld, and the lack of penetration at the root causes the most serious result. Such defects are usually hazardous defects. When the pipeline is put into operation, cracks will occur in the parts that are not completely welded, which will lead to cracks.
(2) Crack analysis of 304 stainless steel flange body. The carbides are found inside the inner cold water pipe, which is relatively small compared to the crystal grains of the 304 stainless steel flange.
From this, it can be found that a large amount of carbide precipitates inside the 304 stainless steel flange body, so the internal strength is greatly reduced. In addition, the crystal stays within the range of the sensitization temperature for a long time during the actual solution treatment, and corrosion occurs.
(3) Analysis of repair cracks. In the process of rework, the interlayer temperature has not been effectively controlled, causing the weld to stay in the sensitization temperature range for a long time, resulting in intergranular corrosion. The particle size inside the inner cooling water pipe is relatively large, but there is no carbide precipitation. However, the 304 stainless steel flange has a tendency to corrode, and the welding operation further aggravates the process, causing the 304 stainless steel flange to crack.

Treatment process

  • (1) 304 stainless steel flange replacement. In order to prevent the phenomenon of intergranular corrosion during the manufacturing process of 304 stainless steel flange, the specific manufacturing process is often limited. It is usually required that the temperature of the final satin must exceed 860℃, and it is necessary to preheat the platform and the actual anvil surface in advance. In the actual forging process, try to prevent carburization as much as possible, and at the same time prevent flame spraying onto the blank. Therefore, it is necessary to increase the carbon of the steel so as to effectively increase the sensitivity of the steel body to intergranular corrosion. After forging, it must be cooled quickly, especially near the sensitization temperature. At the same time, the working temperature of solution treatment usually needs to reach 1050°C to 1200°C. In order to prevent the internal crystal grains from expanding too fast, the temperature of the solid solution should be lower, and the actual holding time should be shorter. In addition, note that rapid cooling is required after the solution is completed.
  • (2) Welding repair. Welding repair work usually uses welding rods to avoid the joints staying at the sensitization temperature during the actual welding process. Using a smaller current and using rapid welding can effectively limit the actual intake of heat, so that the actual width of the weld can be controlled. At the same time, it is also necessary to control the temperature between the layers to ensure that it is below 140°C. If conditions permit, water can be used to force the temperature down, and the welding slag remaining between the layers can be removed by mechanical methods. In order to ensure that the quality of welding can meet the relevant requirements, it can be tested during actual operation. After 24 hours after all welding is completed, conduct a comprehensive inspection to ensure that there is no damage, and then it can be judged as qualified.
  • (3) Conclusion. First, compare the inner cooling water pipe with the 304 stainless steel flange, and it can be found that in the metallographic structure of the 304 stainless steel flange, there is carbide accumulation inside the grain boundary, and it is very obvious. Therefore, it can be inferred that it has a tendency to corrode to a certain extent.

The main reason for the cracking of the weld is that the weld itself is not fully penetrated. In the actual operation of welding rework, if the welding heat input is not effectively controlled, the crack will also occur. At the same time, it will cause large-scale damage to the specific affected area of intergranular corrosion. Therefore, in the process of welding rework, it is necessary to strictly control the actual heat input and the temperature maintained between layers to ensure that the sensitization temperature will not affect the weld.

Conclusion

In summary, 304 stainless steel is a material with a very high application rate in current life. Therefore, it is necessary to analyze the specific causes of their common problems and adopt a plan to deal with them so as to further improve them.

Source: China Stainless Steel Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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