Cause analysis and Countermeasures for incomplete penetration of weldolet

Reciprocating piston compressor, the most commonly used positive displacement compressor, has the advantages of mature technology, wide range of pressure, high thermal efficiency, strong adaptability, large range of exhaust volume regulation, low material requirements, and is widely used in various gas compression airports in the fields of petroleum and chemical industry.
The weldolet is often used for the transition connection between the main pipe and branch pipe of the safety valve, and the threaded weldolet is often used for the transition connection between the main pipe and the temperature and pressure measurement end.
Compared with the traditional connection type, the branch connection has many outstanding advantages, such as safety and reliability, cost reduction, simple construction, improvement of medium flow channel, series standardization, and convenient design and selection.
However, the special structure and weld form of the weldolet increase the difficulty of detection and the complexity of the process. The lack of penetration in the welding process is difficult to detect scientifically and repair effectively. The lack of penetration in the joint has a great influence on the structure. It reduces the effective bearing area of the weld, reduces the strength of the welded joint, causes stress concentration, and increases the safety risk of the unit.

Observation and analysis of inteal appearance of welding joint of weldolet

The appearance of welds on weldolet and main pipe of a unit is shown in Figure 1, and the inteal section is shown in Figure 2.

Figure 1

Figure 2

It can be seen from Fig. 2 that there are a large number of incomplete penetration in the connection weld of weldolet abutment in the actual production process. The possible causes of incomplete penetration are irregular opening, too small assembly gap, improper welding position, too small current, too fast running speed, arc blowing deviation, etc.

Cause analysis

Irregular opening

The weld appearance with mechanical opening is shown in Figure 3. It can be seen from the figure that under the condition that other factors remain unchanged, a large part of the root area of the weld still has incomplete penetration when the mechanical opening is adopted, indicating that the irregular opening is not the main reason for the incomplete penetration of the weld.

Figure 3

Adjust the group clearance

The weld appearance of flame opening mode is shown in Figure 4 after adjusting the assembly clearance, and that of mechanical opening mode is shown in Figure 5. It can be seen from Fig. 4 and Fig. 5 that most areas have been fully welded, indicating that the main reason for lack of penetration is the assembly gap.

Figure 4

Figure 5

The penetration effect of vertical welding and overhead welding position is not good, because these two positions can not promote the droplet away from the end of the welding wire under the same gravity as that of the flat welding position, which is conducive to the weld pool transition.
Manual TIG welding is used for backing, the accessibility of welding gun at the flat position is better, and the weld pool is easy to observe. The welder is more likely to adopt short arc welding, the heat is more concentrated, the heat affected zone is smaller, and the deformation is smaller, which is conducive to improving the welding quality.

The relationship between gap and penetration rate of different pairs is shown in Table 1. It can be seen from table 1 that too small gap and too much gap are not conducive to penetration. The gap is too small, the welding gun is not easy to penetrate into the root of the weld, and the arc can not be well transited to the back of the weld through the gap.

Table 1 Gap and penetration of different pairs

The gap is too large, the melting hole is too large, it is easy to bu through, the back of the weld is too convex, which is not conducive to the gas flow, and is not conducive to the connection of temperature measurement, pressure measurement and other parts. When the gap is 2.5-5mm, the penetration length ratio of single weldolet can be greatly increased.
Under the reasonable clearance, the penetration length of a single weldolet with mechanical opening is slightly longer than that with flame opening, and the weld appearance quality is also better than that with flame opening. This is because adopting mechanical opening is conducive to ensuring the uniform clearance of assembly, and the difference in weld width and residual height is relatively low.

Ensure welding position

On the basis of machining the groove and ensuring the assembly clearance, adjust the welding position and weld the weldment in the flat welding position. The results are shown in Fig. 6 and Fig. 7. It can be seen from the figure that the penetration area of the same weldolet is 100%.

Fig. 6

Fig. 7

The complete sample is radiographed, all areas meet the requirements of domain level, indicating that all welding is complete, and there is no incomplete and inclusion defects.

Parameter validation

The microstructure of the weld after cutting, grinding and corrosion of the qualified test sample is shown in Fig. 8, and the microstructure of the heat affected zone is shown in Fig. 9. Fig. 8 is mainly composed of ferrite and pearlite, and Fig. 9 is mainly composed of ferrite and cordierite.

Fig. 8

Fig. 9

The acicular ferrite nucleates and grows in the austenite grain, which can refine the austenite grain and improve the toughness of weld and HAZ.
Acicular ferrite has fine sub crystal structure and high-density movable dislocation, which is easy to realize multiple slip and is beneficial to the improvement of performance.
Using the same parameters to weld the butt test piece of pipe and pipe, the weld area and heat affected area are tested to get similar structure, the bending sample is tested to meet the standard requirements, and the tensile and impact samples are tested to exceed the standard requirements.

Preventive measures

• (1). The incoming weldolet abutments shall be accepted in strict accordance with their manufacturing standards to ensure that their shapes, dimensions and tolerances meet the requirements, so as to prevent nonstandard manufacturing from affecting the weld quality, and the assembly clearance shall be reasonably controlled during assembly.
• (2). The welder meeting the requirements shall be used for welding to prevent the welder’s skills from affecting the penetration of the weldolet abutment. The prefabricated weldolet abutment and the main pipe, and the welds of the weldolet abutment and the main pipe shall be welded in the flat position, which is conducive to the penetration of the weldolet abutment.
• (3). Clean, process, assemble, weld, record and inspect in strict accordance with the process requirements.

Conclusion

• (1). Too small or too large welding clearance is the main reason to affect the penetration of weldolet, and the clearance between weldolet and main pipe should be 2.5-5mm.
• (2). The weldolet and the main pipe are welded in the flat position, and the weld pool transition is promoted under the gravity, so the accessibility of the welding gun is better, and the welder is easier to observe the weld pool, which is conducive to the penetration of the weldolet.
• (3). Under the reasonable clearance, the mechanical opening method is conducive to improving the penetration length ratio and weld appearance quality of a single weldolet.

Source: China Pipe Fittings Manufacturer – Wilson Pipeline Pipe Industry www.wilsonpipeline.com

(Wilson Pipeline 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. Wilson Pipeline products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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