Welding technology of small diameter thin wall pipe and forged weld neck flange of UHP steel pipe tower

This paper describes in detail the key welding technology of the butt circumferential weld between the small-diameter thin-walled steel pipe and the forged welding neck flange in the steel pipe tower project of UHV AC transmission and transformation project. The butt circumferential weld of small-diameter thin-walled steel pipe and forged flange with neck has achieved beautiful appearance, the pass rate of one-time ultrasonic testing is 99.12%, the welding efficiency is more than 2 times higher than that of manual welding or semi-automatic welding, and the comprehensive welding cost is reduced by more than 40%.

In recent years, with the development of 1000kV UHV transmission line project, automatic welding technology has also been rapid development. At present, the nominal diameter ≥ 273mm, wall thickness ≥ 6.0mm steel pipe and forged welding neck flange butt ring weld has basically been automated or mechanized welding. And small diameter thin-walled steel pipe (diameter <273mm, wall thickness ≤ 6.0mm) and forged welding neck flange butt ring weld basically rely on manual welding or semi-automatic welding process welding. As we all know, with the diameter and wall thickness of the reduction, the welding difficulty increases, the appearance of the weld into poor shape, a flaw detection rate decreases and other problems arise.
To ensure the quality of small-diameter thin-walled steel pipe and forged welding neck flange butt ring weld and production efficiency, our company developed a small-diameter thin-walled steel pipe – forged welding neck flange mechanized welding production line, and the preparation of a special welding process program. After verification of the technology fully meet the quality requirements of the first-class weld seam, welding quality and production efficiency has been significantly improved, and manufacturing costs have been significantly reduced.

1. Raw materials and assembly quality requirements

• (1) The roundness of the steel pipe end within 200mm roundness deviation must be controlled within ± 1.5mm, too much ellipticity will cause the bevel angle, blunt edge size, root gap and misalignment of the alignment exceeds the standard, these problems will seriously affect the quality of welding.
• (2) Bevel angle bevel for V-shaped, bevel angle 30 ° ± 2.5 °, after assembly bevel angle should be 55 ° -65 °.
• (3) Blunt edge size blunt edge size must be controlled within ≤ 1.0mm, too large blunt edge size will reduce the melting penetration, easy to cause imperfections in welding.
• (4) Root group gap root gap shall be controlled in the range of 0.5-2.0mm, too small root gap is easy to cause root penetration defects, too large root gap is easy to cause burn through defects.
• (5) Misalignment edge misalignment shall be controlled in ≤ 2.0mm range, too large misalignment will affect the appearance of the weld forming, will also cause the weld penetration or burn through defects.
• (6) Positioning welding using tungsten arc welding for positioning welding. Positioning weld thickness must be controlled at 2.0-3.0mm, each section of the positioning weld length 25-35mm, and is divided into 3 equal parts for positioning welding.

2. Pre-welding preparation

• First, before welding, remove the rust, oil and other impurities from 20mm on each side of the edge of the bevel to be welded.
• Second, check whether the quality of assembly is in line with the process requirements, when the quality of assembly does not meet the process requirements must be reassembled.
• Third, check whether the roller frame is running normally.
• Fourth, debug welding current, arc voltage, shielding gas flow and welding speed and other parameters, so that it meets the process requirements.

3. Welding order and parameters

Using the internal and external double-sided welding process, first weld the inner ring welding channel and then weld the outer ring welding channel. Inner ring welding channel welding, the torch is five and a half clock position (slightly downhill) torch and workpiece transverse angle of 90 °, torch and workpiece longitudinal angle of 75 ° -85 ° backward, the length of the wire rod body 10-15 mm, and the arc welding at the positioning seam. When the arc runs to the closing position, an overlap of 20mm is added to prevent welding defects. After the welding of the inner ring welding channel, the outer ring welding channel does not need to clear the root, and in another certain position at the arc welding (to avoid the inner ring welding channel and the outer ring welding channel of the arc, the closing arc overlap and cause welding defects), the outer ring welding gun should be in the 12 o’clock position, the gun and the workpiece transverse angle of 90 °, the gun and the workpiece longitudinal angle of 80 ° -90 ° backward, the length of the wire rod body 10-15 mm, when the arc runs to When the arc runs to the closing position to increase the 20mm overlap weld to prevent welding defects. Specific welding parameters are shown in Table 1.

4. Welding process and quality control

Supervision and inspection of the welder’s qualifications, welding site conditions, welding materials and process documentation to ensure that the welding site conditions meet the welding requirements, welding materials used in accordance with the requirements of the code, welding process documentation effectively implemented to ensure the quality of welding.
Implementation of the reward and penalty system. For a non-destructive test pass rate of 100% of the weld, according to the original quota reward 20%; for a non-destructive test pass rate of less than 100% of the weld, according to the original quota down 20%. Through the reward and punishment system to improve the responsibility of the welding operator, improve the quality of welding.

5. Welding equipment

Small-diameter thin-walled steel pipe and necked forged flanges are welded by the company’s self-developed CA-XGHJ1 small-diameter thin-walled steel pipe and necked forged flanges welding production line. The production line consists of an internal welding head, external welding head, DC inverter arc welding power source, welding gun clamping, swing control device, adjustable roller frame, workpiece front and rear adjustment device and workpiece loading and unloading device, mechanized small-diameter steel pipe and forged welding neck flange welding production line as shown in Figure 1.
The welding production line features are as follows:

• (1) Flexible operation of the welding gun can be adjusted along the workpiece longitudinally, the welding gun clamping angle can be 360 ° arbitrary adjustment, the welding gun can be swung laterally, and the swing frequency can be adjusted, the operation is flexible and convenient.
• (2) Stable parameters using the Tangshan Panasonic DC inverter digital display welding power supply. Welding current and arc voltage fluctuation is small.
• (3) Excellent welding quality of the production line welding the circumferential butt weld and traditional manual welding or semi-automatic welding process has a beautiful weld formation, welding spatter less (almost no spatter), low oxidation in the weld area, a high rate of ultrasonic testing (up to 99.85%), small weld residual height (0-2.0mm), small weld width (1.0-2.5mm), weld edge Straightness (deviation of 0-1.0mm) and other characteristics of small diameter thin-walled steel pipe with neck forged flange butt ring weld effect as shown in Figure 2.
• (4) High welding efficiency single-shift production of small-diameter thin-walled steel pipe and forged welding neck flange components 80-120, the efficiency of the traditional manual welding or semi-automatic welding welding process 2-4 times.
• (5) Low production costs of more than 200% reduction in labor costs, welding consumables to reduce the amount of 20%-30%, 30%-50% reduction in post-weld cleaning workload, the overall cost of more than 40% reduction.

6. Welding materials

According to the base metal material selection welding materials. Tower manufacturing welding materials commonly used to select as shown in Table 2. Protective gas using 80% Ar + 20% CO2 argon-rich gas mixture.

7. Conclusion

1000kV UHV transmission project steel pipe tower small-diameter thin-walled steel pipe and forged welding neck flange butt ring welding seam with welding difficulty, a non-destructive testing pass rate is low, low production efficiency and other difficulties. The traditional manual welding or semi-automatic welding process is difficult to meet the requirements of welding quality and production schedule. Mechanized welding technology, the application of small-diameter thin-walled steel pipe and forged welding neck flange butt ring weld welding quality and production efficiency has been greatly improved, production costs have been significantly reduced, achieving high quality, high efficiency, low consumption of good results.
Table.1 Welding parameters

Weld bead level	Torch position	Gas flow/L.min-1	Welding current/A	Arc voltage/V	Welding speed/mm.min-1	Swing/mm	Pauses on both sides	Heat input/kJ.cm-1
Inner ring	Half past five	15-18	160-200	18-22	350-500	0-2	0-2	≤8
Outer ring	Twelve o’clock	15-18	250-270	28-30	300-400	0-2	0-2	≤17

Note: In addition to the root gap is larger, the outer ring welding mouth wider, welding rob should try to avoid swing.

Table.2 Tower manufacturing common welding materials

Base metal material	Welding wire model	Welding wire specification Φ/ mm
Q235、Q345	ER50-6	1.2
Q420、Q460	ER55-D2/ER55-G	1.2
Q345. Welded with Q420 and Q460	ER50-6	1.2

Figure.1 Small diameter steel pipe and forged welding neck flange butt joint mechanized welding production line

Figure.2 Small diameter steel pipe and forged welding neck flange butt joint welding seam

Author: Zhao Liangui

Source: China Weld Neck Flanges Manufacturer – Yaang Pipe Industry Co., Limited (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, and 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.)

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