Welding technology of welding neck flange

What is a welding neck flange?

Welding neck flange is a kind of pipe fitting, which is a kind of flange with neck and transition of round pipe and butt welding connection with pipe. The butt welding flange is not easy to deform, well sealed, widely used, with corresponding rigid and elastic requirements and reasonable butt welding thinning transition. The distance between the welding junction and the joint surface is large, and the joint surface is free from welding temperature deformation. At present, it has been widely used in petrochemical industry, urban water supply and power system. Especially in recent years, with the planning and construction of UHV transmission line project in the State Grid, the demand for UHV steel tube tower has increased dramatically.

As a new form of steel tube tower flange connection, butt welding flange has been proved to be advanced and reliable, which plays an important role in improving the production capacity and product quality of steel tube tower.

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Welding method

There are two kinds of circumferential welding methods for UHP flange and steel pipe, i.e. manual gas shielded welding and full-automatic swing gas shielded welding.
Based on the above two kinds of welding methods, according to the characteristics of the welding process, production process, component shape and other factors, the manufacturers of the special machine for internal and external submerged arc welding are developed to realize the circumferential welding of the welding neck flange and the steel pipe. The process is to use submerged arc welding to weld the workpiece with double-sided penetration, and there is no need to clear the root during the welding process Under the premise of ensuring the welding quality, the welding efficiency is improved to the maximum extent. At present, it has been able to realize the double side submerged arc welding with a minimum diameter of about 300 mm.
With the continuous production of neck butt welding flange, the welding method and welding technology are also constantly improved. According to the specification and size of the actual workpiece, the welding schemes such as manual gas shielded welding, full-automatic swing gas shielded welding or submerged arc welding can be reasonably selected.

Groove design

(1) The weld neck flange and steel pipe of UHP steel pipe tower shall be set according to the corresponding welding process requirements, and a certain weld gap shall be reserved to ensure that the butt weld can be fully penetrated, and a certain blunt edge shall be ensured to prevent the root from burning through. The butt gap of gas shielded welding can be reserved 0-2 mm, and the blunt edge can be reserved 0-2 mm. The butt gap of submerged arc welding can be reserved 0-1mm, and the blunt edge shall be ≥ 7mm.
(2) Groove processing diagram is shown in Fig.3-6.
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Fig.3 V-groove of thin wall pipe
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Figure.4 i-groove
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Figure.5 Y-groove a
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Fig.6 Y-groove B

Basic regulations for welding of UHV butt welding flange

Welding materials

The selection of welding materials shall ensure that the strength, toughness, plasticity and other properties of the weld are matched with those of the base metal. The material of UHV flange is generally Q345B steel, and er50-x welding wire can be used for GMAW. If special steel and welding methods are used, the standard GB 50661-2011 shall be followed.

Welding procedure qualification

Before welding construction, the manufacturer shall carry out welding procedure qualification according to relevant requirements of GB 50661-2011 and DL / T 868-2004, and prepare welding procedure specification.

Weld grade

The circumferential butt weld between UHP butt welding flange and steel pipe shall meet the requirements of level I weld.

Welding process

Tack welding

  • (1) Tack welding must be carried out by welders with corresponding qualification certificates.
  • (2) The surface and both sides of the base metal near the tack weld shall be uniform and smooth, and free from burrs, cracks, oxide skin, rust, grease, water and other impurities.
  • (3) The thickness of tack welding seam shall not be less than 3 mm, and shall not exceed 2 / 3 of the design weld height, the length shall not be less than 40 mm, and the distance between them shall be 200-300 mm. Generally, the number of tack welds shall not be less than 3 and shall be evenly distributed.

Welding environment

  • (1) When gas shielded welding, the wind speed is ≤ 2 m / s.
  • (2) Relative humidity of air ≤ 80%.
  • (3) The surface of weldment shall be dry.
  • (4) The welding environment is better to be at normal temperature, ≥ 0 ℃.

Preheating and interpass temperature control

  • (1) Preheating temperature and interpass temperature shall be determined or tested according to the comprehensive factors such as chemical composition of steel, binding state of joint, heat input size, hydrogen content level of deposited metal and welding method adopted.
  • (2) In the process of welding, the lowest interpass temperature ≥ preheating temperature, and the highest interpass temperature ≤ 230 ℃.

Weld width

The difference between the maximum width Bmax and the minimum width bmin of the weld shall not exceed 4.0mm in any 50 mm weld length range, and the deviation shall not exceed 5.0mm in the whole weld length range. The width of the weld shall be as specified in Table 2, and the weld drawing is shown in Figure 7 and figure 8.
Table.2 Weld width
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Note: 1. B in the table is the assembly clearance, which shall meet the actual assembly value required by GB / T 985.1-2008 and GB / T 985.2-2008; G is the width of groove surface.
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Figure.7 i-groove butt weld
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Fig.8 butt weld of non-I groove


At present, the production technology of welding neck flange has been gradually mature with good overall quality. The overall first-time qualification rate of the weld is 98%. The key quality can meet the production technical requirements of the steel tube tower, but it needs to be further solidified in the process.

Source: China Welding Neck Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.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.)

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Where do you use a welding neck flange?

Weld neck flange are most widely used flanged in process piping. It gives the highest level of joint integrity due to Butt-welded with a pipe. These types of flanges are used in high pressure and temperature application.

How does a weld neck flange work?

A welding neck flange (“WN”)features a long tapered hub that can be welded with a pipe. This flange type is used, normally, in high-pressure and high/low temperatures applications that require an unrestricted flow of the fluid conveyed by the piping system (the bore of the flange matches with the bore of the pipe).

What is long weld neck flange?

Long Neck Welding flanges (abbreviated LWN Flanges) are similar to a standard Welding Neck flange, but the “Neck” is considerably longer. This type is often used as a nozzle for a barrel or column. In addition, there are the type LWN Heavy Barrel (HB) and the Equal LWN Barrel (E); they have a different shape and a thicker “Wall”.

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