Manufacturing Process

Steel tube can be broadly categorized according to manufacturing method as seamless tube made by hot rolling and cold drawn, and welded tube and butt-welded tube made by bending and welding plates.

When seamless tube is made by rolling, the rolling method involves piercing the material while it is being rolled, and is suitable for mass production. The figure shows the manufacturing process used in the Mannesmann plug mill, which is a typical rolling process. The Mannesmann-type piercer reduces the material by rolls that are inclined obliquely to each other. When the round billet is rotated while being compressed in the diametric direction, the central part of the billet becomes loose, which makes it easy to pierce a hole through the center. This is called the Mannesmann effect. The pierced portion is expanded by the elongator, and the wall thickness is then thinned and elongated by the plug mill. The internal and external surfaces are smoothed by the reeler, and the final dimensional adjustments are made by the sizer.

Seamless tube has outstanding homogeneity in the circumferential direction and is thus highly resistant to internal pressure and torsion. Taking advantage of this feature, seamless tube is widely used for drilling and pumping petroleum and natural gas.

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Welded tube is divided into electric-resistance welded (denoted ERW hereinafter) tube and EFW according to the forming and welding method. ERW tube and butt-welded tube are produced by continuously forming a hot-rolled coil into a tubular shape by forming mills. ERW tube is produced by cold forming, and the seam is welded by electric-resistance welding. This type of steel tube is used in large quantities as line tube for transporting petroleum and gas. Butt-welded tube is produced by hot forming after the whole material has been heated, and seams are then butt welded. This type of tube is hot-dip galvanized and used for carrying water and gas.

The outer diameter of ERW tube and butt-welded tube is determined by the width of the material coil.

Seamless Tubes

Seamless tube is Strongest amongst all tubes type as it has a Homogeneous structure throughout tube length.

Seamless tubes are manufactured in a verity of size and schedule. However, there is a Restriction on the manufacturing of large diameter tube. Seamless tubes are widely used in the manufacturing of tube fittings such as bends, elbows, and tees.

• Mandrel Mill Process

• Mannesmann Plug Mill Tube

The seamless tubes manufacturing process involves the following steps:

• Transformation of raw materials into steel bars (Electric arc furnace, ladle furnace, vacuum degassing and continuous casting processes)

• Transformation of steel bars into mother tube, which is manufactured in different types of rolling mills

Each product is manufactured in accordance with customer specifications, including heat treatment for more demanding applications. The tubes are threaded and undergo non-destructive testing before delivery to the customer.

We can also offer cold-drawing for tubes with the diameter and wall thickness required for use in boilers, superheaters, condensers, heat exchangers, automobile production and several other industrial applications.

In Mandrel Mill tube manufacturing process, steel billet is heated to high temperature in the rotary furnace. A cylindrical hollow, which is also known as mother hollow, is produced with the help of a rotary piercer and set of roller arrangement that keeps the piercer at the center of the billet. Outside diameter of piercer is approximately that of the inside diameter of the finished tube. With the help, secondary roller arrangement outside diameter and thickness are achieved.

In the Plug Mill Process, a solid round (billet) is used. It is uniformly heated in the rotary hearth heating furnace and then pierced by a Mannesmann piercer. The pierced billet or hollow shell is roll reduced in outside diameter and wall thickness. The rolled tube simultaneously burnished inside and outside by a reeling machine. The reeled tube is then sized by a sizing mill to the specified dimensions. From this step the tube goes through the straightener. This process completes the hot working of the tube. The tube (referred to as a mother tube) after finishing and inspection, becomes a finished product.

Welded Tubes

Welded Tubes are manufactured from Plate or continues Coil or strips. To manufacture welded tube, first plate or coil is rolled in the circular section with the help of plate bending machine or by a roller in the case of continues process. Once the circular section is rolled from the plate, the tube can be welded with or without filler material. Welded tube can be manufactured in large size without any upper restriction. Welded tube with filler material can be used in the manufacturing of long radius bends and elbow. Welded tubes are cheaper with compared to the seamless tube and also Weak due to the weld

There are different welding methods used to weld the tube.

• ERW- Electric Resistance Welding

• EFW- Electric Fusion Welding

• HFW- High-frequency welding

Less thickness tube, mainly ERW / EFW or HFW welded tube are formed by continues rolling method. In this method, a flat metal strip from the strip coil is feed into the series of roller assembled in line. These rollers gradually form the strip in the circular section. At the end of rolling assembly, this tube is continuously welded by welding machine.

ERW/EFW and HFW are welding methods in which tube is welded without adding filler material. However, EFW welding method can be used with filler material also.

Materials

In ERW welding, two electrodes, usually made from copper, are used to apply pressure and current. The electrodes are disc shaped and rotate as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds.

A welding transformer supplies low voltage, high current AC power. The joint of the tube has high electrical resistance relative to the rest of the circuit and is heated to its melting point by the current. The semi-molten surfaces are pressed together with a force that creates a fusion bond, resulting in a uniformly welded structure.

EFW steel tube is formed by rolling plate and welding the seam. The weld flash can be removed from the outside or inside surfaces using a scarfing blade. The weld zone can also be heat treated to make the seam less visible. Welded tube often has tighter dimensional tolerances than seamless, and can be cheaper if manufactured in the same quantities. It is mainly used for welding dissimilar steel welding sheet or which high power density, metal weldment can rapidly heated to high temperatures, which can melt any refractory metals and alloys. Deep penetration welding fast, heat-affected zone is extremely small, so small performance impact on the joints, the joint almost no distortion. But it has a requirement on a special welding room because welding using X-rays.

High frequency welding (HFW) steel tube is that ERW tube produced with a welding current frequency equal to or greater than 70 kHZ. Through high-frequency current welding resistance, the heat generated in the contact objects, so the objected surface are heated to the plastic state, then with or without forging to achieve a combination of steels. HFW is a solid resistance heat energy. The high frequency current pass through the metal conductor, will produce two peculiar effects, skin effect and proximity effect. And HFW process is to use the skin effect to concentrate on steel object surface, use proximity effects to control the position and the power of the high-frequency electric current flow path. Since the speed is very high, the contacted plate edge could be heated and melted in short time, then extruded through docking process.

In the welding process, HFW steel tubes do not need to add filling meta. So it has fast welding speed and high efficiency in production. HFW tube is widely used in the fields of oil and gas transportation, oil well pipeline, building structure and various kinds of mechanical tube. However, HFW steel tube quality is affected by many factors, such as raw material and process. And the production quality control become difficult. So the yield and welding process still need to be improved continuously.