How to Weld Seamless Steel Tubes?

Welding Process of Seamless Tubes: Preheat Before Welding, Post-Weld Heat Treatment and Quenching

1. Preheating

Preheating seamless tubes prior to arc welding is critical. Maintain the operating temperature for 30 minutes before commencing welding gradually.

This method addresses issues of arc heating and controlled temperature quenching. The temperature control cabinet operates automatically. Far-infrared tracking heat treatment panels are adopted. The system intelligently sets parameters automatically and records temperature trend curves, with accurate temperature measurement via heat transfer probes. During preheating, the distance between heat transfer probe measuring points and the weld edge shall be 15 mm to 20 mm.

2. Welding Methods

1. To effectively prevent welding deformation of seamless tubes, two welders perform symmetrical welding on each column connection joint, welding outward from the center to both sides. Welding is carried out in an open state (the weld closely adheres to the cross beam during open welding). Welding proceeds from the first layer to the third layer of small-size seamless tubes, as arc welding on small specifications greatly contributes to welding deformation. After completing arc welding for the first to third layers, back gouging shall be performed. After air carbon arc gouging, welding equipment shall be polished, and the weld surface shall be nitrided to reveal metallic luster, preventing cracks caused by surface carburization. Complete the outer bore welding in one pass, and all internal thread welding in one pass respectively.
2. When arc welding the second layer of seamless tubes, the welding direction shall be opposite to that of the first layer, and so on. The spacing between butt welds of each layer is 15–20 mm.
3. Consistent welding current, travel speed and number of build-up layers shall be maintained across all welding machines.
4. Arc welding must start on the run-on plate and terminate on the run-off plate. After welding, cut off the plate, grind and clean the weld zone.
5. 

3. Post-Weld Heat Treatment

Quenching shall be conducted within 12 hours after arc welding. If immediate heat treatment of seamless tubes is unavailable, thermal insulation and slow cooling measures shall be implemented. During solution quenching, temperatures at two heat transfer probes shall be precisely measured, with probes welded on both sides of the weld seam.

What Is the Pickling Process for Finished Seamless Tube Products?

A pickling procedure is commonly carried out after seamless tube fabrication to remove surface contaminants and dirt from the tubes, eliminating surface scale formed during production.

Before pickling and passivation of seamless tube surfaces, welding slag, spatter, burrs and debris on the pipe surface must be thoroughly removed. The tube surface shall first be cleaned with gasoline or other solvents. For heavy surface contaminants, steam cleaning or washing with a 3–5% sodium hydroxide (NaOH) aqueous solution is acceptable, followed by rinsing with clean cold water and air drying. Note that the chloride ion (Cl⁻) concentration in cold rinse water shall not exceed 25 mg/L.

Passivation technology can be adopted to clear chemical residues without damaging the tube material properties. A dense oxide film or oxygen adsorption layer forms on the tube surface to isolate the base metal from corrosive chemical media, significantly improving the tube’s surface corrosion resistance.

For light surface rust or stubborn contaminants that cannot be removed by regular cleaning, pickling treatment is applicable. This process removes slight rust on thin-wall stainless seamless tubes without damaging the surface oxide film or base material; surface impurities fade off thoroughly to achieve a clean tube surface.