Horizontal fixed welding of thick-walled stainless steel pipes

Stainless steel pipes are hollow, elongated steel products widely used for conveying fluids such as oil, natural gas, water, steam, and coal gas. Due to their excellent corrosion resistance, high strength-to-weight ratio, and durability, they are also extensively employed in mechanical part manufacturing, structural engineering, and even the production of conventional weapons like gun barrels and shell casings.

Compared with solid steel materials, stainless steel pipes offer the same bending and torsional strength while being lighter in weight. In applications where the pipe needs to withstand internal fluid pressure, thicker wall thickness is required. These pipes must undergo hydraulic pressure testing to ensure they do not leak, bulge, or deform under specified working conditions.

Stainless steel pipes can be divided into seamless and seamed (or welded) types. Among them, seamless stainless steel pipes—produced by perforating steel ingots or solid blanks followed by hot rolling, cold rolling, or cold drawing—are commonly used in high-demand applications. Specifications for seamless pipes are generally indicated as outer diameter × wall thickness in millimeters. A widely used material type is 1Cr18Ni9Ti stainless steel.

Case Example: Horizontal Fixed Welding of 1Cr18Ni9Ti Stainless Steel Pipe (Φ159 mm × 12 mm)

1. Welding Process Analysis

The horizontal fixed butt-welding of large-diameter 1Cr18Ni9Ti stainless steel pipes is commonly used in critical fields such as nuclear power and chemical equipment manufacturing, where both heat resistance and acid resistance are crucial. This type of welding presents high technical difficulty, especially for joints that demand high quality—requiring proper internal bead formation, moderate reinforcement, and avoidance of undercut or concave defects. Post-welding, the joints must pass penetrant testing (PT) and radiographic testing (RT) to ensure integrity.

Traditional welding methods include TIG welding and manual arc welding. TIG, although precise, has low efficiency and high cost, while manual arc welding is labor-intensive and lacks consistency. To strike a balance between quality and efficiency, a hybrid approach is adopted: TIG welding with filler wire is used for root pass welding (inside and outside), followed by MAG welding for filling and capping passes. This combination ensures strong, well-formed joints while maintaining productivity.

It is worth noting that 1Cr18Ni9Ti stainless steel differs significantly from carbon steel in terms of thermal expansion and conductivity. Its molten pool has poor fluidity, which complicates full-position welding. In MAG welding, precise control of welding parameters—such as torch angle, oscillation speed, dwell time at the edges, and wire extension (kept within 10 mm)—is essential to achieve smooth, evenly fused weld seams.

2. Welding Procedure

Material: 1Cr18Ni9Ti stainless steel

Specification: Φ159 mm × 12 mm

Root pass welding: Manual TIG welding

Fill and cap passes: MAG welding with mixed shielding gas (Ar + CO₂)

Welding position: Horizontal fixed, full-position welding

3. Pre-Welding Preparation

Remove oil, dirt, and oxide scale; polish the groove and 10 mm on either side to expose metallic luster.

Check that all water, gas, and electrical supply lines are functioning properly, and verify that all tools and equipment are in good condition.

Assemble the pipes according to design dimensions and use rib plates for tack welding at the 2, 7, and 11 o’clock positions to prevent distortion. Positioning welds inside the groove are also allowed but must be handled carefully to avoid defects.

Considerations for Using Thick-Walled Stainless Steel Pipes

The definition of "thick-walled" steel pipes depends on the ratio between the pipe’s outer diameter and wall thickness rather than an absolute thickness value. For instance, a 50 mm diameter pipe with a 10 mm wall is considered thick-walled, whereas a 219 mm diameter pipe with the same 10 mm wall would be considered thin-walled.

When selecting thick-walled pipes, users should pay attention to the following:

Material specification: Confirm the steel grade to ensure compatibility with intended applications.

Length requirements: Know the required single-pipe length to avoid waste during machining or installation.

Dimensional tolerances: Clearly specify inner and outer diameters, especially for machined components that require allowance.

Pipe type: Determine whether hot-rolled seamless, welded (seamed), cast, or hot-forged thick-walled pipes are suitable. Clarify acceptable alternatives, and emphasize when substitutions are not allowed.

Summary

Stainless steel pipes, especially thick-walled and seamless variants, play a crucial role in high-performance applications across industries. For specialized welding tasks such as the horizontal fixed welding of 1Cr18Ni9Ti pipes, a detailed understanding of welding methods, material properties, and procedural controls is essential to ensure quality and efficiency. Meanwhile, thoughtful selection and specification of thick-walled pipes can optimize usage and reduce waste in downstream machining and assembly processes.