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Welding Techniques and Quality Inspection for Structural Steel Applications

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Types of Welding

There are different types of welding used in steel structures, one of the most common being arc welding. In arc welding, the process uses a welding rod (electrode) to generate heat through an electric arc. The heat melts both the base metal and the tip of the electrode, allowing molten droplets of the electrode to deposit and fuse with the base metal. During this process, the weld area must be protected from atmospheric contamination. This protection is provided by inert gas or a vapor shield produced from the burning of the electrode coating.

If this protective gas shield is not present, oxygen and nitrogen from the air may enter the molten weld metal, resulting in porosity, reduced toughness, and a brittle weld.

Electrodes used in arc welding are classified into two types:

  • Heavily-coated electrodes: Commonly used for structural welding, as they provide adequate shielding to protect the molten weld pool.

  • Lightly-coated electrodes: Provide minimal shielding; therefore, the weld metal is more exposed to atmospheric gases and is not suitable for structural applications.



Submerged Arc Welding (SAW)

Submerged Arc Welding is a semi-automatic or automatic welding process. In SAW, the electrode (filler wire) is continuously fed from a reel, and the welding arc is completely submerged under a layer of granular flux supplied from a hopper. The flux melts to form a protective slag layer, shielding the weld pool from contamination.

SAW offers several advantages, including:

  • High welding speed and productivity

  • Deep penetration with high weld strength

  • Good corrosion resistance and ductility

  • Excellent weld quality with minimal fumes and spatter

SAW is commonly used for bridge construction, large structural members, and long seam welds in plate girders.




Flux-Cored Arc Welding (FCAW)

In Flux-Cored Arc Welding, the electrode is a tubular wire filled with flux and continuously fed from a spool. During welding, the core flux melts and generates a shielding gas (self-shielded FCAW) or may require an external shielding gas (gas-shielded FCAW). The molten flux protects the weld from atmospheric contamination.

FCAW provides high deposition rates and can be used for thick sections, but the electrode size range is limited.




Welding Inspection

To ensure high-quality welding, a proper Welding Procedure Specification (WPS) must be developed, and welding must be performed by qualified welders and supervised by competent inspectors. Welding quality is influenced by several factors such as current, voltage, base and filler material compatibility, welding position, and technique.

Several tests can be carried out to verify weld quality:

1. Visual Inspection (VT)

A competent inspector visually assesses the weld. Through visual inspection, the weld size, profile, fusion, and finish can be evaluated. A uniform color indicates satisfactory cooling, while an overheated weld may appear oxidized or rusty. Visual inspection also provides an indication of possible subsurface defects.

2. Liquid Penetrant Testing (PT)

This method is suitable for detecting surface-breaking cracks. A visible or fluorescent dye is applied to the weld surface and then cleaned. A developer (powder) is applied, which draws out the dye trapped in cracks, making them visible. PT is effective for detecting fine cracks and surface defects.

3. Magnetic Particle Testing (MT)

Used for ferromagnetic materials, this method involves magnetizing the weld. Magnetic fields form north and south poles at the edges of surface cracks. A dry powder or liquid with magnetic particles is then applied, and it accumulates at discontinuities, revealing their location and size. MT can detect surface cracks and shallow subsurface defects.

4. Ultrasonic Testing (UT)

High-frequency sound waves are transmitted into the weld. Discontinuities reflect waves back at different times, helping locate internal defects and assess their severity. UT is effective for carbon steel and low-alloy steel but is less suitable for stainless steel or coarse-grained materials.

5. Radiographic Testing (RT)

This method uses X-ray or gamma rays to inspect the internal structure of welds. It is suitable for butt welds where equipment access is possible. Interpretation of radiographs for fillet welds is more difficult. RT is useful for determining the extent and percentage of defective welds but is relatively expensive.

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