Submerged arc welding
Introduction
Submerged Arc Welding (SAW) is a highly efficient and widely used welding process in the industrial sector, particularly for the joining of thick materials. It is characterized by the use of a continuously fed consumable electrode and a blanket of granular fusible flux, which covers the weld area. This method is predominantly employed in industries such as shipbuilding, pressure vessel manufacturing, and the construction of large steel structures due to its ability to produce high-quality welds with deep penetration and minimal spatter.
Process Overview
The SAW process involves several key components: the power supply, the wire feeder, the welding head, and the flux delivery system. The power supply provides the necessary electrical energy to create an arc between the electrode and the workpiece. The wire feeder continuously supplies the consumable electrode, typically a solid or cored wire, to the welding head. The welding head directs the electrode and flux to the weld area, where the arc is struck beneath a layer of flux, which melts and forms a protective slag over the weld pool.
The flux serves multiple purposes: it protects the molten weld pool from atmospheric contamination, stabilizes the arc, and influences the chemical composition of the weld metal. The slag formed by the flux solidifies on top of the weld bead and is removed after cooling.
Equipment and Materials
Power Supply
The power supply for SAW can be either AC or DC, with DC being more common due to its stable arc characteristics. The choice of current type and polarity (straight or reverse) depends on the specific welding application and desired weld properties.
Electrodes
Electrodes used in SAW are typically solid or metal-cored wires, available in various diameters ranging from 1.6 mm to 6.4 mm. The selection of electrode material is crucial as it affects the mechanical properties and chemical composition of the weld. Common materials include carbon steel, stainless steel, and nickel-based alloys.
Flux
The flux used in SAW is a granular material composed of minerals and other compounds. It can be classified into two main types: fused and bonded. Fused fluxes are melted and solidified into granules, while bonded fluxes are made by mixing the ingredients with a binder and then baking them. The choice of flux affects the weld bead shape, penetration, and mechanical properties.
Advantages and Limitations
Advantages
SAW offers several advantages over other welding processes. It provides high deposition rates, which lead to increased productivity. The process is also highly automated, reducing the need for skilled labor and minimizing operator fatigue. The protective slag and flux blanket result in minimal spatter and a clean weld surface, reducing post-weld cleaning requirements.
Limitations
Despite its advantages, SAW has some limitations. It is primarily suitable for flat or horizontal welding positions, as the flux can fall away in vertical or overhead positions. The process is also limited to ferrous metals and some nickel-based alloys. Additionally, the initial setup cost for SAW equipment can be high, making it less economical for small-scale or one-off projects.
Applications
SAW is extensively used in industries that require high-quality welds on thick materials. In the shipbuilding industry, it is used for welding hulls and other large components. In the construction of pressure vessels, SAW is employed for joining thick steel plates. The process is also used in the manufacturing of pipelines, where it provides the strength and durability required for transporting fluids under high pressure.
Process Variations
Several variations of the SAW process exist to accommodate different welding requirements. These include:
Tandem SAW
Tandem SAW involves the use of two or more electrodes fed into the weld pool simultaneously. This variation increases deposition rates and is used for welding thick sections.
Narrow Gap SAW
Narrow Gap SAW is designed for welding thick plates with a narrow groove, reducing the amount of filler material required and minimizing distortion.
Twin Wire SAW
Twin Wire SAW uses two wires fed through a single welding head, increasing the deposition rate and improving weld quality.
Safety Considerations
While SAW is generally considered a safe welding process due to the absence of visible arc radiation, certain safety precautions are necessary. The flux and slag can produce harmful fumes, so adequate ventilation is essential. Operators should also wear appropriate personal protective equipment, such as gloves and safety glasses, to protect against burns and eye injuries.