Slag inclusion refers to residual slag trapped inside the weld seam. In submerged arc welding (SAW), slag inclusions can weaken the structural integrity of LSAW pipes (Longitudinal Submerged Arc Welded pipes), leading to potential defects. Understanding the causes and prevention methods is crucial for ensuring high-quality pipe production.
From a theoretical perspective, there are three primary reasons for slag inclusion in SAW welds:
1. Excessive impurities in raw materials (base metal, welding wire, and flux).
2. Incomplete interlayer cleaning in multi-pass welding.
3. Improper welding parameters, which hinder slag floating out.
However, in LSAW pipe production, multi-pass welding with insufficient interlayer cleaning is unlikely to be the main cause of slag inclusion.
When slag inclusion occurs due to impurities in the base metal, welding wire, or flux, measures such as pre-weld material inspection and using fresh welding consumables can be taken. However, these steps only slightly reduce slag inclusion at the fusion line, indicating that raw material impurities are not the primary cause.
For thick-walled LSAW pipes, the main cause of slag inclusion at the fusion line is improper welding parameters. Key welding parameters affecting slag formation include:
· Heat input
· Welding current & voltage
· Welding speed
· Wire spacing
· Groove dimensions
From a metallurgical perspective, slag inclusion at the fusion line occurs when the temperature is too low, preventing molten slag from escaping. This can result from:
· Insufficient peak heating temperature
· Excessive cooling rate
Optimizing these parameters ensures proper slag removal and high-quality welds.
The most common method for cutting LSAW pipes is oxy-fuel gas cutting (flame cutting). This process involves:
· Heating the metal to its ignition temperature using an oxygen-acetylene flame.
· Blowing away the molten iron oxide slag with a high-pressure oxygen jet, effectively cutting the pipe.
· High efficiency – Fast cutting speeds.
· Ease of operation – Simple setup and execution.
· Clean cuts – Produces a relatively smooth cut surface.
However, the cut surface develops an oxide layer, which must be removed before welding. Gas cutting is widely used in pipeline construction for cutting large-diameter LSAW pipes, steel plates, and structural sections.
The primary tool for gas cutting is the cutting torch, which comes in two types based on acetylene pressure:
1. Injector (low-pressure) torch – Commonly used.
2. Equal-pressure torch – Requires higher acetylene pressure.
Oxygen is supplied from a cylinder, while acetylene comes from either a cylinder or a generator.
Since gas cutting involves high temperatures and flammable gases, strict safety protocols must be followed. Key considerations include:
o Keep the torch perpendicular to the pipe surface initially.
o After penetration, tilt the torch forward at 70°–80° relative to the cutting direction.
2. Cutting Direction
o For fixed pipes, start cutting from the bottom.
3. Oxygen Pressure & Nozzle Selection
o Adjust based on pipe thickness.
4. Torch-to-Pipe Distance
o Maintain 3–5 mm between the flame’s inner cone and the pipe surface.
5. Post-Cutting Treatment
o Remove slag using a file or grinder for a smooth finish.
o Ensure the pipe end is perpendicular to the centerline.
6. Shutdown Procedure
o Quickly close the cutting oxygen valve, acetylene valve, and preheat oxygen valve after cutting.
Slag inclusion in LSAW pipes primarily stems from incorrect welding parameters rather than material impurities. Optimizing heat input, current, and cooling rates minimizes defects. For cutting, gas cutting remains efficient but requires proper technique and safety measures. By addressing these factors, manufacturers can produce high-quality LSAW pipes with minimal defects.
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