Answer
Checking weld seams on shipboard structures, pressure vessels, and piping requires a combination of Visual Inspection (VT), Non-Destructive Testing (NDT), and Leak/Pressure Testing.
1. Visual Inspection (VT) - Primary Shipboard Check
Visual inspection is performed before, during, and after welding using a Bridge Cam Gauge or Fillet Weld Gauge.
Key Defects to Look For:
- Undercut: A groove melted into the base metal along the toe of the weld (caused by excessive current or improper torch angle).
- Lack of Penetration / Fusion: Incomplete bonding at the root or between weld passes.
- Porosity / Surface Blowholes: Gas pockets trapped inside the cooling weld pool.
- Cracks: Longitudinal, transverse, or toe cracks (the most severe structural failure indicator).
- Misalignment (Hi-Lo): Structural offset between abutting plates/pipes exceeding Class limits (> 1.5 mm).
2. Dye Penetrant Testing (PT / LPI)
Dye Penetrant Testing detects surface-breaking defects on non-porous materials (e.g., stainless steel high-pressure fuel lines, boiler tube welds, turbine blades, deck plating).
Step-by-Step Procedure:
- Surface Cleaning: Mechanically clean the weld seam to remove grease, paint, rust, and slag. Wipe dry with Solvent Cleaner.
- Apply Red Penetrant: Spray liquid red dye penetrant evenly over the weld seam.
- Dwell Time: Allow 10 to 20 minutes dwell time for the dye to seep into microscopic surface cracks via capillary action.
- Remove Excess Penetrant: Wipe off surface penetrant using a clean, lint-free cloth dampened with cleaner. (Never spray cleaner directly onto the weld).
- Apply White Developer: Spray a light, uniform coat of white developer powder over the dry weld seam.
- Inspection: Wait 10 minutes. If surface cracks exist, red dye bleeds out onto the white developer, creating clear red indications marking the defect location and length.
3. Magnetic Particle Inspection (MPI / MT)
MPI detects surface and shallow sub-surface defects in ferromagnetic materials (carbon steel shell plating, engine bedplates, crane pedestals).
- Method: Place an AC/DC magnetic yoke across the weld seam to induce a magnetic field.
- Execution: Spray white contrast paint, magnetize the area, and apply magnetic particles (black ink or fluorescent powder).
- Principle: Cracks disrupt the magnetic flux lines, causing a magnetic leakage field that attracts the particles, highlighting the exact defect geometry.
4. Pressure & Leak Testing Methods for Welds
Testing Method | Application | Procedure |
|---|---|---|
Vacuum Box Testing | Flat seam welds (tank tops, bottom shell, hatch covers) | Brush a soapy water solution over the weld seam. Place a clear acrylic vacuum box over the area and pull a vacuum (approx -0.2 to -0.5 bar). Continuous bubble formation indicates a through-weld leak. |
Air-Soap Bubble Test | Double bottom tanks, cofferdams, pipe sleeve welds | Pressurize the tank or void space with low-pressure compressed air (approx 0.2 bar) and spray soapy water on all external weld seams to check for leaks. |
Hydrostatic Pressure Test | High-pressure fuel oil pipes, steam lines, boiler tubes | Blank the pipe section, fill completely with water, vent air, and pump to 1.5 times design working pressure. Hold for 30 minutes while inspecting welds for pressure drop or weeping. |