Defect Identification and Repair Section (Enriched Version)

1. Combined Application of Multidisciplinary Defect Detection Techniques

Fluorescent Penetrant Testing (PT) for Surface Defects: For non-ferrous metal components such as stainless steel pump casings and aluminum alloy impellers, use fluorescent penetrant testing:

① Spray penetrant (containing fluorescent dye) for 5–10 minutes, with penetrant viscosity controlled at 2–5 mm²/s (excessive viscosity causes residue);

② Rinse the surface with 40°C warm water (pressure 0.3MPa), avoiding water marks that may obscure defects;

③ After drying, spray developer and observe under ultraviolet light (365nm wavelength). Cracks as small as 0.05mm will appear as bright yellow-green fluorescence.

Ultrasonic Testing (UT) for Internal Shrinkage: For thick-walled pump components (e.g., high-pressure pump casings with wall thickness >50mm), use a 2.5MHz probe with water as the couplant. Scan at a speed ≤100mm/s. If defects ≥φ2mm are detected, mark the depth—components with defects >1/3 wall thickness from the surface are difficult to repair and should be scrapped.

2. Material-Specific Welding Repair Processes

Cast Iron (Gray Iron/Ductile Iron) Repair:

Small sand holes (diameter ≤3mm): Use Z308 pure nickel electrode (3.2mm diameter) with 90–110A current. Apply "spot welding + rapid cooling": weld 2–3mm, pause for 10 seconds, to avoid overheating and poor graphitization in the base metal.

Crack repair: Machine a U-shaped groove along the crack (depth ≥2/3 of crack length). Preheat with oxy-acetylene flame to 200°C (monitored by temperature gun). Weld in 3 layers (nickel-based electrode for the first layer, low-carbon steel for the middle, and nickel-based for the surface). Cool slowly in a furnace to room temperature to prevent white iron formation.

Stainless Steel (304/316) Repair:Prioritize TIG welding with a wire matching the base metal (e.g., ER308L for 304). Use argon gas with ≥99.99% purity, 80–100A current, and hold the torch at a 75° angle to the base metal to avoid porosity. After welding, clean the surface with pickling paste (20% hydrochloric acid + 5% nitric acid + 75% water) to remove oxide scale.

High-Chromium Cast Iron (Cr26) Wear Part Repair:For defects >5mm deep, grind to metallic luster first. Preheat to 300°C (monitored by infrared thermometer). Use D707 tungsten carbide electrode (4.0mm diameter) with 140–160A current. Build up welds from the defect edge to the center, with each layer ≤4mm thick. Immediately place in a 600°C oven for 2 hours of slow cooling to prevent martensitic embrittlement.

3. Repair Taboos and Alternative Solutions for Special Components

Impeller Shaft Hole Defect Handling: If shrinkage occurs in the shaft hole (diameter deviation >0.1mm), never repair by welding and reaming directly! Use a "sleeve fitting" method instead: bore the hole 2–3mm larger, press-fit a bushing of the same material (interference fit 0.02–0.05mm), then finish-bore to the design dimension to avoid deformation from welding stress.

Thin-Wall Component (Wall Thickness <5mm) Sand Hole Repair: Use a "brazing + sealant" combination: first fill the hole with silver-based filler metal (e.g., BAg72Cu) at 700–800°C, then apply liquid sealant (e.g., Loctite 5900). After 24 hours of curing, conduct a hydrostatic test (1.5× working pressure). A leakage rate ≤5ml/min is acceptable.

4. Post-Repair Quality Control Loop

All welded pump components must undergo re-inspection:

① Visual inspection: Surface roughness ≤Ra12.5μm, weld height ≤1/3 of base metal thickness;

② Dimensional recheck: Critical dimensions (e.g., impeller outer diameter, pump casing seal cavity ID) must be within ±0.05mm tolerance;

③ Performance retest: For centrifugal pumps, conduct flow-head testing—efficiency reduction must not exceed 3% of the design value.

Critical components (e.g., nuclear power pump casings) require 100% radiographic testing (RT) after repair. Components with Grade II or higher defects must be repaired twice; those failing three repairs are directly scrapped to avoid hidden risks.