CLADDING TECHNOLOGY SHANXI CO., LTDNDT Report System for Cladding and Bimetal Pressure Vessel Inspection
Overview and Inspection Philosophy
The Non-Destructive Testing (NDT) report system for clad and bimetallic pressure vessels is the primary means of verifying weld integrity without compromising the product. For cladding applications, NDT serves a dual purpose: detecting volumetric and planar defects in the weld metal and, critically, verifying the metallurgical bond between the overlay layer and the base metal. Standards such as JB/T 4730, ASME V, EN ISO 17635, and API 934 provide the framework, but the application to cladding requires additional expertise beyond conventional weld inspection.
NDT Methods and Their Application to Cladding
| NDT Method | Application to Cladding | Standard Reference | Typical Acceptance Level |
|---|---|---|---|
| Radiographic Testing (RT) | Volumetric defects in overlay welds | ASME V Art.2 / ISO 17636-2 | Level B (TIG overlay), Level T (SAW overlay) |
| Ultrasonic Testing (UT) | Bond line defects, laminations | ISO 17640 / EN 4179 | No indication > 6 mm equivalent |
| Magnetic Particle Testing (MT) | Surface-breaking cracks in ferromagnetic overlay | ASME V Art.7 / ISO 17638 | No linear indications > 1.5 mm |
| Penetrant Testing (PT) | Surface defects on non-ferromagnetic overlay (Ti, Cu) | ASME V Art.6 / ISO 3452 | No linear indications > 1.0 mm |
| TOFD/PAUT | Bond line mapping, large area screening | EN ISO 17640 / EN ISO 23287 | Calibrated per product-specific procedure |
| Eddy Current Testing (ECT) | Crack detection on conductive overlay | ISO 13588 | Per calibration block |
Bond Line Inspection – The Critical Challenge
The bond line between the overlay and base metal is the most critical inspection zone in any cladding application. For weld overlay cladding, the bond line is essentially the first overlay pass, and its integrity determines the service life of the component. Key challenges include:
- Interface chemistry: The dilution zone at the bond line may contain brittle intermetallic phases (e.g., Fe-Cr sigma phase in stainless steel on carbon steel), which are difficult to detect by conventional NDT methods.
- Geometry effects: On curved surfaces (reactor shells, heat exchanger tubes), the bond line orientation relative to the NDT scan direction affects detection sensitivity.
- Material contrast: For titanium-to-steel or zirconium-to-steel cladding, the acoustic impedance mismatch at the bond line can produce strong reflections that mask actual defects.
Report Documentation Requirements
A complete NDT report for a clad pressure vessel must include the following elements:
- Unique report number cross-referenced to the product serial number and heat/lot numbers
- NDT procedure reference (qualified per ASME V Art.1 or ISO 9712 Level II/III certification)
- Equipment calibration records with date and calibration block identification
- Actual scan coverage mapped to the vessel weld map, showing inspection percentages (e.g., 100% RT for all overlay welds, 20% RT for base metal welds)
- Defect evaluation with sketches, dimensions, and acceptance/rejection decision per the applicable standard
- Inspector qualification (ISO 9712 Level II minimum, Level III for procedure qualification)
- Traceability linking each NDT report number to the specific heat lot, weld number, and WPS number
Engineering Practice – Common Defects and Countermeasures
In cladding NDT practice, the following defects are most frequently encountered:
- Bond line lack of fusion: Typically caused by insufficient preheat, excessive travel speed, or poor flux/wire interaction. Detected by UT with contact method or TOFD. Countermeasure: increase preheat to 100–150 °C, reduce travel speed by 20%, ensure proper flux coverage.
- Porosity in overlay layers: Especially common in GTAW and GMAW overlay on nickel-based alloys due to hydrogen pickup. Countermeasure: use dry flux, ensure proper gas shielding coverage, control moisture in welding consumables.
- Cracking in dilution zone: Often intergranular cracking due to carbon pickup from the base metal into the austenitic overlay. Countermeasure: use hyper-eutectoid composition fillers (e.g., A240-309L instead of A240-304L), control interpass temperature below 150 °C.
- Undercut at overlay base: A geometric defect at the transition from base metal to overlay that can initiate stress corrosion cracking. Countermeasure: proper bead profile control, adequate overlap between successive overlay passes.
Study Insights
The NDT report system is not merely a collection of test results but a structured evidence chain that proves product fitness for service. The most valuable aspect of a well-organized NDT report system is its traceability: every indication, every acceptance decision, and every inspector action is documented and linked to the physical product. In litigation or warranty disputes, this traceability is the difference between a defensible position and an indefensible one. Engineers must ensure that NDT procedures are qualified on product-representative specimens, not generic coupons, and that acceptance criteria are unambiguous and tied to the specific service environment of the vessel.