CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on NB/T 47013.2 Radiographic Testing for Pressure Equipment
Literature Overview
NB/T 47013.2 is the Chinese industry standard for radiographic testing (RT) of pressure equipment, covering both the testing methods and the acceptance criteria for defect evaluation. As part of the NB/T 47013 series, it provides the radiographic examination requirements for welds in pressure vessels, piping, and other pressure-containing components. For clad and bimetal pressure vessels, RT serves as a critical volumetric inspection method capable of detecting internal defects in both the base material welds and the overlay welds.
Core Technical Content
The standard is organized around several key elements: radiographic equipment requirements, film and digital imaging systems, exposure technique, image quality indicators, defect classification, and acceptance levels. Understanding these elements in their proper sequence is essential for effective implementation in clad vessel fabrication.
Equipment and Film Requirements
The standard specifies minimum requirements for X-ray generators, gamma ray sources, and film or digital detector systems. The choice of radiation source depends on material thickness, geometry, and required image quality.
| Material Thickness | Recommended Source | Energy Range |
|---|---|---|
| Up to 20 mm | X-ray tube | 80-150 kV |
| 20-50 mm | X-ray tube or Ir-192 | 150-350 kV or 300-600 keV |
| 50-100 mm | Co-60 or high-energy X-ray | 1.17-1.33 MeV |
| Above 100 mm | Co-60 or linear accelerator | 1.17-1.33 MeV or higher |
For clad plate pressure vessels, the radiographic examination must account for the density difference between the base material and the clad layer. Carbon steel and stainless steel have similar densities, producing good image contrast. However, when examining welds in nickel-based alloy clad plates over carbon steel, the density difference between Inconel 625 (density 8.44 g/cm3) and carbon steel (density 7.85 g/cm3) may produce additional contrast that can be either beneficial or confusing depending on the defect type being sought.
Exposure Technique
The standard prescribes specific exposure parameters including source-to-film distance (SFD), film-to-object distance, exposure time, and geometric unsharpness limits. For butt welds in pressure vessels, the double wall single film technique is commonly used for thin sections, while the single wall technique with internal source or backscatter techniques may be employed for thicker sections.
Geometric unsharpness (Ug) is a critical parameter that must be controlled to ensure defect visibility. The standard limits Ug based on the acceptance level required:
| Acceptance Level | Maximum Ug |
|---|---|
| Level A (most stringent) | 0.10 mm |
| Level B | 0.20 mm |
| Level C (least stringent) | 0.40 mm |
The image quality indicator (IQI) placement and type are also specified. Wire-type IQIs are used for determining the required image quality, with the number of visible wires determining the achieved image quality level. For clad vessel welds, IQI placement on the source side of the weld is mandatory to ensure that defects in both the base and clad layers are detectable.
Defect Classification and Acceptance
NB/T 47013.2 classifies radiographic indications into categories including slag inclusions, porosity, cracks, lack of fusion, lack of penetration, undercut, and others. Each category has specific acceptance criteria based on size, number, and distribution.
| Defect Type | Acceptance Criteria (Level 2 Welds) |
|---|---|
| Slag inclusions | Individual size limited; total area limited |
| Porosity | Cluster porosity limited by area fraction |
| Cracks | Not acceptable |
| Lack of fusion | Not acceptable |
| Lack of penetration | Limited by depth and length |
| Undercut | Limited by depth and length |
For clad vessel applications, the acceptance criteria must be applied with additional consideration for the overlay welds. The overlay weld between the base material and clad layer is a dissimilar material weld that may show different radiographic characteristics than a homogeneous weld. Liner welds in clad vessels, which are longitudinal and circumferential welds in the inner lining, require special attention because any defect in the liner weld directly compromises the corrosion resistance of the vessel interior.
Engineering Practice Considerations
In my experience fabricating hydrogenation reactors with Inconel 625 overlay on 16MnR base plates, RT examination of the overlay welds presents unique challenges. The overlay weld is typically deposited in multiple passes using submerged arc welding or gas metal arc welding, and the radiographic appearance differs significantly from a conventional butt weld. The standard does not specifically address overlay weld RT acceptance, which requires engineering judgment and project-specific procedures.
A practical approach that has proven effective is to develop a project-specific RT procedure that references NB/T 47013.2 for general principles but adds specific acceptance criteria for overlay welds. These criteria typically focus on crack detection and lack of fusion, as these are the most critical defects for overlay welds in pressure vessel applications.
Key Technical Challenges
The primary challenge in RT of clad vessel welds is distinguishing between true defects and benign indications caused by the clad structure. The interface between the base material and clad layer can produce a linear indication on the radiograph that resembles a crack. Experienced radiographic interpreters can distinguish these by observing the characteristic appearance: a true crack shows irregular edges and may branch, while the clad interface appears as a relatively straight, continuous line with consistent density.
Another challenge is the examination of welds at the junction between clad plate and non-clad plate, such as at manway nozzles or tube sheet welds. These transitions create complex radiographic appearances that require careful interpretation and may necessitate supplementary examination methods.
Summary
NB/T 47013.2 provides the essential framework for radiographic examination of pressure equipment welds, including those in clad and bimetal pressure vessels. Effective application requires understanding of exposure technique, image quality control, and defect interpretation in the specific context of clad vessel fabrication. The standard's acceptance criteria must be supplemented with project-specific procedures for overlay welds and dissimilar material welds, as these are not explicitly addressed in the general provisions.