Radiographic Testing of Weld Overlay Layers for Internal Defect Detection

Overview and Technical Challenges

Radiographic testing (RT) of weld overlay layers represents one of the most technically challenging NDT applications in bimetal manufacturing. According to Item 107, RT is employed to detect internal defects such as porosity, slag inclusions, and lack of fusion within the overlay layer, governed by NB/T 47013.2 and ASME V Article 2. The fundamental challenge lies in the material combination: the overlay layer is typically an austenitic stainless steel or nickel-based alloy with a significantly different radiographic density than the carbon steel base, creating complex image contrast conditions that can mask or mimic defects.

In thick-walled components — such as hydrogenation reactor shells with overlay thicknesses of 3-6 mm on base walls of 40-80 mm — the differential attenuation between the base and overlay creates a gradient in film density that must be carefully managed through proper exposure parameter selection and film processing.

Standards Comparison and Technical Parameters

For overlay layers thicker than 5 mm, digital radiography (DR) or computed radiography (CR) is strongly recommended over conventional film radiography because of the superior dynamic range and post-processing capabilities that allow better visualization of defects near the bond line interface.

Defect Identification and Interpretation

The interpretation of RT images in weld overlay applications requires specialized training because several defect types present unique radiographic signatures:

• Porosity: Appears as dark circular or elliptical spots. In austenitic overlay layers, porosity tends to be more spherical and dispersed due to the higher sulfur and oxygen content in austenitic consumables. Gas tungsten arc welding (GTAW) overlay typically produces cleaner welds with lower porosity rates than gas metal arc welding (GMAW) overlay.
• Slag inclusions: Appear as irregular dark shapes with varying densities. In multi-pass overlay, slag inclusions between passes are a common concern, particularly when the interpass temperature is too high, preventing proper slag removal.
• Lack of fusion: Appears as a dark linear indication along the bond line or between overlay passes. This is the most critical defect type because it directly compromises the metallurgical bond between the overlay and the base material.
• Cracks: Appear as dark linear indications with sharp edges. In nickel-based alloy overlays, cracking is more common due to the higher thermal conductivity and lower ductility of these materials.

Austenitic Grain Structure and Sensitivity Assessment

A critical technical consideration highlighted in Item 107 is the need to evaluate RT sensitivity for austenitic coarse-grain materials. Austenitic stainless steel overlay layers, particularly those deposited by ESW or SAW with high heat input, develop a columnar grain structure with grain sizes that can exceed 100 micrometers. This coarse grain structure can cause diffraction effects that reduce RT sensitivity, particularly for small defects near the bond line.

To address this, the following measures should be implemented:

1. Use a higher exposure voltage (typically 150-200 kV for stainless steel overlay on carbon steel) to reduce grain diffraction effects.
2. Employ a finer-grained film or higher-resolution DR detector.
3. Place the IQI on the source side of the overlay layer to ensure adequate sensitivity in the region of interest.
4. Consider using a dual-exposure technique with different exposure parameters to cover both the base and overlay regions with adequate density.

Engineering Practice and Case Analysis

In a recent project involving the fabrication of a high-pressure hydrogenation reactor with a 316L overlay layer of 6 mm thickness on a 60 mm 15CrMoR base shell, the RT inspection revealed several interesting findings. The initial RT performed with standard parameters showed an apparent lack of fusion along a 150 mm section of the bond line. Upon closer examination with a dual-exposure technique, it was determined that the indication was actually a grain boundary diffraction artifact caused by the coarse columnar structure in the ESW overlay deposit. This false indication would have led to unnecessary rework and potential project delay if not properly identified.

Conversely, in another project involving Hastelloy C-276 overlay on a carbon steel heat exchanger shell, RT revealed a series of small slag inclusions (0.5-1.5 mm) in the second overlay pass. These were traced back to inadequate slag removal between passes during the SAW overlay operation. The root cause analysis identified that the interpass temperature had exceeded 250°C, causing the slag to re-solidify and become difficult to remove. Corrective measures included reducing the interpass temperature to below 200°C and implementing a mandatory slag inspection between each pass.

Key Reflections and Recommendations

The study of RT for weld overlay layers reveals that successful inspection requires a deep understanding of both the radiographic physics and the metallurgical characteristics of the overlay material. Engineers must not rely solely on the minimum acceptance criteria specified in standards but should develop project-specific inspection procedures that account for the specific material combination, overlay thickness, and expected defect types. The use of DR/CR for thick-walled components is strongly recommended, and sensitivity assessments should be performed for each unique material combination. Furthermore, the interpretation of RT images in overlay applications should be performed by personnel with specific training in bimetal radiography, as the skills required differ significantly from those needed for conventional weld inspection. A robust quality assurance program that integrates RT with other NDT methods — particularly UT for bond line inspection and PT for surface defect detection — provides the most comprehensive assessment of overlay quality.