Ultrasonic Measurement of Plasma Transferred Arc Powder Cladding Layer Thickness

Literature Overview and Methodological Foundation

This study by Shi Duanhu and colleagues from the Jiangsu Provincial Key Laboratory of Inspection and Control for Large Engineering Equipment, published in 2014, addresses a fundamental metrology challenge in the quality control of plasma transferred arc (PTA) powder cladding operations. The accurate and non-destructive measurement of cladding layer thickness is critical for ensuring that the overlay meets design specifications for corrosion resistance, wear resistance, or other functional requirements. PTA cladding, classified under ASME IX QW-450, is widely used for depositing nickel-based alloys, stainless steels, and high-chromium alloys on substrates such as carbon steel, stainless steel, and nickel-based substrates. The study proposes an ultrasonic-based method for measuring the thickness of the deposited overlay layer, which is particularly valuable for in-process and post-process quality control.

Core Technical Principles and Methodology

The ultrasonic method relies on the principle of acoustic impedance mismatch at the interface between the cladding layer and the substrate. When an ultrasonic pulse is transmitted into the material, reflections occur at interfaces where the acoustic impedance changes. The time delay between the back-wall echo and the overlay-substrate interface echo is directly proportional to the thickness of the overlay layer. The key challenge in this application is the significant difference in acoustic properties between the overlay material (often a nickel-based alloy or high-chromium alloy) and the substrate (often carbon steel or stainless steel), which affects the transmission and reflection of ultrasonic waves.

The study likely developed a calibration procedure using reference blocks with known overlay thicknesses, enabling the conversion of time-of-flight measurements into thickness values. The method must account for the effects of overlay microstructure, which can include columnar grains, carbide precipitates, and porosity, all of which can scatter and attenuate the ultrasonic signal. The attenuation of ultrasonic waves in PTA cladding layers is often higher than in wrought materials due to the presence of fine carbides and possible porosity, which necessitates the use of higher-frequency transducers or immersion techniques to improve signal-to-noise ratio.

Engineering Application and Quality Control Integration

In the context of bimetal pressure vessel fabrication, the thickness of the cladding overlay must be verified to meet the design specification. For example, a hydrogenation reactor with a 316L stainless steel overlay on a carbon steel substrate may require a minimum overlay thickness of 3 mm to ensure adequate corrosion resistance. The ultrasonic method provides a non-destructive means of verifying this requirement without cutting or damaging the component. The method is particularly useful for measuring the thickness of overlay layers on complex geometries such as tubes, pipes, and forgings, where access for destructive thickness measurement is limited.

The integration of ultrasonic thickness measurement into the quality control plan for PTA cladding operations should include:

The study's contribution to the field is significant because it provides a practical, non-destructive method for overlay thickness measurement that can be implemented on the shop floor. The method complements other non-destructive testing techniques such as radiographic testing (RT) for internal defects, magnetic particle testing (MT) for surface cracks, and positive material identification (PMI) for compositional verification. Together, these methods form a comprehensive quality assurance package for PTA cladding operations.

Limitations and Practical Considerations

The ultrasonic method has certain limitations that must be recognized in engineering practice. The presence of coarse columnar grains in the overlay layer can cause significant signal attenuation, particularly at higher frequencies. Porosity and lack of fusion defects can produce spurious echoes that may be mistaken for the overlay-substrate interface. The curvature of the component surface can affect the coupling of the transducer to the surface, requiring the use of angled transducers or immersion techniques for curved surfaces. The study likely addressed these limitations through careful transducer selection, calibration procedures, and signal processing techniques.

The accuracy of the ultrasonic method is also affected by the sound velocity of the overlay material, which can vary depending on the alloy composition, microstructure, and temperature. For nickel-based alloys such as Inconel 625, the sound velocity is approximately 5800-6000 m/s, while for high-chromium alloys it may be slightly lower due to the presence of carbide phases. Temperature compensation may be required for in-process measurements where the overlay temperature is elevated.

This study provides a valuable metrology tool for the quality control of PTA cladding operations, enabling engineers to verify overlay thickness non-destructively and efficiently. The method is particularly relevant for the fabrication of bimetal pressure vessels, heat exchangers, and other components where the overlay thickness is a critical design parameter. Engineers should integrate this ultrasonic measurement method into their quality control plans to ensure that cladding operations meet the required specifications and that the resulting components provide the intended functional performance in service.