Ultrasonic Thickness Measurement of Plasma Arc Powder Surfacing Overlay Layers
1. Definition and Fundamental Principles
Ultrasonic thickness measurement is a non-destructive testing (NDT) method that employs high-frequency sound waves (typically 1 MHz to 25 MHz) to determine the thickness of material layers without causing damage to the substrate or the deposited overlay. In the context of plasma arc powder surfacing (PAPS) — a specialized form of weld overlay cladding — this technique is indispensable for verifying the as-deposited layer thickness, detecting delaminations, measuring dilution depth, and ensuring compliance with specification requirements.
The fundamental principle relies on pulse-echo ultrasonic measurement. A piezoelectric transducer emits a short ultrasonic pulse into the material through a couplant medium. The sound wave propagates through the overlay layer and the base material, reflecting off interfaces (such as the overlay/substrate boundary, the back surface of the substrate, or any internal discontinuities such as delaminations or voids). The time-of-flight between the emitted pulse and the received echo is measured and converted into a thickness value using the known sound velocity in the material:
Thickness = (V × t) / 2
Where V is the longitudinal sound velocity in the material (m/s) and t is the round-trip time of flight (seconds). For plasma arc powder surfacing layers, the sound velocity must be calibrated against known reference standards of the specific overlay material system (e.g., Stellite 6, 309L, 316L, Inconel 625, or hardfacing alloys), as the velocity differs significantly between austenitic stainless steels, nickel-based superalloys, and cast irons.
2. Category and Business Positioning
Ultrasonic thickness measurement of plasma arc powder surfacing layers falls squarely within the Quality Assurance and NDT domain of Cladding Technology Shanxi Co., Ltd's operational framework. It is not a fabrication process per se, but rather a critical enabling capability that underpins the following business functions:
- Process qualification and WPS/PQR development — Establishing reproducible thickness control during welding procedure qualification.
- In-process quality control — Real-time or near-real-time verification during multi-pass surfacing operations.
- Final inspection and acceptance testing — Delivering measurable, documented evidence of layer thickness compliance to customers.
- Traceability and quality records — Generating data packages that satisfy customer, regulatory, and third-party inspection requirements.
Within the company's three technology routes, this measurement capability is most directly applicable to the TIG/MIG weld overlay route, as plasma arc powder surfacing is a variant of arc-based weld overlay. However, the same ultrasonic principles can be adapted for post-fabrication inspection of hydraulic explosive bonded and explosion-welded clad products, particularly for measuring the bonded layer thickness and detecting interface defects.
3. Technical Purpose and Value
3.1 Primary Technical Purposes
- Layer thickness verification: Confirming that the deposited overlay thickness meets the specified minimum and maximum values (e.g., 3.0 ± 0.5 mm or 6.0 ± 1.0 mm as specified in customer drawings or standards such as ASTM A240 or ASME SA-387).
- Interface integrity assessment: Detecting delamination, lack of fusion, or partial bonding at the overlay/base material interface.
- Dilution depth measurement: Estimating the depth of base metal dilution into the overlay layer, which is critical for applications where dilution limits are specified (e.g., <5% for certain corrosion-resistant claddings).
- Weld profile mapping: Generating thickness maps across large surface areas to identify low spots, uneven deposition, or areas requiring additional passes.
- Post-grinding thickness confirmation: Verifying final thickness after machining or grinding operations that reduce the as-deposited layer.
3.2 Business Value
For Cladding Technology Shanxi Co., Ltd, mastery of ultrasonic thickness measurement delivers measurable commercial advantages:
- Reduced rework rates: Early detection of thickness non-conformance prevents costly rework or scrap of large clad plates and pipes.
- Accelerated qualification cycles: Quantitative thickness data accelerates WPS/PQR qualification and reduces the number of trial runs.
- Enhanced customer confidence: Providing ultrasonic thickness maps and data logs demonstrates rigorous quality control, strengthening customer relationships and supporting competitive bidding.
- Regulatory compliance: Meeting NDT requirements mandated by codes such as NB/T 47013, ASME Section V, and API 570 for in-service inspection of clad components.
4. Key Process and Implementation Points
4.1 Transducer Selection and Frequency Optimization
The selection of ultrasonic transducer frequency is a critical parameter that directly affects measurement resolution and accuracy. Higher frequencies provide better resolution for thin layers but suffer from greater attenuation, while lower frequencies penetrate deeper but cannot resolve thin interfaces.
| Overlay Thickness Range | Recommended Frequency | Transducer Type | Resolution Capability |
|---|---|---|---|
| 0.5 – 2.0 mm | 10 – 25 MHz | Single-element, focused | High (sub-0.1 mm) |
| 2.0 – 5.0 mm | 5 – 10 MHz | Single-element, focused | Medium (0.1 – 0.2 mm) |
| 5.0 – 15.0 mm | 2.5 – 5 MHz | Single-element, unfocused | Medium (0.2 – 0.5 mm) |
| 15.0 – 50.0 mm | 1 – 2.5 MHz | Single-element, unfocused | Lower (0.5 – 1.0 mm) |
| Multi-layer (stacked) | 5 – 15 MHz | Phased array | High (individual layer resolution) |
4.2 Couplant Selection
The couplant medium must provide efficient acoustic impedance matching between the transducer and the workpiece surface. Common couplants include:
- Glycerine or petroleum jelly: Suitable for smooth, clean surfaces at ambient temperature.
- Water-based couplants: Environmentally friendly, suitable for automated scanning systems.
- High-temperature couplants (e.g., silicone grease): Required when measuring immediately after surfacing before the component has cooled to ambient temperature.
- Wax-based couplants: For rough or oxidized surfaces where adhesion is a concern.
4.3 Calibration Procedure
Calibration is the most critical step in ensuring measurement accuracy. The following calibration approach should be adopted:
- Sound velocity determination: Calibrate against a reference block of the same overlay material (e.g., a machined coupon of Stellite 6 or 309L with known thickness) to determine the longitudinal sound velocity.
- Time-zero correction: Account for the delay through the transducer backing, wedge (if applicable), and couplant layer.
- Range and gain settings: Set the instrument range to encompass the expected thickness plus margin; adjust gain to achieve a minimum of 80% full-screen height (FSH) on the back-wall echo.
- Verification: Measure a second reference standard of known thickness to confirm accuracy within ±0.1 mm or ±2% of nominal, whichever is greater.
4.4 Measurement Technique for Plasma Arc Powder Surfacing Layers
Plasma arc powder surfacing produces overlay layers that often exhibit a columnar grain structure, high porosity (particularly in single-layer applications), and potential dilution gradients. These features present specific challenges for ultrasonic measurement:
- Porosity-induced attenuation: Gas porosity in the overlay layer scatters and attenuates the ultrasonic signal, reducing the amplitude of the interface echo. Higher gain settings and lower frequencies may be required.
- Columnar grain orientation: The columnar grain structure may cause direction-dependent attenuation. Measurements should be taken from multiple orientations if possible.
- Interface echo identification: In multi-pass surfacing, the individual pass boundaries may produce weak echoes. The primary interface of interest is the overlay/base material boundary, which typically produces a strong reflection due to the acoustic impedance mismatch between the overlay alloy and the base steel.
- Surface roughness: As-deposited surfacing layers may have surface roughness of Ra 12.5 – 25 μm, which can scatter the ultrasonic beam. Pre-cleaning or light grinding of the surface may be necessary before measurement.
4.5 Multi-Layer and Dilution Zone Measurement
For thick overlay layers built up in multiple passes (e.g., 6 – 15 mm of 309L transition layer followed by 3 – 6 mm of 316L or Inconel 625 overlay), phased array ultrasonic testing (PAUT) offers significant advantages:
- Layer-by-layer thickness resolution: Phased array systems can independently measure the thickness of each deposited layer by focusing the ultrasonic beam at different depths.
- A-scan and B-scan visualization: Provides a visual representation of the layer structure, dilution zone, and any internal defects.
- Automated scanning: Enables rapid thickness mapping of large surfaces (e.g., full plate width) with consistent data acquisition.
5. Applicable Standards and Acceptance Criteria
5.1 Ultrasonic Testing Standards
| Standard | Title / Scope | Relevance to Overlay Thickness Measurement |
|---|---|---|
| GB/T 11344-2013 | Non-destructive testing — Ultrasonic thickness measurement of metallic materials | Primary Chinese standard for ultrasonic thickness measurement methodology |
| GB/T 5940-2008 | Steel and steel products — Ultrasonic testing of flat steel products | Applicable to clad plate thickness verification |
| NB/T 47013.3-2015 | Non-destructive testing of pressure vessels — Ultrasonic testing | Ultrasonic testing procedures for pressure vessel clad components |
| ASME Section V, Article 9 | Nondestructive Examination — Ultrasonic Examination | Ultrasonic thickness measurement procedures for ASME-coded components |
| ASTM E797-19 | Standard Practice for Determining Thickness of Metallic Materials by Ultrasonic Pulse-Echo Method | International standard for ultrasonic thickness measurement |
| ASTM E1877-22 | Standard Guide for Ultrasonic Measurement of Coating Thickness | Specifically addresses ultrasonic measurement of coatings and overlay layers |
| ISO 16810:2017 | Non-destructive testing of materials — Ultrasonic thickness measurement | International standard for ultrasonic thickness measurement |
| API RP 570 | In-service Inspection of Pressure Vessels | Ultrasonic thickness measurement for in-service monitoring of clad components |
5.2 Weld Overlay and Cladding Standards
| Standard | Title / Scope | Thickness Requirements |
|---|---|---|
| ASTM A240 / ASME SA-240 | Stainless steel plate, sheet, and strip for general applications | Specifies minimum overlay thickness for clad products (e.g., Type 316L overlay ≥ 1.6 mm) |
| ASME SA-387 | Chromium and nickel-chromium alloy steel plates for fusion-welded pressure vessel components | Specifies clad thickness and dilution limits for clad plates |
| ASTM A490 / ASME SA-490 | High-yield-strength alloy steel plate for pressure vessels | Overlay thickness and hardness requirements |
| NB/T 47093-2013 | Welding cladding and surfacing of pressure vessels | Chinese standard for cladding/surfacing procedures including thickness verification |
| EN 15614-1:2017 | Welding recommendations — Weld overlay | European standard for weld overlay qualification including thickness requirements |
5.3 Typical Acceptance Criteria
- Overlay thickness: Minimum thickness as specified in the customer drawing or applicable standard (e.g., ≥ 3.0 mm for corrosion-resistant overlay, ≥ 6.0 mm for erosion-resistant overlay).
- Thickness uniformity: Variation from the specified nominal thickness should not exceed ±10% or ±0.5 mm, whichever is greater, across the full surface area.
- Measurement accuracy: Ultrasonic thickness measurement uncertainty should be within ±0.1 mm for layers ≥ 1.0 mm thick and ±0.05 mm for layers < 1.0 mm thick.
- Interface integrity: No delamination or lack of fusion detected at the overlay/base material interface, as evidenced by the presence of a strong, continuous interface echo.
- Dilution depth: For applications with dilution limits (e.g., < 5% for certain corrosion-resistant claddings), the dilution zone depth should be measured and documented.
6. Common Risks and Controls
6.1 Measurement Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Overestimation of thickness | Use of incorrect sound velocity (e.g., using base material velocity for overlay measurement) | Calibrate against reference blocks of the actual overlay material; document sound velocity for each material system |
| Underestimation of thickness | Signal attenuation due to porosity, coarse grain structure, or surface roughness | Use lower frequency transducers; increase gain; apply surface preparation; use immersion testing for severe cases |
| Interface echo misidentification | Confusion between pass boundaries and the true overlay/base material interface | Use phased array for layer-by-layer visualization; cross-reference with known deposition geometry; use dual-element transducers |
| Inconsistent couplant thickness | Manual application of couplant with varying thickness | Use automated scanning systems with controlled couplant application; standardize manual technique with documented procedures |
| Temperature effects | Sound velocity changes with temperature, particularly for austenitic stainless steels | Measure at controlled ambient temperature; apply temperature compensation if measuring hot components |
| Geometric effects | Curved surfaces (pipes, tubes) causing beam divergence or refraction | Use focused transducers matched to the surface curvature; apply geometric correction factors |
6.2 Process Risks Related to Thickness Control
- Insufficient overlay thickness due to process instability: Plasma arc powder surfacing can be sensitive to gas flow rates, powder feed rates, and travel speed variations. Control: Implement closed-loop powder feed control and monitor arc parameters in real time.
- Excessive dilution reducing effective overlay thickness: High heat input dilutes the overlay alloy with base metal, effectively reducing the corrosion-resistant layer thickness. Control: Optimize welding parameters to minimize heat input; use low-heat-input processes for the first pass; verify dilution depth by ultrasonic measurement.
- Delamination due to improper preheating or interpass temperature: Thermal stresses from rapid cooling can cause interface delamination. Control: Maintain appropriate preheat and interpass temperatures; perform ultrasonic inspection to detect delamination.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
Ultrasonic thickness measurement is most directly applicable to the TIG/MIG weld overlay route, which includes plasma arc powder surfacing as a specialized variant. Key applications include:
- Post-build thickness verification: After completing a multi-pass overlay build (e.g., 309L transition + 316L overlay + Stellite 6 hardfacing), ultrasonic measurement confirms that each layer meets the specified thickness.
- In-process thickness monitoring: Measuring the thickness after each pass or pass group to ensure the build is on track, preventing over- or under-deposition.
- Post-grinding final thickness: After machining the overlay to the final specified thickness, ultrasonic measurement verifies compliance and detects any over-grinding that may have exposed the base material.
- WPS/PQR qualification: During welding procedure qualification, ultrasonic thickness measurement provides quantitative data to demonstrate that the procedure produces consistent, repeatable overlay thicknesses within the specified range.
7.2 Hydraulic Explosive Bonding Route
For hydraulic explosive bonding (a high-pressure, room-temperature bonding process), ultrasonic thickness measurement serves the following purposes:
- Bonded layer thickness measurement: Confirming that the bonded overlay layer (e.g., Hastelloy C-276, titanium, or aluminum) meets the specified thickness after bonding and any subsequent machining.
- Interface integrity verification: Detecting unbonded areas, partial bonding, or delamination at the interface between the bonded layers. The ultrasonic interface echo should be strong and continuous across the full bonded area.
- Post-machining thickness confirmation: Hydraulic explosive bonding typically produces a thicker bonded layer than required, which is then machined to the final thickness. Ultrasonic measurement verifies the final thickness and ensures no over-machining has occurred.
7.3 Explosion Welding Route
For explosion welding (explosive cladding), ultrasonic thickness measurement is applied as follows:
- Clad layer thickness verification: Explosion welding produces clad layers typically in the range of 1.5 – 6.0 mm. Ultrasonic measurement confirms that the clad thickness meets the specified minimum (e.g., ≥ 2.0 mm for corrosion-resistant cladding per ASTM A240).
- Wavy interface characterization: The characteristic wavy interface produced by explosion welding can be visualized using phased array ultrasonic testing, providing insight into the bonding quality and interface geometry.
- Post-fabrication inspection: After rolling, forging, or machining of explosion-welded clad plates, pipes, or tubes, ultrasonic measurement verifies final dimensions and detects any defects introduced during fabrication.
- In-service thickness monitoring: For explosion-welded components in service (e.g., pressure vessels, heat exchangers), periodic ultrasonic thickness measurement monitors overlay consumption due to corrosion or erosion, enabling predictive maintenance.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR data packages: Ultrasonic thickness measurement data is an integral part of welding procedure qualification records. By demonstrating consistent thickness control, the company builds a library of qualified procedures that can be rapidly deployed for customer projects.
- NDT procedure qualification: Developing and qualifying ultrasonic thickness measurement procedures (per NB/T 47013.3 or ASME Section V) demonstrates the company's NDT competence and supports third-party inspection acceptance.
- Personnel certification: Training and certifying NDT personnel in ultrasonic thickness measurement (Level II per NB/T 47013 or SNT-TC-1A) builds institutional capability and supports customer audits.
8.2 Product Delivery
- Quality documentation: Providing customers with ultrasonic thickness maps and data logs for each delivered component demonstrates rigorous quality control and supports acceptance inspection.
- Reduced rejection rates: Early detection of thickness non-conformance through ultrasonic measurement prevents delivery of non-conforming products, reducing customer complaints and warranty claims.
- Accelerated delivery cycles: Efficient ultrasonic measurement procedures enable rapid inspection, reducing the time between fabrication completion and final delivery.
8.3 Customer Value
- Reliability assurance: Quantitative thickness verification ensures that the overlay layer will provide the intended corrosion, erosion, or wear protection throughout the component's service life.
- Compliance support: Ultrasonic thickness measurement data supports customer compliance with regulatory requirements (e.g., API 570 for in-service inspection, ASME Section VIII for pressure vessel construction).
- Lifecycle cost reduction: Accurate thickness measurement enables predictive maintenance planning, reducing unplanned shutdowns and extending component service life.
9. Conclusion
Ultrasonic thickness measurement of plasma arc powder surfacing layers is a foundational NDT capability that underpins the quality assurance, qualification, and delivery functions of Cladding Technology Shanxi Co., Ltd. By mastering this technique — including transducer selection, calibration procedures, couplant management, and data interpretation — the company ensures that every clad component delivered to customers meets the specified overlay thickness requirements with documented, traceable evidence. This capability directly supports the company's three technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding) and contributes to qualification building, product quality, and customer satisfaction. Continuous investment in ultrasonic measurement technology, personnel training, and procedure development will further strengthen the company's competitive position in the cladding and overlay manufacturing market.