Zhuojin Pipe Fitting Co., LtdUltrasonic Method for Measuring Plasma Arc Powder Overlay Layer Thickness
Literature Overview
This study by Shi Duanhu and colleagues from the Jiangsu Key Laboratory of Inspection and Control for Large Engineering Equipment at Xuzhou Institute of Technology addresses a persistent challenge in overlay welding quality control: the accurate, non-destructive measurement of overlay layer thickness. Published in 2014 in the journal Hot Working Technology (Volume 43, Issue 17, pages 197-200), the paper proposes a mathematical model based on ultrasonic thickness measurement principles to determine the true overlay layer thickness in plasma arc powder cladding applications. The work was supported by the Jiangsu Provincial Natural Science Foundation (12KJD430003) and the Key Laboratory open project (JSKLEDC201213).
Core Technical Approach
The fundamental difficulty in measuring overlay layer thickness lies in the fact that the overlay layer is deposited on top of a base material, and conventional methods such as vernier calipers can only measure the total height from the base material surface to the top of the overlay layer. The true overlay thickness must account for the dilution zone—the region where base metal has melted and mixed with the overlay material. The authors propose a three-step measurement methodology:
- Measure the base material thickness using ultrasonic testing with a straight probe calibrated on the unclad surface.
- Measure the combined thickness of the base material plus the overlay layer from the opposite side.
- Examine metallographic micrographs to determine the dilution depth—the thickness of the melted base metal that has been incorporated into the weld zone.
The mathematical model then calculates the true overlay layer thickness by subtracting both the base material thickness and the dilution zone thickness from the total measured thickness. This approach effectively separates the three distinct zones: the unmelted base material, the dilution zone, and the true overlay layer.
Process Parameters and Measurement Calibration
| Parameter | Typical Value | Notes |
|---|---|---|
| Ultrasonic probe type | Straight (normal incidence) | Calibrated for longitudinal wave propagation |
| Frequency range | 2.5-5 MHz | Selected based on material thickness |
| Couplant | Glycerin or petroleum jelly | Ensures acoustic impedance matching |
| Calibration blocks | Standard step wedges | For zero-offset and velocity calibration |
| Plasma arc power | 20-40 kW | Typical for powder cladding |
| Powder feed rate | 5-15 kg/h | Depends on required layer thickness |
| Travel speed | 100-500 mm/min | Controls dilution ratio |
The calibration procedure is critical to measurement accuracy. The ultrasonic flaw detector must be zero-offset calibrated on a reference block of known thickness, and the sound velocity in the base material must be determined accurately. Any error in the base material sound velocity propagates directly into the thickness calculation, so precise material identification is essential before measurement.
Engineering Practice Insights
In practical overlay welding operations, particularly for components such as pump impellers, valve seats, and rotating shafts, the overlay layer thickness directly determines the service life of the component. An overlay layer that is too thin will wear through prematurely, while an excessively thick layer wastes expensive alloy material and may introduce cracking risks due to higher residual stresses. The ultrasonic method described in this paper offers several practical advantages:
- It is non-destructive, preserving the component for continued service.
- It can access difficult-to-reach locations where calipers cannot fit, such as internal cavities or recessed surfaces.
- It provides faster measurement cycles compared to destructive sectioning and metallographic preparation.
- It reduces the physical labor intensity for inspection personnel, particularly when measuring multiple components in production batches.
However, the method has limitations that must be acknowledged. The accuracy depends on the ability to obtain a reliable metallographic cross-section for dilution zone determination. If the dilution zone varies significantly across the overlay surface, a single dilution measurement may not represent the entire overlay. Additionally, the ultrasonic measurement assumes a planar interface between the base material and the overlay, which may not hold true for irregular surface geometries. For curved or contoured surfaces, the measurement results should be interpreted with caution.
Key Reflections
This study represents a practical engineering solution to a real measurement problem. The three-step methodology is logically sound and leverages existing ultrasonic testing infrastructure that is already available in most welding inspection facilities. The mathematical model is straightforward and does not require specialized equipment beyond a standard ultrasonic flaw detector and a metallographic microscope. The approach is particularly valuable in production environments where rapid quality verification of overlay thickness is needed before the component proceeds to the next manufacturing step or enters service.