CLADDING TECHNOLOGY SHANXI CO., LTDDimensional and Geometric Tolerance Inspection of Weld Overlay Components
Literature Overview
The dimensional and geometric accuracy of weld overlay components is critical for ensuring proper fit, function, and performance in service. Whether it is a sealing surface on a valve, a wear ring in a pump, or a clad plate for pressure vessel fabrication, the overlay layer must meet specified thickness, flatness, and surface finish requirements. This study note examines the inspection methods, acceptance criteria, and practical challenges associated with dimensional and geometric tolerance verification of weld overlay components.
Core Technical Principles
Key Dimensional Parameters
| Parameter | Definition | Typical Tolerance |
|---|---|---|
| Overlay thickness | Thickness of the deposited overlay layer | ±0.5 mm or ±10% of nominal |
| Minimum thickness | Minimum acceptable thickness at any point | Specified minimum (e.g., 3 mm for Stellite) |
| Flatness | Deviation from a perfect plane | 0.5 mm per 100 mm (typical for sealing surfaces) |
| Thickness variation | Variation in overlay thickness across the surface | ±0.3 mm (tight); ±1.0 mm (general) |
| Surface roughness (Ra) | Arithmetic average surface roughness | Ra 1.6 µm (general); Ra 0.8 µm (sealing); Ra 0.4 µm (precision) |
| Circularity | Deviation from a perfect circle | 0.1 mm diameter (typical for sealing surfaces) |
| Concentricity | Deviation of overlay centre from base centre | 0.1 mm (typical for rotating components) |
Inspection Methods
| Method | Parameter Measured | Accuracy | Application |
|---|---|---|---|
| Ultrasonic thickness gauge | Overlay thickness | ±0.1 mm | Non-destructive, in-process monitoring |
| Caliper / micrometre | Thickness at specific points | ±0.01 mm | Final verification |
| Coordinate measuring machine (CMM) | Flatness, circularity, concentricity | ±0.005 mm | Final dimensional verification |
| Surface roughness tester | Ra, Rz, Rmax | ±0.01 µm | Surface finish verification |
| Optical comparator | Profile, flatness | ±0.01 mm | Visual verification of contours |
| Dye penetrant + micrometre | Overlay depth at specific locations | ±0.05 mm | Spot checking of overlay depth |
| Sectioning + optical measurement | Overlay thickness at interface | ±0.05 mm | Destructive verification of interface thickness |
Interpretation of Technical Points
Ultrasonic Thickness Measurement of Overlay Layers
Ultrasonic thickness gauges are the most widely used method for non-destructive measurement of overlay layer thickness. The principle is based on the time-of-flight of an ultrasonic pulse through the material, with the thickness calculated from the sound velocity and the time delay. However, several factors can affect the accuracy of ultrasonic thickness measurement of weld overlay layers:
- Surface roughness: A rough surface scatters the ultrasonic beam, reducing the signal amplitude and increasing the measurement uncertainty. The surface must be ground smooth (Ra < 3.2 µm) before measurement.
- Material attenuation: Weld overlay materials, particularly nickel-based alloys, have higher ultrasonic attenuation than base metals, which can reduce the signal-to-noise ratio and increase the measurement uncertainty.
- Layer interfaces: The interface between the overlay layer and the base metal can reflect the ultrasonic pulse, creating multiple echoes that must be correctly interpreted. The operator must identify the correct echo corresponding to the back wall of the overlay layer.
- Temperature effects: The sound velocity in the material is temperature-dependent, and the thickness gauge must be calibrated at the measurement temperature or corrected for temperature differences.
For accurate ultrasonic thickness measurement of weld overlay layers, the following practices are recommended:
- Calibrate the gauge on a reference block of the same material and thickness as the overlay layer.
- Use a high-frequency transducer (5–10 MHz) for thin overlay layers (< 5 mm) and a lower frequency (2.5–5 MHz) for thicker layers.
- Apply a generous amount of couplant to ensure good acoustic coupling between the transducer and the surface.
- Measure at multiple points across the overlay surface, particularly at the edges where thinning is most likely.
- Record the minimum thickness and the location of the minimum thickness for comparison with the specification.
Minimum Overlay Thickness Requirements
The minimum overlay thickness is a critical specification that ensures adequate wear resistance, corrosion resistance, and mechanical integrity. The minimum thickness is typically specified as an absolute value (e.g., 3 mm) or as a percentage of the nominal thickness (e.g., 80% of nominal). The minimum thickness must be verified at multiple locations, particularly at the edges of the overlay where thinning is most likely due to the spray angle of the welding torch and the edge effects of the molten pool.
For weld overlay applications, the minimum thickness is typically verified using ultrasonic thickness measurement at a grid of points across the overlay surface. The grid spacing is typically 25–50 mm for general applications and 10–25 mm for critical sealing surfaces. The minimum thickness must be verified at all locations, and any area below the minimum specification must be reworked or the component rejected.
Surface Roughness of Sealing Surfaces
The surface roughness of weld overlay sealing surfaces is critical for ensuring a proper seal and preventing leakage. The surface roughness is typically specified as an arithmetic average (Ra) value, with typical requirements ranging from Ra 0.4 µm for precision sealing surfaces to Ra 3.2 µm for general wear surfaces.
The surface roughness of weld overlay layers is influenced by:
- Welding parameters: Higher current and voltage produce a wider, flatter weld bead with lower surface roughness. Lower current and voltage produce a narrower, more convex bead with higher surface roughness.
- Travel speed: Higher travel speed produces a narrower bead with higher surface roughness. Lower travel speed produces a wider bead with lower surface roughness.
- Post-weld machining: Grinding, honing, or lapping can reduce the surface roughness to the required specification. The amount of material removed must be sufficient to eliminate the weld bead profile and achieve the required flatness and roughness.
For critical sealing surfaces, the surface roughness must be verified using a surface roughness tester at multiple points across the surface. The measurement must be performed in accordance with the applicable standard (ISO 4287, ASTM E1927), and the results must be within the specified tolerance at all measurement locations.
Engineering Practice Cases
In a project involving a high-pressure valve seat with Stellite 6 overlay on a 316 stainless steel body, the overlay thickness was specified as 3.0 ± 0.5 mm with a minimum of 2.5 mm. Ultrasonic thickness measurement revealed that the overlay thickness at the centre of the seat was 3.2 mm, but at the edge, the thickness was only 2.1 mm, below the minimum specification. This thinning at the edge was attributed to the spray angle of the welding torch, which was not optimised for the curved geometry of the valve seat. The solution was to rework the overlay by adding an additional pass at the edge, adjusting the torch angle to compensate for the curvature, and then re-measuring the thickness. After rework, the minimum thickness was verified to be 2.7 mm, within the specification.
The surface roughness of the valve seat was specified as Ra 0.8 µm. After grinding the overlay surface, the surface roughness was measured at 12 points across the seat. The results showed Ra values ranging from 0.6 to 1.2 µm, with the higher values located at the edge where the grinding wheel had less contact. The solution was to perform a final lapping operation using a fine abrasive (6 µm) to reduce the surface roughness to Ra 0.8 µm at all locations.
Key Questions and Reflections
- How to verify overlay thickness at the interface? Ultrasonic thickness measurement provides the total thickness of the overlay layer, but it does not distinguish between the overlay material and the dilution zone at the interface. For critical applications where the dilution zone thickness must be controlled, destructive sectioning and optical measurement are required. A cross-section is prepared, and the overlay thickness is measured from the interface to the surface using an optical microscope or a scanning electron microscope.
- What is the effect of post-weld machining on overlay properties? Post-weld machining (grinding, honing, lapping) removes a layer of material from the overlay surface, which can affect the hardness, microstructure, and residual stress state. The amount of material removed must be carefully controlled to ensure that the overlay thickness remains within specification and that the surface finish meets the requirement. Excessive machining can reduce the overlay thickness below the minimum specification, while insufficient machining can leave surface roughness above the maximum specification.
- How to handle dimensional non-conformance? If the overlay thickness is below the minimum specification, the component must be reworked by adding additional overlay material. If the flatness or circularity is out of tolerance, the component must be machined to correct the geometry.