Infrared Thermometer and Thermal Imaging for Interpass Temperature Monitoring in Weld Overlay Cladding

Technical Principles and Advantages

Infrared (IR) thermometers and thermal imaging cameras offer a non-contact method for measuring interpass temperature in weld overlay cladding operations. The fundamental principle is based on the detection of infrared radiation emitted by the surface of the weld, which is proportional to the surface temperature according to Planck's law. The measured temperature is a function of the emissivity of the surface, which is a material-specific property that must be set correctly in the instrument to obtain accurate readings. For oxidized steel surfaces, the emissivity is typically in the range of 0.75–0.90, while for polished stainless steel surfaces, it can be as low as 0.20–0.40.

The primary advantages of IR measurement are speed, non-contact operation, and the ability to monitor large areas simultaneously with thermal imaging cameras. A single-point IR thermometer (commonly referred to as an IR gun) can provide a temperature reading within 1–2 seconds, making it ideal for rapid spot checks of interpass temperature. A thermal imaging camera can capture the temperature distribution across the entire weld area, providing a comprehensive view of the thermal field and enabling the identification of hot spots and cold spots that may not be detected by point measurements.

Comparison of IR Thermometers and Thermal Imaging Cameras

Single-point IR thermometers are the most practical and economical option for routine interpass temperature monitoring in production environments. They are portable, easy to use, and require minimal training. However, they only measure the temperature at a single point, which may not be representative of the overall interpass temperature if there are significant temperature gradients across the weld. Thermal imaging cameras provide a more comprehensive view of the temperature distribution, but they are more expensive and require more training to operate effectively.

Emissivity Considerations and Measurement Accuracy

The accuracy of IR temperature measurement is critically dependent on the correct setting of the emissivity. If the emissivity is set incorrectly, the measured temperature can be significantly different from the actual temperature. For example, if the emissivity is set to 0.95 for a polished stainless steel surface with an actual emissivity of 0.30, the measured temperature can be 50–100°C lower than the actual temperature. This error can lead to incorrect decisions about whether the interpass temperature is within the acceptable range.

To mitigate the risk of emissivity-related errors, the following practices should be implemented:

1. Surface Preparation: The weld surface should be cleaned to remove oxide, spatter, and debris before measurement. A consistent surface finish should be maintained across the measurement area.
2. Emissivity Verification: The emissivity of the surface should be verified periodically using a reference method, such as a contact thermocouple. The measured temperature from the IR thermometer should be compared with the contact thermocouple reading, and the emissivity setting should be adjusted accordingly.
3. Use of Emissivity Tape: High-emissivity tape (emissivity > 0.95) can be applied to the weld surface to provide a consistent and known emissivity. This method is particularly useful for polished or reflective surfaces where the emissivity is low and variable.
4. Distance-to-Spot Ratio: The distance-to-spot ratio (DSR) of the IR thermometer should be considered when selecting the measurement distance. A DSR of 50:1 means that the spot size is 1/50th of the distance from the thermometer to the surface. For a 100 mm distance, the spot size would be 2 mm, which is suitable for measuring small welds. For larger welds, a lower DSR may be acceptable.

Integration with Welding Equipment and Automated Monitoring

One of the most advanced applications of thermal imaging in weld overlay cladding is the integration of the thermal imaging camera with the welding equipment to provide real-time monitoring and automatic control of the interpass temperature. In this configuration, the thermal imaging camera is mounted on the welding head or on a fixed position above the weld, and the temperature data is fed to the welding controller, which can automatically adjust the welding parameters or stop the welding process if the interpass temperature exceeds the specified limit.

This type of automated monitoring system has been successfully implemented in robotic welding cells for the cladding of large-scale components such as pressure vessel heads and heat exchanger tubesheets. The system provides continuous, real-time monitoring of the interpass temperature across the entire weld area, and can trigger an alarm or an automatic shutdown if the temperature exceeds the limit. This approach significantly improves the quality and consistency of the overlay layer, and reduces the risk of defects due to excessive interpass temperature.

However, the implementation of automated thermal monitoring systems requires significant investment in equipment and software, and the system must be carefully calibrated and validated to ensure reliable performance. The emissivity of the weld surface must be continuously monitored and adjusted, and the system must be protected from environmental factors such as smoke, spatter, and ambient temperature variations.

Common Defects and Countermeasures

The use of IR thermometers and thermal imaging cameras in weld overlay cladding can reveal several types of defects and anomalies that may not be detected by other methods:

1. Hot Spots: Localized areas of elevated temperature that can indicate excessive heat input, poor thermal conductivity, or the presence of a defect such as a slag inclusion or a porosity. Hot spots should be investigated and corrected before proceeding with the next layer.
2. Cold Spots: Localized areas of lower temperature that can indicate poor thermal contact, inadequate preheating, or the presence of a cold weld. Cold spots can lead to cold cracking and poor fusion, and should be addressed by adjusting the preheat temperature or the welding parameters.
3. Temperature Gradients: Large temperature gradients across the weld width can indicate asymmetric heat input, which can lead to distortion and residual stresses. The welding parameters should be adjusted to minimize the temperature gradient, and the weld should be monitored for signs of distortion.
4. Emissivity Variations: Variations in the emissivity of the weld surface can lead to errors in the IR temperature measurement. The emissivity should be verified periodically, and the surface should be prepared to provide a consistent emissivity.

Study Insights and Engineering Recommendations

Infrared thermometers and thermal imaging cameras offer a powerful and versatile tool for interpass temperature monitoring in weld overlay cladding operations. They provide rapid, non-contact measurement that is suitable for both routine production monitoring and advanced automated control systems. However, the accuracy of IR measurement is critically dependent on the correct setting of the emissivity, and the results should be verified periodically using contact thermocouples. Engineers should ensure that IR measurement is used as a complementary tool rather than a replacement for contact thermocouples, and that the emissivity is verified and documented as part of the quality assurance process. The key insight is that IR measurement provides a unique perspective on the thermal field of the weld that can reveal defects and anomalies that are not visible to the naked eye, and that this information can be used to optimize the welding process and improve the quality of the overlay layer.