Contact Thermocouple Measurement of Interpass Temperature in Weld Overlay Cladding

Technical Foundation and Standards Requirements

Contact thermocouples are the gold standard for interpass temperature measurement in weld overlay cladding operations, particularly for procedure qualification testing and critical product fabrication. The use of contact thermocouples is mandated by most major welding standards, including NB/T 47014, ASME IX, and AWS D1.1, which require that interpass temperature be recorded continuously during procedure qualification testing. A K-type thermocouple (chromel-alumel) is the most commonly used type for welding applications, owing to its wide temperature range (up to 1200°C), good stability, and relatively low cost. The thermocouple is typically point-welded or mechanically clamped to the edge of the groove or the previously deposited weld layer, with the junction point positioned at a location that is representative of the interpass temperature.

The thermocouple signal is connected to a data acquisition system or a portable temperature recorder, which continuously logs the temperature as a function of time. This continuous recording provides a complete thermal history of the weld, including the peak temperature, the cooling rate, and the interpass temperature at each layer. The data obtained from contact thermocouples is essential for metallurgical analysis, as the cooling rate from the peak temperature directly influences the microstructure and hardness of the weld metal and HAZ.

Thermocouple Installation and Configuration

The correct installation of the thermocouple is critical to obtaining accurate and representative temperature measurements. The following considerations must be addressed:

The junction point should be positioned at a location that is representative of the interpass temperature, typically on the edge of the groove or on the surface of the previously deposited weld layer. For multi-layer welds, the thermocouple should be repositioned after each layer to ensure that it is in contact with the most recently deposited metal. For thick sections, multiple thermocouples should be used at different depths to capture the temperature gradient through the thickness.

The attachment method must be selected based on the material and the welding process. Point welding is the most common method for carbon steel and low-alloy steel, as it provides a reliable electrical and thermal connection. However, point welding is not suitable for stainless steel or nickel-based alloys, as the heat input from the point welding process can damage the base material. In such cases, mechanical clamping using a thermocouple clamp or adhesive bonding with a high-temperature adhesive should be used.

Data Interpretation and Metallurgical Correlation

The temperature data obtained from contact thermocouples provides valuable information for metallurgical analysis and process optimization. The following parameters can be extracted from the temperature curve:

1. Peak Temperature: The maximum temperature reached in the weld metal and HAZ during welding. This parameter is related to the heat input and the thermal conductivity of the base material.
2. Cooling Rate: The rate at which the temperature decreases from the peak temperature to 800°C (t800) and to 500°C (t500). The cooling rate is a critical parameter for predicting the microstructure and hardness of the weld metal and HAZ. A cooling rate of less than 10°C/s is generally considered slow, while a cooling rate greater than 50°C/s is considered fast.
3. Interpass Temperature: The temperature of the previously deposited weld layer immediately before the deposition of the next layer. This parameter must be within the specified range to ensure that the microstructure and mechanical properties of the overlay layer are acceptable.
4. Temperature Gradient: The difference in temperature between different depths of the weld, which is particularly important for thick sections where the cooling rate can vary significantly through the thickness.

In a procedure qualification test for the cladding of a 16MnR pressure vessel with a 304L overlay layer, the temperature data obtained from contact thermocouples revealed that the cooling rate from 800°C to 500°C was 12°C/s, which was within the acceptable range for preventing cold cracking. The interpass temperature was maintained at 120°C, which was below the 150°C limit specified for austenitic stainless steel overlay. The resulting overlay layer showed a fully austenitic microstructure with no delta ferrite, and the intergranular corrosion resistance was confirmed by a ASTM A923 practice E test.

Quality Assurance and Common Pitfalls

The use of contact thermocouples requires a robust quality assurance system to ensure that the temperature data is accurate and reliable. The following common pitfalls should be avoided:

1. Thermocouple Junction Damage: The junction point of the thermocouple can be damaged by arc spatter, mechanical deformation, or overheating. Regular inspection of the thermocouple and the junction point is essential to ensure data integrity.
2. Electrical Noise: The thermocouple signal can be corrupted by electrical noise from the welding power source, particularly in high-current welding processes such as submerged arc welding (SAW). The use of shielded cables and proper grounding can minimize this issue.
3. Thermal Mass Effects: The thermocouple wire itself has a finite thermal mass that can lag behind the actual temperature of the weld surface. This lag can be minimized by using thin-gauge thermocouple wire and by ensuring good thermal contact between the junction point and the weld surface.
4. Calibration: Thermocouples must be calibrated periodically to ensure accuracy. Calibration should be performed in accordance with the applicable standard, such as ISO/IEC 17025 or ASTM E220, and the calibration certificate should be retained as part of the quality documentation.

Study Insights and Engineering Recommendations

Contact thermocouples are the most reliable and comprehensive method for interpass temperature measurement in weld overlay cladding operations. They provide continuous, high-accuracy temperature data that is essential for procedure qualification testing, metallurgical analysis, and process optimization. However, their use requires careful attention to installation, data acquisition, and quality assurance to ensure that the temperature data is accurate and representative. Engineers should ensure that thermocouples are properly installed, calibrated, and protected, and that the temperature data is interpreted in the context of the metallurgical requirements of the overlay layer. The key insight is that contact thermocouples are not merely a measurement tool but a diagnostic instrument that provides valuable information about the thermal history of the weld, which can be used to optimize the welding process and ensure the integrity of the overlay layer.