Multi-Point Thermocouple Temperature Field Verification Test for Cladding Process Qualification

Literature Overview and Technical Context

In the fabrication of clad-plate pressure vessels and weld-overlay components, temperature control constitutes one of the most critical process variables that directly governs the metallurgical quality of the overlay layer, the base metal heat-affected zone (HAZ), and the metallurgical bond at the cladding interface. The multi-point thermocouple temperature field verification test is a mandatory validation activity conducted during process qualification (PQR) and first-article stages to confirm that the designed temperature control scheme—comprising preheat, interpass temperature maintenance, and post-weld heat treatment (PWHT)—is both effective and reproducible. This test serves as the empirical backbone of the entire thermal management strategy and is typically submitted as an annex to the process qualification report.

Core Technical Objectives and Test Configuration

The primary objective of this verification test is threefold: (1) to measure the actual spatial uniformity of preheat temperature across the workpiece; (2) to capture the layer-to-layer temperature distribution during multi-pass overlay welding; and (3) to record the cooling rate curves from each pass down to the lower interpass temperature limit. These three measurement categories collectively validate whether the furnace or induction heating system, the thermal insulation strategy, and the interpass monitoring protocol are capable of maintaining the prescribed thermal window throughout the entire cladding operation.

Thermocouple Placement Strategy

Thermocouple placement is not arbitrary—it must follow a systematic grid pattern that captures the thermal gradient across the full cross-section and along the weld length. The following table summarizes the typical placement configuration for a multi-pass ESW or SAW overlay cladding operation:

The number of thermocouples is determined by the size of the workpiece, the number of cladding passes, and the applicable code requirements. For large-diameter hydrogenation reactor shells, the placement density may exceed 30 thermocouple points to ensure adequate spatial resolution of the temperature field.

Cooling Rate Analysis

The cooling rate from 800 °C to 500 °C (t₈₀₀₋₅₀₀) is a critical parameter for assessing the susceptibility of the HAZ to hardening and hydrogen-induced cracking (HIC). During the multi-point thermocouple test, the cooling rate is derived from the recorded temperature-time curves at each measurement point. The following criteria are typically applied:

• For carbon steel base metals with Ceq ≤ 0.40%, the target t₈₀₀₋₅₀₀ should be no less than 15 s to minimize martensitic transformation risk.
• For low-alloy steels such as 16Mn or 15CrMo, the cooling rate should be controlled to avoid excessive hardness in the HAZ, typically requiring t₈₀₀₋₅₀₀ ≥ 20 s.
• For stainless steel overlay layers (e.g., 304L, 316L), the cooling rate is less critical from a cracking standpoint but must be monitored to assess grain growth and sensitization risk.

Preheat Uniformity Acceptance Criteria

The preheat uniformity is evaluated by the maximum temperature deviation across all measurement points at the time when preheat is declared complete. The acceptance criterion is typically expressed as:

• Maximum deviation from the target preheat temperature ≤ ±25 °C for furnace preheat.
• Maximum deviation ≤ ±35 °C for induction or flame preheat.
• All measurement points must reach the minimum preheat temperature simultaneously before welding commences.

Interpretation of Test Results and Defect Correlation

Temperature Field Non-Uniformity and Its Consequences

In practice, temperature field non-uniformity is the most common finding from multi-point thermocouple verification tests. The root causes include insufficient furnace capacity, poor thermal insulation on the back side of the cladding panel, uneven flame distribution, and inadequate preheat duration. The consequences of non-uniform preheat are severe: regions with lower-than-specified preheat temperatures experience higher cooling rates, leading to increased HAZ hardness, potential cracking, and reduced toughness. Regions with higher-than-specified temperatures may suffer from excessive grain growth, reduced strength, and in the case of austenitic stainless steel cladding, increased sensitization risk due to prolonged exposure in the 450–850 °C sensitization range.

Cooling Rate Anomalies and Process Adjustment

When the recorded cooling rate curves show that t₈₀₀₋₅₀₀ falls below the specified minimum, the corrective actions include: increasing the preheat temperature, adding thermal insulation blankets to the back of the panel, reducing the welding heat input (which paradoxically increases cooling rate and may require the opposite action), or introducing interpass heating between passes. The key insight from the test data is that cooling rate is not solely a function of heat input—it is equally governed by the thermal mass of the base metal, the ambient temperature, and the convective/radiative heat loss from the workpiece surface.

Integration with Engineering Practice

Case Study: Hydrogenation Reactor Shell Overlay Cladding

During the qualification of a 316L overlay cladding process on a 16Mn base plate for a hydrogenation reactor shell (inner diameter 3200 mm, wall thickness 85 mm, cladding thickness 6 mm), a multi-point thermocouple test was conducted with 24 thermocouple points arranged in a 4×6 grid on the preheat surface, 8 points on the back side, and 6 points on the cladding layer surface. The test revealed that the bottom 150 mm of the shell experienced preheat temperatures 30 °C below the target, attributable to radiation shielding by the support saddles. The corrective measure was to install additional induction heating coils near the saddle regions and increase the preheat duration by 40 minutes. After re-testing, the maximum deviation was reduced to ±18 °C, meeting the acceptance criterion. The cooling rate at the back side of the shell was measured at 18 s (t₈₀₀₋₅₀₀), which satisfied the requirement for the 16Mn base metal. This case demonstrates that the multi-point thermocouple verification test is not merely a compliance exercise—it is an indispensable tool for identifying and resolving thermal control deficiencies before they manifest as weld defects in production.

PDCA Cycle Application

The multi-point thermocouple verification test fits naturally into the PDCA (Plan-Do-Check-Act) quality management cycle. In the Plan phase, the thermocouple layout, instrument calibration certificates, and data acquisition parameters are defined. In the Do phase, the test is executed according to the WPS thermal parameters. In the Check phase, the recorded temperature data are analyzed against the acceptance criteria. In the Act phase, deviations are addressed through process adjustments, and the updated WPS is re-qualified if necessary. This cyclical approach ensures continuous improvement of the temperature control scheme and builds a robust technical foundation for production-scale cladding operations.

Key Questions and Reflections

A recurring question in engineering practice is: How many thermocouple points are sufficient? The answer depends on the geometry, the thermal gradient expected, and the code requirements. For a flat plate, a grid pattern with spacing no greater than 300 mm is generally adequate. For cylindrical shells, the placement must account for both axial and circumferential gradients. Another important consideration is the response time of the thermocouples—K-type thermocouples with a 1 mm diameter have a response time of approximately 5–10 s, which is acceptable for most cladding applications but may be insufficient for rapid thermal cycling scenarios. Furthermore, the calibration status of each thermocouple must be verified before the test, as even a ±5 °C calibration error can lead to incorrect process decisions when the interpass temperature window is narrow.

Study Insights and Implications

The multi-point thermocouple temperature field verification test is far more than a data collection exercise—it is a systematic engineering tool that bridges the gap between theoretical thermal modeling and actual manufacturing conditions. The test data provide irreplaceable empirical evidence for the process qualification report, support the engineering judgment required for WPS optimization, and create a traceable record that satisfies code inspectors and end users. From a broader perspective, the insights gained from temperature field verification tests contribute to the development of more sophisticated thermal control strategies, including real-time feedback control of induction heating systems and predictive cooling rate monitoring based on data analysis algorithms. However, the fundamental principle remains unchanged: understanding and controlling the temperature field is the cornerstone of producing high-quality clad components that meet both metallurgical and mechanical performance requirements. Engineers must approach each test with rigorous attention to detail, systematic data analysis, and a willingness to adjust processes based on empirical evidence rather than assumptions.