Infrared Thermographic Welding Temperature Monitoring for Clad Overlay Processes

1. Definition and Operating Principles

Infrared thermographic welding temperature monitoring is a non-contact thermal sensing technology that employs high-speed infrared cameras (thermal imagers) to capture real-time two-dimensional temperature field distributions across the weld bead, heat-affected zone (HAZ), and base substrate during clad overlay operations. The system continuously acquires thermal radiation data across the mid-wave (3–5 μm) or long-wave (8–14 μm) infrared spectral bands, converts radiant flux into quantitative surface temperature maps, and applies automated threshold logic to determine preheat adequacy, interpass temperature compliance, and cooldown progression.

The fundamental physical basis is Planck's radiation law: every surface above absolute zero emits electromagnetic radiation whose spectral radiance is a function of temperature and emissivity. The thermal imager captures this radiation through a germanium or zinc selenide lens onto a microbolometer focal plane array (FPA), typically of resolution 640 × 480 pixels or higher. An on-board microprocessor performs non-uniformity correction (NUC), applies emissivity compensation based on material-specific calibration curves, and outputs a calibrated temperature map at frame rates of 30–60 Hz (or higher for high-speed models). This real-time data stream feeds into a dedicated quality control software platform that executes automated decision logic for interpass temperature management.

The technology replaces the traditional manual approach of using contact-type temperature measurement pens (thermocouple probes or pyrometers), which provide only single-point spot readings, require operator intervention at discrete intervals, and are inherently subject to human error, response lag, and inconsistency. By contrast, infrared thermography delivers continuous, spatially resolved, fully automated thermal surveillance across the entire weld zone.

2. Category and Business Positioning

Within the company's capability architecture, infrared thermographic temperature monitoring is classified under the category of "Weld Pool Camera and Quality Control Software" (熔池相机与质控软件), representing the intersection of optical sensing, embedded computing, and process control automation. This positioning is significant because it situates the technology not merely as a monitoring tool but as an integral component of the digital quality control ecosystem that underpins the company's value-added cladding services.

From a business perspective, this technology serves three strategic functions:

3. Technical Purpose and Value Proposition

The primary technical purpose of this system is to achieve "real-time interpass temperature management" (层温实时化) — replacing the reactive, periodic, manual verification paradigm with a proactive, continuous, automated monitoring and intervention system. The specific functional objectives include:

  1. Preheat verification: Automatically confirm that the base material surface has reached the required preheat temperature (as specified in the WPS) before welding commences. The system verifies both the magnitude and spatial uniformity of preheat across the entire weld zone, not merely at a single spot.
  2. Interpass temperature monitoring: Continuously track the temperature of the completed weld layer and the adjacent HAZ during the interpass interval. The system determines whether the temperature has cooled to within the specified interpass range before the next pass is initiated.
  3. Over-temperature alarm and arc-stopping interlock: If the interpass temperature exceeds the upper limit defined in the WPS, the system triggers an audible and visual alarm and, through a hard-wired or networked interlock signal, commands the welding power source to stop arc output. This prevents the deposition of metal under non-compliant thermal conditions.
  4. Post-weld cooldown monitoring: Track the cooling rate (ΔT/Δt) of the completed overlay, which is a critical parameter for hydrogen cracking susceptibility in high-carbon equivalents and for residual stress development.
  5. Thermal record generation: Produce time-stamped thermal maps and temperature-time curves that serve as permanent quality records for traceability, audit, and dispute resolution.

The value proposition is further amplified by the elimination of the "temperature measurement pen trend" (替代人工测温笔趋势) — the systematic reduction of reliance on manual thermometry in favor of automated, objective, and repeatable sensing. This shift is consistent with Industry 4.0 and smart manufacturing trends in the welding and fabrication sector.

4. Key Process and Implementation Points

4.1 System Architecture and Component Selection

A complete infrared thermographic welding temperature monitoring system comprises the following functional modules:

4.2 Critical Implementation Parameters

Parameter Typical Specification Rationale
Imager resolution ≥640 × 480 pixels Ensures sufficient spatial resolution to distinguish weld bead, HAZ, and base material temperature gradients
NETD (Noise-Equivalent Temperature Difference) ≤50 mK Provides adequate sensitivity to detect interpass temperature changes at the 1–2 °C resolution required by WPS specifications
Spectral range 3–5 μm (MWIR) for hot monitoring; 8–14 μm (LWIR) for cool monitoring MWIR offers superior signal at high temperatures; LWIR is more sensitive at low temperatures
Frame rate ≥30 Hz Ensures real-time tracking of rapid temperature transients during welding and interpass cooling
Temperature measurement range -20 °C to 1200 °C (or higher) Covers the full range from ambient through preheat, welding, interpass, and cooldown
Emissivity setting 0.80–0.95 (material-dependent) Must be calibrated for the specific base material and surface condition; oxide layers, paint, and scale significantly affect emissivity
Standoff distance 100–500 mm Optimized to balance field of view (covering the entire weld zone) against spatial resolution
Temperature accuracy ±2 °C or ±2% of reading (whichever is greater) Meets the tolerance requirements of most WPS interpass temperature specifications
Response time (alarm to arc-stop) ≤500 ms Ensures that the welding power source ceases arc output before significant metal deposition occurs under non-compliant thermal conditions
Logging frequency ≥1 Hz (full thermal map); ≥10 Hz (ROI temperature trace) Provides sufficient temporal resolution to reconstruct temperature-time curves for qualification documentation

4.3 Emissivity Calibration Methodology

Emissivity is the single most critical calibration parameter for infrared thermography accuracy. The emissivity of metal surfaces varies significantly with material type, surface finish, oxidation state, and temperature. The following calibration methodology is recommended:

  1. Reference standard method: Apply a high-emissivity reference tape (ε = 0.95 ± 0.01, e.g., 3M 861 or equivalent) to a representative area of the base material. Simultaneously measure the reference area temperature with a calibrated contact thermocouple and the infrared imager. Adjust the imager's emissivity setting until the two readings converge within ±2 °C.
  2. Blackbody comparison method: Use a calibrated blackbody radiation source at known temperatures to verify the imager's absolute temperature accuracy. This method validates the imager itself but does not account for material-specific emissivity.
  3. Field calibration at operating temperature: Perform emissivity calibration at the actual operating temperature range (e.g., preheat at 200 °C, interpass at 150–300 °C) rather than at room temperature, because emissivity is temperature-dependent for oxidized metal surfaces.
  4. Periodic re-verification: Re-verify emissivity settings at the start of each production shift and whenever the surface condition changes (e.g., after grinding, after oxide removal, after paint application).

4.4 Software Logic and Decision Architecture

The quality control software implements a state-machine logic for interpass temperature management:

  1. Preheat verification state: The system monitors the base material temperature and determines whether the preheat temperature has been reached and stabilized across the entire weld zone (not merely at a single point). The system confirms preheat adequacy when the minimum temperature in the defined ROI exceeds the WPS-specified preheat temperature for a sustained period (e.g., ≥2 minutes).
  2. Welding state: During arc-on welding, the system records the thermal field but does not enforce interpass temperature limits (since the temperature is inherently elevated by the arc).
  3. Interpass cooling state: After arc-out, the system continuously monitors the ROI temperature. When the temperature drops to within the WPS-specified interpass range (e.g., 150–250 °C for a given WPS), the system signals "ready for next pass."
  4. Over-temperature alarm state: If, during the interpass cooling state, the temperature exceeds the upper interpass limit (e.g., 350 °C for a given WPS), the system triggers an alarm and sends an arc-stopping signal to the welding power source. The system remains in alarm state until the temperature drops below the limit.
  5. Post-weld cooldown state: After the final pass, the system monitors the cooling curve and records the cooling rate at specified temperature intervals (e.g., 800→600 °C cooling time, which is a key parameter for hydrogen cracking assessment).

5. Applicable Standards and Acceptance Criteria

The implementation and use of infrared thermographic welding temperature monitoring must be aligned with the following standards and qualification frameworks:

Standard / Specification Relevant Requirement Application
ASME Section IX, QW-11 (Welding Procedure Qualification) Preheat and interpass temperature limits must be maintained within the qualified range Thermal monitoring provides objective evidence of compliance with qualified WPS temperature parameters
ASME Section IX, QW-251 (Procedure Qualification Records) PQR shall include a record of welding parameters, including preheat and interpass temperatures Automated thermal logging produces continuous, timestamped temperature records that satisfy PQR documentation requirements
ASME BPV Code Section VIII Div. 1, UW-36 (Post-Weld Heat Treatment) Temperature uniformity and ramp rates for PWHT must be controlled and recorded Infrared thermography can extend to PWHT monitoring, verifying uniform heating and cooling rates across the weld zone
ASME BPV Code Section VIII Div. 3, UW-36 (PWHT for Div. 3) Similar requirements as Div. 1 but with additional constraints for nuclear applications Enhanced thermal monitoring with automated alarm and interlock is particularly valuable for nuclear-grade cladding
NB/T 20901-2018 (Welding Procedure Qualification Rules for Pressure Vessels) Preheat and interpass temperature control requirements for pressure vessel fabrication Thermal monitoring ensures compliance with NB standards for Chinese pressure vessel code
GB/T 985.1-2008 (Non-destructive Testing of Welds — General Principles) General framework for NDT and process monitoring in welding Infrared thermography serves as an in-process monitoring technique complementary to post-weld NDT
ISO 9712 (Non-Destructive Testing — Qualification and Certification of NDT Personnel) Personnel qualification requirements for NDT methods Operators of the thermal monitoring system should be trained and qualified; while infrared thermography is not traditionally classified as NDT, its use in quality assurance warrants formal operator qualification
ISO 15614-1:2017 (Specification and Qualification of Welding Procedures for Metallic Materials — General Rules) WPS and PQR requirements including thermal cycle parameters Thermal monitoring data supports WPS qualification and ongoing compliance verification
API 579 (Fitness-for-Service) Thermal cycle history is relevant for assessment of remaining life and fitness-for-service Accurate thermal records contribute to fitness-for-service assessments of clad components
NACE MR0175 / ISO 15156 (Materials for Use in H₂S-Containing Environments) Hardness and microstructure control in H₂S service requires thermal cycle management Interpass temperature control prevents excessive grain growth and hardness excursions in weld overlay deposits
EN ISO 13919 (Welding — Guidance on Welding Procedures for Steels) Thermal cycle control requirements for different steel grades Provides guidance on preheat and interpass temperature ranges that the monitoring system enforces

Acceptance criteria for the thermal monitoring system itself should include:

6. Common Risks and Controls

Risk Consequence Control Measure
Emissivity miscalibration Systematic temperature measurement error; false alarms or missed alarms Perform emissivity calibration at the start of each shift using reference tape; document calibration records; re-verify when surface condition changes
Optical obstruction (spatter, fume, shielding gas) Signal loss or degraded image quality; incomplete thermal data Position the camera with adequate standoff and shielding from spatter; use a sapphire or zirconia window; implement signal-loss detection with automatic alarm
Viewing angle deviation Apparent temperature error due to cosine effect and reflected radiation Maintain viewing angle ≤30° off-normal; apply cosine correction in software; shield the camera from reflected radiation sources (e.g., nearby hot surfaces)
Arc radiation interference Camera sensor saturation or damage during arc-on welding; thermal blooming Use a neutral density filter or bandpass filter matched to the IR spectral band; implement automatic gain control; use a camera with sufficient dynamic range (≥120 dB)
False interlock triggering Unintended arc stopping; production disruption Implement a time-delay threshold (e.g., over-temperature must persist for ≥2 seconds before triggering arc-stop); implement a manual override with documented authorization
Environmental factors (wind, rain, temperature extremes) Degraded measurement accuracy; camera malfunction Use a weather-protected enclosure with IR-transparent window; implement temperature compensation algorithms; specify cameras with wide operating temperature range (-20 °C to +55 °C)
Data integrity and cybersecurity Tampering with temperature records; unauthorized modification of alarm thresholds Implement role-based access control; use encrypted data storage; maintain audit trails for all parameter changes; use tamper-evident data logging
Camera drift over time Progressive degradation of measurement accuracy Implement periodic calibration schedules (e.g., quarterly or semi-annual); use built-in NUC and self-test functions; monitor calibration drift trends

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay (Thermal Spray Welding)

In TIG (GTAW) and MIG (GMAW) weld overlay processes, infrared thermographic temperature monitoring is directly applicable and provides the highest value. The thermal process in weld overlay involves multi-pass deposition of overlay metal onto a base substrate, with each pass requiring strict control of preheat and interpass temperatures. The monitoring system is particularly valuable in the following scenarios:

7.2 Hydraulic Explosive Bonding (Hydrodynamic Explosive Cladding)

In hydraulic explosive bonding (also known as explosive welding or hydrodynamic explosion welding), the bonding process itself is a high-velocity impact event that does not involve a sustained thermal process amenable to interpass temperature monitoring in the traditional sense. However, infrared thermographic monitoring is applicable in the following supporting contexts:

  • Post-bonding thermal assessment: After the explosive bonding event, the clad laminate may exhibit localized temperature elevations due to the kinetic energy conversion. Infrared thermography can be used to map the residual temperature distribution across the bonded surface, identifying any anomalous hot spots that may indicate incomplete bonding or microstructural anomalies.
  • Post-bonding stress relief annealing: Explo sively clad laminates often require stress relief annealing to reduce residual stresses induced by the bonding process. Infrared thermography can monitor the heating and cooling rates during annealing, ensuring compliance with the WPS or heat treatment specification (e.g., uniform heating at ≤100 °C/h, holding at specified temperature, cooling at ≤100 °C/h).
  • Subsequent weld overlay on explosively bonded substrates: When a weld overlay layer is deposited on top of an explosively bonded clad laminate (a common configuration for corrosion-resistant cladding on structural steel), the infrared thermographic monitoring system is fully applicable to the weld overlay process, ensuring proper preheat and interpass temperature control on the composite substrate.
  • Quality verification of the bonding interface: While infrared thermography cannot directly inspect the bonding interface (which requires NDT such as ultrasonic testing or metallographic examination), it can provide indirect evidence of bonding quality by detecting temperature anomalies that may correlate with interface defects.

7.3 Explosion Welding (Explosive Cladding)

Explosion welding (explosive cladding) is a solid-state joining process that uses controlled detonation of explosive charges to accelerate a cladding plate onto a base plate at high velocity, creating a metallurgical bond through plastic deformation and jetting. Similar to hydraulic explosive bonding, the primary bonding event is not a thermal process, but infrared thermographic monitoring is applicable in the following contexts:

  • Explosive cladding of large plates and complex geometries: For large-format explosive cladding (e.g., 6 m × 12 m plates), the detonation wave propagates across the plate, and the temperature distribution during and after the event may be non-uniform. Infrared thermography can capture the thermal evolution during the detonation event (using high-speed IR cameras) and the subsequent cooldown, providing data for process optimization and quality assessment.
  • Post-cladding heat treatment monitoring: Explosively clad plates often require stress relief annealing. Infrared thermography provides a non-contact, spatially resolved method for monitoring the temperature distribution during annealing, ensuring uniform heating and cooling across the entire plate surface. This is particularly important for large plates where conventional contact thermocouples may not provide adequate spatial coverage.
  • Weld overlay on explosively clad components: When explosively clad components are subsequently machined and require repair welding or additional overlay layers, the infrared thermographic monitoring system is fully applicable to these welding operations.
  • Process development and research: Infrared thermography is a valuable research tool for studying the thermal effects of the explosion welding process, including the temperature rise at the bonding interface, the cooling rate, and the thermal gradients that influence the microstructure of the bonded interface.

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

Infrared thermographic welding temperature monitoring directly supports the company's qualification building efforts in several ways:

  • WPS/PQR documentation: The automated thermal records produced by the system provide continuous, timestamped, spatially resolved temperature data that satisfy the documentation requirements of ASME Section IX, ISO 15614-1, and NB/T 20901-2018. This reduces the administrative burden of manual temperature logging and eliminates gaps in the thermal record.
  • Demonstration of process control capability: When bidding for projects that require advanced process control (e.g., nuclear, LNG, offshore), the capability to demonstrate automated interpass temperature monitoring with alarm and interlock functionality is a significant differentiator. It shows the client that the company has invested in process control technology that goes beyond the minimum requirements of the applicable code.
  • Support for advanced WPS development: The thermal data collected by the system can be used to develop and refine WPS parameters, including optimal preheat temperatures, interpass temperature ranges, and cooling rates for different material combinations and joint configurations.

8.2 Product Delivery

The technology contributes to product delivery quality and efficiency through:

  • Reduced rework rates: By preventing welding under non-compliant thermal conditions, the system reduces the incidence of defects related to excessive interpass temperature (e.g., grain coarsening, reduced hardness, hydrogen cracking). This directly translates into lower rework rates and higher first-pass yield.
  • Faster production cycles: Automated interpass temperature monitoring eliminates the need for operators to manually check temperatures with a measurement pen, reducing the non-value-added time between passes. The system provides instant "ready for next pass" signals, enabling more efficient production scheduling.
  • Consistent quality across shifts and operators: The automated system eliminates the variability introduced by different operators' manual temperature checking practices, ensuring consistent thermal cycle control regardless of operator experience or fatigue.
  • Traceability and audit readiness: The complete thermal records produced by the system provide a permanent, tamper-evident quality record that satisfies customer audit requirements and regulatory inspection requirements.

8.3 Customer Value

The technology delivers tangible value to the company's customers through:

  • Enhanced confidence in product quality: Customers receive comprehensive thermal cycle documentation with their clad products, providing evidence that the welding process was performed under controlled thermal conditions. This is particularly valuable for customers in regulated industries (nuclear, pharmaceutical, food processing) who require extensive quality documentation.
  • Reduced lifecycle risk: By ensuring proper thermal cycle control, the technology reduces the risk of in-service failures due to thermal cycle-related defects (e.g., hydrogen cracking, intergranular corrosion, stress corrosion cracking). This extends the service life of clad components and reduces the customer's total cost of ownership.
  • Competitive advantage in bidding: The capability to offer automated thermal monitoring as a standard feature of the cladding service differentiates the company from competitors who rely on manual temperature verification, enabling the company to win higher-value contracts.
  • Alignment with Industry 4.0 expectations: Increasingly, customers expect digital quality documentation and real-time process monitoring. The infrared thermographic monitoring system positions the company as a technology-forward provider that meets these expectations.

9. Conclusion

Infrared thermographic welding temperature monitoring represents a significant advancement in the process control capabilities of clad overlay manufacturing. By replacing manual, periodic, single-point temperature measurement with continuous, automated, spatially resolved thermal surveillance, the technology enables a fundamental shift in the quality assurance paradigm — from reactive verification to proactive control. Its application across the company's three technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding) demonstrates its versatility as a quality control tool that transcends any single process technology. When integrated into the company's quality management system, the technology directly supports WPS/PQR qualification, reduces rework rates, enhances product traceability, and delivers measurable value to customers in the form of higher-quality clad products with lower lifecycle risk.