Interlayer Temperature Measurement via Infrared Thermometry and Thermal Imaging in Bimetallic Cladding Processes
1. Definition and Fundamental Principles
Interlayer (interpass) temperature measurement using infrared thermometers and thermal imaging cameras is a non-contact thermal monitoring technique employed to continuously or intermittently assess the surface temperature of substrate and deposited layers during bimetallic cladding operations. The technology is rooted in the Stefan-Boltzmann radiation law, which states that all objects above absolute zero emit electromagnetic radiation proportional to the fourth power of their absolute temperature. Infrared pyrometers and thermal imagers capture this emitted radiation in the mid- to long-wave infrared spectrum (typically 0.7–14 μm) and convert it into a temperature reading.
In the context of cladding manufacturing—whether weld overlay, hydraulic explosive bonding, or explosion welding—the interlayer temperature is a critical process parameter that governs solidification microstructure, dilution, residual stress development, hydrogen-induced cracking susceptibility, and ultimate metallurgical compatibility between dissimilar materials. The entry under discussion (No. 349) specifies two operational modes:
- Single-point infrared gun (spot pyrometer): Rapid spot-check measurement at discrete locations along the weld overlay bead or clad surface, suitable for interpass temperature verification between successive passes.
- Full-field thermal imager: Continuous two-dimensional temperature mapping of the entire welding zone, enabling real-time monitoring, trend analysis, and automated interlock with welding equipment for over-temperature alarm and shutdown.
The description explicitly cautions against emissivity setting error, which is the single largest source of measurement inaccuracy in infrared thermometry applied to metallic surfaces. Oxidized, spattered, or molten surfaces present variable emissivity values that, if not correctly compensated, can lead to systematic under- or over-estimation of true surface temperature.
2. Category and Business Positioning
This capability falls under the major category of Process Temperature Control and Cooling (过程温控与降温), subcategory Layer Temperature Measurement (层温测量), with the technical purpose of achieving full-field non-contact monitoring (全场非接触). It is referenced in linkage with Entry No. 324, which pertains to the broader interpass temperature control strategy and welding sequence planning.
Within the company's quality management architecture, this capability serves as the measurement backbone for interpass temperature compliance. It bridges the gap between WPS (Welding Procedure Specification) requirements for maximum interpass temperature and the actual field execution, providing auditable data trails for NQA-1, ISO 3834, or customer-specific quality audits.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Real-time interpass temperature verification: Ensure that the substrate surface temperature between successive weld overlay passes does not exceed the WPS-specified maximum (commonly 250°C for austenitic stainless steels, 200°C for duplex grades, and 150°C for sensitization-sensitive applications).
- Full-field thermal mapping: Identify hot spots, thermal accumulation zones, and temperature gradients that single-point measurement cannot detect, particularly in multi-pass overlay builds or thick-section clad plates.
- Automated over-temperature interlock: Integrate thermal imager output with the welding power source or robotic controller to trigger alarm, pause, or automatic shutdown when temperature thresholds are breached, preventing dilution excursions and microstructural degradation.
- Process data acquisition for qualification: Generate continuous temperature logs that serve as objective evidence during WPS qualification, procedure performance qualification, and customer witness inspections.
3.2 Business Value
- Qualification building: Demonstrates to certification bodies (e.g., NB/T 47014 for pressure vessel welder qualification, ASME Section IX) that interpass temperature is controlled within specified limits, supporting both WPS qualification and PWHT exemption claims.
- Product delivery confidence: Reduces rework rates by catching thermal excursions in real time rather than through post-weld NDT rejection, directly improving schedule adherence and cost efficiency.
- Customer value: Provides customers in oil & gas, power generation, and chemical processing with documented proof of process control, reducing inspection burden and accelerating approval of critical components such as clad pressure vessels, heat exchanger tubesheets, and pipeline fittings.
- Knowledge capture: Thermal imaging data enables post-process analysis of heat input distribution, supporting continuous improvement of welding parameters and sequence optimization.
4. Key Process and Implementation Points
4.1 Instrument Selection and Specification
| Parameter | Single-Point Infrared Gun | Thermal Imager (Full-Field) |
|---|---|---|
| Measurement principle | Spot pyrometry, single pixel or small area | Multi-pixel infrared detector array (e.g., microbolometer) |
| Temperature range | −50°C to 1800°C (typical industrial grade) | −20°C to 500°C (standard) / 1000°C (high-temp) |
| Spatial resolution | Spot size determined by distance-to-spot ratio (DSR), typically 10:1 to 50:1 | Detector pixel count (e.g., 384×288, 640×480) |
| Measurement speed | 1–5 readings per second | Up to 60 Hz frame rate (real-time video) |
| Accuracy (typical) | ±1.5°C or ±1.5% of reading | ±2°C or ±2% of reading |
| Data output | Point value, manual log or serial output | Full-frame image + CSV/JSON temperature matrix, network integration (Ethernet, Wi-Fi) |
| Welding integration | Manual spot-check between passes | Automated interlock with welding controller, PLC, or robotic system |
| Typical cost | $200–$2,000 | $5,000–$50,000+ |
4.2 Emissivity Management—The Critical Control Variable
Emissivity (ε) is the ratio of radiation emitted by a surface to that emitted by a perfect blackbody at the same temperature. For metallic surfaces in cladding operations, emissivity is highly variable:
| Surface Condition | Typical Emissivity (ε) | Impact on Measurement |
|---|---|---|
| Clean stainless steel (304/316L) | 0.25–0.35 | Low emissivity; high reflectivity from surroundings causes significant error |
| Oxidized stainless steel | 0.75–0.95 | High emissivity; more reliable measurement |
| Carbon steel (mill finish) | 0.60–0.80 | Moderate; oxidation state dependent |
| Molten weld pool | 0.85–0.95 | Generally reliable, but spatter and arc radiation interfere |
| Spattered/porous surface | 0.70–0.90 | Variable; localized measurement uncertainty |
| Carbon steel (machined, polished) | 0.10–0.20 | Extremely reflective; requires tape or coating for accurate measurement |
Control measures for emissivity error:
- High-emissivity tape application: Apply ε ≈ 0.95 tape (e.g., Kapton, electrical tape, or specialized IR tape) at designated measurement points prior to welding. This is the most reliable method for accurate interpass temperature reading on low-emissivity substrates.
- Two-color (ratio) pyrometry: For molten or semi-transparent materials, two-color pyrometers measure at two wavelengths and compute temperature from the ratio, eliminating emissivity dependence.
- Reflective target correction: For low-emissivity surfaces, measure reflected radiation from a known-temperature target (e.g., blackbody cavity or calibrated reflector) and apply correction algorithms.
- Software emissivity calibration: For thermal imagers, perform empirical emissivity calibration by comparing IR readings against a contact thermocouple at multiple temperatures and surface conditions, then set the instrument's emissivity parameter accordingly.
4.3 Measurement Procedure—Single-Point Infrared Gun
- Pre-weld preparation: Apply high-emissivity tape at predetermined measurement locations (typically at the toe of the preceding bead, the center of the preceding bead, and the intended start point of the next bead).
- Instrument calibration: Verify the infrared gun against a calibrated blackbody furnace or reference thermometer within the operating range. Document calibration certificate number and date.
- Distance verification: Confirm measurement distance is within the instrument's specified DSR range to ensure the spot size does not exceed the intended measurement area.
- Measurement execution: Between each pass, allow the operator to measure surface temperature at all designated points. Record the maximum reading and the time since the preceding pass completed.
- Decision logic: If any reading exceeds the WPS-specified maximum interpass temperature, initiate forced cooling (air blast, water spray, or passive cooling) and re-measure after a defined cooling interval.
- Documentation: Log all readings in a welding logbook or electronic data capture system, including instrument ID, calibration date, operator ID, and pass number.
4.4 Measurement Procedure—Thermal Imager with Welding Integration
- System setup: Mount the thermal imager on a fixed or robotic position with a clear line of sight to the entire welding zone. Ensure the field of view encompasses the current weld area plus a margin of at least 50 mm beyond the active bead.
- Emissivity configuration: Set the thermal imager's emissivity parameter based on pre-calibrated values for the specific substrate material and surface condition. For multi-material cladding (e.g., carbon steel substrate with stainless overlay), configure zone-specific emissivity settings or apply measurement tape.
- Threshold configuration: Define maximum interpass temperature limits in the thermal imager's software or the connected PLC. Set alarm thresholds at 80%, 90%, and 100% of the maximum allowable temperature for graduated warning.
- Integration with welding controller: Establish a communication link (e.g., Modbus TCP, Ethernet/IP, or proprietary protocol) between the thermal imager and the welding power source or robotic controller. Program the interlock logic: if the maximum pixel temperature in the defined ROI (Region of Interest) exceeds the threshold, the system triggers a warning alarm; if it exceeds a critical threshold, the system automatically pauses the welding cycle.
- Real-time monitoring: During welding, the thermal imager captures continuous frames. The software extracts the maximum temperature, mean temperature, and temperature gradient across the ROI. This data is displayed on a monitoring screen and logged to a time-series database.
- Post-weld analysis: After welding completion, export the full thermal imaging dataset for analysis. Generate temperature distribution maps, cooling rate curves, and heat accumulation profiles for process optimization and quality records.
4.5 Recommended Instrument Parameters for Cladding Applications
| Application | Recommended Temperature Range | Recommended Spectral Range | Recommended Emissivity Setting | Key Feature |
|---|---|---|---|---|
| TIG/MIG weld overlay interpass monitoring | 0–500°C | 7.5–14 μm (long-wave) | 0.95 (with tape) or calibrated value | Real-time video, ROI alarm, data logging |
| Explosion welding surface temperature check | 0–200°C | 7.5–13 μm | 0.85–0.95 (oxidized surfaces) | Full-field imaging for bond line inspection |
| Post-weld PWHT temperature uniformity | 100–900°C | 7.5–14 μm | 0.90–0.95 | High resolution, uniformity mapping |
| Molten pool monitoring (research) | 1500–2000°C | 1.5–2.5 μm (short-wave, two-color) | Not applicable (ratio method) | High-speed capture, arc radiation immunity |
5. Applicable Standards and Acceptance Criteria
5.1 Standards Governing Interpass Temperature Control
- NB/T 47014—2011 (Qualification testing of welding procedures for pressure vessels): Specifies that interpass temperature shall not exceed the value specified in the WPS. The WPS must state the maximum interpass temperature based on material type and thickness.
- ASME Section IX (Welding, Brazing, Fusing and Qualifying Rules): QW-401 requires that the welding procedure shall specify the maximum interpass temperature. For austenitic stainless steels (Group P-No. 8), the maximum interpass temperature is typically limited to 250°C (482°F) to prevent sensitization and intergranular corrosion.
- GB/T 985.1—2008 (Non-destructive testing of welds—Radiographic testing): While primarily an NDT standard, it references the importance of proper thermal control to minimize defects detectable by radiography.
- ASME Section VIII, Division 1 (Rules for Construction of Pressure Vessels): UW-13 requires that welding procedures be qualified and followed, including interpass temperature limits. UW-40 specifies that the actual welding procedure used must conform to the qualified WPS, including interpass temperature.
- ISO 15614-1:2017 (Qualification testing of welding procedures for metallic materials—General rules): Requires documentation of interpass temperature control as part of the WPS qualification evidence.
- NACE SP0169 / ISO 15589 (Control of corrosion during construction): Addresses temperature control during welding of coated or lined surfaces to prevent coating degradation.
5.2 Standards Governing Infrared Thermometry Instruments
- IEC 60904-1:2004 (Infrared radiation thermometers—Part 1: General requirements and definitions): Specifies accuracy, reproducibility, and environmental conditions for infrared thermometers.
- IEC 60904-2:2004 (Infrared radiation thermometers—Part 2: Specific requirements for optical pyrometers): Applies to high-temperature infrared instruments.
- ANSI/ASME BPV Section V (Nondestructive Examination): While primarily for NDT methods, the principles of instrument calibration and traceability apply to infrared thermometry used as a process control parameter.
- ISO 50436:2019 (Energy efficiency—Thermal imaging): Provides guidelines for the use of thermal imaging in industrial applications, including process monitoring.
- GB/T 19146—2019 (Infrared thermometers—General requirements): Chinese national standard equivalent to IEC 60904-1.
5.3 Acceptance Criteria for Infrared Temperature Measurement Systems
| Acceptance Criterion | Requirement | Verification Method |
|---|---|---|
| Instrument calibration | Calibrated within ±1.0°C or ±1.0% of reading against a traceable reference | Calibration certificate from accredited laboratory (CNAS/ISO 17025) |
| Emissivity setting | Empirically verified against contact thermocouple within ±5°C at operating temperatures | Side-by-side comparison test at 100°C, 200°C, 300°C, 400°C |
| Interpass temperature compliance | All recorded readings within WPS-specified maximum interpass temperature | Review of temperature log data for each weld pass |
| Alarm function | System triggers alarm within 2 seconds of threshold exceedance | Functional test with heated target exceeding alarm setpoint |
| Data integrity | Continuous, unbroken temperature log with timestamp for entire welding operation | Review of exported data file for gaps or anomalies |
6. Common Risks and Controls
6.1 Measurement Accuracy Risks
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Emissivity mismatch | Incorrect emissivity setting for actual surface condition | Systematic temperature error (up to ±50°C on polished steel) | Apply high-emissivity tape; perform empirical calibration; use two-color pyrometry |
| Atmospheric absorption | Water vapor, CO₂, and particulates between sensor and target | Underestimation of true temperature | Minimize measurement distance; use short-wave IR for long distances; apply atmospheric correction |
| Reflected radiation | Hot surroundings (welding arc, furnace walls) reflected off low-emissivity surface | Overestimation of true temperature | Use emissivity tape; shield sensor from arc radiation; use reflectance correction |
| Window contamination | Spatter, dust, or smoke on the thermal imager lens or infrared gun window | Reduced signal, inaccurate readings | Protective window with purge air; regular cleaning schedule |
| Instrument drift | Aging detector, thermal drift of reference thermometer | Gradual accuracy degradation | Regular calibration schedule (every 6–12 months); daily reference checks |
6.2 Process Control Risks
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Excessive interpass temperature | High travel speed, insufficient cooling time, high heat input | Dilution increase, sensitization, intergranular corrosion, HAZ softening | Real-time thermal imager monitoring with automated alarm; enforced cooling intervals | Insufficient interpass temperature | Over-cooling, excessive air blast | Hard microstructure, hydrogen cracking, poor weld fusion | Minimum temperature alarm; controlled cooling methods |
| Thermal accumulation in multi-pass builds | Sequential deposition without adequate heat dissipation | Progressive temperature rise, final passes exceed limit | Full-field thermal mapping to identify accumulation zones; adaptive sequence planning |
| False alarm / nuisance shutdown | Incorrect threshold setting, arc radiation interference | Welding disruption, productivity loss | Shield imager from arc; set graduated alarm levels; use ROI masking |
6.3 Quality System Risks
- Documentation gap: If infrared measurement data is not properly integrated into the welding log and quality records, it cannot serve as evidence during audits or customer inspections. Control: Implement automated data logging with electronic signature and time-stamped records.
- Calibration traceability: Instruments not calibrated against traceable standards cannot support qualification claims. Control: Maintain calibration certificates from CNAS-accredited laboratories and implement a calibration management system.
- Operator competency: Infrared measurement requires trained operators who understand emissivity effects, instrument limitations, and data interpretation. Control: Implement training and qualification programs for operators using infrared thermometry.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay
In TIG (GTAW) and MIG (GMAW) weld overlay cladding, interpass temperature control is arguably the most critical process parameter after heat input. The application of infrared thermometry in this route is extensive and well-established:
- Single-point infrared gun usage: Operators perform spot-check measurements at 2–3 locations along the preceding bead between each pass. The measurement is taken at the point of maximum expected temperature (typically the center of the preceding bead, 10–20 mm from the weld toe). This is the most common and cost-effective method for routine production.
- Thermal imager usage: For high-value, high-integrity applications (e.g., clad pressure vessel heads, nuclear-grade components), a thermal imager is mounted above the welding zone to provide continuous full-field monitoring. The imager captures the entire overlay build-up, enabling detection of thermal accumulation patterns that would be missed by spot measurements. The imager's data output is integrated with the welding controller to enforce interpass temperature limits automatically.
- Linkage with Entry No. 324: Entry 324 (interpass temperature control strategy) defines the welding sequence, pass order, and cooling methodology. Entry 349 provides the measurement capability to verify that the strategy is being executed correctly. Together, they form a closed-loop process control system: the strategy defines the target, and the measurement verifies compliance.
- Typical parameters: Maximum interpass temperature of 250°C for austenitic stainless overlay (e.g., 309L, 316L, 321) on carbon steel substrate; 200°C for duplex stainless overlay; 150°C for sensitization-critical applications. Infrared gun measurement accuracy of ±5°C is generally sufficient for these applications when emissivity is properly managed.
7.2 Hydraulic Explosive Bonding
Hydraulic explosive bonding (also known as hydraulic shock bonding or hydraulic explosion welding) is a solid-state bonding process that uses controlled hydraulic shock to achieve metallurgical bonding between dissimilar metal plates. While the process itself does not involve welding heat input in the traditional sense, infrared thermometry plays important roles in pre-processing, post-processing, and quality verification:
- Pre-bonding surface preparation verification: After shot blasting or grinding preparation of the flyer and base plates, infrared thermography can detect residual stress-induced temperature variations or subsurface defects that may affect bond quality. Uniform surface temperature indicates uniform preparation.
- Post-bonding temperature assessment: The hydraulic shock event generates localized heating at the bond interface due to plastic deformation and adiabatic shear. Infrared thermography immediately after bonding can map the temperature distribution across the bonded surface, providing indirect information about the uniformity of the shock event and potential bond quality variations.
- Cooling monitoring: After bonding, the plates must cool to ambient temperature before handling and inspection. Infrared thermography provides non-contact monitoring of cooling progress, ensuring that the plates have reached a safe handling temperature and that no residual thermal gradients remain that could cause distortion.
- Process development: During process parameter optimization (hydraulic pressure, flyer velocity, impact angle), infrared thermography serves as a rapid feedback tool. By correlating post-bonding temperature patterns with bond quality (verified by NDT), process engineers can establish temperature signatures indicative of good bonding.
7.3 Explosion Welding
Explosion welding is a high-energy solid-state bonding process that uses detonation of a shaped explosive charge to accelerate a flyer plate onto a base plate at high velocity, achieving metallurgical bonding through plastic instability (jet formation) and adiabatic shear. Infrared thermometry is applied in several critical aspects of this process:
- Explosive charge temperature monitoring: Prior to detonation, the explosive charge and surrounding environment must be at controlled temperatures. Infrared thermography can verify that no thermal anomalies exist in the charge assembly that could cause premature detonation or uneven energy release.
- Post-explosion surface temperature mapping: Immediately after detonation, the bonded plates exhibit a complex temperature distribution resulting from the high-velocity impact, plastic deformation, and jet formation. High-speed thermal imaging (if available) or rapid-frame thermal imager capture can record the temperature evolution in the seconds following detonation. This data is invaluable for:
- Process qualification: Temperature mapping provides evidence of uniform shock energy distribution across the bonded area, supporting qualification of the explosion welding procedure.
- Defect detection: Non-uniform temperature patterns may indicate incomplete bonding, void formation, or delamination at the bond interface. Correlation of temperature maps with subsequent NDT results (ultrasonic testing, magnetic particle inspection) establishes temperature signatures for defect identification.
- Subsequent welding operations: Explosion-welded clad plates often require subsequent welding operations (e.g., edge welding, repair welding, or transition layer welding). Infrared thermography is used to monitor interpass temperatures during these subsequent operations, ensuring that the explosion-welded bond interface is not thermally compromised.
- Post-weld heat treatment: If post-explosion heat treatment is required (e.g., stress relief), infrared thermography monitors the temperature uniformity and ramp/soak/cool rates across the clad plate, ensuring compliance with the specified heat treatment cycle.
8. Integration with Entry No. 324 and System-Level Process Control
The explicit reference to linkage with Entry No. 324 underscores that infrared temperature measurement is not a standalone capability but an integral component of a broader interpass temperature control system. Entry 324 likely encompasses:
- Welding sequence planning and pass order optimization
- Cooling methodology selection (passive cooling, air blast, water spray, controlled cooling rate)
- Interpass temperature limits by material combination and thickness
- Operator procedures for temperature verification and corrective action
Entry 349 provides the measurement and monitoring capability that enables the control strategy defined in Entry 324 to be executed and verified. The relationship is analogous to a thermostat (Entry 324) and a thermometer (Entry 349): the thermostat defines the target and control logic, while the thermometer provides the feedback signal. Without accurate, reliable, and timely temperature measurement, the interpass temperature control strategy cannot be validated or audited.
9. Implementation Roadmap and Recommendations
9.1 Short-Term Actions (0–6 months)
- Instrument procurement: Acquire a minimum of two industrial-grade infrared guns (e.g., Fluke 62 MAX, Testo 875, or equivalent) with calibration certificates, and one mid-range thermal imager (e.g., FLIR A-series, FLIR T-series, or equivalent) with 384×288 or higher resolution.
- Emissivity calibration program: Establish a calibration procedure for each substrate material used in production. Perform side-by-side comparison tests with calibrated contact thermocouples at representative temperatures (100°C, 200°C, 300°C, 400°C) and document the optimal emissivity setting for each material/surface condition combination.
- Operator training: Train all welding operators and quality inspectors on infrared thermometer operation, emissivity management, data recording, and interpretation. Include practical exercises with known-temperature targets.
- Procedural documentation: Update welding work instructions and quality procedures to include infrared temperature measurement as a required step for interpass temperature verification. Specify instrument ID, calibration status, measurement location, and recording requirements.
9.2 Medium-Term Actions (6–18 months)
- Thermal imager integration: Install a thermal imager with welding controller integration on the primary weld overlay production line. Develop interlock logic for over-temperature alarm and automatic pause. Validate the integration with test welds.
- Data management system: Implement a data logging and management system for infrared temperature data. Ensure data is stored with proper metadata (weld ID, material, WPS number, operator ID, instrument ID, calibration date) and is retrievable for audit and customer inspection.
- Process optimization: Use accumulated thermal imaging data to identify patterns in thermal accumulation, optimize welding sequences, and refine cooling methodologies. Develop predictive models for interpass temperature based on welding parameters, ambient conditions, and substrate thickness.
9.3 Long-Term Actions (18–36 months)
- Advanced thermal monitoring: Investigate high-speed thermal imaging for molten pool monitoring and real-time weld quality assessment. Explore the integration of thermal imaging with machine learning algorithms for predictive defect detection.
- Multi-sensor fusion: Combine infrared thermography with other process monitoring sensors (current, voltage, travel speed, arc height) in a comprehensive welding monitoring system. Develop a digital twin of the welding process that incorporates thermal data for real-time simulation and optimization.
- Industry leadership: Publish technical papers and present at industry conferences on the application of infrared thermometry in bimetallic cladding. Contribute to standard development for infrared-based process monitoring in welding and cladding operations.
10. Conclusion
Interlayer temperature measurement via infrared thermometry and thermal imaging is a foundational capability for ensuring metallurgical quality, process compliance, and qualification integrity in bimetallic cladding manufacturing. The transition from manual, contact-based temperature measurement to non-contact, full-field infrared monitoring represents a significant advancement in process control maturity. By implementing the capabilities described in Entry No. 349 in conjunction with the interpass temperature control strategy of Entry No. 324, Cladding Technology Shanxi Co., Ltd. can achieve:
- Real-time process control with automated over-temperature protection, reducing rework and scrap.
- Auditable documentation of interpass temperature compliance, supporting WPS qualification, NQA-1 certification, and customer inspections.
- Process optimization through data-driven analysis of thermal behavior, leading to improved efficiency and quality.
- Competitive differentiation through demonstrated capability in advanced process monitoring, appealing to high-integrity customers in nuclear, aerospace, and offshore oil & gas sectors.
The critical success factor is disciplined emissivity management. Without proper emissivity calibration and verification, even the most sophisticated thermal imager will produce unreliable data. Investment in calibration procedures, high-emissivity measurement targets, and operator training is essential to realize the full value of this capability.