Electromagnetic Induction Heating for Preheating and Post-Heating in Steam Generator Tube Sheet Weld Overlay

Literature Overview

This technical paper by Sun Guohui and Wang Xiaohui from Harbin Electric Group (Qinhuangdao) Heavy Equipment Co., published in 2014, addresses the application of electromagnetic induction heating for preheating and post-heating during the weld overlay repair of nuclear power steam generator tube sheets. Steam generators are critical components in nuclear power plants, and their tube sheets are subject to severe thermal and mechanical loading during normal operation and transients. When tube sheet weld overlay is required, the control of thermal input and residual stresses is paramount to ensure the structural integrity of this safety-critical component. The use of electromagnetic induction heating offers a precise, efficient, and controllable method for managing the thermal conditions during the overlay process.

Technical Background

Steam Generator Tube Sheet Function and Requirements

The steam generator tube sheet is a critical structural component in pressurized water reactor (PWR) nuclear power plants. It serves as the closure for the primary and secondary pressure boundaries, sealing the ends of thousands of heat transfer tubes. The tube sheet must withstand:

• Primary side pressure: 15–17 MPa at operating temperature.
• Secondary side pressure: 5–7 MPa at operating temperature.
• Thermal cycling: Temperature variations of ±100 °C during load follow and transients.
• Vibration: Tube vibration due to flow-induced forces, which can cause fretting at the tube-to-tube sheet joint.
• Corrosion: Exposure to primary coolant (BWR) or secondary water (PWR) with potential for stress corrosion cracking.

The tube sheet is typically fabricated from low-alloy steel (e.g., SA-266 Gr. B or equivalent) with weld overlay layers on the primary and/or secondary sides to provide corrosion resistance and compatibility with the heat transfer tubes.

Weld Overlay Requirements for Tube Sheets

The weld overlay on steam generator tube sheets must meet stringent requirements:

Challenges of Tube Sheet Weld Overlay

The weld overlay of steam generator tube sheets presents several unique challenges:

1. Large component size: Tube sheets can have diameters of 1500–2500 mm and thicknesses of 100–200 mm, requiring extensive thermal management.
2. High residual stress: The thick section and large diameter create significant welding stresses that can lead to distortion and cracking.
3. Safety-critical application: Any defect in the overlay can compromise the pressure boundary integrity, leading to potential nuclear safety concerns.
4. Regulatory requirements: The repair must comply with nuclear safety regulations, including ASME NQA-1, RBP-NP-1, and other applicable codes.
5. Limited access: The tube sheet is often installed within the steam generator vessel, limiting access for welding and heating equipment.

Electromagnetic Induction Heating Technology

Principles of Electromagnetic Induction Heating

Electromagnetic induction heating operates on the principle of electromagnetic induction, where an alternating current passes through a coil, creating a time-varying magnetic field. When this magnetic field interacts with a conductive workpiece, it induces eddy currents within the material. The resistance of the material to these eddy currents generates heat, which is concentrated in the region near the coil.

The key advantages of electromagnetic induction heating for weld overlay applications include:

• Rapid heating: The heating rate can be several hundred degrees per minute, significantly faster than conventional gas or electric resistance heating.
• Precise temperature control: The power input and frequency can be adjusted to achieve and maintain the desired temperature with high accuracy.
• Localized heating: The heat can be concentrated in specific regions, such as the weld zone, without heating the entire component.
• Non-contact: The heating coil does not physically contact the workpiece, reducing the risk of damage or contamination.
• Automation compatibility: The heating process can be easily integrated into automated welding sequences.

Induction Heating System Configuration

The induction heating system for steam generator tube sheet weld overlay typically includes the following components:

The coil geometry is designed to match the shape of the tube sheet, ensuring uniform heating across the weld zone. For circular tube sheets, a circular coil with a diameter slightly larger than the weld zone is used. The coil is positioned close to the workpiece surface, typically within 5–20 mm, to maximize heating efficiency.

Application to Preheating and Post-Heating

Preheating Strategy

Preheating is essential for reducing the cooling rate in the heat-affected zone (HAZ) and preventing cold cracking. For steam generator tube sheet weld overlay, the preheating strategy involves:

1. Target temperature: 200–300 °C, depending on the base metal composition and the welding procedure specification.
2. Heating rate: 100–200 °C per minute, to achieve rapid heating without thermal shock.
3. Temperature uniformity: The temperature variation across the preheated zone should be within ±20 °C to ensure consistent thermal conditions.
4. Holding time: The preheated zone is held at the target temperature for 15–30 minutes to ensure thermal equilibrium.

The induction heating system is programmed to ramp up the power to achieve the target temperature, then maintain the temperature through feedback control. Temperature sensors are placed at multiple locations around the weld zone to monitor temperature uniformity.

Post-Heating Strategy

Post-heating (post-weld heat treatment) is critical for relieving residual stresses and tempering hard phases in the weld and HAZ. For steam generator tube sheet weld overlay, the post-heating strategy involves:

1. Target temperature: 550–650 °C, to temper martensite and relieve residual stresses.
2. Heating rate: 100–150 °C per minute, to avoid thermal shock.
3. Holding time: 1–2 hours, depending on the thickness of the weld zone.
4. Cooling rate: Controlled cooling to ambient temperature, typically < 10 °C per minute, to prevent re-hardening.

The post-heating is performed immediately after welding, before the weld zone has cooled significantly. The induction heating system is used to raise the temperature of the weld zone to the target temperature, hold it for the specified time, and then cool it at a controlled rate.

Process Integration

The integration of induction heating with the welding process is critical for achieving consistent results. The following process sequence is typically employed:

1. Preheat: Induction heating to 200–300 °C, hold for 15–30 minutes.
2. Weld overlay: Multi-pass welding using SAW or GTAW, with interpass temperature control.
3. Post-heat: Induction heating to 550–650 °C, hold for 1–2 hours, controlled cooling.
4. Inspection: NDT and mechanical testing to verify weld quality.

The interpass temperature during welding is maintained at 200–250 °C using a combination of induction heating and thermal insulation. The induction heating system can be used to quickly raise the interpass temperature if it drops below the minimum specified value.

Quality Control and Inspection

Non-Destructive Testing

The weld overlay on steam generator tube sheets must undergo rigorous NDT to ensure weld quality:

Residual Stress Measurement

Residual stress measurement is critical for safety-critical components. The following methods may be employed:

• X-ray diffraction (XRD): Non-destructive measurement of surface residual stresses.
• Hole drilling method: Semi-destructive measurement of residual stresses at specific locations.
• Neutron diffraction: Non-destructive measurement of bulk residual stresses (requires specialized facilities).

The residual stress in the weld zone should be ≤ 150 MPa in both the longitudinal and transverse directions to minimize the risk of stress corrosion cracking.

Engineering Practice and Lessons Learned

Case Study: Steam Generator Tube Sheet Repair

In a typical nuclear power plant scenario, a steam generator tube sheet with a diameter of 2000 mm and a thickness of 150 mm requires weld overlay repair on the primary side due to corrosion damage. The repair is performed in accordance with the applicable nuclear safety regulations and the owner's repair procedure.

The repair procedure involves the following steps:

1. Damage assessment: The extent of corrosion damage is assessed using UT and visual inspection. The damaged area is marked and the repair boundaries are defined.
2. Surface preparation: The damaged area is ground back to sound metal, removing all corroded material. The grinding is performed with a sequence of progressively finer abrasives to ensure a smooth surface.
3. Preheating: The repair zone is preheated to 250 °C using electromagnetic induction heating. The temperature is monitored at multiple locations to ensure uniformity.
4. Weld overlay: Multiple passes of overlay alloy are deposited using SAW, following a symmetrical sequence to minimize distortion. The interpass temperature is maintained at 200–250 °C.
5. Post-heat treatment: The weld zone is heated to 600 °C using electromagnetic induction heating, held for 2 hours, and cooled at a controlled rate.
6. Inspection: The repaired area is inspected using RT, UT, MT, and PT. Residual stress measurements are performed using XRD.
7. Documentation: All repair activities are documented in accordance with nuclear quality assurance requirements.

Process Optimization Insights

The study highlights several process optimizations that were critical to the success of the tube sheet repair:

• Coil design: The induction coil was designed with a geometry that matched the circular shape of the tube sheet, ensuring uniform heating across the repair zone. The coil was water-cooled to prevent overheating and to allow prolonged use.
• Power control: The power supply was programmed to ramp up the power gradually, avoiding thermal shock that could lead to cracking. The power was then reduced to maintain the target temperature through feedback control.
• Temperature monitoring: Multiple temperature sensors were placed around the repair zone to monitor temperature uniformity. The sensors were connected to the controller, which adjusted the power input to maintain the target temperature.
• Cooling control: The cooling rate after post-heat treatment was controlled by adjusting the power input and using thermal insulation. The cooling rate was kept below 10 °C per minute to prevent re-hardening of the weld and HAZ.

Study Insights and Reflections

This study provides a compelling demonstration of the advantages of electromagnetic induction heating for weld overlay applications on safety-critical components. The most significant lesson is that precise thermal control is essential for ensuring weld quality and long-term reliability. In nuclear power applications, where the consequences of failure are catastrophic, the use of advanced heating technologies is not optional but mandatory.

The emphasis on process integration is particularly instructive. The induction heating system is not used in isolation but is integrated into the overall welding sequence, with careful coordination between heating, welding, and cooling operations. This level of integration requires a thorough understanding of both the welding process and the heating technology, as well as the ability to program and control the heating system to achieve the desired thermal profile.

The case also highlights the importance of documentation and traceability in nuclear applications. Every step of the repair process must be documented, from the initial damage assessment to the final inspection and release. This documentation is essential for regulatory compliance and for demonstrating that the repair was performed in accordance with the applicable codes and standards.

From a technical perspective, the study reinforces the principle that the quality of the weld overlay is determined not only by the welding process itself but also by the thermal management that surrounds it. Preheating, interpass temperature control, and post-weld heat treatment are all critical to achieving a reliable overlay joint. The use of electromagnetic induction heating provides a powerful tool for managing these thermal conditions, but it requires careful design, programming, and operation to achieve the desired results.

In summary, this study provides valuable insights into the application of electromagnetic induction heating for weld overlay repair of nuclear power steam generator tube sheets. The emphasis on precise thermal control, process integration, and quality assurance is particularly relevant for engineers working in nuclear safety-critical applications. The findings can be directly applied to improve repair procedures for similar components in other industries, including power generation, petrochemicals, and aerospace.