Strip Electrode Submerged Arc Surfacing of Nuclear Pressurizer Hemispherical Heads

Literature Overview and Nuclear Grade Requirements

The 2008 paper by Liao Guoping from Dongfang Boiler Group, published in the journal Pressure Vessel, documents the development and implementation of a strip electrode submerged arc surfacing process for applying a large-area ultra-low carbon austenitic stainless steel corrosion-resistant layer to the internal surface of nuclear power plant pressurizer hemispherical heads. This work represents a significant engineering achievement because it addresses one of the most demanding surfacing applications in the nuclear industry. The pressurizer is a critical pressure boundary component in the reactor primary coolant system, and its internal surface must resist corrosion from the high-temperature, high-pressure, and chemically aggressive primary coolant environment.

The nuclear industry imposes exceptionally stringent requirements on surfacing quality. The surfacing layer must be free of any defects that could initiate stress corrosion cracking, must have adequate thickness to ensure long-term corrosion resistance, and must maintain mechanical integrity under cyclic thermal and pressure loading. Standards such as ASME Section VIII, RBP-NB, and various national nuclear codes govern the design, fabrication, and inspection of these components. The surfacing procedure must be qualified in accordance with applicable codes, and all welding operators must be certified to nuclear grade standards.

Process Development and Equipment Configuration

The process development involved the design and fabrication of a specialized assembly jig and auxiliary curved backing plate to accommodate the complex hemispherical geometry of the pressurizer head. The 100-ton welding positioner provided the necessary rotational capability to access all areas of the internal surface, while the ESAB welding machine delivered the precise current and voltage control required for consistent surfacing quality.

The use of a specialized jig is critical because the hemispherical geometry creates significant challenges for maintaining consistent weld parameters throughout the surfacing operation. As the positioner rotates the head, the effective stickout length and arc angle change continuously, which can lead to variations in heat input, bead geometry, and dilution. The auxiliary curved backing plate serves to support the slag and prevent sagging in overhead and vertical positions, which is essential for maintaining sound weld metal quality.

Welding Parameter Optimization

The paper discusses the influence of five key parameters on surfacing quality: welding current, welding voltage, welding speed, weld bead overlap, and workpiece horizontal orientation. Each parameter was systematically varied to determine its effect on bead geometry, dilution rate, and weld quality.

Welding current directly controls heat input and penetration depth. Higher current increases dilution, which is undesirable for maintaining the corrosion resistance of the austenitic stainless steel overlay. Welding voltage affects arc length and bead width, with higher voltage producing wider, flatter beads. Welding speed inversely affects heat input, with slower speeds producing deeper penetration but increasing the risk of distortion. Bead overlap must be carefully controlled to ensure complete fusion between adjacent passes while avoiding excessive heat input from overlapping. Workpiece horizontal orientation affects slag flow and penetration, with the positioner programmed to maintain optimal slag flow direction throughout the surfacing sequence.

The optimization process identified a parameter window that balanced adequate deposition rate with controlled dilution and minimal distortion. The final procedure achieved a dilution rate within the specified limit, ensuring that the corrosion resistance of the ultra-low carbon austenitic stainless steel overlay was maintained. The successful completion of surfacing on four hemispherical heads for two pressurizers demonstrates the repeatability and reliability of the qualified procedure.

Quality Control and Standards Compliance

Quality control in nuclear surfacing operations is comprehensive and multi-layered. Visual inspection is performed after each pass to verify bead geometry, surface finish, and absence of obvious defects. Dimensional inspection confirms that the surfacing layer thickness meets the minimum specification throughout the entire surface area. Non-destructive testing includes magnetic particle testing for surface and near-surface defects, ultrasonic testing for internal discontinuities, and radiographic testing for volumetric defects in critical areas.

Hardness testing is performed to verify that the surfacing layer microstructure is within the specified range, ensuring adequate corrosion resistance and mechanical properties. Chemical analysis of the weld metal confirms the dilution rate and verifies that the overlay composition meets the specified requirements. In some cases, corrosion testing such as crevice corrosion and stress corrosion cracking tests are performed on coupon samples to validate the long-term performance of the surfacing layer.

The qualification of the surfacing procedure in accordance with applicable nuclear codes is a prerequisite for production work. This includes procedure qualification welding, destructive testing of the qualification welds, and documentation of all parameters and results. The procedure qualification record must be maintained and referenced for all production surfacing operations.

Study Insights

This paper exemplifies the integration of process engineering, equipment design, and quality management that is required for successful surfacing in the nuclear industry. The development of a specialized jig and backing plate demonstrates that successful surfacing of complex geometries often requires custom tooling and fixtures. The emphasis on parameter optimization through systematic experimentation highlights the importance of understanding the fundamental relationships between process variables and weld quality. For engineers working in nuclear fabrication, this paper serves as a valuable reference for developing surfacing procedures for complex curved geometries, and it underscores the critical importance of thorough process qualification and rigorous quality control in safety-critical applications.