Overlay Welding of Austenitic Stainless Steel Erosion-Resistant Layer on Nuclear Power High-Pressure Outer Cylinder

Literature Overview

This 2006 publication by Guo Wei and Li Zhihong, published in Hot Working Technology, addresses one of the most demanding applications in the cladding field: the overlay welding of austenitic stainless steel erosion-resistant layers on the high-pressure outer cylinder of a nuclear power steam turbine. The high-pressure outer cylinder is a critical pressure-retaining component in the Rankine cycle, and the erosion layer protects against high-velocity steam flow at temperatures exceeding 500°C and pressures above 10 MPa. This application is governed by the most stringent quality requirements in the pressure vessel industry.

Technical Requirements and Design Specifications

The high-pressure outer cylinder of a nuclear power steam turbine operates under extreme conditions that impose rigorous requirements on the erosion-resistant overlay layer:

The overlay material must withstand continuous high-temperature erosion by high-velocity steam without spalling, cracking, or excessive thinning. The bond between the overlay and the Cr-Mo base steel must maintain integrity under cyclic thermal loading throughout the component's design life of 30–40 years.

Welding Process Selection and Parameters

For nuclear-grade applications, the welding process must be qualified under ASME Section IX or the equivalent Chinese national standard (NB/T 47014). The process selection is governed by the requirement for low dilution, low hydrogen content, and excellent weldability with the Cr-Mo base material.

Typical welding parameters for the SAW overlay process include:

• Current: 350–500 A (DCEN)
• Voltage: 28–34 V
• Travel speed: 200–350 mm/min
• Wire: ER310 or ER347 stainless steel wire, diameter 1.6–2.4 mm
• Flux: Low-hydrogen stainless steel flux (e.g., HJ312 equivalent)
• Preheat: 150–200°C for the Cr-Mo base steel
• Interpass temperature: Maximum 250°C

The dilution ratio between the overlay and base metal must be carefully controlled. For erosion-resistant applications, dilution should be limited to below 20% to maintain the austenitic structure and corrosion/erosion resistance of the overlay. Excessive dilution introduces ferrite into the overlay, which degrades high-temperature oxidation resistance and increases the risk of intergranular corrosion.

Quality Control and Inspection Requirements

Nuclear-grade overlay welding requires the most comprehensive quality control regime in pressure vessel fabrication:

1. Welder qualification: All welders must be qualified under ASME Section IX or NB/T 47014 with the specific process, material combination, and position.
2. Welding procedure qualification (WPQ): The welding procedure must be qualified with full mechanical testing, including tensile, bend, and hardness tests on the overlay and heat-affected zone (HAZ).
3. Material certification: All consumables must have full chemical and mechanical property certificates traceable to the heat number.
4. Non-destructive testing (NDT):

• Magnetic particle testing (MT) or penetrant testing (PT) on every pass
• Ultrasonic testing (UT) for lack of fusion and cracks after every 2–3 passes
• Radiographic testing (RT) on 100% of the overlay weld for critical applications
• Final UT of the completed overlay for thickness measurement and defect detection

1. Dye penetrant testing: 100% of the final overlay surface for surface-breaking defects.
2. Hardness testing: The overlay hardness must be 20–30 HV below the base metal to prevent stress concentration at the overlay edge.

Metallurgical Considerations

The weld metal deposited on a Cr-Mo low-alloy steel base with an austenitic stainless steel overlay creates a dissimilar weld joint with significant metallurgical complexity:

• Dilution zone: The first 0.5–1.0 mm of the overlay will contain significant dilution from the base metal, resulting in a microstructure that is a mixture of austenite, ferrite, and possibly martensite. This zone must be thick enough to be ground away or must be acceptable for the service conditions.
• Heat-affected zone (HAZ): The Cr-Mo base steel HAZ will experience tempering, which reduces strength but improves toughness. The tempering depth depends on the peak temperature and cooling rate.
• Residual stress: The coefficient of thermal expansion difference between austenitic stainless steel (17–18 × 10⁻⁶/°C) and Cr-Mo steel (12–13 × 10⁻⁶/°C) creates significant residual stresses at the overlay interface. These stresses can be tensile in the overlay and compressive in the base metal.
• Creep behavior: At operating temperatures above 500°C, the overlay and HAZ are subject to creep deformation. The weld metal must have adequate creep strength for the design life.

Study Insights and Engineering Implications

This paper represents the intersection of nuclear safety requirements and practical welding engineering. The key insight is that nuclear-grade overlay welding is not simply a matter of depositing a corrosion-resistant layer—it is a multidisciplinary challenge that integrates metallurgy, thermomechanical analysis, quality assurance, and regulatory compliance.

Several engineering lessons emerge from this application:

1. Process qualification is the foundation: Without a properly qualified welding procedure, no amount of operator skill can guarantee the required quality level. The WPQ must be designed to simulate the actual welding conditions as closely as possible.
2. Dilution control is the critical parameter: The erosion resistance of the overlay is directly related to its chemical composition, which is determined by the dilution ratio. Process parameters must be optimized to achieve the target dilution while maintaining adequate bond strength.
3. Inspection must be comprehensive and traceable: Nuclear applications require full traceability from raw material to final inspection. Every weld pass must be documented, inspected, and recorded.
4. Post-weld heat treatment (PWHT) must be carefully controlled: The PWHT for the Cr-Mo base steel (typically 740–770°C) must not cause excessive grain growth or sensitization in the austenitic overlay. The overlay material must be selected to withstand the PWHT temperature without detrimental phase transformations.

The practical challenge of applying a multi-pass overlay to a large cylindrical component with restricted access is significant. The high-pressure outer cylinder is typically welded in a horizontal position, and the overlay is applied to the interior surface, which requires specialized fixtures and torch positioning equipment. The geometry also creates challenges for NDT access, particularly for UT and RT.