Nickel-Based Alloy Strip Electroslag Welding Overlay in Nuclear Power Equipment

Literature Overview

The study authored by Li Shuangyan from Shanghai Electric Nuclear Equipment Co., Ltd. (2011) addresses a critical manufacturing challenge in the nuclear power industry: the application of nickel-based alloy strip electroslag welding (ESW) overlay to pressure vessels. Nuclear power plant pressure vessels, particularly reactor pressure vessels (RPVs) and steam generators, operate under extreme conditions of high temperature, high pressure, and corrosive coolant environments. The base materials are typically low-alloy steels such as SA-508 Gr.3 Class 1, while the overlay layers must provide superior corrosion resistance using nickel-based alloys such as Inconel 625 or Hastelloy C-276. This literature focuses on the process development, qualification, and engineering implementation of strip ESW overlay for these demanding nuclear applications.

Core Technical Points

The electroslag welding overlay process is selected for nuclear pressure vessels due to its exceptional deposition rate, which can reach 80–150 kg/h compared to only 3–8 kg/h for conventional submerged arc welding (SAW). For large-diameter pressure vessel heads and shells, the efficiency advantage of ESW is decisive. The strip electrode system, typically using ERNiCrMo-3 or equivalent consumables, provides consistent dilution control and uniform microstructure across the overlay layers.

Key process parameters identified in the study include:

Parameter	Typical Range	Notes
Arc voltage	28–34 V	Depends on strip width and thickness
Welding current	800–1400 A	DC polarity (electrode positive)
Travel speed	80–160 mm/min	Adjusted per layer thickness
Strip thickness	2.0–3.0 mm	Standard for ESW overlay
Preheat temperature	150–250 °C	Per NB/T 47014 qualification
Interpass temperature	≤250 °C	To prevent excessive grain growth
Dilution rate	5–15%	Must be controlled per design specification
Overlay thickness	3–6 mm	Typically 3 layers minimum

The dilution control is the most critical quality parameter. For nuclear applications, the dilution of base metal into the overlay layer must not exceed the design-specified maximum, as excessive dilution compromises the corrosion resistance of the nickel-based alloy. The study emphasizes that strip ESW inherently provides better dilution control than wire ESW because the strip geometry maintains a more stable molten pool profile.

Process Analysis and Standards Compliance

Nuclear equipment fabrication in China follows the NB/T 47002 series standards, which govern the design and fabrication of nuclear pressure vessels. The welding procedure qualification must comply with NB/T 47014, and the welder performance qualification follows NB/T 47015. For ESW overlay specifically, the procedure must demonstrate consistent dilution control, acceptable microstructure, and adequate mechanical properties across the entire overlay thickness.

The metallurgical considerations are significant. The nickel-based alloy overlay on a low-alloy steel base creates a dissimilar metal joint where the coefficient of thermal expansion mismatch (approximately 14×10⁻⁶/°C for Inconel 625 versus 12×10⁻⁶/°C for SA-508 Gr.3) generates substantial residual stresses. The study discusses the importance of post-weld heat treatment (PWHT) to relieve these stresses while avoiding sensitization of the nickel-based alloy. The PWHT temperature is typically limited to 425–450 °C for Inconel 625 overlays, below the sensitization threshold of 480 °C, while still achieving stress relief in the base material.

Non-destructive testing requirements are stringent for nuclear applications. The overlay layers must be inspected by ultrasonic testing (UT) per JB/T 4730 or equivalent, with acceptance criteria that are more severe than those for conventional pressure vessels. Surface defects such as cracks, lack of fusion, and excessive porosity are not permitted.

Engineering Practice and Defect Analysis

From practical experience, the most common defects encountered in ESW overlay on nuclear pressure vessels include:

Defect Type	Root Cause	Countermeasure
Undercut at strip edge	Excessive travel speed or insufficient current	Reduce travel speed by 10–20%, increase current
Incomplete fusion between layers	Low interpass temperature or poor strip alignment	Maintain interpass temperature at 150–200 °C
Cracking in heat-affected zone	High cooling rate, hydrogen embrittlement	Increase preheat to 200 °C, use low-hydrogen flux
Excessive dilution	High current, low travel speed, thick strip	Optimize current/speed ratio, verify strip thickness
Porosity	Moisture in flux or contaminated base metal	Dry flux at 300 °C for 2 h, clean base metal thoroughly

The study highlights that the transition from laboratory qualification to production welding on large pressure vessel components introduces additional challenges related to positional welding, joint fit-up, and thermal distortion control. For large-diameter pressure vessel shells, the circumferential welding sequence must be carefully planned to minimize cumulative distortion, and the overlay welding sequence must ensure that each layer is fully solidified and cooled before the next layer is deposited.

Study Insights and Implications

This literature provides valuable insight into the practical challenges of applying ESW overlay technology to nuclear-grade pressure vessels. The emphasis on dilution control, process parameter stability, and comprehensive NDT coverage reflects the inherent conservatism of nuclear safety culture. For engineers working in this field, the key takeaway is that process qualification is not merely a regulatory formality but a fundamental engineering requirement that directly impacts the long-term integrity and safety of nuclear pressure vessels. The study also underscores the importance of consumable quality control, as the strip electrode composition and mechanical properties directly determine the overlay layer performance. Engineers should always verify consumable certificates against the qualified procedure specifications before production welding commences.