Research on Wide-Strip Submerged Arc and Electroslag Overlay Welding

Literature Overview

The paper by Li Pengfei and Wang Jiantao from Xi'an Nuclear Equipment Co., Ltd. presents research on wide-strip submerged arc welding (SAW) and electroslag welding (ESW) overlay processes for nuclear-grade pressure vessels and heat exchangers. Published in the journal "Welding Machine" in 2010, this work addresses the specific challenges of overlay welding in nuclear equipment manufacturing, where the requirements for weld quality, traceability, and non-destructive examination are exceptionally stringent. The study focuses on the development and optimization of wide-strip overlay processes that can achieve high deposition rates while maintaining the high quality standards required for nuclear applications.

Core Technical Content

Nuclear equipment, including reactor pressure vessels, steam generators, and primary coolant piping, often requires stainless steel or nickel-based alloy overlay to provide corrosion resistance in high-temperature water environments. The overlay layer must be free from defects that could initiate stress corrosion cracking (SCC) or intergranular corrosion (IGC), and must have a consistent composition and microstructure throughout its thickness. The wide-strip SAW and ESW processes are designed to deposit large areas of overlay material efficiently while maintaining the quality standards required for nuclear applications.

The wide-strip SAW process uses a strip electrode of width 20–50 mm and thickness 1.0–2.0 mm, fed into the arc through a contact shoe. The strip is submerged in a layer of flux that provides shielding and contributes to the weld composition. The wide strip allows for a deposition rate of 15–25 kg/h, which is 2–3 times higher than conventional wire SAW. The wide-strip ESW process uses a strip of similar dimensions but relies on the electrical resistance of the slag to generate heat, with deposition rates of 40–60 kg/h.

Process Development and Optimization

The development of wide-strip overlay processes for nuclear applications requires careful optimization of process parameters to ensure consistent weld quality. The key parameters include strip feed rate, travel speed, current, voltage, and flux composition. The strip feed rate and travel speed must be precisely controlled to maintain a consistent weld bead profile and penetration. Deviations in these parameters can lead to undercut, overlap, or insufficient fusion, all of which are unacceptable in nuclear applications.

The flux composition is critical for both processes, as it provides shielding, contributes to the weld composition, and influences the slag properties. For stainless steel overlay, the flux must be designed to produce a slag with adequate fluidity, low gas absorption, and appropriate composition to avoid excessive dilution or contamination of the weld metal. The flux must also be compatible with the strip composition to ensure consistent weld metal chemistry.

The process optimization is guided by a systematic approach that includes:

1. Development of a baseline procedure with standard parameters
2. Parameter variation studies to identify the process window
3. Quality assessment through NDT and metallurgical examination
4. Refinement of parameters based on quality feedback
5. Validation through full-scale weld qualification tests

Quality Assurance for Nuclear Applications

The quality assurance requirements for overlay welds on nuclear equipment are governed by standards such as ASME Section VIII Division 2, ASME Section IX, and the applicable national nuclear codes. The overlay weld must be free from any indications that could compromise the integrity of the corrosion-resistant barrier. The acceptance criteria for overlay welds in nuclear applications are typically more stringent than those for conventional pressure vessels, with zero tolerance for linear indications such as cracks, lack of fusion, or inclusions.

The non-destructive examination (NDE) program for overlay welds includes magnetic particle testing (MT) for surface and near-surface defects, ultrasonic testing (UT) for subsurface defects, and in some cases, phased array ultrasonic testing (PAUT) for enhanced detection capability. The NDE must be performed by certified personnel in accordance with the applicable qualification standards, and the results must be documented and traceable.