Carbon Dilution Behavior in Austenitic Stainless Steel Surfacings on Hualong One Safety Injection Tank

Literature Overview

This 2018 paper by Liao Guoping, Liu Zijun, Zhang Tao, and Liu Hongjie from Dongfang Electric Group and Dongfang Boiler Co., Ltd. addresses a critical quality issue encountered during the surfacing welding of the safety injection tank (SIT) for the Hualong One nuclear power plant. The chemical analysis of the surfacing layer on the tank shell and heads repeatedly showed unacceptable carbon content levels, drawing significant attention from the project owner. The authors conducted a systematic investigation examining welding material composition, base material composition, surfacing process parameters, sampling procedures, and weld overlap control to identify the root causes of carbon dilution. The study concluded that the original chemical composition of the welding consumables and the overlap ratio between weld passes had the most significant impact on carbon dilution in the surfacing layer, while adjustments to other factors yielded negligible results.

Carbon Dilution Mechanism in Stainless Steel Surfacings

Carbon dilution is a well-known phenomenon in stainless steel surfacing welding, particularly when depositing austenitic stainless steel layers on carbon or low-alloy steel substrates. The base material typically contains carbon levels ranging from 0.15% to 0.30%, while the austenitic stainless steel filler metal is designed with carbon content below 0.03% (for low-carbon grades like 304L or 316L) or below 0.08% (for standard grades like 304 or 316). When the filler metal melts and mixes with the base material at the fusion boundary, the resulting dilution increases the carbon content of the deposited layer.

The carbon dilution problem becomes particularly critical in nuclear applications where the safety injection tank must withstand severe service conditions including high temperature, high pressure, and potential irradiation. Elevated carbon content in austenitic stainless steel can lead to sensitization, where chromium carbides precipitate at grain boundaries during the heat treatment or welding thermal cycle, reducing the corrosion resistance of the material. This is unacceptable for a nuclear safety component that must maintain integrity over decades of service.

Investigation Methodology and Key Findings

The authors' systematic approach to identifying the root causes of carbon dilution is exemplary of rigorous engineering investigation. The following table summarizes the factors examined and their relative influence:

The finding that welding material composition and weld overlap ratio are the dominant factors has important implications for process design. The weld overlap ratio determines the degree to which each new pass dilutes with the previously deposited material. If the overlap is too large, the new pass incorporates more of the previous layer's carbon content, compounding the dilution effect. Conversely, if the overlap is too small, there may be gaps or incomplete coverage between passes, compromising the integrity of the surfacing layer.

Process Optimization and Quality Control

Based on the study findings, several process optimization strategies can be implemented to control carbon dilution in stainless steel surfacings. First, the welding consumables should be selected with the lowest practical carbon content, such as ER308L or ER316L wire, to minimize the initial carbon contribution from the filler metal. Second, the weld overlap ratio must be carefully controlled to balance dilution effects with adequate coverage. Third, the number of surfacing passes should be optimized to achieve the desired dilution level without excessive thermal cycles that could promote sensitization.

Quality control for nuclear-grade surfacing welds must include comprehensive chemical analysis of the surfacing layer, with particular attention to carbon content verification. The sampling technique must be carefully controlled to ensure representative specimens are obtained from the correct depth within the surfacing layer. Additionally, metallographic examination should be performed to verify the absence of sensitization and to confirm the microstructural integrity of the surfacing layer.

Engineering Practice and Standard Compliance

The Hualong One project represents China's advanced nuclear technology, and the quality requirements for safety-critical components such as the safety injection tank are exceptionally stringent. The surfacing layer must comply with applicable nuclear standards including NB/T 47016, ASME Section VIII Division 3, and the specific nuclear construction codes applicable to the project. The carbon dilution issue identified in this study highlights the challenges of achieving compliant surfacing layers on large-diameter pressure vessels where the base material carbon content may vary across different sections of the tank.

This research provides valuable guidance for engineers working on similar nuclear or high-integrity surfacing applications. The systematic investigation methodology, which isolated each potential factor and evaluated its contribution to the carbon dilution problem, is a model for root cause analysis in welding quality issues. The emphasis on welding material composition and weld overlap control as the primary levers for managing carbon dilution is a practical and actionable finding that can be directly applied to process design and quality control procedures.