Carbon Element Dilution Behavior in Austenitic Stainless Steel Surfacing Layers for Hualong One Safety Injection Tanks

Problem Statement and Quality Concern

The paper by Liao Guoping, Liu Zijun, Zhang Tao, and Liu Hongjie from Dongfang Electric Group and Dongfang Boiler Co., Ltd. addresses a significant quality issue encountered during the manufacturing of safety injection tank shells and heads for the Hualong One nuclear power plant. The chemical analysis of the austenitic stainless steel surfacing layer repeatedly showed carbon content exceeding the specified limit, drawing heightened attention from the project owner. In nuclear applications, carbon content in austenitic stainless steel is critically important because excessive carbon can lead to chromium carbide precipitation at grain boundaries, resulting in sensitization and intergranular corrosion susceptibility, which is unacceptable in the high-purity water environment of a safety injection system.

Systematic Investigation of Carbon Dilution Factors

The authors conducted a comprehensive investigation into the factors influencing carbon dilution in the surfacing layer, examining welding material composition, base metal composition, surfacing process parameters, sampling procedures, and weld pass overlap control. Through systematic experimental analysis, they identified that the original chemical composition of the welding consumable and the overlap quantity between adjacent weld passes were the two factors with significant influence on carbon dilution in the deposited metal.

The finding that weld pass overlap quantity is a critical factor is particularly important for practical implementation. When adjacent weld passes overlap excessively, a greater proportion of previously deposited metal is remelted and mixed with the new filler metal, effectively increasing the dilution rate. This remelted metal, which may have already undergone carbon pickup from the base metal during the previous pass, reintroduces carbon into the subsequent layer, creating a cumulative dilution effect that compounds across multiple passes.

Carbon Dilution Mechanism in Austenitic Surfacing

The dilution of carbon in austenitic stainless steel surfacing layers occurs through several mechanisms. During the welding process, the molten pool absorbs carbon from the base metal, which is typically a carbon steel or low-alloy steel with higher carbon content than the austenitic overlay. The degree of carbon pickup depends on the heat input, weld pool volume, and the time available for elemental diffusion between the base metal and the filler metal. In surfacing applications, where the overlay must be deposited on a dissimilar base material, the dilution effect is inherently more pronounced than in welding applications where both sides are the same material.

The control of carbon dilution requires a multi-faceted approach. First, the welding consumable must be selected with a carbon content sufficiently below the specification limit to accommodate the expected dilution. Second, the overlap between passes must be carefully controlled to minimize the remelting of previously deposited metal. Third, the welding parameters should be optimized to minimize base metal melting while maintaining adequate fusion. The authors demonstrated that adjusting process parameters such as current, voltage, and travel speed had relatively minor effects compared to consumable selection and pass overlap control.

Engineering Practice and Quality Assurance Implications

For nuclear-grade manufacturing, the carbon content specification for austenitic stainless steel surfacing layers is typically very stringent, often limited to 0.03 percent or below for low-carbon grades and 0.08 percent or below for standard grades. The repeated failures reported in this paper underscore the difficulty of meeting these specifications in practice, particularly for complex geometries such as tank shells and heads where access and positioning are challenging.

The investigation methodology employed by the authors, which systematically varied each factor while controlling others, provides a model for quality problem resolution in nuclear manufacturing. The clear identification of the two dominant factors allows for targeted process improvement without unnecessary changes to the overall welding procedure. For similar applications in the nuclear industry, this paper serves as an important reference for understanding and controlling elemental dilution in dissimilar metal surfacing operations.

The practical takeaway is that carbon dilution control in austenitic surfacing is primarily a matter of consumable specification and weld pass layout design, rather than process parameter optimization. Engineers should prioritize the selection of low-carbon filler metals and the development of weld sequence plans that minimize pass overlap, while using process parameters as a secondary means of control.