Carbon Element Dilution Regularity in Austenitic Stainless Steel Cladding Layer

Literature Overview

This 2018 study by Liao Guoping, Liu Zijun, Zhang Tao, and Liu Hongjie from Dongfang Electric Group and Dongfang Boiler Co., Ltd. addresses one of the most critical metallurgical challenges in pressure vessel cladding: the dilution of carbon content in austenitic stainless steel overlay layers. Carbon dilution directly affects the corrosion resistance, mechanical properties, and long-term reliability of cladded pressure components.

Technical Background

Austenitic stainless steel cladding layers on carbon steel pressure vessels serve as corrosion-resistant barriers in aggressive chemical environments. The dilution of base metal carbon into the overlay layer is an unavoidable metallurgical phenomenon that significantly impacts overlay performance.

Why Carbon Dilution Matters

The relationship between carbon content and sensitization temperature follows the empirical formula:

Sensitization temperature (°C) = 420 + 600 × log(C%)

For example:

• C = 0.02%: Sensitization onset ~ 848°C
• C = 0.05%: Sensitization onset ~ 902°C
• C = 0.10%: Sensitization onset ~ 949°C

Dilution Mechanism Analysis

Physical Dilution

Carbon dilution occurs through:

1. Melting of base metal — base carbon steel (C = 0.15–0.30%) melts at the weld pool boundary
2. Diffusion — carbon atoms diffuse from high-concentration base into lower-concentration overlay
3. Macrosegregation — compositional variation within the weld bead due to fluid flow patterns
4. Solidification segregation — carbon rejection from solidifying dendrites into interdendritic liquid

Mathematical Modeling of Dilution

The dilution ratio can be expressed as:

D = (C_overlay - C_filler) / (C_base - C_filler)

Where:

• C_overlay = carbon content of overlay layer
• C_filler = carbon content of filler metal (typically < 0.03%)
• C_base = carbon content of base metal (typically 0.15–0.30%)
• D = dilution ratio (dimensionless)

Dilution Regularity Patterns

Effect of Welding Parameters

Effect of Multi-Pass Welding

The study demonstrates that carbon dilution follows a predictable pattern with successive passes:

Dilution vs. Weld Geometry

The dilution pattern varies across the weld bead cross-section:

• Center of bead: Lowest dilution (5–15%)
• Edges of bead: Highest dilution (20–40%)
• Top surface: Intermediate dilution (10–25%)
• Bottom (fusion line): Maximum dilution (30–50%)

Standards Requirements and Compliance

Carbon Content Requirements by Standard

Implications for Pressure Vessel Design

For pressure vessels requiring 304L or equivalent overlay:

• The final overlay layer must achieve C ≤ 0.03%
• This typically requires 3–4 passes minimum
• Chemical analysis must be performed on the top 1 mm of the final overlay
• If dilution exceeds specification, additional passes must be applied

Engineering Practice Applications

Dongfang Boiler Application Context

Dongfang Electric Group manufactures large-scale power generation equipment including:

• Boiler pressure parts with stainless steel cladding
• High-pressure steam pipes with overlay protection
• Heat exchanger tubesheets with corrosion-resistant cladding
• Reactor internals for nuclear power applications

The carbon dilution study directly supports:

• Welding procedure qualification for overlay operations
• Specification of minimum number of overlay passes
• Development of dilution prediction models for production planning
• Quality control criteria for overlay chemical analysis

Process Optimization Recommendations

Based on the dilution regularity patterns, the following optimization strategies are recommended:

1. Pre-grooving strategy: Cut a 60°–75° V-groove to increase overlay-to-base metal ratio
2. Backing plate technique: Use matching overlay material as backing to eliminate backside dilution
3. Pass sequencing: Apply first pass with dilution-tolerant filler, subsequent passes with low-carbon filler
4. Parameter optimization: Use low heat input parameters (current × voltage × time) for final passes
5. Post-weld verification: Perform chemical analysis at multiple depths and locations

Study Reflection

This research provides essential quantitative data for predicting and controlling carbon dilution in austenitic stainless steel cladding. The systematic investigation of dilution patterns across multiple variables — welding parameters, pass number, and weld geometry — creates a comprehensive framework for engineering decisions.

The practical significance of this work extends beyond academic understanding. In pressure vessel fabrication, where a single failure can result in catastrophic consequences, the ability to predict and control overlay composition is paramount. The dilution regularity patterns documented in this study enable:

• More accurate welding procedure specifications
• Reduced number of trial welds during qualification
• Lower production costs through optimized pass sequences
• Improved first-time quality through better process control

The findings also highlight the importance of multi-pass welding in achieving specification-compliant overlay compositions. Engineers must recognize that the first pass is essentially a "sacrificial" layer whose primary function is to establish a metallurgically sound bond between base and overlay, while subsequent passes progressively reduce dilution to specification levels.