Experimental Study on Stainless Steel Cladding of Pressure Vessel Head Inner Walls

Literature Overview and Context

This study note examines the 2017 paper by Du Ning of Harbin Electric Group (Qinhuangdao) Heavy Equipment Co., Ltd., which presents an experimental study on stainless steel cladding of the inner wall of pressure vessel heads. The research addresses a specific and challenging engineering problem: achieving reliable stainless steel overlay on the complex curved geometry of pressure vessel heads, which are critical components in chemical processing, petrochemical, and power generation industries. The study is published in the journal "China Chemical Equipment" and reflects the practical needs of Chinese chemical equipment manufacturing.

Technical Background and Challenges

Pressure vessel heads are typically fabricated from carbon steel or low-alloy steel (e.g., Q345R, 16MnR, 15CrMoR) for structural strength, but the process medium inside the vessel often requires a corrosion-resistant inner surface. Stainless steel cladding of the head inner wall is a cost-effective alternative to using fully stainless steel heads, as it combines the strength of the carbon steel base with the corrosion resistance of the stainless overlay.

The primary challenges in cladding pressure vessel heads include:

1. Complex geometry: Heads have varying curvature (elliptical, hemispherical, torispherical), which affects weld bead geometry and dilution.
2. Positional welding: The cladding must be applied in multiple positions (flat, horizontal, vertical, overhead), which affects arc stability and penetration.
3. Residual stress management: The cladding process introduces residual stresses that interact with the primary structural stresses of the pressure vessel.
4. Bond strength verification: The bond strength between the stainless overlay and the carbon steel base must be verified according to NB/T 47002 and ASME VIII Div.1 requirements.
5. NDE accessibility: The curved geometry and confined spaces make non-destructive testing challenging.

Material Selection

The selection of 309L as the transition layer is critical. The high chromium (23–25%) and nickel (13–15%) content of 309L provides excellent crack resistance when welding dissimilar metals. The 316L overlay layer provides superior corrosion resistance, particularly in chloride-containing environments, due to the 2–3% molybdenum content.

Experimental Procedure and Process Parameters

The experimental study employed gas metal arc welding (GMAW) with solid stainless steel wire, which was selected for its high productivity, good arc stability, and suitability for automated and semi-automated welding. The experimental parameters were optimized through a systematic approach involving single-variable optimization followed by confirmation testing.

The cladding procedure involves the following steps:

1. Surface preparation: The head inner wall is ground to remove oxide scale, paint, and contaminants. The surface roughness should be Ra 3.2–6.3 μm to ensure good wetting.
2. Transition layer welding: A single pass of 309L wire is applied at the optimized parameters. The transition layer thickness should be 1.5–2.5 mm.
3. Overlay layer welding: Two to three passes of 316L wire are applied, with the total overlay thickness of 3–5 mm. The last pass should be a dressing pass with slightly lower current to produce a smooth surface.
4. Post-weld treatment: The cladded surface is ground smooth to Ra 1.6 μm for corrosion resistance and visual inspection.

Quality Verification and Results

The quality of the cladding was verified through the following tests, in accordance with NB/T 47014 and ASME IX requirements:

The bond strength results of 22–28 MPa exceed the minimum requirement of 15 MPa specified in NB/T 47014, indicating good metallurgical bonding between the overlay and the base metal. The intergranular corrosion test results confirm that the overlay layer is resistant to sensitization, which is critical for service in corrosive environments.

Defect Analysis and Countermeasures

The study specifically addresses the challenge of cladding in the knuckle region of elliptical heads, where the curvature changes rapidly. In this region, the contact angle between the welding torch and the surface varies significantly, which can lead to inconsistent penetration and dilution. The recommended approach is to use a shorter arc length and a slightly lower current in the knuckle region, with a travel speed reduction of 10–15% to compensate for the reduced contact angle.

Engineering Practice and Standards Compliance

The study emphasizes compliance with relevant standards for pressure vessel cladding:

• NB/T 47002: Specifies the design, fabrication, and inspection requirements for clad pressure vessels.
• NB/T 47014: Specifies the qualification procedure for welding procedures and welders.
• JB/T 4730: Specifies the NDE methods and acceptance criteria.
• GB/T 150: Specifies the design and fabrication requirements for pressure vessels.
• ASME VIII Div.1: Provides the international reference for pressure vessel code requirements.

The paper notes that the cladding procedure must be qualified according to NB/T 47014, which requires testing of mechanical properties, microstructure, and NDE of the qualified weld. The qualified procedure establishes the applicable range of parameters, within which the procedure can be used without requalification.

Study Insights and Practical Implications

This 2017 study provides practical, field-tested guidance for stainless steel cladding of pressure vessel heads. The systematic approach to parameter optimization, combined with comprehensive quality verification, provides a reliable methodology that can be adapted to similar applications. The emphasis on the transition layer is particularly important, as the transition layer is the critical interface between the dissimilar metals and is the most susceptible to cracking and poor bonding.

From a standards perspective, the study highlights the importance of NB/T 47014 qualification, which is often overlooked in practice. Many fabricators apply cladding procedures based on experience rather than formal qualification, which can lead to quality issues that are not detected until the vessel is in service. The study's emphasis on formal qualification and comprehensive NDE is a valuable contribution to engineering practice.

One area for further development is the application of hot-wire TIG cladding, which can achieve lower dilution rates (10–20%) compared to GMAW (25–35%). This would allow the use of 316L as both the transition and overlay layer, simplifying the procedure and reducing the risk of cracking at the interface. However, the lower productivity of hot-wire TIG (2–4 kg/h vs. 8–12 kg/h for GMAW) must be weighed against the quality benefits.

Conclusion

The 2017 study by Du Ning provides a comprehensive and practical guide to stainless steel cladding of pressure vessel head inner walls. The systematic approach to process optimization, the emphasis on formal qualification according to NB/T 47014, and the comprehensive quality verification program provide a reliable methodology for engineers working in the field. The study's practical focus, validated through experimental testing, makes it a valuable reference for fabricators and inspectors involved in clad pressure vessel fabrication. The fundamental principles of transition layer design, dilution control, and standards compliance remain essential for ensuring the integrity and service life of cladded pressure vessel components.