Inner Wall Surfacing Technology for Thick-Walled Pressure Vessels

Literature Overview

The paper by Wang Jiachun, Zhu Qi, and Sun Dunwu (1998), published in Welding (No. 12, pp. 2-5), addresses the technical challenges of inner wall surfacing for thick-walled pressure vessels. The authors, affiliated with the Harbin Welding Research Institute, discuss the key technical issues including performance requirements, surfacing material and process selection, and critical technical considerations. This paper, while published in 1998, remains highly relevant to modern engineering practice because the fundamental metallurgical and process challenges of thick-walled vessel surfacing have not materially changed.

Technical Performance Requirements

Thick-walled pressure vessels require inner wall surfacing primarily for corrosion resistance, wear resistance, or to provide a compatible material interface for specific service conditions. The technical performance requirements for such surfacing operations are stringent and must be carefully defined before the surfacing operation begins.

The following table summarizes the typical performance requirements for thick-walled pressure vessel inner wall surfacing:

The paper emphasizes that the dilution rate is the most critical parameter for corrosion-resistant surfacing applications. When surfacing austenitic stainless steel (such as 304, 316, or 321) over carbon steel or low-alloy steel, the dilution rate must be controlled to ensure that the final surfacing layer composition maintains sufficient austenite stability and corrosion resistance. A dilution rate exceeding 30% can significantly reduce the chromium and nickel content of the surfacing layer, compromising its corrosion resistance.

Surfacing Material and Process Selection

The selection of surfacing material and process is governed by several factors including the base material, the required surfacing thickness, the service environment, and the available equipment. The paper presents a systematic approach to this selection:

The paper highlights that for thick-walled vessels (wall thickness > 50 mm), the thermal mass of the base material creates significant challenges. The large heat sink effect of the thick wall tends to increase the cooling rate at the weld interface, which can lead to:

• Increased hardness and reduced toughness in the heat-affected zone
• Higher susceptibility to hydrogen-induced cracking in high-strength steels
• Increased residual stress levels due to constrained cooling

To address these challenges, the paper recommends the following measures:

1. Preheating the base material to 150-250°C (depending on carbon equivalent)
2. Maintaining interpass temperatures between 150-300°C
3. Using low-hydrogen fluxes or shielding gases with controlled moisture content
4. Applying post-weld stress relief at temperatures appropriate for the surfacing material (typically 620°C for austenitic stainless steel)

Critical Technical Considerations

The paper identifies several critical technical issues that must be addressed in thick-walled pressure vessel inner wall surfacing:

1. Surface preparation: The base surface must be cleaned to bare metal with a minimum roughness of Ra 3.2 μm. Any scale, rust, or contamination must be completely removed to ensure proper surfacing layer bonding.
2. Welding sequence: For large-diameter vessels, the welding sequence must be planned to minimize distortion. A symmetric welding pattern, starting from the center and progressing outward, is recommended. The sequence should also account for the vessel's support structure to minimize constraint-induced distortion.
3. Multi-pass surfacing strategy: For thick surfacing layers (> 5 mm), a multi-pass approach is essential. The first pass should use a transition alloy (such as 309L) to provide a low-dilution interface, followed by intermediate passes with the target composition, and a final pass with the specified surfacing material.
4. Residual stress management: Thick-walled vessels accumulate significant residual stresses during surfacing. The paper recommends a combination of thermal stress relief (PWHT) and mechanical vibration stress relief (VSR) to achieve acceptable residual stress levels.
5. Quality inspection: Comprehensive inspection is required at each stage:

• Visual inspection (VT) after each pass
• Magnetic particle testing (MT) after the final pass
• Ultrasonic testing (UT) for lack-of-bond defects
• Hardness testing at representative locations
• Chemical composition analysis of the final surfacing layer

Engineering Practice and Lessons Learned

In my engineering practice, I have found that the most challenging aspect of thick-walled vessel inner wall surfacing is the management of residual stress and distortion. The large thermal gradients inherent in surfacing operations on thick walls can produce residual stresses approaching the yield strength of the base material, which can lead to delayed cracking, distortion, or even catastrophic failure under service conditions. The paper's recommendation for comprehensive stress relief is well-founded, but engineers should also consider the effects of stress relief on the surfacing layer properties. For example, stress relief at 620°C for austenitic stainless steel surfacing layers can cause carbide precipitation at grain boundaries, potentially reducing corrosion resistance. This trade-off must be carefully managed through proper heat treatment cycle design.

Another practical consideration is the access to the inner wall of the vessel. For large-diameter vessels, manual welding processes (SMAW, GTAW) may be the only practical option for certain areas, while mechanized processes (SAW, PAW) can be used for accessible flat or cylindrical surfaces. The paper's discussion of process selection is therefore directly applicable to real-world planning.

Summary

This paper provides a comprehensive technical framework for inner wall surfacing of thick-walled pressure vessels. The key technical insights include the critical importance of dilution control for corrosion-resistant surfacing, the need for systematic multi-pass surfacing strategies, and the necessity of comprehensive residual stress management. For engineers working on pressure vessel surfacing projects, this paper serves as a valuable reference for process planning, material selection, and quality assurance. The fundamental principles presented remain applicable to modern practice, and the systematic approach advocated by the authors is essential for achieving reliable, long-lasting surfacing layers on critical pressure-containing components.