Weld Overlay Technology for Internal Surfaces of Thick-Walled Pressure Vessels

Literature Overview

This study note addresses the technical challenges and methodologies associated with weld overlaying the internal surfaces of thick-walled pressure vessels, as investigated by Wang Jiachun, Zhu Qi, and Sun Dunwu from the Harbin Welding Research Institute in 1998. The work sits at the intersection of heavy pressure vessel fabrication and corrosion-resistant overlay engineering, targeting components such as hydrogenation reactors, high-pressure hydrogen-containing service vessels, and ammonia synthesis loops where the base material provides structural strength while the overlay layer delivers corrosion and hydrogen resistance. The publication predates many of the modern standards governing overlay qualification, making its process insights particularly valuable for understanding the foundational thinking behind current practices in NB/T 47014 and ASME IX overlay qualification.

Core Technical Challenges of Thick-Wall Internal Overlay

The primary difficulty in overlaying thick-walled pressure vessel internals lies in heat input management. Thick sections typically range from 60 mm to over 200 mm in wall thickness, creating extreme thermal mass that promotes slow cooling rates, excessive dilution of the overlay layer, and distortion of the vessel shell during sequential weld passes. The base material is usually a low-alloy steel such as 12Cr1MoV, 14Cr1Mo, or 0.5Cr-0.5Mo-0.25V, which further complicates the process by introducing hydrogen-induced cracking susceptibility in the heat-affected zone (HAZ).

The internal geometry of the vessel creates additional access constraints. Unlike external overlay operations where the welder has unrestricted access, internal cladding requires the welder to work inside a confined cylindrical or spherical shell, often at elevated temperatures to maintain proper preheat levels. This geometric limitation restricts the choice of welding process and electrode positioning, making manual submerged arc welding (SAW) and gas metal arc welding (GMAW) the most commonly employed methods, with electroslag welding (ESW) sometimes used for the initial buildup layers on very thick sections.

Key Process Parameters and Material Selection

The following table summarizes the typical process windows and material combinations encountered in thick-walled internal overlay operations:

Dilution Control Strategies

Dilution is the single most critical quality parameter in overlay welding. Excessive dilution reduces the chromium and nickel content of the overlay layer below the threshold required for corrosion resistance, potentially introducing intergranular sensitization zones at the overlay-substrate interface. The study emphasizes several strategies for dilution control:

1. First-pass dilution management: The first overlay pass always experiences the highest dilution because the full heat input is applied to the base material with no prior overlay to act as a thermal buffer. The recommended approach is to use a low-dilution filler composition for the first pass, followed by subsequent passes with the target overlay composition.
2. Wire feed rate and travel speed optimization: Increasing wire feed rate relative to travel speed increases the heat input per unit length and generally increases dilution. Conversely, a higher travel speed with adequate wire feed reduces dilution but may compromise fusion and bond strength. The optimal window is typically found through coupon qualification testing per NB/T 47014 or ASME IX.
3. Multi-pass overlay with alternating directions: Alternating the welding direction between passes helps distribute heat more uniformly and reduces the cumulative thermal distortion of the thick section.
4. Backing layer technique: For very thick sections, a thin austenitic stainless steel backing layer may be deposited first to reduce the thermal conductivity mismatch and provide a controlled dilution environment for subsequent overlay passes.

Quality Assurance and Non-Destructive Testing

The quality assurance program for thick-walled internal overlay must address both the overlay layer integrity and the bond strength at the overlay-base interface. The following NDT methods are typically specified:

The study highlights that UT bond strength testing is particularly critical for thick-walled vessels because the high thermal mass and slow cooling rates can produce incomplete fusion at the overlay-base interface, especially at the toes of the weld where the thermal gradient is steepest. A 100% bonded interface is mandatory for pressure-retaining applications, and any lack of fusion must be ground out and rewelded.

Engineering Practice Insights

From a practical standpoint, several lessons emerge from this body of work that remain relevant to current engineering practice:

• Preheat uniformity is essential: In thick-walled vessels, achieving uniform preheat across the entire internal surface is challenging due to the confined geometry. Infrared thermometers and portable gas heating systems are recommended, with preheat verified at multiple locations before welding begins.
• Post-weld heat treatment (PWHT) interaction: Thick-walled vessels typically require PWHT to relieve residual stresses. The overlay layer must survive the PWHT cycle without cracking or excessive grain growth. Austenitic stainless steel overlays generally tolerate PWHT well, but Ni-based alloy overlays may require careful selection of the PWHT temperature to avoid sigma phase precipitation or solid solution strengthening degradation.
• Welder position and access: Internal overlay often requires welders to work in overhead or vertical positions inside the vessel, which increases the risk of slag inclusion and porosity. Special electrode holders and wire feeding arrangements are necessary to maintain consistent arc characteristics.
• Distortion control: Sequential welding of internal overlay around the vessel circumference can cause significant ovality distortion in cylindrical shells. Skip welding patterns and symmetric welding sequences are recommended to minimize distortion.

Key Questions and Reflections

The 1998 study predates the widespread adoption of hot-wire TIG overlay and laser cladding, both of which have since become important tools for dilution control in thick-walled applications. Hot-wire TIG overlay, in particular, allows precise control of heat input and dilution by adding a preheated filler wire that reduces the required arc power. This represents a significant evolution from the SAW and GMAW methods discussed in the original study.

Another question that arises is the applicability of the dilution control strategies described to modern duplex stainless steel overlays. Duplex stainless steels have a narrower composition window and are more sensitive to dilution than austenitic grades. The strategies of first-pass dilution management and multi-pass overlay remain applicable, but the filler metal selection and process parameters must be adjusted accordingly.

The study also raises the question of how to balance overlay thickness with dilution control. Thicker overlays require more passes and more heat input, which increases dilution in the first few passes. However, thicker overlays provide better protection against mechanical damage and erosion. The optimal overlay thickness is therefore a trade-off between corrosion protection, mechanical durability, and dilution control, and must be determined on a case-by-case basis through coupon qualification testing.

Study Insights and Implications

This literature provides a foundational understanding of the thermal, metallurgical, and geometric challenges inherent in thick-walled internal overlay welding. The emphasis on dilution control, preheat management, and bond strength verification remains directly applicable to current engineering practice, even as new welding processes have expanded the available process windows. The work underscores the importance of systematic qualification testing and the integration of NDT methods into the quality assurance program for overlay operations on thick-walled pressure vessels. For engineers involved in the fabrication of hydrogenation reactors, high-pressure corrosion-resistant vessels, and similar critical equipment, the principles described here serve as a reliable basis for process development and quality control.