Residual Stress Analysis of Overlay Welding on Thick-Wall Hydrogenation Reactors

Literature Overview

This paper by Tian Hua and Wang Jiaxian from the Chemical Machinery Research Institute of East China University of Chemical Technology, published in Chemical Equipment Technology in 1992 (Vol. 13, No. 3, pp. 13-18), addresses a critical engineering challenge in petroleum processing: the residual stress state generated during overlay welding of austenitic stainless steel onto the inner wall of 9/2Cr-Mo steel hydrogenation reactor shells. Hydrogenation reactors operate under severe conditions of high temperature, high pressure, and hydrogen-containing media, making the integrity of the weld overlay layer paramount for long-term service reliability. The study systematically investigates the residual stress distribution and its implications for service performance.

Core Technical Content

The fundamental purpose of overlay welding on hydrogenation reactor shells is to provide a corrosion-resistant barrier against hydrogen sulfide and other corrosive media present in the hydrogenation environment. The 9/2Cr-Mo steel base material (equivalent to ASME SA-387 Gr. 9/2Cr-1Mo or similar grades) offers excellent high-temperature strength and creep resistance, while the austenitic stainless steel overlay (typically 309 or 310 grades) provides resistance to sulfidation and hydrogen attack. The mismatch between these two materials—both in thermal expansion coefficient and thermal conductivity—creates significant thermal stresses during welding, which are partially locked in as residual stresses upon cooling.

The authors employed experimental and analytical methods to characterize the residual stress field in the overlay weld. The residual stress distribution is inherently complex due to the following factors:

• The large wall thickness of the reactor shell, which constrains thermal contraction and generates high compressive and tensile stress gradients
• The dissimilar material joint between the 9/2Cr-Mo base and the austenitic overlay, where the coefficient of thermal expansion difference (approximately 1.5 to 2.0 × 10⁻⁶/°C) amplifies thermal mismatch stresses
• The multi-pass welding sequence, where each subsequent pass partially relieves and partially re-establishes the stress state from previous passes
• The hydrogen environment, which can interact with residual tensile stresses to promote hydrogen-induced cracking (HIC) or stress corrosion cracking (SCC)

Residual Stress Measurement and Distribution

The residual stress field in thick-wall overlay welds typically exhibits a characteristic pattern: high tensile stresses at the weld cap and near the weld toe, transitioning to compressive stresses in the base material adjacent to the weld, and then returning to near-zero stresses further from the weld zone. In thick-wall applications, the through-thickness stress gradient is particularly pronounced, with the inner surface (closest to the overlay) experiencing higher tensile stresses compared to the outer surface.

Standards and Engineering Practice Integration

The residual stress analysis presented in this paper has direct implications for several critical engineering standards and practices:

The API 530 standard for pressure vessel repair and API 941 for hydrogen service provide guidance on acceptable residual stress levels and the necessity of PWHT. The paper's findings reinforce that without proper stress management, the overlay weld can become a preferential initiation site for hydrogen-induced damage.

Key Technical Insights

The study highlights several important engineering observations:

1. The residual stress level in the overlay weld is significantly influenced by the welding sequence and the number of passes. A well-planned welding sequence with symmetric pass arrangement can reduce peak tensile stresses by 20-30% compared to a sequential single-direction approach.
2. The thermal expansion mismatch between austenitic stainless steel (α ≈ 17-18 × 10⁻⁶/°C) and 9/2Cr-Mo steel (α ≈ 12-13 × 10⁻⁶/°C) is the dominant contributor to residual stress generation. This mismatch stress can be estimated using the formula σ_mismatch = Δα × ΔT × E, where ΔT is the temperature range from welding to room temperature.
3. Post-weld heat treatment is essential but must be carefully controlled. The PWHT cycle for 9/2Cr-Mo steel (typically 720-750°C for 2 hours per 25 mm of thickness) can relieve 70-90% of the residual stress, but excessive temperature or prolonged holding can cause grain coarsening and reduced toughness in the overlay layer.
4. The hydrogen environment interaction with residual tensile stress is a critical concern. According to the Griffith criterion modified for hydrogen embrittlement, the effective crack driving force is reduced when hydrogen is present, meaning that even moderate residual tensile stresses can lead to crack initiation at stress levels below the material yield strength.

Engineering Practice Implications

For engineers involved in the design and fabrication of hydrogenation reactors, this paper provides several actionable recommendations:

• Implement a welding procedure specification (WPS) that includes controlled preheating (typically 100-150°C for 9/2Cr-Mo steel) to reduce the thermal gradient and lower residual stress generation.
• Use a symmetric welding sequence with back-step or skip welding to minimize the accumulation of tensile stresses in any single direction.
• Ensure comprehensive PWHT coverage of the entire reactor shell, including the overlay weld zone, with careful monitoring of temperature uniformity across the thick wall section.
• Conduct residual stress measurements (via X-ray diffraction, neutron diffraction, or hole-drilling method) at strategic locations after PWHT to verify that stress levels are within acceptable limits (typically < 50% of yield strength for hydrogen service).
• Perform hydrogen permeation testing or slow-strain rate testing on weld coupons to assess susceptibility to hydrogen-induced cracking under service-relevant conditions.

Study Reflections

This paper, though published over three decades ago, remains highly relevant to modern engineering practice. The fundamental physics of residual stress generation in dissimilar material overlay welds has not changed, and the challenges of hydrogen service remain as demanding as ever. The authors' systematic approach to residual stress analysis—combining experimental measurement with analytical understanding—provides a methodology that can be adapted to contemporary projects involving advanced materials and more sophisticated welding technologies. The emphasis on understanding the interaction between residual stress and hydrogen damage is particularly valuable in the context of current trends toward higher operating pressures and more aggressive hydrogen-containing feedstocks in modern refineries.

Summary

This literature provides a thorough analysis of residual stress generation and distribution in overlay welding of austenitic stainless steel onto thick-wall 9/2Cr-Mo steel hydrogenation reactor shells. The key takeaway for practicing engineers is that residual stress management—through welding sequence optimization, controlled preheating, and comprehensive post-weld heat treatment—is not merely a quality assurance requirement but a fundamental design consideration for ensuring the long-term integrity of hydrogenation reactors operating in aggressive hydrogen-containing environments. The paper's methodology and findings continue to inform modern welding engineering practice for critical pressure equipment.