Hydrogenation Reactor Tube Sheet Surfacing Process and Anti-Deformation Measures

Literature Overview

This 2024 paper by Jia Na and Wu Jingjing from Xi'an Aerospace Huawei Chemical and Biological Engineering Co., Ltd., published in "China Chemical Equipment" (Volume 26, Issue 2), addresses a highly specialized and challenging welding problem: the surfacing of large-diameter, thin-wall Cr-Mo steel tube sheets in hydrogenation reactors using an E309+2209 duplex stainless steel overlay structure. The paper proposes a systematic process approach and anti-deformation measures, validated by ferrite content measurements using multiple methods. The classification TG455 indicates the focus on welding processes and deformation control.

Technical Background and Challenge Analysis

Hydrogenation reactors operate under extreme conditions—high pressure, high temperature, and hydrogen-containing environments—that necessitate advanced materials and rigorous fabrication standards. The tube sheet is a critical pressure-containing component that must withstand these conditions while maintaining dimensional accuracy and structural integrity. The Cr-Mo steel base provides good high-temperature strength, while the E309+2209 overlay offers corrosion resistance against hydrogen attack and sulfur-containing compounds.

The fundamental challenges are:

1. Geometric challenges: Large diameter combined with thin wall creates a high aspect ratio that amplifies any angular distortion from welding heat input.
2. Metallurgical challenges: The dissimilar metal weld between Cr-Mo steel and austenitic/duplex stainless steel introduces thermal expansion mismatch and potential cracking susceptibility.
3. Quality challenges: Ferrite content in the overlay must be controlled to prevent intergranular corrosion and ensure adequate ductility.

Surfacing Process Design

The proposed surfacing process employs a multi-pass approach with careful sequence planning to minimize residual stresses. The E309 layer serves as a transition buffer between the Cr-Mo base and the 2209 duplex overlay, accommodating thermal expansion differences and reducing the risk of interfacial cracking.

Process Sequence

1. Surface preparation: The tube sheet surface is ground to remove scale and oxide, revealing bare metal. The surface is cleaned with acetone to remove residual oils.
2. Preheating: The base metal is preheated to 150-250°C depending on the carbon equivalent of the Cr-Mo steel. This reduces the cooling rate and minimizes the risk of hydrogen-induced cracking in the heat-affected zone.
3. E309 transition layer deposition: Using SMAW or SAW with E309 electrodes/wire, a transition layer of 2-4 mm is deposited. The welding sequence follows a symmetric pattern—starting from the center and working outward in alternating directions—to balance thermal input.
4. 2209 duplex overlay deposition: The final overlay is applied using SAW or PAW with ER2209 wire. Multiple passes are used to achieve the required thickness (typically 3-6 mm). The welding parameters are optimized to maintain a ferrite content in the 5-40 FE% range.
5. Post-weld heat treatment: A stress-relief anneal is performed at 550-650°C for 1-2 hours per inch of thickness, followed by controlled cooling. This reduces residual stresses and stabilizes the microstructure.

Anti-Deformation Measures

The paper proposes several anti-deformation strategies:

• Symmetric welding sequence: Welding proceeds from the center of the tube sheet outward in a spiral or radial pattern, ensuring balanced heat distribution.
• Back-plate constraint: A rigid backing plate is clamped to the back of the tube sheet during welding to resist through-thickness deformation.
• Controlled heat input: The welding parameters (current, voltage, travel speed) are adjusted to minimize total heat input per pass. Lower currents with higher travel speeds are preferred to reduce the depth of heat penetration.
• Interpass temperature monitoring: Thermocouples are placed at strategic locations to ensure the interpass temperature does not exceed 250°C, preventing excessive softening of the base metal.
• Post-weld straightening: Any residual deformation is corrected through mechanical straightening or thermal straightening, followed by dimensional verification using laser scanning or coordinate measurement.

Ferrite Content Analysis and Verification

The paper employs multiple methods to measure ferrite content in the 2209 overlay:

All measurement methods confirmed that the ferrite content met the design and standard requirements, validating the process parameters.

Engineering Practice Integration

In the context of hydrogenation reactor fabrication, the tube sheet surfacing operation is a critical path activity that directly impacts project schedule and cost. The proposed process approach represents a practical solution to a well-known industry problem. Key lessons for engineering practice include:

• The importance of process qualification (PQR) and welder qualification (WPQ) specific to the tube sheet geometry and material combination.
• The value of multi-method ferrite verification to ensure reliability, as single-method measurements may be affected by surface conditions or local microstructural variations.
• The necessity of integrating deformation control measures into the overall fabrication plan, rather than treating them as afterthoughts.

The PDCA cycle applies naturally to this process: Plan (process design and parameter selection), Do (execution with monitoring), Check (ferrite measurement, dimensional verification, NDT), and Act (parameter adjustment for subsequent batches).

Study Insights

This paper exemplifies the integration of materials science, welding metallurgy, and mechanical engineering in solving a complex fabrication problem. The systematic approach to process design, combined with rigorous verification, provides a template for tackling similar challenges in other high-pressure equipment fabrication. The emphasis on multi-method ferrite analysis is particularly noteworthy, as it demonstrates the value of redundancy in quality assurance for safety-critical components.