Double-Sided Overlay Welding Thick Tube Sheet Manufacturing Technology

Literature Overview

The publication by Yang Meikun and Zhang Bin from Xi'an Nuclear Equipment Co., Ltd., published in China Chemical Equipment (Volume 23, Issue 5, 2021, pp. 38-42), describes the manufacturing technology for double-sided overlay welded thick tube sheets used in demineralized water preheaters for coal chemical equipment. The article provides a comprehensive overview of the structural design, functional requirements, and manufacturing process for a critical component in the coal chemical industry.

Thick tube sheets are among the most challenging components to manufacture in pressure equipment because they combine multiple material types, require precise dimensional tolerances, and must withstand demanding operating conditions. The double-sided overlay welding approach addresses the need for corrosion-resistant surfaces on both sides of the tube sheet while maintaining the structural integrity of the base material.

Core Technical Content

Equipment Structure and Functional Requirements

The demineralized water preheater is a heat exchanger used in coal chemical processes to recover heat from process streams. The tube sheet serves as the structural separator between the tube side and shell side, providing tube support, sealing, and pressure containment. The operating conditions require resistance to high-temperature demineralized water, potential chemical attack from trace contaminants, and mechanical loading from pressure differentials.

Design Parameter	Typical Requirement
Tube Sheet Material	Carbon steel or low-alloy steel base
Overlay Material	Corrosion-resistant alloy (stainless steel or alloy steel)
Overlay Thickness	Sufficient for corrosion allowance
Operating Temperature	Process-dependent (typically 100-300°C)
Operating Pressure	Process-dependent
Corrosion Environment	Demineralized water with trace chemicals

Manufacturing Process Overview

The double-sided overlay welding process involves the following key steps:

Base Material Preparation: The thick tube sheet blank is machined to the required dimensions with adequate stock for overlay welding on both sides. Surface preparation includes grinding and cleaning to ensure proper weld adhesion.
Overlay Welding - First Side: The corrosion-resistant overlay material is deposited on one side of the tube sheet using a selected welding process. The process selection, heat input, and layer build-up strategy are critical for achieving the required dilution rate and microstructure.
Intermediate Machining: The overlay weld on the first side is machined to the required thickness and surface finish. This step also removes any surface defects and prepares the surface for the subsequent drilling operation.
Tube Hole Drilling: The tube holes are drilled through the tube sheet, including the overlay layer. The drilling process must be carefully controlled to avoid damaging the overlay material and to maintain hole dimensional accuracy.
Overlay Welding - Second Side: The corrosion-resistant overlay material is deposited on the second side of the tube sheet, covering the tube holes. The welding parameters must be optimized to ensure proper fusion with the existing overlay material and base metal.
Final Machining and Inspection: The second side overlay is machined to the required thickness, and the tube holes are reamed or finished to the final dimensional tolerances. Comprehensive inspection is performed to verify weld quality, dimensional accuracy, and corrosion resistance.

Key Process Parameters and Quality Control

Process Parameter	Control Objective	Quality Impact
Heat Input	Minimize dilution, prevent base metal overheating	Dilution rate, microstructure
Welding Current	Control penetration and fusion	Dilution rate, weld geometry
Travel Speed	Maintain consistent weld bead profile	Dilution rate, surface quality
Interpass Temperature	Prevent excessive grain growth	Microstructure, toughness
Layer Thickness	Control dilution in each layer	Overall dilution rate
Preheat Temperature	Reduce residual stress, prevent cracking	Residual stress, cracking susceptibility
Post-Weld Heat Treatment	Reduce residual stress, improve toughness	Residual stress, mechanical properties

Dilution Rate Control

The dilution rate is a critical parameter in overlay welding that determines the composition of the weld metal and, consequently, its corrosion resistance. The dilution rate is defined as the volume fraction of base metal in the weld metal. For overlay welding applications, the dilution rate must be minimized to maintain the corrosion resistance of the overlay material.

The article emphasizes the use of reasonable process technology methods to reduce welding deformation and determine correct, standardized, and reliable manufacturing process technical parameters. The selection of welding process (such as electroslag welding, which is mentioned in the keywords) and the optimization of welding parameters are essential for achieving the target dilution rate while maintaining weld quality.

Engineering Practice Implications

Deformation Control

Thick tube sheets are particularly susceptible to welding deformation due to the large thermal gradients and high residual stresses developed during multi-pass welding. The article highlights the importance of deformation control through:

Symmetric Welding Sequences: Alternating welding on opposite sides of the tube sheet to balance thermal inputs and reduce angular and bow distortion.
Back Bar and Back Plate Use: Employing rigid back bars or back plates to constrain the tube sheet during welding and reduce out-of-plane distortion.
Stress Relief Heat Treatment: Applying post-weld stress relief to reduce residual stresses and improve dimensional stability.
Controlled Cooling Rates: Managing the cooling rate after welding to reduce thermal stresses and prevent cracking.

Quality Assurance and Inspection

The quality assurance program for double-sided overlay welded tube sheets must include:

Material Certification: Verification of the chemical composition and mechanical properties of both the base material and the overlay material.
Welding Procedure Qualification: Qualification of the welding procedures in accordance with applicable codes (ASME, EN, or relevant Chinese standards) including demonstration of acceptable dilution rates.
Weld Inspection: Non-destructive testing of the overlay welds using methods such as magnetic particle testing, ultrasonic testing, and radiographic testing.
Dilution Rate Verification: Metallographic examination of the overlay welds to measure the dilution rate and verify compliance with the design requirements.
Corrosion Testing: Verification of the corrosion resistance of the overlay welds through laboratory testing or coupon testing under simulated service conditions.
Dimensional Inspection: Verification of the tube sheet thickness, tube hole dimensions, and surface finish in accordance with the design specifications.

Ferrite Content Control

The keyword "ferrite" in the article indicates attention to the ferrite content in the overlay welds. For stainless steel overlay welds, the ferrite content is a critical microstructural parameter that affects corrosion resistance, cracking susceptibility, and mechanical properties. The ferrite content is typically controlled through:

Weld Metal Composition: Selection of filler metal with appropriate Cr/Ni ratio to achieve the target ferrite content (typically 5-35% for duplex or austenitic-ferritic weld metals).
Dilution Rate: Control of the dilution rate to ensure that the effective weld metal composition falls within the acceptable range.
Welding Sequence: Optimization of the welding sequence to control the thermal history and, consequently, the microstructural evolution.

Key Questions and Reflections

The article provides a practical overview of the manufacturing technology for double-sided overlay welded tube sheets, but several aspects warrant further discussion. First, the specific welding process used for the overlay welding is not fully detailed. The keyword mentions electroslag welding, which is suitable for thick-section welding but may present challenges for overlay welding due to the high heat input and potential for excessive dilution. The selection of welding process must be carefully considered based on the tube sheet thickness, overlay thickness, and required dilution rate.

Second, the article does not provide detailed information on the specific materials used for the base and overlay materials. The selection of overlay material depends on the specific corrosion environment, and different materials (such as 309L, 316L, 625, or 2205) offer different levels of corrosion resistance and mechanical properties. The compatibility between the base and overlay materials, including thermal expansion mismatch and galvanic coupling effects, must be carefully evaluated.

A further reflection concerns the long-term performance of the overlay welds under cyclic thermal and mechanical loading. The interface between the overlay and base metal is a potential site for crack initiation, particularly in the presence of thermal cycling. Fatigue testing of the overlay weld interface under simulated service conditions would provide valuable data for life prediction and design optimization.

Study Insights and Implications

The most significant insight from this research is the demonstration that double-sided overlay welding is a viable manufacturing approach for thick tube sheets requiring corrosion resistance on both sides. The comprehensive approach to process development, including deformation control, dilution rate management, and quality assurance, provides a practical framework for the manufacturing of similar components in other industrial applications.

This work contributes practical manufacturing knowledge to the field of pressure equipment fabrication and demonstrates the importance of integrated process design, quality control, and material selection in achieving reliable performance of critical components. The methodology described is applicable to other thick-section components requiring corrosion-resistant surfaces, including heat exchanger tube sheets, reactor internals, and pressure vessel heads.