CLADDING TECHNOLOGY SHANXI CO., LTDDouble-Sided Weld Overlay Thick Tube Sheet Manufacturing Technology
Literature Overview and Technical Context
This 2021 publication by Yang Meikun and Zhang Bin from Xi'an Nuclear Equipment Co., Ltd., published in the journal China Chemical Equipment, addresses a critical manufacturing challenge in the production of thick tube sheets for nuclear and chemical pressure vessels. Thick tube sheets, with thicknesses typically ranging from 100 mm to 400 mm, are essential components in heat exchangers, hydrogenation reactors, and nuclear steam generators that require both structural strength and corrosion resistance. The double-sided weld overlay technology presented in this literature provides a solution for applying a corrosion-resistant overlay on both sides of the thick tube sheet while maintaining the structural integrity of the base material.
The significance of this work is underscored by the stringent quality requirements for nuclear equipment manufacturing. Any defect in the weld overlay of a nuclear tube sheet can compromise the safety and reliability of the entire pressure vessel, leading to catastrophic consequences. The literature provides detailed process parameters, quality control procedures, and defect analysis that are directly applicable to the manufacturing of high-integrity tube sheets for nuclear and chemical applications.
Core Technical Points on Double-Sided Overlay Process
The double-sided weld overlay process involves depositing a corrosion-resistant alloy on both the front and back surfaces of the thick tube sheet. The following table summarizes the key process parameters for the overlay of 316L stainless steel on a P92 base material with a thickness of 200 mm:
| Parameter | Front Side | Back Side | Notes |
|---|---|---|---|
| Welding process | SAW (Submerged Arc Welding) | SAW | High deposition rate required |
| Filler wire | ER316L, φ3.2 mm | ER316L, φ3.2 mm | Low-carbon to prevent sensitization |
| Flux | Rutile-type, dry at 250°C for 2 h | Rutile-type, dry at 250°C for 2 h | Low hydrogen content |
| Welding current | 450-550 A | 450-550 A | DCEN polarity |
| Voltage | 30-35 V | 30-35 V | Stable arc |
| Travel speed | 80-120 mm/min | 80-120 mm/min | Controlled heat input |
| Number of passes | 3-4 | 3-4 | First pass with lower current |
| Overlay thickness | 6-8 mm | 6-8 mm | Minimum 5 mm for corrosion resistance |
| Preheat temperature | 200-250°C | 200-250°C | Reduces cracking susceptibility |
| Interpass temperature | < 300°C | < 300°C | Prevents grain growth |
The critical challenge in double-sided overlay is the management of thermal stresses that develop as the front and back overlays are deposited sequentially. When the front overlay is deposited, it generates compressive stresses in the base metal. When the back overlay is subsequently deposited, it generates additional compressive stresses that can accumulate and lead to distortion or cracking. The literature addresses this challenge through a carefully designed welding sequence that alternates between the front and back sides to distribute the thermal stresses more uniformly.
Welding Sequence and Thermal Management
The recommended welding sequence for a 200 mm thick tube sheet with double-sided 316L overlay is as follows:
- Surface preparation: Both surfaces are ground to a smooth finish with a Ra of less than 6.3 μm. The edges are beveled to a 45° angle to facilitate the first weld pass.
- Front side first pass: A single pass of ER316L wire is deposited at a reduced current of 350 A to establish a sound fusion bond with the base metal. The travel speed is increased to 150 mm/min to minimize the heat input and reduce the risk of dilution.
- Front side subsequent passes: Additional passes are deposited at the full current of 500 A, building up the overlay to the required thickness of 6-8 mm. The interpass temperature is maintained below 300°C using a thermocouple monitoring system.
- Thermal equilibration: After the front side overlay is complete, the tube sheet is allowed to cool to 100°C before starting the back side overlay. This equilibration period allows the residual stresses from the front side to partially relax.
- Back side first pass: The back side overlay is deposited following the same sequence as the front side, with the first pass at a reduced current and subsequent passes at full current.
- Post-weld stress relief: The completed tube sheet is stress-relieved at 620°C for 2 hours per 25 mm of thickness, followed by air cooling. This treatment reduces residual stresses by 70-80% and eliminates the risk of delayed cracking.
The thermal management strategy is critical for preventing distortion. The literature reports that the maximum distortion observed in a 200 mm thick tube sheet with double-sided overlay was 1.5 mm per 1000 mm of length, which is well within the acceptable tolerance of 2.0 mm per 1000 mm specified by NB/T 47002.
Defect Analysis and Countermeasures
The literature identifies several common defects in double-sided thick tube sheet overlay and provides detailed countermeasures:
| Defect | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Cracking at fusion boundary | Excessive dilution, high residual stress | MT, PT | Preheat to 250°C, reduce first pass current by 30% |
| Porosity | Flux contamination, moisture absorption | RT, UT | Dry flux at 250°C for 2 h, use low-hydrogen flux |
| Lack of fusion | Insufficient heat input, poor fit-up | UT, RT | Increase current by 15%, ensure surface cleanliness |
| Excessive dilution | High heat input, thin first pass | Hardness testing, macrograph | Use smaller wire for first pass, increase travel speed |
| Distortion | Asymmetric thermal input | Dimensional inspection | Alternate front/back welding, use clamping fixtures |
| Undercut | Excessive current, slow travel speed | Visual inspection | Reduce current by 10%, increase travel speed by 20% |
Non-Destructive Testing Requirements
The literature specifies the following NDT requirements for double-sided thick tube sheet overlay, which are consistent with the requirements of NB/T 47002 and ASME VIII Division 1:
| NDT Method | Coverage | Acceptance Criteria | Standard Reference |
|---|---|---|---|
| Visual inspection (VT) | 100% | No cracks, porosity > 1.0 mm, undercut > 0.5 mm | NB/T 47002.1 |
| Magnetic particle testing (MT) | 100% of overlay and HAZ | No linear indications > 1.5 mm | NB/T 47002.4 |
| Ultrasonic testing (UT) | 100% of overlay thickness | No indications above reference level | NB/T 47002.5 |
| Radiographic testing (RT) | 10% of overlay joints | No porosity > 1.0 mm, no slag > 1.5 mm | NB/T 47002.2 |
| Hardness testing | 5 points per 100 mm² | Overlay: 150-250 HV; HAZ: < 300 HV | NB/T 47002.7 |
| Intergranular corrosion test | 1 specimen per batch | No intergranular corrosion per ASTM A923 Method C | ASTM A923 |
The intergranular corrosion test is particularly important for 316L overlay on P92 base metal, as the high carbon content of P92 can lead to chromium depletion at the grain boundaries of the overlay if the dilution ratio is excessive. The literature recommends a maximum dilution ratio of 10% for the first pass and 5% for subsequent passes to prevent sensitization.
Engineering Practice Integration
The literature provides a detailed case study of the manufacturing of a 300 mm thick tube sheet for a nuclear steam generator, with the following specifications:
- Base material: P92 (9Cr-1Mo-V), thickness 300 mm
- Overlay material: 316L stainless steel, thickness 8 mm on each side
- Total overlay volume: Approximately 15 kg of ER316L wire
- Welding time: 48 hours for both sides
- Stress relief: 620°C for 24 hours
The quality assurance results for this tube sheet were as follows:
| QA Parameter | Result | Acceptance Criteria |
|---|---|---|
| Overlay hardness | 185 HV | 150-250 HV |
| HAZ hardness | 280 HV | < 300 HV |
| Dilution ratio (first pass) | 8% | < 10% |
| Dilution ratio (subsequent passes) | 4% | < 5% |
| MT indications | None | No linear > 1.5 mm |
| UT indications | None | No indications above reference |
| IGC test | Pass | No intergranular corrosion |
| Distortion | 1.2 mm/1000 mm | < 2.0 mm/1000 mm |
| Hydrostatic test | Pass | No leakage at 1.5× design pressure |
The successful manufacturing of this tube sheet demonstrates the feasibility and reliability of the double-sided weld overlay technology for thick nuclear tube sheets. The key to success was the disciplined control of welding parameters, the careful management of thermal stresses through the welding sequence, and the rigorous application of quality assurance procedures.
Cost and Time Analysis
The literature also provides a cost and time analysis for the double-sided overlay process:
| Cost Component | Cost (CNY) | Percentage |
|---|---|---|
| Base material (P92 plate) | 120,000 | 45% |
| Overlay material (ER316L wire) | 15,000 | 6% |
| Flux and consumables | 5,000 | 2% |
| Welding labor | 30,000 | 11% |
| NDT and testing | 25,000 | 9% |
| Stress relief and heat treatment | 20,000 | 8% |
| Machining and finishing | 35,000 | 13% |
| Total | 250,000 | 100% |
The total manufacturing time for the tube sheet, including preparation, welding, stress relief, testing, and machining, was approximately 120 hours. The overlay operation itself accounted for 48 hours of the total time, with the remaining time allocated to preparation, stress relief, testing, and machining.
Study Insights and Independent Reflection
This literature provides a comprehensive and practical guide to the manufacturing of double-sided thick tube sheets with weld overlay, and its recommendations are directly applicable to the production of high-integrity nuclear and chemical equipment. The detailed process parameters, welding sequence, and quality assurance procedures provide a valuable reference for engineers and technicians involved in tube sheet manufacturing.
One of the most important insights from this work is the emphasis on thermal management through the welding sequence. The practice of alternating between the front and back sides, combined with the thermal equilibration period between sides, is a simple but effective strategy for minimizing distortion and residual stresses. This approach is consistent with the general principle of thermal symmetry that is applied in all types of welding operations, but its application to thick tube sheets with double-sided overlay is particularly important due to the large thermal mass and the potential for significant distortion.
Another important observation is the role of the first weld pass in determining the overall quality of the overlay. The first pass establishes the fusion bond between the overlay and the base metal, and any defect in the first pass can propagate through the subsequent passes and compromise the entire overlay. The literature's recommendation to use a reduced current and increased travel speed for the first pass is a practical and effective strategy for minimizing dilution and ensuring a sound fusion bond.
I also note that the literature does not extensively discuss the effect of the overlay on the mechanical properties of the base metal. For P92 base metal, which has a yield strength of approximately 550 MPa, the overlay operation can cause localized softening of the heat-affected zone due to the thermal cycling. The literature reports a HAZ hardness of 280 HV, which is slightly below the base metal hardness of 300 HV, indicating some degree of softening. While this softening is within the acceptable range, it warrants consideration for applications where the mechanical properties of the HAZ are critical.
A further reflection is that the double-sided overlay technology presented in this literature can be extended to other base materials and overlay materials. The same principles of thermal management, welding sequence optimization, and quality assurance can be applied to the overlay of nickel-based alloys on carbon steel, or titanium on steel, with appropriate modifications to the welding parameters and heat treatment conditions. The key is to understand the metallurgical interaction between the base metal and the overlay material and to design the process to minimize the formation of brittle interfacial phases.
In conclusion, this literature provides a comprehensive and practical guide to the manufacturing of double-sided thick tube sheets with weld overlay. The detailed process parameters, welding sequence, and quality assurance procedures are directly applicable to the production of high-integrity nuclear and chemical equipment. The key takeaway is that the success of double-sided overlay depends on the disciplined control of thermal stresses through the welding sequence, the careful management of dilution through the first pass, and the rigorous application of quality assurance procedures. As the demand for high-integrity tube sheets continues to grow in the nuclear and chemical industries, the principles presented in this literature will remain essential for ensuring the safety and reliability of critical pressure vessel components.