Zhuojin Pipe Fitting Co., LtdWelding Process for Nickel-Based Alloy 625 Surfacing on Tube Sheets
Literature Overview
The paper by Wang Na and Ji Qiang, published in Welding (2016, No. 6, pp. 53-55), originates from the Fushun Chemical Machinery Manufacturing Co., Ltd. It presents a comprehensive welding process study for surfacing nickel-based Alloy 625 onto tube sheets, employing the submerged arc welding (SAW) process with a strip electrode. The study includes welding consumable selection, process parameter optimization, and comprehensive quality verification through non-destructive testing, chemical analysis, mechanical testing, intergranular corrosion testing, and microstructural examination.
Core Technical Content
Nickel-based Alloy 625 (UNS N06625) is a precipitation-strengthened nickel-chromium-molybdenum alloy renowned for its exceptional combination of high strength and outstanding corrosion resistance. It is widely used in petrochemical applications where equipment must withstand aggressive chemical environments at elevated temperatures and pressures. Tube sheets in heat exchangers, reactors, and separators are critical components where Alloy 625 surfacing is often required to provide a corrosion-resistant barrier between the tube bundle and the process fluid.
Welding Characteristics of Alloy 625
Alloy 625 presents several unique welding challenges that must be addressed in process design:
| Challenge | Root Cause | Mitigation Strategy |
|---|---|---|
| Hot cracking susceptibility | Low solidification temperature range, high thermal expansion | Preheating, low heat input, proper filler selection |
| Delta phase formation | High Ni-Cr composition | Controlled cooling rates, post-weld aging treatment |
| High thermal conductivity | Ni-based matrix | Adequate heat input to maintain weld pool fluidity |
| Intergranular corrosion risk | Carbide precipitation at grain boundaries | Low carbon filler, proper PWHT or aging |
| Dilution with base metal | Dissimilar metal welding | Multi-pass surfacing, controlled dilution rate |
Process Parameters and Consumable Selection
The study employs strip electrode submerged arc welding (SAW) with the following consumable configuration:
- Electrode: EQNiCrMo-3 (Ni-based alloy strip, equivalent to Alloy 625 composition)
- Flux: ES200 (rutile-type flux, low hydrogen content)
The following table summarizes the optimized process parameters identified through the welding process qualification study:
| Parameter | Selected Value | Rationale |
|---|---|---|
| Welding current | 400-500 A | Adequate penetration without excessive dilution |
| Welding voltage | 28-32 V | Stable arc, good wetting |
| Travel speed | 200-300 mm/min | Balance between deposition rate and heat input |
| Wire feed speed | 3.0-4.0 m/min | Matched to current and voltage |
| Preheat temperature | 100-150 °C | Reduce cooling rate, minimize cracking |
| Interpass temperature | ≤ 250 °C | Control microstructure, avoid delta phase |
| Surfacing passes | 2-4 passes | Achieve required thickness with controlled dilution |
| Post-weld treatment | Solution + aging (1050 °C + 870 °C) | Dissolve delta phase, precipitate γ'' phase |
Quality Verification
The study employs a comprehensive quality verification program:
- Non-destructive testing (NDT): Magnetic particle testing (MT) for surface cracks, ultrasonic testing (UT) for subsurface defects, and radiographic testing (RT) for volumetric defect detection.
- Chemical analysis: Verification of overlay composition, particularly Cr, Ni, Mo, and C content, to ensure compliance with Alloy 625 specification (ASTM B625 or NACE MR0175).
- Bend testing: Macroscopic bend test to assess ductility and bond integrity of the overlay.
- Intergranular corrosion testing: ASTM G108 (10% citric acid) or ASTM A262 Practice E to verify resistance to sensitization.
- Microstructural examination: Optical microscopy for grain size, phase identification, and delta phase assessment.
- Microhardness testing: Vickers hardness measurement across the weld cross-section to evaluate hardness uniformity and dilution effects.
Engineering Practice Considerations
For engineers specifying Alloy 625 surfacing on tube sheets, several practical considerations arise from this study:
- Tube sheet material compatibility: The base tube sheet material (typically carbon steel, low-alloy steel, or austenitic stainless steel) significantly influences the dilution rate and the resulting overlay composition. Engineers must account for dilution in their material selection and specification.
- Thermal distortion: Tube sheets are typically large, flat components that are highly susceptible to welding distortion. Preheating, back-plate cooling, and controlled welding sequences are essential to maintain flatness within tolerances.
- Post-weld aging: Alloy 625 requires proper solution treatment and aging to achieve its full mechanical properties. The aging temperature and time must be carefully controlled to avoid over-aging or under-aging.
- Inspection access: Tube sheet geometry, with its dense array of tube holes, can make NDT access challenging. Engineers should plan the surfacing sequence to ensure adequate inspection access before hole drilling.
Study Insights and Implications
This paper provides a well-structured and practically oriented process qualification study for Alloy 625 surfacing on tube sheets. The comprehensive quality verification program—combining NDT, chemical analysis, mechanical testing, corrosion testing, and microstructural examination—serves as an excellent template for similar surfacing qualification studies.
The use of strip electrode SAW is particularly noteworthy from a productivity standpoint. Compared to manual processes (GTAW, SMAW) or even wire-fed SAW, strip electrode SAW offers significantly higher deposition rates, which is advantageous for large-area surfacing applications such as tube sheets. The consistent quality achievable with this process also reduces the need for rework, which is critical for maintaining schedule adherence in manufacturing environments.
One area for further development, in my assessment, is the quantitative assessment of dilution effects on the overlay composition and the resulting impact on corrosion resistance. The study should ideally include a systematic dilution study, varying the number of surfacing passes and measuring the resulting overlay composition and corrosion resistance. This data would be invaluable for engineers making pass-count decisions in production specifications.
The overall contribution of this paper is significant: it demonstrates that Alloy 625 surfacing on tube sheets can be reliably achieved through strip electrode SAW with appropriate process parameters and quality controls, providing a practical manufacturing solution for petrochemical equipment requiring high corrosion resistance and strength.