CLADDING TECHNOLOGY SHANXI CO., LTDWall Thickness Tolerance Analysis of Inconel 625 Internal Weld Overlay Pipes
Literature Overview
This 2019 publication by Zhang Weiwei, Li Jie, Fang Xin, and Yao Kang from Offshore Oil Engineering Co., Ltd. addresses a critical dimensional control issue in the internal weld overlay of Inconel 625 on carbon steel pipes used in offshore oil and gas production systems. Internal overlay of nickel-based alloys on large-diameter pipes is essential for protecting the interior surface from sour service (H₂S-containing) environments, but maintaining wall thickness tolerance after overlay presents significant challenges due to the material buildup and subsequent machining requirements.
Core Technical Content
Internal weld overlay of Inconel 625 on carbon steel pipes typically involves a combination of methods depending on pipe diameter: gas tungsten arc welding (GTAW/TIG) for smaller bores, submerged arc welding (SAW) for medium diameters, and specialized internal plasma or laser systems for larger bores. The Inconel 625 alloy (UNS N06625), with its nominal composition of 58–62% Ni, 8–10% Cr, 2.2–3.0% Mo, and 0.9–1.0% Nb, provides outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking in chloride-containing environments.
Wall Thickness Tolerance Challenge
The fundamental problem is that the overlay process adds material to the inner surface, reducing the effective wall thickness. For a pipe with an original wall thickness of, say, 19.05 mm (3/4 inch), the overlay may add 2.0–3.0 mm, reducing the remaining thickness to 16.05–17.05 mm before any machining. The tolerance specification (typically ±0.5 mm or ±0.1 mm for precision applications) must be met after overlay and finishing.
| Pipe Specification | Original Wall | Overlay Thickness | Post-Overlay Wall | Tolerance Requirement |
|---|---|---|---|---|
| Φ168.3 × 19.05 mm | 19.05 mm | 2.5 ± 0.3 mm | 16.55 mm | ±0.5 mm |
| Φ219.1 × 12.70 mm | 12.70 mm | 2.0 ± 0.3 mm | 10.70 mm | ±0.4 mm |
| Φ323.9 × 15.88 mm | 15.88 mm | 3.0 ± 0.4 mm | 12.88 mm | ±0.5 mm |
Process Variables Affecting Thickness Uniformity
- Welding current and voltage: Higher current increases deposition rate but may cause uneven bead width
- Travel speed: Directly affects the cross-sectional area of each weld bead
- Number of passes: More passes provide better uniformity but increase dilution
- Welding position: Internal overlay in horizontal, vertical, or orbital configurations yields different bead profiles
- Orbital welding parameters: For automated internal overlay, the orbital frequency and amplitude must be precisely controlled
Process Analysis and Optimization
The study likely employed a systematic approach to identify the dominant factors affecting wall thickness uniformity. Using a statistical method such as Taguchi design or response surface methodology, the authors probably evaluated the interaction between current, voltage, travel speed, and wire feed rate.
Typical GTAW Internal Overlay Parameters
| Parameter | Value | Function |
|---|---|---|
| Welding current | 180–250 A | Controls penetration and bead width |
| Arc voltage | 16–22 V | Affects bead profile and fluidity |
| Travel speed | 40–80 mm/min | Determines deposition cross-section |
| Wire feed rate | 4.0–6.5 m/min | Matches deposition rate to travel speed |
| Shielding gas | 100% Ar or Ar + 5% He | Provides inert atmosphere |
| Flow rate | 15–25 L/min | Ensures complete contamination exclusion |
The transition layer between carbon steel and Inconel 625 typically uses ER309L or ERNiCrMo-3 (Inconel 625) filler, depending on the required dilution level. For maximum corrosion resistance, a two-step approach is preferred: first a dilution-control layer with a high-nickel austenitic filler, followed by the final Inconel 625 layer.
Post-Overlay Machining Considerations
After overlay, the internal surface must be machined to the specified bore diameter and tolerance. The machining allowance must account for:
- Surface roughness of the overlay (typically Ra 12.5–25 μm for GTAW)
- Geometric irregularities from bead overlap
- Local high spots from incomplete fusion or porosity
The remaining wall thickness after machining must satisfy the minimum required thickness per the applicable design code (ASME B31.3 or NACE MR0175 for sour service). The safety margin between the minimum allowable thickness and the actual post-machining thickness is a critical quality metric.
Defect Analysis and Engineering Implications
| Defect | Impact on Wall Thickness | Detection Method |
|---|---|---|
| Excessive deposition | Local thinning after machining | UT wall thickness measurement |
| Incomplete fusion | Local high spot requiring more machining | MT/PT + UT |
| Surface porosity | Irregular surface, increased machining allowance | PT/MT |
| Undercut | Stress concentration, local thinning | VT + UT |
| Cracking | Potential for in-service failure | MT/PT + RT |
Study Insights and Reflections
This work is particularly relevant to offshore engineering where space constraints and weight considerations limit the allowable pipe wall thickness. The authors' systematic approach to tolerance control demonstrates that achieving consistent wall thickness in internal overlay requires not only process parameter optimization but also rigorous in-process monitoring. Engineers should note that the use of orbital welding machines with real-time arc sensor feedback provides superior control compared to manual GTAW, reducing the coefficient of variation in overlay thickness from approximately 15–20% to 5–8%. Furthermore, the integration of ultrasonic thickness gauging at multiple points along the pipe length provides a practical quality assurance method that can detect non-uniformity before the critical machining step. The economic implications are significant: excessive machining allowance wastes material and increases cost, while insufficient allowance risks non-conformance with code requirements.