CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electrode Submerged Arc Weld Overlay of Stainless Steel on Spherical Head Inner Walls
Literature Overview
This 1992 publication by Qi Bingzhi from Hefei Chemical Machinery Factory addresses a highly specific and practically important fabrication challenge: applying a stainless steel cladding layer to the inner walls of spherical pressure vessel heads using strip electrode submerged arc welding (SAW). Spherical heads are commonly used in pressure vessels for their favorable stress distribution, and their curved internal geometry presents unique challenges for weld overlay operations. The study is a direct engineering solution to a real manufacturing problem, making it particularly valuable for fabrication engineers working in the pressure vessel industry.
Technical Background and Challenges
Spherical heads, typically fabricated from carbon steel or low-alloy steel (such as 16Mn or Q345R), require internal stainless steel cladding when the vessel contents are corrosive. The key challenges include:
- Geometric complexity: The curved surface requires precise control of the welding gun position relative to the seam.
- Accessibility: The internal surface of a large spherical head is difficult to access for welding equipment.
- Dilution control: The thick base plate and the strip electrode process tend to produce high dilution, potentially compromising the corrosion resistance of the overlay.
- Residual stress: The confined geometry of the spherical head can lead to high residual stresses and distortion.
Process Description
The study employs a strip electrode submerged arc welding process, which is particularly suited for thick overlay layers on large components. The process involves:
- A continuous stainless steel strip serves as the consumable electrode.
- The strip is fed through a guide shoe that maintains contact with the workpiece.
- A flux blanket protects the molten pool and provides additional alloying.
- The welding torch moves along a pre-defined path on the internal surface of the spherical head.
| Process Parameter | Typical Value | Notes |
|---|---|---|
| Strip thickness | 1.0–1.5 mm | Thicker strips increase dilution |
| Strip width | 30–50 mm | Wider strips increase deposition rate |
| Welding current | 600–900 A | Higher current increases penetration |
| Arc voltage | 25–35 V | Controls arc length and strip feed |
| Travel speed | 150–300 mm/min | Depends on desired overlay thickness |
| Flux composition | Rutile-basic type | Provides alloying and protection |
| Preheat temperature | 100–200°C | Reduces cold cracking risk |
| Interpass temperature | <250°C | Controls thermal cycle |
Microstructural Considerations
The fusion zone between the carbon steel base and the stainless steel overlay is the most critical region from a metallurgical standpoint. The dilution in the first pass is typically 20–40%, producing a martensitic microstructure that can be susceptible to cracking. The study recommends:
- Using a multi-pass approach where the first pass is designed for bonding and subsequent passes progressively enrich the stainless steel content.
- Selecting a strip composition with higher chromium content (e.g., 304 or 316 grade) to compensate for dilution.
- Applying post-weld stress relief at 600°C to reduce residual stresses in the overlay and heat-affected zone.
The overlay microstructure in the final passes is typically a fully austenitic or austenitic-ferritic structure, depending on the strip composition and dilution level. For corrosion resistance, the final overlay should contain at least 18% Cr and 8% Ni after accounting for dilution.
Defect Prevention
| Defect | Cause | Prevention |
|---|---|---|
| Lack of fusion | Insufficient current, excessive travel speed | Optimize current-to-speed ratio |
| Undercut | Excessive arc voltage | Reduce voltage, adjust gun angle |
| Excessive dilution | High current, slow travel | Reduce current, increase travel speed |
| Cracking at fusion line | High cooling rate, martensitic structure | Preheat, use appropriate filler composition |
| Distortion | Excessive heat input | Use multi-pass approach, control interpass temperature |
| Porosity | Flux contamination, moisture | Control flux storage and drying |
Engineering Practice Application
The study's practical value lies in its direct application to pressure vessel fabrication. The following engineering recommendations emerge:
- Welding sequence: The spherical head should be welded in a radial pattern starting from the pole and working outward, or in a spiral pattern, to minimize distortion.
- Fixture design: The spherical head should be firmly clamped to prevent movement during welding.
- Inspection: Each pass should be inspected by magnetic particle testing (MT) before the next pass is applied.
- Final testing: The completed overlay should undergo dye penetrant testing (PT) and, if required, eddy current testing (ET) to verify bond strength.
- Corrosion testing: A ferric chloride immersion test or electrochemical corrosion test should be performed on a coupon to verify the overlay's corrosion resistance.
Key Questions and Reflections
The 1992 publication predates many modern process control technologies, but its fundamental principles remain valid. A question that arises is whether modern automated strip electrode systems with real-time parameter monitoring could further improve the consistency and quality of spherical head cladding. The geometric challenges of curved surfaces have not been eliminated by technology, but they have been made more manageable through improved robotic path planning and sensor feedback.
Another reflection is the trade-off between dilution control and deposition efficiency. The strip electrode process is inherently less efficient than processes such as plasma transferred arc (PTA) or laser cladding in terms of dilution control, but it offers significantly higher deposition rates and lower equipment costs. For large spherical heads requiring thick overlay layers, the strip electrode SAW process remains a practical and economical choice.
Summary
This 1992 study represents a practical engineering solution to a specific fabrication challenge: stainless steel cladding on the internal surfaces of spherical pressure vessel heads using strip electrode submerged arc welding. Its value lies in its direct applicability to real-world manufacturing, providing concrete process parameters, defect prevention strategies, and inspection requirements. While the technology has advanced since publication, the fundamental metallurgical principles and process considerations remain relevant and continue to guide modern cladding operations on curved pressure vessel components.