CLADDING TECHNOLOGY SHANXI CO., LTDStainless Steel Strip Electroslag Welding Overlay on Spherical Heads
Literature Overview and Context
This 1992 study by Qi Bingzhi from the Hefei Chemical Machinery Factory, published in the journal Welding, presents a practical engineering approach to depositing stainless steel overlay layers on the inner walls of spherical heads using the strip electroslag welding (strip ESW) process. Spherical heads are critical components in pressure vessel fabrication, serving as end closures for cylindrical shells and providing superior pressure containment due to their uniform stress distribution. The need for corrosion-resistant inner surfaces on spherical heads arises in chemical processing, petrochemical, and pharmaceutical applications where the vessel interior is exposed to aggressive media.
The strip electroslag welding process is particularly well-suited for this application because it offers high deposition rates, deep penetration, and the ability to deposit thick overlay layers in a single pass. The process uses a continuous strip of filler metal as both the electrode and the source of deposited material, with the molten slag pool providing thermal insulation and atmospheric protection. This makes strip ESW an economical and efficient choice for large-diameter spherical heads where productivity is a primary concern.
Core Technical Points
Process Description and Equipment Configuration
Strip electroslag welding involves the continuous feeding of a flat strip electrode (typically 15 to 25 mm wide and 1.5 to 3 mm thick) into a molten slag pool that forms between the strip and the substrate. The welding current passes through the strip and the slag pool, generating resistive heating that melts both the strip and the substrate surface. The molten metal accumulates at the bottom of the slag pool and solidifies as the torch travels along the joint.
For spherical head inner wall overlay, the equipment must be configured to accommodate the curved geometry of the head. This typically involves a multi-torch system with a torch tracking mechanism that follows the curvature of the spherical surface. The torch travel speed, strip feed rate, and welding current must be synchronized to maintain a stable slag pool and consistent weld quality throughout the overlay operation.
| Process Parameter | Typical Value | Notes |
|---|---|---|
| Strip material | 304, 316L stainless steel | Matched to corrosion requirements |
| Strip width | 15–25 mm | Determines weld bead width |
| Strip thickness | 1.5–3.0 mm | Affects deposition rate |
| Welding current | 600–1200 A | DC, strip positive |
| Welding voltage | 25–40 V | Depends on slag composition |
| Travel speed | 100–300 mm/min | Adjusted for head diameter |
| Slag composition | CaF2-CaO-SiO2 system | Controls slag viscosity and fluidity |
| Preheating temperature | 150–250 °C | For low-alloy steel substrates |
Challenges of Spherical Head Geometry
The curvature of spherical heads presents several unique challenges for strip electroslag welding overlay. The varying curvature along the travel path means that the contact angle between the strip and the substrate changes continuously, which affects the stability of the slag pool and the uniformity of the weld bead. Near the pole of the sphere, where the curvature is most severe, the strip may lose contact with the substrate, leading to defects such as lack of fusion and porosity.
To address these challenges, the study recommends the use of a multi-torch approach, where multiple torches are arranged around the circumference of the spherical head and welded simultaneously. This approach reduces the heat input per unit length and minimizes distortion, while also allowing for more uniform coverage of the curved surface. The torches are synchronized using a central control system that adjusts the travel speed and strip feed rate based on the local curvature of the head.
Weld Quality and Inspection
The quality of the strip ESW overlay on spherical heads is assessed through a combination of visual inspection, radiographic testing (RT), ultrasonic testing (UT), and mechanical property testing. The key quality criteria include:
- Bond strength: The overlay layer must be fully bonded to the substrate, with no lack of fusion or delamination. This is verified by UT or by performing bend tests on coupon samples.
- Overlay thickness: The deposited thickness must be uniform and meet the minimum specified thickness, typically 2 to 5 mm for corrosion protection applications.
- Weld defects: The overlay welds must be free from significant defects such as porosity, slag inclusion, and cracks. RT and UT are used to detect volumetric and planar defects, respectively.
- Corrosion resistance: The overlay layer must provide adequate corrosion resistance in the intended service environment. This is verified through immersion testing or corrosion potential measurements.
Engineering Practice and Process Optimization
Preheating and Interpass Temperature Control
For low-alloy steel substrates such as 16Mn or Q345R, preheating is essential to prevent cold cracking in the heat-affected zone (HAZ). The recommended preheating temperature ranges from 150 to 250 °C, depending on the carbon equivalent of the substrate material. The interpass temperature must also be maintained within a controlled range to avoid excessive grain growth and to minimize residual stresses.
In the case of spherical heads, the preheating challenge is compounded by the geometry of the component. Uniform preheating of a large spherical head requires either a gas-fired heating system or an induction heating system that can cover the entire surface area. The temperature distribution across the head must be monitored using thermocouples placed at representative locations to ensure that the preheating is uniform and adequate.
Distortion Control
One of the most significant practical challenges in overlay welding spherical heads is the control of distortion. The thermal input from the welding process can cause the spherical head to deform, leading to out-of-roundness and dimensional inaccuracies that are unacceptable for pressure vessel fabrication. The study recommends several strategies for distortion control:
- Symmetric welding sequence: Welding is performed in a symmetric pattern around the circumference of the head to minimize asymmetric thermal distortion.
- Low heat input per pass: The welding parameters are optimized to minimize the heat input while maintaining adequate penetration and bond strength.
- Mechanical restraint: The spherical head is clamped or fixture-mounted to limit the degree of freedom for distortion.
- Post-weld straightening: If distortion occurs, it can be corrected by thermal or mechanical straightening methods, although this should be a last resort as it may introduce additional residual stresses.
Multi-Layer Overlay Strategy
For thick overlay layers, a multi-layer approach is recommended. The first layer is deposited with a lower current and slower travel speed to ensure good bonding with the substrate. Subsequent layers are deposited with progressively higher current and faster travel speed to increase productivity. The number of layers required depends on the target overlay thickness and the deposition rate per layer.
| Layer | Current (A) | Travel Speed (mm/min) | Deposition Rate (mm) |
|---|---|---|---|
| Layer 1 (bonding) | 600–800 | 100–150 | 1.5–2.0 |
| Layer 2 | 800–1000 | 150–200 | 2.0–2.5 |
| Layer 3 | 1000–1200 | 200–300 | 2.5–3.0 |
Key Questions and Reflections
This study provides valuable practical insight into the application of strip electroslag welding for spherical head overlay. However, several questions remain open. First, the long-term performance of the overlay layer under cyclic pressure loading and thermal cycling is not extensively addressed. The bond between the stainless steel overlay and the carbon steel substrate is a potential failure site under fatigue loading, and the fatigue resistance of the overlay-substrate interface should be characterized through fatigue testing.
Second, the study does not extensively discuss the effect of the slag composition on the weld quality and the metallurgical properties of the overlay layer. The slag composition in electroslag welding is a critical parameter that affects the fluidity of the slag pool, the heat input, and the chemical composition of the weld metal. Systematic optimization of the slag composition for different stainless steel grades would be valuable for improving the consistency and reliability of the overlay process.
Third, the economic aspects of the strip ESW process for spherical head overlay deserve further consideration. While strip ESW offers high deposition rates, the initial investment in equipment and the cost of the strip electrode may be higher than alternative processes such as submerged arc welding (SAW) or gas metal arc welding (GMAW) for smaller spherical heads. A cost-benefit analysis comparing different overlay processes for different head sizes would be useful for engineering decision-making.
Study Insights and Implications
The study by Qi Bingzhi represents an important contribution to the practical application of electroslag welding technology in pressure vessel fabrication. The use of strip ESW for spherical head overlay demonstrates the versatility of this process and its potential for high-productivity overlay welding in industrial settings.
For pressure vessel engineers, this study highlights the importance of process selection based on the specific geometry and requirements of the component. The strip ESW process is particularly advantageous for large-diameter spherical heads where high deposition rates are required and where the geometry allows for the use of a multi-torch configuration. However, for smaller heads or for applications requiring very precise control of the overlay thickness, alternative processes may be more appropriate.
The study also underscores the importance of process qualification and quality control in overlay welding. The rigorous inspection and testing protocols recommended in the study should be adopted as standard practice to ensure the reliability and safety of overlay-welded pressure vessel components.