Zhuojin Pipe Fitting Co., LtdStainless Steel Strip Electrode Electroslag Overlay Welding Process on 16MND5 Steel
Literature Overview
This paper by Zhang Mingliang, Xin Yu, and Han Guoming, published in Welding Technology (2010, Vol. 39, No. 9, pp. 57-59), presents the results of process development trials for 0.4 mm × 50 mm stainless steel strip electrode electroslag overlay welding on 16MND5 low-alloy steel. The work was conducted by the Welding Technology Department of China First Heavy Industries Co., Ltd. Nuclear Power and Petrochemical Division, in collaboration with the Navy Representative Office and Tianjin University. The primary application is the overlay welding of stainless steel corrosion-resistant layers on reactor pressure vessel components, where the combination of mechanical strength and corrosion resistance is critical.
Electroslag overlay welding (ESOW) is a high-deposition-rate process that is particularly well-suited for applying thick overlay layers on large components. The use of a thin stainless steel strip electrode (0.4 mm × 50 mm) in conjunction with an auxiliary magnetic field device represents an innovative approach to achieving uniform, high-quality overlay welds with controlled dilution and excellent metallurgical properties.
Core Technical Content
Process Principles and Configuration
Electroslag welding operates on the principle of electric resistance heating of a molten slag pool. The electric current passes through the molten slag, which acts as a heating element and flux, creating a molten pool that solidifies into the weld. The strip electrode is fed into the slag pool, where it melts and contributes to the weld metal.
The key innovation in this process is the use of an auxiliary magnetic field device to control the position and shape of the arc and molten pool. The magnetic field exerts a Lorentz force on the current-carrying molten metal, which can be used to:
- Control the arc position relative to the strip electrode
- Influence the shape of the molten pool
- Reduce spatter and improve weld bead profile
- Control the dilution ratio between the strip electrode and base metal
| Parameter | Typical Value |
|---|---|
| Strip electrode thickness | 0.4 mm |
| Strip electrode width | 50 mm |
| Base material | 16MND5 low-alloy steel |
| Overlay material | Stainless steel (304L or 316L) |
| Slag flux coverage | 15-25 mm |
| Strip electrode stickout | 15-25 mm |
| Overlap | 10-15 mm |
| Eccentricity | 5-10 mm |
| Magnetic control current | 100-300 A |
Process Parameter Optimization
The paper describes a systematic approach to optimizing the welding parameters through trial and error:
- Welding current and voltage: These parameters determine the heat input and deposition rate. Higher current increases the deposition rate but also increases dilution and residual stress.
- Strip electrode stickout length: Affects the resistance heating of the strip and the arc stability. Too short a stickout can cause arc instability, while too long a stickout can cause excessive burning of the strip.
- Overlap: The overlap between adjacent weld passes affects the uniformity of the overlay layer and the risk of lack of fusion.
- Eccentricity: The offset of the strip electrode relative to the weld centerline is used to control the dilution and weld bead profile.
- Slag flux coverage: The thickness of the slag flux layer affects the heat distribution and the protection of the molten pool.
- Magnetic control current: The auxiliary magnetic field current is used to fine-tune the arc position and molten pool shape.
Metallurgical Considerations
The overlay weld metal is a mixture of the stainless steel strip electrode and the 16MND5 base metal. The dilution ratio directly affects the chemical composition and mechanical properties of the overlay layer:
| Dilution (%) | Carbon Equivalent | Hardness (HV) | Corrosion Resistance |
|---|---|---|---|
| 0-10 | Low | 150-200 | Excellent |
| 10-20 | Moderate | 200-250 | Good |
| 20-30 | High | 250-300 | Reduced |
| >30 | Very high | >300 | Poor |
The target dilution for nuclear power applications is typically below 20%, which requires careful control of the welding parameters and the use of the auxiliary magnetic field to minimize base metal melting.
Engineering Practice Integration
Application to Reactor Pressure Vessels
The primary application of this process is the overlay welding of stainless steel layers on reactor pressure vessel components, such as:
- Head-to-shell weld joints
- Nozzle-to-shell weld joints
- Internal components exposed to corrosive coolant
The 16MND5 base material provides the required mechanical properties for pressure containment, while the stainless steel overlay provides corrosion resistance against the reactor coolant environment. The overlay thickness is typically 3-6 mm, applied in multiple passes using the electroslag process.
Quality Control Requirements
For nuclear power applications, the overlay weld must meet stringent quality requirements:
| Requirement | Acceptance Criteria |
|---|---|
| Dilution ratio | <20% |
| Chemical composition | Within specified limits |
| Mechanical properties | Meets ASME/NB/T requirements |
| NDT | 100% RT or UT, no defects |
| Surface finish | Ra ≤ 3.2 μm |
| Thickness | Within ±0.5 mm of nominal |
The quality control process includes:
- Pre-weld inspection of base metal and consumables
- In-process monitoring of welding parameters
- Post-weld NDT of each pass
- Final NDT of the completed overlay
- Mechanical property testing on coupon samples
- Chemical analysis of the overlay metal
Comparison with Other Overlay Processes
| Process | Deposition Rate | Dilution Control | Equipment Cost | Flexibility |
|---|---|---|---|---|
| Electroslag (strip) | Very high | Good with magnetic field | High | Low (large components) |
| TIG welding | Low | Excellent | Low | High |
| Submerged arc welding | High | Moderate | Moderate | Moderate |
| Flux-cored arc welding | High | Moderate | Moderate | Moderate |
The electroslag process with strip electrode offers the highest deposition rate, making it the most economical choice for thick overlay layers on large components. The auxiliary magnetic field device provides the dilution control needed for nuclear power applications, making this process suitable for critical safety-related components.
Key Questions and Reflections
The paper raises several important questions about the scalability and reliability of this process. First, how does the process performance change with component geometry? The trial was conducted on a flat coupon, but actual reactor pressure vessel components have complex geometries with curved surfaces and tight access. The process parameters optimized on a flat coupon may not be directly transferable to curved surfaces.
Second, what is the long-term performance of the overlay weld under reactor service conditions? The dilution ratio and metallurgical properties of the overlay weld can change over time due to thermal cycling and irradiation effects. Long-term performance data is essential for validating the process for nuclear power applications.
Third, how does the auxiliary magnetic field device affect the residual stress distribution in the overlay weld? The magnetic field can influence the cooling sequence and solidification pattern of the weld, which in turn affects the residual stress. High residual stress can lead to stress corrosion cracking in the overlay weld, particularly in corrosive environments.
In my experience, the electroslag overlay welding process with strip electrode is a powerful tool for applying thick corrosion-resistant overlay layers on large components. However, the process requires careful parameter optimization and strict quality control to achieve the required performance. The auxiliary magnetic field device is a valuable addition that provides the dilution control needed for critical applications, but it also adds complexity to the process that must be managed.
Study Insights and Implications
This paper demonstrates the successful application of electroslag overlay welding with a thin stainless steel strip electrode on 16MND5 low-alloy steel for reactor pressure vessel applications. The use of an auxiliary magnetic field device to control the welding process represents a significant advancement in overlay welding technology, enabling the production of high-quality overlay layers with controlled dilution and excellent metallurgical properties.
The key implication for engineering practice is that electroslag overlay welding is a viable and economical process for applying thick corrosion-resistant overlay layers on large nuclear power components. The process offers high deposition rates, good dilution control, and excellent weld quality when properly optimized. However, the process requires significant investment in equipment and operator training, and the parameters must be carefully controlled to achieve the required performance.
The paper also highlights the importance of systematic process development and optimization. The trial approach described in the paper, with careful variation of parameters and detailed evaluation of results, is essential for developing reliable welding procedures for critical applications. This approach should be adopted as a standard practice for any new welding process development in the nuclear power industry.
In conclusion, the electroslag overlay welding process with strip electrode and auxiliary magnetic field is a promising technology for the overlay welding of stainless steel layers on reactor pressure vessel components. The process offers the combination of high deposition rate, good dilution control, and excellent weld quality that is required for nuclear power applications.