CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Single-Layer Electroslag Welding Overlay Material Properties and Application in Pressure Vessels
Literature Overview
This paper, published in 2020 in the journal China Chemical Equipment, authored by Ren Jinshuo of Shanxi Yangmei Chemical Machinery (Group) Co., Ltd., addresses a highly practical engineering problem: the performance characterization and application of single-layer electroslag welding (ESW) overlay cladding on pressure vessel components. Single-layer ESW overlay is a critical technology for fabricating large-scale clad pressure vessels, particularly hydrogenation reactors and high-pressure equipment where the base material requires structural strength while the corrosion-resistant overlay must meet stringent chemical resistance requirements. The paper investigates how overlay material properties are influenced by the single-layer ESW process parameters and evaluates their suitability for industrial service conditions.
Core Technical Points and Process Analysis
Single-layer electroslag welding overlay differs fundamentally from multi-pass SAW overlay in that the entire overlay thickness is deposited in a single pass, which imposes severe constraints on heat input control, solidification behavior, and microstructural development. The ESW process generates extremely high heat input, typically in the range of 30 to 80 kJ/mm, which results in slow cooling rates and coarse grain structures if not properly managed. The key challenge is achieving adequate dilution control between the base steel and the overlay material while maintaining the required corrosion resistance and mechanical properties of the single-layer overlay.
| Parameter | Typical Range for Single-Layer ESW Overlay | Impact on Overlay Quality |
|---|---|---|
| Heat input | 30–80 kJ/mm | Controls grain size and dilution rate |
| Electrode diameter | 12–25 mm | Affects current density and deposition rate |
| Travel speed | 150–400 mm/min | Influences solidification rate and microstructure |
| Slag composition | CaF2-CaO-Al2O3 based | Controls heat distribution and fluidity |
| Base metal preheat | 150–250 °C | Reduces cracking susceptibility |
| Interpass temperature | 200–400 °C | Controls thermal stress accumulation |
| Overlay thickness (single layer) | 5–15 mm | Must meet design specification requirements |
| Dilution rate (target) | < 10–15% | Critical for maintaining corrosion resistance |
The single-layer approach eliminates the need for multiple passes, which simplifies the fabrication sequence and reduces the risk of interpass defects. However, it demands that the selected overlay material has a sufficiently wide solidification range and good hot-cracking resistance to tolerate the large thermal gradients inherent in the process. The dilution rate is a critical parameter because even small amounts of base metal contamination can significantly degrade the corrosion resistance of nickel-based or stainless steel overlays. For example, in a 316L overlay on a carbon steel base, a dilution rate exceeding 15% can reduce the chromium content below the minimum threshold for passivity, leading to intergranular corrosion susceptibility.
Material Selection and Microstructural Considerations
The overlay material selection for single-layer ESW must account for the expected dilution. Common overlay materials include:
- Stainless steel 316L: Suitable for moderate corrosion environments; dilution tolerance up to approximately 12%.
- Inconel 625: Excellent corrosion resistance in aggressive environments; can tolerate dilution up to 15–20% due to its high nickel content.
- Monel 400: Preferred for sulfuric acid service; dilution must be kept below 10% to maintain resistance.
- Hastelloy C276: For highly aggressive chemical environments; dilution control is critical at below 8%.
The microstructure of single-layer ESW overlay typically consists of columnar dendrites growing from the fusion boundary toward the surface, with some equiaxed grains near the center if the cooling rate is sufficiently slow. The columnar grain structure can be detrimental because it creates continuous paths for intergranular corrosion and stress corrosion cracking. Post-weld heat treatment (PWHT) at 1050–1150 °C can partially homogenize the microstructure and relieve residual stresses, but it must be carefully controlled to avoid excessive grain growth.
Engineering Application and Quality Control
In engineering practice, single-layer ESW overlay is most commonly applied to large-diameter pressure vessels where the overlay area is extensive and the use of clad plates would be prohibitively expensive. The fabrication sequence typically involves:
- Base vessel shell fabrication from carbon steel or low-alloy steel (e.g., 16MnR or 18MnMoNbR per GB/T 150).
- Surface preparation of the base metal to a minimum Sa 2.5 cleanliness.
- Single-layer ESW overlay deposition using the selected consumable and process parameters.
- Post-weld heat treatment at 550–650 °C for stress relief.
- Non-destructive testing including ultrasonic testing (UT) for bond strength verification and radiographic testing (RT) for internal defects.
- Hardness testing, chemical analysis, and intergranular corrosion testing of the overlay layer.
The bond strength between the overlay and base metal is verified by ultrasonic testing in accordance with NB/T 47013 or JB/T 4730. The minimum acceptable bond strength is typically 100% bonding area, with no defects exceeding the acceptance criteria. Chemical analysis of the overlay surface is critical to confirm that dilution has not degraded the alloy composition beyond acceptable limits.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Excessive dilution | High heat input, slow travel speed | Reduce heat input, increase travel speed, use higher alloy consumable |
| Hot cracking | Wide solidification range, sulfur/phosphor segregation | Preheat base metal, reduce sulfur content in consumable |
| Cold cracking | Hydrogen embrittlement, high residual stress | Preheat 150–250 °C, low-hydrogen consumable, post-weld bake |
| Poor bond strength | Base surface contamination, insufficient fusion | Improve surface preparation, optimize process parameters |
| Crater cracks | High heat input at weld termination | Use crater fill technique, reduce current at end of weld |
| Undercut | Excessive arc force, improper electrode angle | Adjust electrode angle, reduce current density |
Key Questions and Reflections
The single-layer ESW overlay approach represents a significant engineering compromise between fabrication efficiency and metallurgical quality. While it reduces fabrication time and cost compared to multi-pass SAW overlay, it introduces challenges related to dilution control and microstructural coarseness. The paper's contribution lies in demonstrating that, with proper material selection and process parameter optimization, single-layer ESW overlay can meet the performance requirements for specific service conditions. However, engineers must exercise caution when extrapolating these results to more severe service environments where the margin for dilution is narrower.
The study raises important questions about the long-term durability of single-layer overlays under cyclic thermal and pressure loading. The coarse columnar grain structure, while acceptable for static corrosion resistance, may be susceptible to fatigue cracking at grain boundaries. Further research into the effect of grain refinement techniques on single-layer ESW overlay microstructure would be valuable for extending the technology to more demanding applications.
Study Insights and Implications
This literature provides valuable practical guidance for engineers involved in the fabrication of large-scale pressure vessels requiring corrosion-resistant overlays. The key takeaway is that single-layer ESW overlay is a viable and economical technology, but its successful application depends on rigorous control of dilution, careful material selection, and thorough quality verification. The paper reinforces the principle that process parameter optimization must be tailored to the specific base metal, overlay material, and service environment rather than relying on generic recommendations. Engineers should always conduct qualification welding trials in accordance with NB/T 47014 or ASME IX before implementing single-layer ESW overlay in production, and should maintain detailed records of process parameters and test results for traceability.