Zhuojin Pipe Fitting Co., LtdStainless Steel Strip Electrode SAW and Electroslag Overlay Welding on Hydrogenation Reactors
Literature Overview
This paper by Zhou Weiyu and Fan Zheng (1990), published in Boiler Technology (Vol. 21, No. 11, pp. 14-17), presents experimental results on overlay welding of stainless steel on 2¼Cr-1Mo pressure vessel steel for hydrogenation reactors in the petrochemical industry. The authors, affiliated with Sandvik, investigated single-layer and double-layer strip electrode submerged arc welding (SAW) using Sandvik 24.13.LNb and a combination of Sandvik 24.13.LNb with 19.9.LNb, together with sintered Sandvik flux 10SW.
Technical Context
Hydrogenation reactors in petrochemical processing are subjected to extremely harsh conditions: high temperature (up to 450°C), high pressure (up to 30 MPa), and hydrogen-containing environments. These conditions necessitate corrosion-resistant overlay layers on the internal surfaces of reactor shells and heads. The base material, 2¼Cr-1Mo steel (ASTM A387 Grade 22 or equivalent), provides the necessary high-temperature strength, while the stainless steel overlay protects against hydrogen attack and sulfidation corrosion.
Material Specifications
| Material | Composition | Application |
|---|---|---|
| Sandvik 24.13.LNb | 347H type (18Cr-12Ni-0.25Ti-0.07Nb) | Single-layer overlay |
| Sandvik 19.9.LNb | 316L type (16-18Cr-10-12Ni-0.07Nb) | Second layer in double-layer scheme |
| Sandvik flux 10SW | Sintered flux, low sulfur/phosphorus | SAW flux for both strip electrode and electrode |
| Base material | 2¼Cr-1Mo (A387 Gr. 22) | Pressure vessel shell |
The use of niobium-stabilized stainless steels (24.13.LNb and 19.9.LNb) is significant. Niobium stabilization prevents intergranular chromium carbide precipitation during welding and subsequent heat exposure, which is critical for maintaining corrosion resistance in the HAZ and weld metal.
Process Analysis
Strip Electrode Submerged Arc Welding (SAW)
Strip electrode SAW is a high-deposition-rate process well-suited for thick overlay layers on large pressure vessel components. The process offers excellent weld quality, consistent dilution control, and high productivity.
| Parameter | Single Layer (24.13.LNb) | Double Layer (24.13.LNb + 19.9.LNb) |
|---|---|---|
| Strip thickness | 3-4 mm | 3-4 mm (1st), 2-3 mm (2nd) |
| Current | 600-800 A | 600-700 A (1st), 400-500 A (2nd) |
| Voltage | 28-32 V | 28-30 V (1st), 26-28 V (2nd) |
| Travel speed | 200-300 mm/min | 200-250 mm/min (1st), 250-350 mm/min (2nd) |
| Flux coverage | 5-8 mm | 5-8 mm |
| Preheating | 100-150°C | 100-150°C |
| Interpass temp | ≤ 250°C | ≤ 250°C |
Electroslag Overlay Welding
Electroslag welding (ESW) provides even higher deposition rates than SAW and is suitable for very thick overlay builds. The electroslag process operates at higher temperatures with slower cooling rates, which can be advantageous for reducing residual stresses but requires careful control to avoid excessive grain growth.
Key advantages of ESW for overlay welding:
- Extremely high deposition rate (up to 20-30 kg/h)
- Excellent mechanical properties due to slow cooling
- Reduced residual stresses compared to SAW
- Suitable for thick single-pass deposits (15-25 mm)
Dilution Control
One of the most critical aspects of overlay welding on alloy steel substrates is controlling dilution—the mixing of base metal into the overlay layer. Excessive dilution reduces the corrosion resistance of the overlay by introducing chromium-depleted zones.
| Layer Configuration | Dilution (1st layer) | Dilution (2nd layer) | Effective Cr in overlay |
|---|---|---|---|
| Single layer (24.13.LNb) | 15-25% | N/A | ~14-16% |
| Double layer (24.13.LNb + 19.9.LNb) | 15-25% | 5-10% | ~16-18% (surface) |
The double-layer approach is superior for corrosion-critical applications because the second layer experiences lower dilution from the first layer (which already contains significant chromium), resulting in a more corrosion-resistant surface.
Engineering Considerations
Hydrogen Attack Mitigation
In hydrogenation service, the overlay layer must resist both hydrogen blistering and high-temperature hydrogen attack (HTHA). The 347H-type overlay (24.13.LNb) provides adequate protection against sulfidation corrosion, while the 316L-type second layer (19.9.LNb) offers enhanced resistance to pitting and crevice corrosion in chloride-containing environments.
Post-Weld Treatment
Pressure vessel overlay welds typically require post-weld heat treatment (PWHT) at 750-780°C for 2-4 hours per 25 mm of wall thickness. This serves multiple purposes:
- Stress relief of the overlay welds
- Tempering of any martensitic phases in the 2¼Cr-1Mo HAZ
- Homogenization of the overlay microstructure
The challenge is that the overlay layer is not designed for high-temperature exposure, and excessive PWHT temperatures can cause sensitization. The niobium stabilization in 24.13.LNb and 19.9.LNb mitigates this risk, but PWHT parameters must be carefully controlled.
Study Reflections
This paper demonstrates the systematic approach to overlay welding qualification for critical pressure vessel applications. The Sandvik material system (24.13.LNb with 10SW flux) represents a mature, well-characterized overlay welding solution that has been proven in industrial service.
The double-layer overlay strategy is particularly instructive. From a metallurgical perspective, the first layer serves as a transition zone that accommodates the thermal expansion mismatch between the ferritic base material and the austenitic overlay, while the second layer provides the ultimate corrosion resistance. This layered approach balances metallurgical compatibility with performance requirements.
For modern engineering practice, this work remains relevant as the fundamental principles of dilution control, layer design, and PWHT management continue to apply to current overlay welding operations in the petrochemical and power generation industries. The selection of niobium-stabilized consumables also reflects an early understanding of sensitization prevention that remains valid today.