Stainless Steel Strip Submerged Arc Cladding and Electroslag Cladding of Hydrogenation Reactors

Literature Overview

This 1990 technical paper by Zhou Weiyu and Fan Zheng, published in a boiler technology journal, documents the application of stainless steel strip submerged arc welding (SAW) and electroslag welding (ESW) cladding for hydrogenation reactors. The authors' affiliation with Sandvik Company is particularly significant: Sandvik was one of the world's leading manufacturers of strip cladding systems and consumables, and this paper represents the transfer of Sandvik's proprietary technology to Chinese pressure vessel fabrication. The classification under "boiler technology" reflects the historical categorization of pressure vessel and heat exchanger technology under the broader umbrella of boiler engineering.

Core Technical Context

Hydrogenation reactors are among the most demanding pressure vessels in the petrochemical industry. They operate at high pressures (typically 10-30 MPa) and elevated temperatures (300-450 °C) in a hydrogen-rich environment. The combination of high hydrogen partial pressure and elevated temperature leads to hydrogen damage mechanisms including:

• Hydrogen blistering: Hydrogen atoms diffuse into the steel and accumulate at inclusions and microvoids, forming blisters.
• Hydrogen-induced cracking (HIC): Blister growth and coalescence lead to crack formation.
• Sulfide stress cracking (SSC): In the presence of H2S, hydrogen-induced cracking can occur even at lower temperatures.
• Tempering embrittlement: Prolonged exposure at elevated temperatures can cause embrittlement of the base metal.

To protect against these hydrogen damage mechanisms, the inner surface of the reactor must be clad with a corrosion-resistant and hydrogen-resistant material, typically a stainless steel such as 304L, 316L, 321, or 347, or a nickel-based alloy such as Inconel 625.

Cladding Process Comparison

Process Details and Technical Points

Strip Submerged Arc Welding (Strip SAW)

Strip SAW uses a stainless steel strip (typically 0.3-1.5 mm thick, 50-100 mm wide) as the filler material, fed through a submerged arc welding torch. The flux covers the weld pool, and the strip melts into the base metal. The key advantages of strip SAW include:

• High deposition rate compared to wire SAW
• Good surface finish of the overlay
• Ability to produce uniform overlay thickness
• Relatively low equipment cost

The process parameters for strip SAW cladding of hydrogenation reactors typically include:

• Current: 400-700 A
• Voltage: 25-35 V
• Travel speed: 100-300 mm/min
• Strip width: 50-100 mm
• Strip thickness: 0.5-1.0 mm
• Flux: Special low-hydrogen flux for stainless steel overlay
• Preheat: 150-250 °C
• Interpass temperature: <250 °C

Electroslag Welding (ESW) Cladding

ESW cladding uses a consumable electrode (typically a stainless steel rod or strip) and a flux that melts to form a slag pool. The heat is generated by the electrical resistance of the slag, which is more uniform and intense than an arc. The key advantages of ESW cladding include:

• Very high deposition rate
• Excellent metallurgical quality (slow cooling rate promotes grain refinement)
• Ability to clad thick sections in a single pass
• Excellent surface finish

The process parameters for ESW cladding typically include:

• Current: 500-1500 A
• Voltage: 30-50 V
• Travel speed: 50-200 mm/min
• Electrode diameter: 10-25 mm
• Flux: Special high-basicity flux for stainless steel
• Preheat: 200-350 °C
• Interpass temperature: <350 °C
• Post-weld heat treatment: Essential for stress relief

Metallurgical Considerations for Hydrogenation Reactor Cladding

The cladding of hydrogenation reactors requires special attention to metallurgical issues:

1. Base metal: Typically a low-alloy steel such as 15CrMo, 12Cr1MoV, or 1.25Cr-0.5Mo. These steels have good high-temperature strength but limited hydrogen resistance.
2. Overlay material: 304L, 316L, 321, or 347 stainless steel. The choice depends on the specific hydrogen damage mechanism and the operating conditions.
3. Dilution: The dilution rate in the first pass must be controlled to ensure that the overlay meets the required corrosion resistance. A dilution rate above 30% can significantly reduce the corrosion resistance of the overlay.
4. Bond strength: The bond between the base metal and the overlay must be strong enough to resist the mechanical and thermal stresses during operation. The bond strength is typically verified by a peel test or a tensile test according to NB/T 47014.
5. Microstructure: The overlay microstructure should be austenitic or austenitic-ferritic, with a grain size that promotes corrosion resistance. Excessive grain growth can reduce corrosion resistance.
6. Residual stress: The residual stress in the cladding weld can cause distortion and cracking. Post-weld heat treatment (PWHT) is essential to relieve residual stress.

Quality Control and Inspection

The quality of the cladding weld in a hydrogenation reactor is critical to the safety and performance of the vessel. The following inspections are typically required:

Engineering Practice and Case Study

The Sandvik technology for strip SAW and ESW cladding was introduced to China in the late 1980s and early 1990s, coinciding with the construction of several large-scale hydrogenation reactors for the petrochemical industry. The typical application involved:

• Reactor shell: Carbon steel or low-alloy steel shell with a stainless steel cladding layer on the inner surface.
• Cladding thickness: 3-6 mm (strip SAW) or 5-10 mm (ESW).
• Cladding material: 304L or 316L stainless steel.
• Welding sequence: The cladding is applied after the shell is fabricated and welded, typically after the PWHT of the base metal welds.
• Inspection: Dilution rate, bond strength, UT, MT, and PT are performed after each cladding pass.

Typical Welding Procedure Specification (WPS) Parameters

Key Questions and Reflections

Several important questions arise from studying this topic:

1. How does the choice between strip SAW and ESW affect the overall cost and quality of the cladding? Strip SAW is generally less expensive and more versatile, while ESW offers higher deposition rates and better metallurgical quality for thick sections. The choice depends on the specific application, the required cladding thickness, and the available equipment.
2. What is the impact of the welding sequence on the distortion and residual stress of the reactor shell? The cladding welding sequence must be carefully planned to minimize distortion. A symmetric welding sequence, welding from the top of the shell downward or in a spiral pattern, can minimize distortion.
3. How does the PWHT affect the cladding weld quality? PWHT is essential to relieve residual stress and improve the toughness of the overlay. However, excessive PWHT temperature or time can cause grain growth and reduce the corrosion resistance of the overlay. The PWHT parameters must be carefully controlled.

Study Insights and Implications

This 1990 paper represents a significant milestone in the development of hydrogenation reactor cladding technology in China. The introduction of Sandvik's strip SAW and ESW cladding technology provided Chinese pressure vessel fabricators with access to world-class cladding processes and consumables. The paper's publication in a boiler technology journal reflects the interdisciplinary nature of pressure vessel fabrication, which draws on welding, materials science, mechanical engineering, and chemical engineering.

For today's engineers, the lessons from this work are that the cladding of hydrogenation reactors is a highly specialized application that requires careful process selection, metallurgical understanding, and thorough quality control. The principles of low dilution, controlled heat input, and post-weld stress relief remain as relevant today as they were in 1990. The continued development of advanced cladding processes such as PTA and laser cladding has expanded the options available to engineers, but the fundamental challenges of hydrogen damage, dilution control, and metallurgical bonding remain unchanged.