Single-Layer Cladding Trial Study on the Inner Wall of Hydrogenation Equipment

Literature Overview

This study, published in 2009 in the journal Pipeline Technology and Equipment, was conducted by Song Lifeng from Sany Heavy Equipment Co., Ltd., Wang Lili from Shenyang Luzheng Cold and Hot Equipment Co., Ltd., and Liu Wei from the Shenyang Safety Engineering Research and Training Center. The work focuses on the practical application of single-layer weld overlay cladding on the internal wall of hydrogenation reactors — a critical class of pressure vessels operating under severe conditions of high temperature, high hydrogen partial pressure, and corrosive media.

Core Technical Context

Hydrogenation reactors in the petroleum refining and chemical industries are typically fabricated from carbon steel or low-alloy steel (such as 15CrMoR or 12Cr1MoV) for their excellent mechanical properties at elevated temperatures. However, the internal surfaces are exposed to hydrogen-containing gases, aqueous amine solutions, and acidic species at temperatures ranging from 300 °C to 450 °C and pressures up to 15 MPa. The base material alone cannot provide adequate resistance to hydrogen damage mechanisms including hydrogen blistering, hydrogen-induced cracking (HIC), and sulfide stress cracking (SSC). A protective overlay layer — typically austenitic stainless steel such as 304L, 316L, or a nickel-based alloy — is therefore applied to the internal wall.

The study specifically investigates the feasibility and quality control of a single-layer cladding scheme, which is a cost-effective alternative to the more common multi-layer (two or three layer) approach.

Key Technical Points

Single-Layer vs. Multi-Layer Cladding Scheme

The primary engineering trade-off addressed in this study is between economy and metallurgical quality. Multi-layer cladding (typically a transition layer of 2507 or 309L followed by a corrosion-resistant layer of 304L or 316L) provides better dilution control and reduced cracking susceptibility. However, single-layer cladding reduces material consumption by approximately 40-50% and shortens fabrication cycle time significantly.

Process Parameters and Welding Method Selection

For hydrogenation reactor internal wall cladding, the following welding methods are commonly employed:

• Submerged Arc Welding (SAW): High deposition rate, suitable for thick overlay layers on large-diameter vessels. Wire diameter typically 1.6–2.4 mm, current 350–550 A, voltage 28–36 V, travel speed 200–400 mm/min.
• Flux-Cored Arc Welding (FCAW): Good for field application and repair, wire diameter 1.2–1.6 mm.
• Gas Metal Arc Welding (GMAW): Versatile for both transition and overlay layers, particularly for thinner single-layer applications.

The study emphasizes that for single-layer cladding, the welding consumable selection is critical. A low-dilution consumable — such as a duplex 2205-based wire or a high-nickel 316L wire with reduced carbon content — is recommended to minimize the dilution effect from the carbon steel substrate while maintaining adequate toughness in the overlay layer.

Preheat and Interpass Temperature Control

Preheating of the base metal is essential to prevent cold cracking in the overlay weld metal. For carbon steel substrates with carbon equivalent (CE) values above 0.45%, a preheat temperature of 150–250 °C is recommended. The interpass temperature should be maintained below 250 °C to avoid excessive grain growth in the overlay layer.

Quality Control and Inspection

The following non-destructive testing (NDT) methods are applicable for quality assurance of the cladding layer:

Engineering Practice Reflections

From a practical standpoint, the single-layer cladding approach presents several challenges that must be carefully managed:

1. Dilution control: The dilution ratio is the single most important factor determining the corrosion and hydrogen resistance of the overlay layer. With single-layer cladding, the dilution can reach 30–40%, which may push the overlay composition into a region where martensite formation occurs, compromising corrosion resistance.
2. Bond strength: The metallurgical bond between the overlay layer and the base metal must be verified. A common failure mode is interfacial decohesion, particularly in the presence of hydrogen. The bond strength test per ASTM A263 requires that at least 75% of the tensile test specimen fracture in the base metal rather than at the interface.
3. Residual stress management: The residual stresses in the overlay layer, combined with the hydrogen environment, can promote delayed cracking. Post-weld heat treatment (PWHT) or stress-relief annealing may be necessary, though this must be carefully controlled to avoid sensitization of the overlay layer.

Study Insights and Implications

This study provides valuable engineering data for situations where economic constraints make multi-layer cladding impractical. However, the engineer must exercise caution: single-layer cladding should only be employed when the service conditions are not at the most severe end of the hydrogenation spectrum. For reactors operating above 400 °C with hydrogen partial pressures exceeding 6.9 MPa, a multi-layer approach with a dedicated transition layer is strongly recommended. The study's contribution lies in establishing practical process windows and quality criteria that enable a defensible single-layer cladding solution for moderately severe service conditions.