Single-Layer Strip Electrode Electroslag Cladding for Hydrotreating Reactor Manufacturing

Literature Overview and Research Context

The paper by Liu Yan, Ma Xiaobing, Wang Junjie, Li Fufu, An Tianyou, and Wang Xiping, published in Chemical Engineering and Machinery (2020, Vol. 47, No. 3, pp. 393–396), documents the application of single-layer strip electrode electroslag cladding technology for manufacturing hydrotreating reactors. The authors from Xinjiang Lanchi Heavy Energy Engineering Co., Ltd., conducted a comprehensive welding procedure qualification (WPQ) for 12Cr2Mo1R stainless steel cladding on reactor shells and heads, and successfully applied the qualified procedure in production. This work addresses a critical manufacturing challenge in the petroleum refining industry, where large-diameter pressure vessels require corrosion-resistant internal surfaces to withstand hydrogen-containing environments at elevated temperatures and pressures.

Core Technical Analysis

The hydrotreating reactor is one of the most demanding pressure vessels in petroleum refining, operating at temperatures of 350–450°C and pressures of 10–20 MPa in the presence of hydrogen, hydrogen sulfide, and other corrosive species. The internal surface of the reactor must be clad with a corrosion-resistant alloy to protect the base material from hydrogen attack, sulfidation, and stress corrosion cracking. The 12Cr2Mo1R stainless steel, with its 12% chromium and 2% molybdenum composition, provides excellent resistance to these degradation mechanisms while maintaining adequate mechanical properties at elevated temperatures.

The single-layer strip electrode electroslag cladding process is a specialized variant of electroslag welding (ESW) adapted for cladding applications. In this process, a strip electrode (flat wire) is fed into a molten slag pool that forms between the electrode and the substrate. The electrical resistance of the slag generates intense heat, which melts the electrode and a small amount of the substrate, creating a weld pool that solidifies as the slag pool moves. The single-layer approach means that the entire cladding thickness is deposited in one pass, which is efficient for thick cladding layers but requires careful control of process parameters to ensure uniform composition and microstructure throughout the layer thickness.

The welding procedure qualification included evaluation of ferrite content, chemical composition, bend test performance, corrosion resistance, and hydrogen blistering resistance. Each of these tests addresses a specific failure mode that could compromise the integrity of the cladding layer in service.

Process Parameters and Manufacturing Considerations

The single-layer strip electrode electroslag cladding process requires precise control of several parameters: electrode feed rate, travel speed, slag composition, electrical current, and voltage. The strip electrode geometry (width, thickness, and alloy composition) directly influences the heat input and the final weld bead geometry. The slag composition must be optimized to provide adequate fluidity for proper slag pool formation, sufficient deoxidation to prevent inclusions, and appropriate electrical resistance for efficient heat generation.

For the 12Cr2Mo1R cladding alloy, the ferrite content is a critical microstructural parameter. Too little ferrite (below 5%) increases susceptibility to solidification cracking and intergranular corrosion, while too much ferrite (above 20%) reduces corrosion resistance and increases susceptibility to σ-phase precipitation at elevated temperatures. The optimal ferrite content for 12Cr2Mo1R is typically in the range of 5–20%, which provides adequate weldability while maintaining corrosion resistance in the hydrotreating environment.

The hydrogen blistering test is particularly important for hydrotreating reactors, as the high partial pressure of hydrogen at elevated temperatures can cause hydrogen atoms to diffuse into the steel and form molecular hydrogen at internal defects, leading to blister formation and eventual cracking. The cladding layer must be free of internal defects such as inclusions, porosity, and laminations that could serve as hydrogen accumulation sites. The electroslag process, with its deep penetration and slow cooling rates, is generally favorable for producing dense, inclusion-free weld metal, but the process parameters must be carefully controlled to avoid gas porosity from hydrogen or nitrogen absorption.

From a manufacturing perspective, the application of this technology to both reactor shells and heads demonstrates its versatility. The shell cladding is performed on cylindrical surfaces, while the head cladding requires accommodation of the curved geometry of the hemispherical or ellipsoidal head. The process parameters may need adjustment for different surface curvatures to maintain consistent weld bead geometry and composition.

Study Insights and Reflections

This work represents a practical application of electroslag welding technology to a critical industrial application. The single-layer approach offers significant productivity advantages over multi-pass processes, as it eliminates the need for interpass cleaning, flux replacement, and parameter adjustments between passes. For large-diameter reactors with extensive internal cladding areas, the productivity gain translates to substantial cost savings in fabrication.

The comprehensive qualification approach—covering ferrite content, chemistry, mechanical properties, corrosion resistance, and hydrogen resistance—reflects the rigorous quality assurance standards required for pressure vessel manufacturing. Each test addresses a specific failure mode, and the combination provides confidence that the cladding layer will perform reliably throughout the service life of the reactor. The hydrogen blistering test, in particular, is a distinguishing requirement for hydrotreating applications that is not typically required for other cladding applications.

The use of 12Cr2Mo1R as the cladding alloy is well-established in the hydrotreating industry, with extensive field experience confirming its resistance to hydrogen attack and sulfidation. The alloy's composition—12% Cr, 2% Mo, with appropriate balances of Mn, Si, and other elements—provides a favorable combination of corrosion resistance, strength, and weldability. The Mo addition is particularly important for resistance to chloride-induced stress corrosion cracking, which can be a concern in some hydrotreating environments where trace chlorides are present.

From a standards perspective, this work would be evaluated against applicable codes including ASME Section VIII Division 1 for pressure vessel construction, which includes requirements for cladding and overlay welding. The relevant welding procedure qualification would follow ASME Section IX, and the material specifications would reference ASTM A213 for tubing or A312 for pipe if applicable to the reactor construction. National standards such as GB/T 150 for pressure vessels and NB/T standards for nuclear-related equipment may also be applicable depending on the jurisdiction.

This literature demonstrates the successful integration of advanced welding technology with rigorous qualification procedures to meet the demanding requirements of hydrotreating reactor manufacturing, and the single-layer strip electrode electroslag cladding process offers a viable solution for producing high-quality, corrosion-resistant internal surfaces on large-diameter pressure vessels.