Study Note on Domestic Nickel-Based Welding Strip Single-Layer Cladding Technology for Hot High-Pressure Separator Vessels

Literature Overview

This paper published in China Chemical Equipment (2026, Vol. 28, No. 2, pp. 16-18) by An Tianyou and colleagues from Xinjiang Lanchi Heavy Energy Engineering Co., Ltd. addresses a critical manufacturing challenge: the internal wall cladding of Inconel 625 nickel-based alloy on the shell of a hot high-pressure separator vessel used in a waste lubricating oil hydrotreating regeneration unit. The authors propose and validate a single-layer cladding approach using domestic welding materials—specifically H625 wire (50×0.4 mm) with SJ82B flux—replacing the conventional multi-layer cladding practice. The central claim is that this single-layer technique maintains product quality while substantially improving production efficiency. This work is particularly relevant to engineers working on high-pressure chemical equipment where nickel-based overlay cladding is required for corrosion resistance under aggressive hydrocarbon service conditions.

Core Technical Content

The hot high-pressure separator vessel operates under severe conditions involving high temperature, high pressure, and corrosive media typical of hydrotreating units. The internal surface must be clad with Inconel 625 (UNS N06625) to resist oxidation, sulfidation, and pitting corrosion. Traditionally, achieving a sound nickel-based overlay on carbon or low-alloy steel substrates requires multiple layers: a transition layer to mitigate dilution and prevent cracking, followed by one or more functional layers. This multi-pass approach is time-consuming, increases heat input, and raises the risk of interlayer defects.

The authors' approach consolidates the cladding into a single layer using a submerged arc welding (SAW) process with a domestically produced H625 welding wire (50 mm × 0.4 mm) paired with SJ82B flux. The wire diameter of 50 mm (likely referring to a 5.0 mm diameter or possibly a misprint for 5.0 mm based on standard SAW wire sizes) with 0.4 mm coating or cladding thickness suggests a relatively thin single-pass deposit designed to achieve full coverage in one operation.

Key Technical Parameters and Process Analysis

The single-layer approach eliminates the need for a separate transition layer, which is possible only when the dilution rate from the base metal into the cladding layer is sufficiently controlled. For Inconel 625 cladding on carbon steel, dilution is a well-known concern because carbon and manganese from the base metal can cause brittle intermetallic phases (such as Ni₃Fe, Ni₄Mo, and Fe₂Mo) in the cladding layer, potentially leading to hot cracking or reduced corrosion resistance. The use of SJ82B flux likely serves a dual purpose: providing adequate slag coverage and potentially diluting the base metal contribution through flux chemistry.

Engineering Practice Considerations

Dilution Control

The dilution rate in single-layer SAW cladding is a critical parameter. For a wire diameter of 5.0 mm and a single-pass deposit, typical dilution rates range from 15% to 30% depending on preheating, welding speed, and base metal preparation. The authors must have validated that the resulting cladding composition still meets the minimum nickel and chromium content required for corrosion resistance in the service environment.

Preheating and Interpass Temperature

Preheating of the base metal is essential to reduce residual stress and prevent cold cracking, particularly at the fusion boundary where carbon diffusion from the steel substrate into the nickel alloy can create a hard, brittle zone. Typical preheat temperatures for Inconel 625 cladding on carbon steel range from 150°C to 250°C, depending on the carbon equivalent of the base metal.

Quality Verification

Post-cladding quality verification should include:

• Visual inspection for surface quality and uniformity
• Magnetic particle testing (MT) for surface and near-surface defects
• Penetrant testing (PT) for surface discontinuities
• Hardness testing across the cladding layer and transition zone
• Chemical analysis of the cladding layer to confirm composition within Inconel 625 specification (ASTM B366/B670)
• Corrosion testing per ASTM G27 (sulfidation) or ASTM G48 (pitting) as applicable

Study Insights and Reflections

This paper represents a pragmatic engineering optimization that aligns with the broader trend of reducing manufacturing costs through domestic material substitution and process simplification. The shift from multi-layer to single-layer cladding is not merely an efficiency gain—it also reduces the total heat input, which minimizes thermal distortion of the vessel shell and reduces the risk of hydrogen-induced cracking in the heat-affected zone.

However, from a quality assurance perspective, single-layer cladding demands tighter process control. The margin for error is smaller because there is no subsequent layer to "heal" defects from a previous pass. Engineers implementing this approach should establish a robust process qualification procedure per ASME Section IX or NB/T 47014, including welder performance qualification and procedure qualification with documented dilution control data.

The use of domestic H625 wire and SJ82B flux also raises questions about batch-to-batch consistency of the welding consumables. In my experience, domestic consumables have improved significantly, but certification traceability and incoming inspection protocols remain essential for critical applications.

This work is particularly valuable for Chinese chemical equipment manufacturers seeking to reduce dependence on imported welding consumables while maintaining compliance with international standards such as ASME B31.3, NORSOK M-501, or API 941 for overlay cladding.

Reference Value and Outlook

The study provides a validated pathway for single-layer Inconel 625 cladding on chemical vessel shells, with clear efficiency benefits. Future work should address the long-term corrosion performance of single-layer versus multi-layer cladding under actual service conditions, particularly in hydrotreating environments where chloride-induced stress corrosion cracking (CSCC) and sulfidation resistance are critical concerns. Engineers should also investigate whether the single-layer approach can be extended to other nickel-based alloys such as Hastelloy C-276 or Alloy 625 variants for different service scenarios.