Single-Layer Overlay Welding Technology with Domestic Nickel-Based Welding Strips

Literature Overview

This paper, published in China Chemical Equipment (2026, Vol. 28, No. 2, pp. 16-18) by An Tianyou and colleagues from Xinjiang Lanshi Heavy Equipment Energy Engineering Co., Ltd., reports on the application of domestic welding materials for single-layer overlay welding of Inconel 625 nickel-based alloy on the inner wall of a hot high-pressure separator drum in a waste lubricating oil hydrocracking regeneration unit. The study demonstrates that using domestic H625 (50×0.4 mm) and SJ82B welding strips in a single-layer overlay configuration can achieve acceptable quality while significantly improving production efficiency compared to conventional multi-layer overlay approaches.

Technical Background and Engineering Challenge

The hot high-pressure separator drum in a waste lubricating oil hydrocracking regeneration unit operates under conditions of high temperature, high pressure, and corrosive hydrocarbon environments. The inner wall requires overlay welding with a nickel-based alloy such as Inconel 625 to provide corrosion resistance against acidic and sulfidic species present in the process stream. Conventionally, multi-layer overlay welding is employed to ensure adequate alloy content in the final surface layer and to mitigate dilution from the carbon steel base material. However, multi-layer overlay welding is time-consuming, labor-intensive, and costly—particularly for large-diameter vessels where the overlay area can be extensive.

The engineering challenge is to reduce the number of overlay layers from the conventional multi-layer approach (typically 2-3 layers) to a single layer while maintaining adequate corrosion resistance and mechanical integrity. This requires careful selection of welding materials and process parameters to ensure that the single layer achieves sufficient nickel, chromium, and molybdenum content despite dilution from the base metal.

Welding Materials and Process Configuration

The use of domestic welding strips (H625 and SJ82B) represents a strategic shift from imported welding consumables, which have historically been the standard for nickel-based alloy overlay welding in China. The domestic materials are designed to mimic the composition and welding characteristics of imported Inconel 625 consumables while offering cost advantages and supply chain security. The 50×0.4 mm strip dimension is a relatively thick single-layer configuration, which is critical for achieving adequate alloy deposition in a single pass.

Single-Layer Overlay Technique and Quality Assurance

Single-layer overlay welding presents several technical challenges that must be addressed to ensure quality. The primary concern is dilution: in a single-layer configuration, the molten weld metal is in direct contact with the carbon steel base metal, resulting in significant dilution that reduces the nickel, chromium, and molybdenum content in the final overlay layer. If dilution is excessive, the corrosion resistance of the overlay may be compromised, particularly in aggressive environments where high nickel and chromium content are essential for passive film stability.

To mitigate dilution, the welding process must be carefully controlled. Key parameters include heat input, travel speed, and electrode/strip positioning. Lower heat input reduces the volume of base metal melted, thereby limiting dilution. However, excessively low heat input may result in poor wetting and incomplete fusion. The optimal heat input window must be determined through trial welding and microchemical analysis of the overlay layer. The use of SJ82B as a complementary welding material may provide additional alloying elements or process advantages that help compensate for dilution effects.

The quality assurance program for single-layer overlay welding must include:

1. Visual inspection of the overlay surface for uniformity, porosity, and undercut.
2. Hardness testing to verify that the overlay hardness is within the expected range for Inconel 625 (typically 200-250 HV).
3. Chemical analysis of the overlay layer to confirm adequate Ni, Cr, and Mo content.
4. Non-destructive testing (PT or MT) to detect surface and near-surface defects.
5. Corrosion testing (potentiodynamic polarization or immersion testing) to verify corrosion resistance performance.

Production Efficiency Implications

The primary benefit reported in this study is the significant improvement in production efficiency. Multi-layer overlay welding of large-diameter vessel inner walls can require hundreds of welding hours, with each layer requiring careful preparation, welding, and inspection. Reducing this to a single layer can reduce welding time by 50-70%, which translates directly to reduced fabrication cost and shorter delivery schedules. In the context of large-scale chemical equipment manufacturing, where project schedules are tight and cost competitiveness is critical, this efficiency gain is substantial.

However, engineers must be cautious about over-generalizing this result. The applicability of single-layer overlay welding depends on the specific application conditions, including the severity of the corrosive environment, the operating temperature and pressure, and the required design life. For highly aggressive environments or critical safety applications, the margin provided by multi-layer overlay welding may be necessary to ensure long-term reliability. The single-layer approach should be validated through comprehensive qualification testing that includes corrosion testing under simulated service conditions.

Study Insights and Practical Recommendations

This study contributes to the growing trend of domestic substitution of imported welding consumables in China's chemical equipment manufacturing industry. The successful application of domestic H625 and SJ82B welding strips demonstrates that domestic materials can achieve quality comparable to imported alternatives, provided that proper process control and quality assurance measures are implemented. Engineers should approach domestic material qualification systematically, following established qualification procedures that include weld procedure qualification (WPQ), welder performance qualification (WPQ), and production weld testing.

The single-layer overlay approach represents a pragmatic engineering solution that balances quality requirements with production efficiency. It is particularly suitable for applications where the corrosive environment is moderate to severe but not extreme, and where the operating conditions do not demand the maximum corrosion resistance that multi-layer overlay welding provides. For engineers evaluating this technique for their own applications, a thorough risk assessment should be conducted, considering the consequences of overlay failure in the specific service environment.