Study Note on 15CrMo Tube Sheet Surfacing Process

Literature Overview

The paper by Zou Yuqing and Zhang Shu, published in Hot Working Technology in 2015 (Vol. 44, No. 11, pp. 231-232), investigates the surfacing process for corrosion-resistant overlay on 15CrMo tube sheets. Authored by researchers from Jilin Electronic Information Vocational and Technical College and the Jilin Machinery Manufacturing Branch of CNPC Northeast Refining and Chemical Engineering Co., Ltd., this work addresses a critical challenge in pressure vessel and heat exchanger manufacturing: providing corrosion resistance on alloy steel tube sheets that must also maintain structural integrity.

The study is classified under TG455 and focuses on strip electrode surfacing (带极堆焊) as the primary process. The 15CrMo base material is a chromium-molybdenum alloy steel widely used in high-temperature and high-pressure applications in the petrochemical and power generation industries.

Core Technical Points

15CrMo Base Material Characteristics

15CrMo is a low-alloy steel containing approximately 0.15% carbon, 0.9-1.2% chromium, and 0.25-0.30% molybdenum. Its properties make it suitable for elevated temperature service but also present challenges for surfacing operations:

Strip Electrode Surfacing Process

Strip electrode surfacing (also known as submerged arc surfacing with strip electrode) is a highly productive process suitable for large-area overlay applications. The process uses a continuous strip of filler metal as the electrode, submerged under a layer of flux, with the arc traveling along the strip to deposit uniform overlay layers.

Surfacing Process Development

Process Development Methodology

The authors employed a systematic approach to develop the surfacing process, following a PDCA-type methodology:

1. Plan: Define surfacing objectives, select filler material, and establish process parameters based on material compatibility
2. Do: Conduct trial surfacing operations with varying parameter combinations
3. Check: Evaluate overlay properties through hardness testing, metallographic examination, and mechanical testing
4. Act: Optimize parameters based on evaluation results and establish the final process specification

Filler Material Selection

The selection of filler material for 15CrMo tube sheet surfacing must consider:

• Corrosion resistance requirements of the service environment
• Compatibility with the 15CrMo base material
• Weldability and susceptibility to cracking
• Mechanical property requirements for the overlay layer

Common filler materials for corrosion-resistant surfacing on 15CrMo include:

Process Parameters Optimization

The optimization of strip electrode surfacing parameters involves balancing competing requirements:

• Heat input: Higher heat input increases productivity but increases dilution and may cause base material degradation
• Travel speed: Higher speed reduces heat input but may lead to incomplete fusion
• Arc voltage: Higher voltage increases arc width but may reduce penetration
• Strip thickness: Thicker strips deposit more material per pass but require higher current

Quality Control and Inspection

Inspection Requirements

Quality control for 15CrMo tube sheet surfacing includes:

Defect Analysis

Engineering Practice Integration

Application in Heat Exchangers

15CrMo tube sheets are widely used in heat exchangers operating at elevated temperatures and pressures in petrochemical and power generation applications. Corrosion-resistant surfacing extends the service life of tube sheets exposed to corrosive process fluids while maintaining the structural integrity required for pressure containment.

Manufacturing Considerations

For production applications, the following considerations are important:

1. Surface preparation: Tube sheet surfaces must be machined to bare metal, free of scale, oil, and contaminants
2. Preheating: Uniform preheating to 150-250°C is required to prevent cold cracking in the 15CrMo base material
3. Interpass temperature control: Maintain interpass temperature below 250°C to prevent excessive grain growth
4. Post-weld stress relief: Stress relief annealing at 620-680°C is recommended to reduce residual stresses
5. Dimensional control: Account for overlay thickness in final dimensional tolerances

Cost-Benefit Analysis

Key Questions and Reflections

Several technical questions arise from this study:

1. How does the surfacing overlay perform under thermal cycling conditions typical of heat exchanger operation?
2. What is the maximum number of surfacing passes achievable before the accumulated heat input degrades the base material properties?
3. How does the dilution profile vary with tube sheet thickness and geometry?

The study demonstrates that strip electrode surfacing is a viable and economical approach for providing corrosion resistance on 15CrMo tube sheets, offering a practical alternative to complete component replacement.

Study Insights and Implications

The Zou and Zhang study exemplifies the systematic approach to welding process development, applying engineering methodology to optimize surfacing parameters for a specific application. The integration of academic research with industrial practice—evidenced by the collaboration between a technical college and a manufacturing company—provides a model for technology transfer in welding engineering.

The emphasis on strip electrode surfacing highlights the importance of process selection based on production requirements. For large-area overlay applications such as tube sheets, strip electrode surfacing offers superior productivity compared to manual or mechanized arc welding processes, while maintaining acceptable quality levels.

In conclusion, this study provides a validated process for corrosion-resistant surfacing of 15CrMo tube sheets, offering engineers a practical solution for extending the service life of critical pressure vessel components in demanding industrial environments.