Band Electrode Submerged Arc Surfacing Process for 16MnR Tube Sheets

Literature Overview and Background

The technical paper by Hu Wei and Liu Rongjun from the Second Construction Company of China National Petroleum Corporation, published in China Chemical Equipment (2005, Vol. 7, No. 1, pp. 16-19), presents a practical welding procedure for band electrode submerged arc surfacing of 16MnR tube sheets with stainless steel overlay layers. Tube sheets are critical components in heat exchangers and reactors, where they must withstand both mechanical loading from pressure differentials and corrosion from process fluids. The 16MnR grade, a low carbon low alloy steel with good weldability and moderate strength, is commonly used for tube sheet applications in petrochemical and chemical processing industries. This paper addresses the key technical challenges of achieving a corrosion-resistant overlay with appropriate microstructure while controlling welding distortion in a thick, rigid component.

Core Technical Analysis

The primary objectives of the surfacing operation are twofold: to deposit a corrosion-resistant stainless steel overlay with a specific microstructure (austenite plus 3-12% ferrite), and to control welding distortion within acceptable limits for subsequent machining and assembly. These objectives require careful selection of welding materials, process parameters, and welding sequence.

Material Selection and Metallurgical Requirements

The selection of welding consumables is critical for achieving the required overlay properties:

The austenite-ferrite dual-phase microstructure is essential for the overlay's performance. The ferrite phase provides resistance to solidification cracking, while the austenite phase provides corrosion resistance and ductility. The ferrite content must be maintained within the 3-12% range to avoid excessive cracking susceptibility at the low end or intergranular corrosion susceptibility at the high end.

Welding Procedure Development

The welding procedure involves several critical steps:

1. Surface preparation: Thorough cleaning of the tube sheet surface to remove rust, oil, and contaminants. Surface roughening may be applied to improve overlay adhesion.
2. Preheating: Moderate preheating (typically 100-150°C) to reduce thermal stresses and prevent cold cracking in the base metal. Excessive preheating must be avoided to prevent excessive dilution and microstructural softening.
3. Surfacing passes: Multiple passes are typically required to achieve the desired overlay thickness. The first pass may involve a transition layer to minimize dilution effects, followed by subsequent passes of the final overlay material.
4. Interpass temperature control: Maintaining interpass temperature within specified limits (typically below 250°C) to prevent excessive grain growth and softening in the overlay.
5. Post-weld treatment: Depending on the application requirements, post-weld heat treatment may be applied to relieve residual stresses and optimize the microstructure.

Distortion Control

Welding distortion is a significant concern for tube sheets due to their large flat surface area and the thermal stresses induced by the welding process. The paper addresses several distortion control measures:

• Welding sequence optimization: Using a balanced welding sequence that alternates between different regions of the tube sheet to distribute thermal stresses evenly.
• Back-up plate application: Using a copper back-up plate to control the back side of the weld and reduce through-thickness distortion.
• Clamping and restraint: Applying appropriate clamping forces to restrain the tube sheet during welding, although excessive restraint can lead to cracking.
• Symmetric welding: Welding from both sides of the tube sheet in a symmetric pattern to minimize angular and bowing distortion.

Dilution Rate Control

The dilution rate, defined as the percentage of base metal alloyed into the overlay weld metal, is a critical parameter affecting overlay performance. High dilution rates can compromise the corrosion resistance of the overlay by introducing excessive amounts of base metal elements. The following factors influence dilution rate:

Engineering Practice Implications

The successful application of band electrode submerged arc surfacing to 16MnR tube sheets requires careful attention to several practical aspects:

• Equipment capability: The welding equipment must be capable of precise control of current, voltage, and travel speed. Automatic feed mechanisms are essential for maintaining consistent weld bead geometry.
• Flux handling: Flux must be properly dried and stored to prevent moisture-induced defects. The flux composition must be compatible with the stainless steel overlay material to ensure proper slag properties and weld metal chemistry.
• Inspection requirements: Visual inspection, magnetic particle testing, and potentially radiographic testing should be performed to verify overlay quality. Chemical analysis of the overlay surface should confirm the required composition.
• Dimensional control: Post-weld machining is typically required to achieve final dimensional tolerances. Allowance for machining should be included in the overlay thickness specification.

Quality Assurance Considerations

The quality of the overlay weld directly impacts the service life and reliability of the tube sheet. Key quality indicators include:

• Overlay thickness uniformity: Must meet specification requirements with appropriate tolerances
• Microstructural verification: Metallographic examination should confirm the austenite-ferrite balance
• Corrosion resistance testing: Immersion testing or electrochemical testing should verify corrosion resistance in the intended service environment
• Adhesion testing: The overlay must maintain metallurgical bonding with the base metal throughout the service life

Study Insights and Reflections

This paper represents practical engineering knowledge gained from industrial experience in petrochemical equipment manufacturing. The challenges addressed—achieving the required overlay microstructure, controlling dilution, and managing distortion—are common to many surfacing applications and provide valuable lessons for engineers working on similar projects.

The emphasis on the austenite-ferrite dual-phase microstructure highlights the importance of metallurgical understanding in overlay welding. The ferrite content is not merely a metallurgical curiosity but a critical design parameter that directly impacts the overlay's resistance to cracking and corrosion. Engineers must ensure that welding procedures are qualified to produce the required microstructure, and that production welding maintains these conditions through proper parameter control and monitoring.

The distortion control measures described in the paper reflect the practical reality that welding thick, flat components presents unique challenges. Tube sheets are often large, flat components that are sensitive to thermal distortion. The welding sequence and restraint strategies must be carefully planned to minimize distortion while avoiding excessive residual stresses that could lead to cracking.

For contemporary practice, this paper serves as a reminder that fundamental welding metallurgy and process control principles remain relevant even as welding technology advances. While modern equipment may offer more precise control and monitoring capabilities, the underlying metallurgical requirements and engineering considerations remain the same. Engineers should build on the knowledge presented here, supplementing it with modern techniques such as numerical simulation for distortion prediction, advanced monitoring systems for real-time process control, and non-destructive testing methods for comprehensive quality verification.

This comprehensive review of five technical papers spanning from 1991 to 2016 demonstrates the evolution of welding technology and metallurgical understanding in the field of steel pipe, fitting, and overlay welding. Each paper addresses specific technical challenges while contributing to the broader body of knowledge that engineers rely upon in their professional practice. The common thread across all five studies is the recognition that welding quality is determined not only by the welding process itself but also by the careful control of metallurgical factors, process parameters, and quality assurance measures. Engineers who study and apply these principles will be better equipped to develop reliable welding procedures, solve production problems, and ensure the long-term performance of welded components in demanding service environments.