CLADDING TECHNOLOGY SHANXI CO., LTDResearch on the Welding Process for Tube Sheet Cladding
Literature Overview
This 2010 paper by Wang Hengxiang from Luoyang Longhui Petrochemical Engineering Co., Ltd., published in Henan Chemical Industry, focuses on the development and optimization of welding procedures for overlay cladding on heat exchanger tube sheets in the petrochemical industry. Tube sheets in petrochemical service are often subjected to corrosive media such as sour gas (containing H2S), amines, and hydrocarbons, necessitating the application of corrosion-resistant overlay layers. The paper addresses process selection, parameter optimization, and quality assurance for tube sheet cladding, drawing on practical fabrication experience in the petrochemical sector.
Process Selection and Parameter Optimization
The authors evaluate several cladding processes applicable to tube sheets, including submerged arc welding (SAW), gas metal arc welding (GMAW), and flux-cored arc welding (FCAW). The selection depends on the required overlay thickness, production rate, and the geometry of the tube sheet. SAW is preferred for thick overlays (> 3 mm) due to its high deposition rate, while GMAW offers better flexibility for thin overlays and complex geometries.
The following table presents the process comparison:
| Process | Deposition Rate | Flexibility | Equipment Cost | Typical Application |
|---|---|---|---|---|
| SAW | High | Low | Medium | Thick overlays, flat surfaces |
| GMAW | Medium | High | Medium | Thin overlays, complex geometry |
| FCAW | High | Medium | Medium | Thick overlays, outdoor work |
| Oxy-fuel | Low | High | Low | Thin overlays, field repair |
The optimization of welding parameters is critical to achieving a sound overlay with good bond strength and minimum dilution. The authors recommend a multi-pass approach with a root pass using a lower current to ensure good fusion with the base metal, followed by fill passes with higher current for deposition efficiency, and a cap pass with controlled parameters to achieve a smooth, defect-free surface.
Quality Control and Defect Prevention
The paper emphasizes the importance of pre-weld preparation and in-process monitoring. The base metal surface must be cleaned to a near-white metal finish using grinding or wire brushing to remove scale, rust, and contaminants. Any porosity or lack of fusion in the root pass can compromise the bond strength of the entire overlay. The authors recommend the following quality control measures:
- Visual inspection after each pass to detect surface defects such as cracks, porosity, and undercuts.
- Magnetic particle testing (MT) of the root pass to detect subsurface cracks and lack of fusion.
- Ultrasonic testing (UT) of the final overlay to verify bond integrity and detect internal defects.
- Hardness testing across the overlay thickness to ensure uniform microstructure and detect excessive base metal dilution.
- Intergranular corrosion testing (ASTM A923 Practice E) for stainless steel overlays to verify adequate carbon content and proper heat treatment.
Engineering Practice Considerations
In petrochemical applications, tube sheets are often fabricated from 16MnR or Q345R carbon steel with a 316L or 321 stainless steel overlay. The weld procedure qualification (WPQ) must comply with NB/T 47014 and ASME IX, and the welding operator must be certified for the specific process, material, and position. The interpass temperature is a critical parameter: if it exceeds 200 °C, the risk of sensitization in the stainless steel overlay increases, leading to intergranular corrosion susceptibility. Conversely, if it is too low, the HAZ of the base metal may experience excessive hardening and cracking.
I have found in practice that the tube hole drilling and reaming must be performed after the overlay welding is complete and stress-relieved. Drilling through a stressed overlay can initiate cracks that propagate into the overlay layer. If drilling must be performed before cladding (as is sometimes done for assembly purposes), the holes must be carefully inspected for damage after the overlay is applied.
Key Reflections
This paper provides a solid foundation for understanding the practical aspects of tube sheet cladding in the petrochemical industry. The emphasis on process qualification and quality control is particularly valuable, as it aligns with the rigorous regulatory requirements of the petrochemical sector. One area that could be further explored is the use of laser cladding or plasma transferred arc (PTA) welding for tube sheet overlay, which offer lower dilution rates and better microstructural control. These advanced processes are increasingly used in high-value applications where overlay quality is critical.
In conclusion, this publication offers practical guidance on welding process selection, parameter optimization, and quality assurance for tube sheet cladding in petrochemical service, and its recommendations remain relevant for engineers designing and fabricating heat exchangers in corrosive environments.