Zhuojin Pipe Fitting Co., LtdDelamination of Overlay Layer on Thick Tube Sheet and Preventive Measures
Literature Overview
This paper by Chen Xiaochang, published in Welding Technology (2009, Vol. 38, Issue 12, pp. 70-71), addresses a critical quality issue in the fabrication of heat exchanger tube sheets: the delamination or spalling of the overlay weld layer on thick tube sheets. The author, representing Changsha Weizhong Chemical Machinery Co., Ltd., conducted a detailed investigation involving chemical composition analysis and stress analysis at the interface between the base material and the transition layer to identify the root causes of delamination and propose preventive measures.
Technical Context
Tube sheets are integral components of heat exchangers, and in many applications, they require overlay welding to provide corrosion resistance to the tube-side or shell-side fluid. The overlay typically consists of a multi-layer system: a base layer (directly on the substrate), a transition layer (to manage dilution and metallurgical compatibility), and a final overlay layer (providing the required corrosion or wear resistance). On thick tube sheets, the thermal mass of the base material creates unique challenges for the welding process.
The delamination of the overlay layer is a serious defect because it compromises the integrity of the corrosion barrier and can lead to premature failure of the heat exchanger. Unlike surface cracks that may be detectable and repairable, delamination is a subsurface defect that is difficult to detect and challenging to repair.
Root Cause Analysis
The author's investigation focused on the chemical composition and stress state at the interface between the base material and the transition layer. The analysis revealed that delamination was caused by a combination of factors:
- Chemical incompatibility at the interface: The transition layer material was not properly selected to bridge the metallurgical gap between the base material and the overlay material. Incompatibility in terms of dilution behavior, thermal expansion coefficient, and phase stability can lead to the formation of brittle phases or weak interfaces.
- Residual stress concentration: Thick tube sheets have high thermal mass, which affects the cooling rate and residual stress distribution during welding. The thermal mismatch between the overlay and the thick base material can generate significant residual stresses at the interface. If these stresses exceed the interfacial bond strength, delamination occurs.
- Insufficient heat input or improper preheating: Thick sections require adequate preheating to ensure uniform temperature distribution and to slow the cooling rate. Without proper preheating, the cooling rate can be too high, leading to hard and brittle microstructures at the interface that are susceptible to cracking and delamination.
Preventive Measures
The author proposed several measures to prevent delamination:
| Measure | Description | Rationale |
|---|---|---|
| Proper transition layer material selection | Choose transition layer wire with composition that ensures good metallurgical compatibility | Minimizes dilution effects and brittle phase formation |
| Controlled welding parameters | Optimize heat input, travel speed, and arc voltage | Ensures adequate melting and proper microstructure |
| Adequate preheating | Preheat thick tube sheets to appropriate temperature | Reduces cooling rate and thermal stress |
| Controlled interpass temperature | Maintain interpass temperature within specified range | Prevents overheating and underheating between passes |
| Post-weld stress relief | Apply PWHT to relieve residual stresses | Reduces risk of stress-induced delamination |
Engineering Practice Considerations
The case of thick tube sheet overlay welding presents several challenges that are not commonly encountered in thinner section welding:
- Thermal mass effect: Thick tube sheets act as heat sinks, drawing heat away from the weld zone and potentially causing rapid cooling. This can lead to martensitic transformation in the weld metal and HAZ, resulting in hard, brittle microstructures.
- Residual stress accumulation: Each weld pass adds to the residual stress field. In thick sections, the stress distribution is complex and can lead to high stress concentrations at the weld interface.
- Inspection challenges: Delamination defects are difficult to detect by conventional NDT methods. Ultrasonic testing may be effective but requires careful technique and skilled operators. Radiographic testing is generally ineffective for detecting planar delamination defects.
Study Insights and Reflections
This paper, while brief, addresses a practically important problem that can have significant consequences in the heat exchanger industry. The approach of combining chemical composition analysis with stress analysis is a sound engineering methodology that goes beyond simple defect observation to identify the underlying physical mechanisms. The emphasis on transition layer material selection is particularly important because the transition layer is the critical interface between dissimilar materials, and its composition and microstructure determine the long-term integrity of the overlay system. For engineers involved in the design and fabrication of heat exchangers, this study reinforces the importance of proper welding procedure qualification, especially for thick sections where the thermal and mechanical challenges are amplified. The preventive measures proposed are straightforward in principle but require careful implementation in practice, particularly the selection of appropriate transition layer materials and the control of welding parameters and thermal cycles.