Zhuojin Pipe Fitting Co., LtdDelamination of Overlay Layers on Thick Tube Sheets and Prevention Measures
Literature Overview
The paper by Chen Xiaochang (Welding Technology, Vol. 38, No. 12, 2009) addresses a critical quality issue in the overlay welding of thick tube sheets: delamination at the interface between the base material and the transition layer. The study conducts a systematic analysis of the chemical composition and stress distribution at the base-to-transition layer interface, identifies the root causes of delamination, and proposes specific countermeasures including transition layer material selection and process parameter optimization. This work is directly relevant to the manufacture of heat exchangers, reactors, and pressure vessels with large-diameter thick tube sheets where overlay cladding is applied for corrosion resistance.
Technical Analysis of Delamination Mechanisms
Root Cause Investigation
The delamination failure at the base-to-transition layer interface can be attributed to the following factors:
- Thermal stress mismatch — Thick tube sheets (typically 50-200 mm) experience significant thermal gradients during overlay welding, creating tensile stresses at the interface that can exceed the bond strength.
- Chemical incompatibility — Inadequate transition layer material selection can result in brittle intermetallic compound formation at the interface, reducing adhesion strength.
- Residual stress accumulation — Multi-pass overlay welding on thick sections accumulates residual stresses that can cause progressive delamination during subsequent welding passes or post-weld heat treatment.
- Inclusion and slag entrapment — Poor surface preparation or inadequate slag removal between passes can create weak interfaces susceptible to delamination.
Material Selection for Transition Layer
| Base Material | Recommended Transition Layer | Overlay Material | Rationale |
|---|---|---|---|
| Carbon steel (Q345R) | E309L (309L equivalent) | 316L or 321 | Gradual alloy transition, reduces thermal stress |
| Low-alloy steel (15CrMoR) | E309L or E347 | 321 or 347 | Accommodates thermal expansion difference |
| Cr-Mo steel (12Cr1MoV) | E309MoL | 316L | Molybdenum compatibility, prevents cracking |
| Austenitic SS | — (direct overlay) | Same grade or higher | No transition needed for similar materials |
Process Control Parameters
| Parameter | Recommended Range | Effect on Delamination |
|---|---|---|
| Preheat temperature | 150-250°C (for thick sections) | Reduces thermal gradient, minimizes residual stress |
| Interpass temperature | 100-200°C | Prevents excessive cooling between passes |
| Heat input | Moderate to low | Limits HAZ width and thermal stress |
| Welding sequence | Spiral or segmented | Distributes thermal load uniformly |
| Layer thickness | 3-5 mm per pass | Ensures adequate penetration and bond strength |
| Surface preparation | Grinding to bare metal + degreasing | Eliminates contaminants at interface |
FMEA Analysis of Delamination Risk
Applying Failure Mode and Effects Analysis (FMEA) methodology to identify and prioritize delamination risks:
| Failure Mode | Cause | Effect | Severity (S) | Occurrence (O) | Detection (D) | RPN |
|---|---|---|---|---|---|---|
| Interface delamination | Insufficient preheat | Loss of overlay integrity | 9 | 6 | 5 | 270 |
| Brittle cracking at interface | Wrong transition material | Structural failure | 10 | 4 | 6 | 240 |
| Partial bond loss | Slag/inclusion entrapment | Reduced service life | 7 | 5 | 7 | 245 |
| Progressive delamination | Excessive residual stress | Delayed failure | 8 | 5 | 6 | 240 |
Engineering Practice Integration
Prevention Measures for Thick Tube Sheet Overlay
- Preheat strategy: Apply uniform preheating at 200-250°C for tube sheets exceeding 80 mm thickness, using induction heating or gas flame with temperature verification at multiple points.
- Welding sequence optimization: For large-diameter tube sheets, employ a spiral welding sequence starting from the center and working outward, or use segmented welding with controlled overlap to minimize thermal distortion.
- Transition layer qualification: Perform full qualification testing of the transition layer weld procedure, including:
- Bond strength testing (shear test per ASTM E23 or equivalent)
- Metallographic examination of interface (500x magnification, checking for intermetallics)
- Radiographic testing for interface quality
- Hardness survey across the interface
- Post-weld inspection protocol: Implement a multi-method NDT approach:
- UT thickness measurement to detect delamination
- MT or PT for surface indication of bond loss
- Tap testing for large flat areas (audible method)
- Dye penetrant for final surface verification
- PWHT consideration: Post-weld heat treatment should be applied with controlled heating rates (1.5°C/mm of section thickness) to relieve residual stresses without introducing new stresses. The maximum PWHT temperature should not exceed the lower of the overlay or base material reheat temperature limit.
Case Study Application
In the manufacture of large heat exchanger tube sheets (typically 100-200 mm thick, with 316L overlay for chloride-containing process service), the following protocol has proven effective in practice:
- Base material: 16MnR or Q345R carbon steel tube sheet
- Transition layer: 2-3 passes of E309L with heat input of 1.5-2.5 kJ/mm
- Overlay layer: 3-4 passes of E316L with heat input of 1.0-2.0 kJ/mm
- Preheat: 200°C uniform, verified at 6 points around circumference
- Interpass: Maximum 200°C, with thermocouple monitoring
- Post-weld: PWHT at 620°C × 2 h with controlled heating/cooling rates
- Inspection: UT + PT + hardness survey + metallographic cross-section
Key Questions and Reflections
The paper raises an important question about the fundamental nature of the delamination problem: is it primarily a metallurgical issue (incompatible materials, brittle phases) or a process issue (thermal management, residual stress)? In my assessment, both factors are significant, but the process control aspect is often more critical in practice because it can be managed through proper procedure design and execution. The chemical composition analysis of the interface provides valuable data for understanding the metallurgical compatibility, but the stress analysis is equally important for predicting the mechanical stability of the bond.
Another reflection point is the economic aspect: delamination failure in a thick tube sheet can result in complete component rejection, with costs ranging from several hundred thousand to millions of yuan depending on the equipment size. The cost of prevention (proper material selection, qualified procedures, adequate inspection) is always significantly lower than the cost of failure. This is a classic example where investment in quality assurance at the manufacturing stage yields substantial returns in terms of avoided downtime and replacement costs.
Study Insights and Implications
This literature provides a practical framework for addressing overlay delamination on thick tube sheets, combining metallurgical analysis with process engineering solutions. The key takeaway for engineers is that delamination prevention requires a holistic approach: correct material selection for the transition layer, rigorous process parameter control (particularly preheat and interpass temperature), systematic welding sequence planning, and comprehensive post-weld inspection. The FMEA analysis approach applied to this problem highlights the importance of systematic risk assessment in overlay welding operations, and the specific countermeasures proposed are directly applicable to manufacturing environments producing large pressure vessels and heat exchangers.