CLADDING TECHNOLOGY SHANXI CO., LTDPeeling of Overlay Layer on Thick Tube Sheet and Prevention Measures
Literature Overview
This 2009 publication by Chen Xiaochang from Changsha Weizhong Chemical Machinery Co., Ltd., published in Welding Technology, documents a critical quality issue encountered during the fabrication of large-diameter heat exchanger tube sheets. The paper describes the phenomenon of overlay layer peeling (delamination) on thick tube sheets and proposes comprehensive prevention measures. This topic is of considerable practical importance in the pressure vessel and heat exchanger fabrication industry, where tube sheets with overlay layers are commonly used in sour service environments requiring corrosion-resistant tube-to-tubesheet joints.
Thick tube sheets (typically exceeding 50 mm in thickness) present unique challenges for overlay welding due to the high thermal mass of the base material, the significant residual stresses developed during welding, and the complex stress state imposed by the subsequent tube-to-tubesheet welding operations. The peeling failure mode represents a catastrophic defect that can lead to immediate loss of containment integrity and must be prevented through rigorous process control.
Core Technical Content
Peeling Failure Mode Characterization
The overlay layer peeling observed in this case study manifested as a sudden separation of the entire overlay layer from the base tube sheet surface during or immediately after the tube-to-tubesheet welding operation. The failure occurred at the overlay-to-base interface, indicating an interfacial bond failure rather than a cohesive failure within the overlay or base material. The affected tube sheets were carbon steel (16MnR) with a thickness of 65-80 mm, overlaid with 304 stainless steel using submerged arc welding (SAW) with a total overlay thickness of 3-4 mm.
Root Cause Analysis
The root cause analysis, conducted using a systematic approach combining metallographic examination, mechanical testing, and welding residual stress measurement, identified the following contributing factors:
| Contributing Factor | Mechanism | Evidence |
|---|---|---|
| Excessive residual stress | Tensile stress at interface exceeds bond strength | Residual stress measurement: 280-320 MPa |
| Inadequate preheat | High cooling rate causes martensitic transformation in HAZ | Microhardness profile: 380-420 HV in HAZ |
| Excessive dilution | Weak metallurgical bond at interface | Dilution rate measured at 15-20% (excessive) |
| Tube welding heat input | Additional thermal cycling weakens already stressed interface | Tube welding heat input: 12-18 kJ/mm |
| Insufficient PWHT | Residual stress not adequately relieved | Stress relief effectiveness: only 40% reduction |
Critical Technical Parameters
The study identifies several critical process parameters that must be controlled to prevent overlay peeling on thick tube sheets:
| Parameter | Critical Limit | Measurement Method |
|---|---|---|
| Preheat temperature | ≥ 200°C (for thickness > 50 mm) | Thermocouple at weld center |
| Interpass temperature | 150-250°C | Infrared pyrometer |
| Dilution rate | < 10% (target < 7%) | Metallographic analysis |
| Overlay residual stress | < 200 MPa (target < 150 MPa) | X-ray diffraction method |
| PWHT temperature | 580-620°C | Furnace thermocouple |
| PWHT holding time | ≥ 2 h per 25 mm thickness | Time-temperature record |
| Cooling rate (800→500°C) | < 20°C/s | Embedded thermocouple |
Prevention Measures
The following comprehensive prevention measures are proposed:
- Optimized overlay welding procedure: Use a multi-layer approach with a dedicated transition layer (309L composition) deposited by GTAW with low heat input (6-8 kJ/mm), followed by 2-3 passes of 304L overlay by SAW with controlled heat input (10-14 kJ/mm). The transition layer thickness should be 1.5-2.0 mm, with each subsequent layer being 1.0-1.5 mm thick.
- Adequate preheat and interpass temperature control: For tube sheets exceeding 50 mm in thickness, the preheat temperature should be maintained at 200-250°C, applied within a radius of 300 mm from the weld area. The preheat should be applied gradually over a minimum of 30 minutes to avoid thermal shock. Interpass temperature should be monitored at multiple points and maintained between 150-250°C throughout the welding sequence.
- Dilution rate control: The dilution rate must be verified through metallographic examination of a witness coupon welded with the same parameters. If the dilution exceeds 10%, the welding parameters must be adjusted (increase current, decrease travel speed) or an additional transition layer must be added.
- Post-weld stress relief before tube welding: A local stress relief treatment should be applied to the overlay area before the tube-to-tubesheet welding operation. This can be achieved by heating the overlay area to 550-600°C using induction heating, holding for 1 hour, and allowing air cooling. This step reduces the residual stress in the overlay to below 100 MPa, providing a safety margin during subsequent tube welding.
- Tube welding heat input control: The tube-to-tubesheet welding heat input should be limited to 8-12 kJ/mm (compared to the typical 12-18 kJ/mm for non-overlay tube sheets). This can be achieved by reducing the welding current or increasing the travel speed. The tube welding sequence should start from the center of the tube sheet and proceed radially outward to minimize thermal distortion.
- Thorough PWHT: The final PWHT should be performed on the complete assembly (tube sheet with tubes welded) at 580-620°C, with a holding time calculated as 2 hours per 25 mm of maximum section thickness. For an 80 mm thick tube sheet, the holding time should be at least 6.4 hours. The heating rate should be limited to 200°C/h to prevent thermal gradients that could induce additional stresses.
Inspection Protocol for Overlay Quality
| Inspection Method | Inspection Stage | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | After each pass | No undercut, porosity, or lack of fusion |
| Magnetic particle testing (MT) | After final overlay pass | No linear indications ≥ 3 mm |
| Ultrasonic testing (UT) | After overlay completion | No delamination; bond quality per API 934 |
| Hardness testing | After PWHT | 150-250 HV (304L overlay); gradient smooth |
| Dilution measurement | Witness coupon | < 10% (target < 7%) |
| Residual stress measurement | After PWHT | < 150 MPa (target < 100 MPa) |
Engineering Practice Integration
This case study highlights the importance of considering the entire fabrication sequence when designing overlay welding procedures for thick tube sheets. The overlay layer is not merely a surface coating but an integral part of the structural component that will be subjected to additional thermal and mechanical loading during subsequent fabrication steps. The tube-to-tubesheet welding operation imposes significant additional thermal cycles and mechanical constraints on the overlay layer, and this must be anticipated in the overlay procedure design.
For engineering practice, the following quality control checklist is recommended:
- Verify base material heat number and mechanical properties before overlay welding
- Perform a welding procedure qualification (WPQ) per NB/T 47014 or ASME IX with the actual base material thickness
- Conduct a dilution test on a witness coupon before production welding
- Monitor and record preheat and interpass temperatures throughout the welding sequence
- Perform UT inspection of the overlay-to-base bond before tube welding
- Apply local stress relief before tube welding for tube sheets exceeding 50 mm
- Limit tube welding heat input and control the welding sequence
- Perform final PWHT with adequate holding time
- Conduct final NDT including UT for delamination detection
Study Insights and Implications
The peeling failure of overlay layers on thick tube sheets is a preventable defect that results from inadequate consideration of the interaction between overlay welding residual stresses and subsequent fabrication operations. The fundamental lesson is that overlay welding on thick sections requires a fundamentally different approach than overlay welding on thin sections, with significantly higher preheat temperatures, more conservative heat input, and mandatory stress relief steps. Engineers should recognize that the bond strength of an overlay layer is not a fixed property but a dynamic quantity that changes with thermal cycling, and the cumulative effect of multiple thermal cycles (overlay welding, tube welding, PWHT) must be evaluated. The prevention measures proposed in this study, particularly the local stress relief before tube welding and the dilution rate control, should be considered as minimum requirements for any thick tube sheet overlay application. Failure to implement these measures risks catastrophic in-service failure, which in pressure vessel applications can have severe safety and environmental consequences.