Delamination of Overlay Layer on Thick Tube Sheets and Prevention Measures

Literature Overview and Problem Statement

Delamination of the overlay layer on thick tube sheets is a critical failure mode that can lead to catastrophic loss of pressure containment in heat exchangers and pressure vessels. Thick tube sheets, typically defined as those with thickness exceeding 50 mm, present unique challenges for overlay welding due to the high cooling rates, significant residual stress buildup, and the increased likelihood of interfacial cracking. This study note examines the metallurgical mechanisms of delamination, the contributing process factors, and the preventive measures discussed in the literature.

Metallurgical Mechanisms of Delamination

Delamination in thick tube sheet overlays occurs primarily through two mechanisms: hydrogen-induced cracking (HIC) and thermal stress cracking at the bond interface. In carbon steel and low-alloy steel substrates, the high cooling rates associated with thick sections promote the formation of untempered martensite in the heat-affected zone (HAZ). This martensite is brittle and susceptible to hydrogen embrittlement, particularly when the base steel has high hardenability (carbon equivalent CE > 0.5%).

The literature identifies three critical microstructural features that predispose the bond interface to delamination:

Network-type carbides: In high-carbon austenitic stainless steel overlays, chromium carbides (Cr23C6) precipitate at grain boundaries during slow cooling, creating a network that reduces intergranular fracture resistance.
Sigma phase: In nickel-based alloy overlays, sigma phase (Cr-Ni-Fe intermetallic) can form when the overlay is held in the 600–900°C range for extended periods, reducing ductility and promoting intergranular cracking.
Lack of fusion: Incomplete melting of the base metal at the bond interface creates a weak boundary that can separate under tensile stress.

Microstructural Feature	Formation Condition	Effect on Delamination Resistance
Network Cr23C6 carbides	Slow cooling through 850–550°C	Reduces intergranular toughness by 50–70%
Sigma phase	Dwell at 600–900°C for >1 hour	Embrittles grain boundaries, promotes intergranular fracture
Untempered martensite in HAZ	High cooling rate, high CE steel	Promotes HIC and cold cracking
Lack of fusion	Insufficient heat input, contamination	Creates weak interface, direct delamination path

Contributing Process Factors

The literature identifies several process-related factors that contribute to overlay delamination on thick tube sheets:

Preheat and interpass temperature control: Thick tube sheets require preheat of 150–300°C depending on the carbon equivalent of the base steel. Without adequate preheat, the cooling rate at the bond interface exceeds the critical rate for martensite formation, leading to HAZ embrittlement. The interpass temperature must be maintained above 100°C to prevent excessive cooling between passes.

Welding heat input: The heat input per pass must be sufficient to ensure complete melting of the base metal at the bond interface but not so high as to cause excessive grain growth or sensitization in the overlay. For thick tube sheets, a heat input of 25–45 kJ/cm is typically recommended for the first pass, with subsequent passes at 20–35 kJ/cm.

Welding sequence: A systematic welding sequence that minimizes拘束 stress is essential. The literature recommends a center-outward sequence with cross-tie passes to distribute heat input uniformly and reduce the risk of edge cracking.

Post-weld heat treatment: Stress relief at 550–650°C for 1 hour per 25 mm of thickness is critical for thick tube sheets. This treatment reduces residual stresses, transforms untempered martensite to tempered martensite, and dissolves network carbides in austenitic overlays.

Preventive Measures and Engineering Controls

The literature proposes a multi-layered approach to prevent overlay delamination:

Material selection: Use low-carbon or stabilized overlay alloys (304L, 316L, 321, 347) to minimize carbide precipitation. For nickel-based overlays, select alloys with controlled carbon content (< 0.08% C) to reduce sigma phase risk.
Welding procedure optimization: Develop and qualify a WPS that specifies preheat, interpass temperature, heat input, and welding sequence. The procedure qualification test must include bond line metallographic examination and intergranular corrosion testing per ASTM A263.
Post-weld stress relief: Apply stress relief at 550–650°C with a soak time of 1 hour per 25 mm of thickness. The cooling rate from the stress relief temperature must be controlled to avoid re-formation of brittle phases.
Non-destructive testing: Implement 100% ultrasonic testing of the bond line using phased array ultrasonic testing (PAUT) for improved sensitivity to planar defects. The acceptance criteria for bond line inspection should follow NB/T 47013.2 or ASME V, with no delamination indications exceeding 20% of the overlay area.
Design modifications: For critical applications, consider using a clad plate tube sheet (explosive cladding or roll-bonded cladding) instead of weld overlay, as the metallurgical bond in clad plates is more reliable for thick sections.

Case Study and Lessons Learned

The literature presents a case study of a thick tube sheet (80 mm thick, carbon steel base with 316L overlay) that experienced delamination during hydrostatic testing. The investigation revealed that the welding procedure lacked adequate preheat (only 50°C was applied instead of the recommended 200°C), and the interpass temperature was not monitored. Metallographic examination showed extensive untempered martensite in the HAZ and network carbides at the bond interface. The corrective action involved re-welding with proper preheat, applying stress relief at 620°C for 4 hours, and re-inspecting with PAUT. The reworked tube sheet passed all inspections and has been in service for over 5 years without incident.

Study Reflections

The delamination of overlay layers on thick tube sheets is a preventable failure mode that requires a systematic approach to process design, material selection, and quality control. The literature reinforces that thick sections are not simply "larger versions" of thin sections but present fundamentally different metallurgical challenges that demand tailored procedures. The engineer must recognize that the bond line is a metallurgical boundary with its own unique requirements, and that the welding procedure must be designed to create a strong, ductile, and corrosion-resistant interface. This understanding is essential for the safe and reliable fabrication of thick tube sheet pressure vessels in demanding service environments.