Delamination of Weld Overlay Layers on Thick Tubesheets and Prevention Measures

Literature Overview and Background

The literature under study addresses one of the most persistent and costly failure modes encountered in the fabrication of large-thickness tubesheets used in heat exchangers and pressure vessels — delamination (peeling or spalling) of the weld overlay layer from the base metal substrate. Tubesheets in hydrogenation reactors, ammonia converters, and high-pressure heat exchangers commonly require a corrosion-resistant overlay of stainless steel or nickel-based alloy on carbon steel or low-alloy steel substrates with thicknesses ranging from 60 mm to over 200 mm. The combination of high thermal mass, significant residual stresses from thick-section welding, and the metallurgical incompatibility between austenitic overlay and ferritic base metal creates a highly susceptible condition for interface cracking and eventual delamination during service.

The study focuses on root cause analysis of delamination mechanisms, which include thermal mismatch cracking, hydrogen-induced cracking (HIC) at the weld interface, solidification cracking in the overlay, and stress corrosion cracking (SCC) initiated at the bond line. Understanding these mechanisms is essential for specifying appropriate welding procedures, preheat and interpass temperature regimes, post-weld heat treatment (PWHT) schedules, and inspection protocols.

Core Technical Analysis

Mechanisms of Delamination

Delamination in thick-section tube overlay is rarely a single-mode failure. The primary mechanisms identified in the literature can be categorized as follows:

Thermal Mismatch and Residual Stress

The thermal expansion coefficient mismatch between austenitic stainless steel overlay (e.g., 304, 316, 321) and ferritic base metals (e.g., SA-516 Gr.70, SA-387 Gr.11 Cl.2, 12Cr1MoV) is the fundamental driver of interface stress. During welding, the overlay is deposited at temperatures exceeding 1400 °C and cools to ambient, while the thick base plate retains significant thermal mass. In tubesheets exceeding 100 mm thickness, the cooling rate at the bond line can be as low as 0.5–2.0 °C/s, which paradoxically allows hydrogen diffusion and accumulation rather than rapid quenching that would minimize HIC.

The residual stress state in the overlay bond line region is typically in the range of 200–400 MPa in the transverse direction and 100–300 MPa in the longitudinal direction for thick sections. When combined with the yield strength of sensitized 304 stainless steel (approximately 205 MPa in the annealed condition), the local stress state can exceed the material's capacity to resist cracking, particularly in the presence of hydrogen or corrosive media.

Process Controls and Prevention Measures

Preheat and Interpass Temperature Management

Adequate preheat is the single most effective control measure for preventing HIC and thermal mismatch cracking in thick-section overlay welding. The literature recommends the following preheat regimes based on base metal type and thickness:

For overlay materials such as E309L or E309L-16 electrodes used with SAW, the low-carbon variant (E309L) is strongly preferred to minimize carbon pickup from the base metal and reduce sensitization risk. The carbon content in the overlay should be maintained below 0.030% to prevent chromium carbide precipitation at grain boundaries.

Welding Procedure Selection

The choice of welding process significantly impacts the quality of the bond line. The literature emphasizes the following considerations:

• Submerged Arc Welding (SAW) with flux-cored wire (e.g., E309L-16) provides high deposition rates suitable for thick sections but requires careful flux selection to control hydrogen content. Rutile-type fluxes with low moisture content (< 0.5%) are recommended.
• Gas Tungsten Arc Welding (GTAW/TIG) is preferred for the first pass (bond line pass) due to superior control over heat input and penetration. A typical heat input for the first pass on a 100 mm tubesheet is 1.0–1.5 kJ/mm.
• Hot-Wire TIG can reduce the number of passes and minimize interpass reheating cycles, which is beneficial for reducing sensitization.
• Plasma Transferred Arc (PTA) Powder Cladding offers precise compositional control but is limited in deposition rate for thick sections.

Post-Weld Heat Treatment (PWHT)

PWHT is mandatory for thick-section overlay welds per ASME Section VIII Division 1 (UG-95) and NB/T 47002. For austenitic overlay on ferritic base metal, the PWHT temperature must be carefully controlled to avoid sensitization of the overlay while achieving stress relief in the base metal. The recommended PWHT parameters are:

The critical issue is that PWHT at 620 °C or below is insufficient for stress relief of 12Cr1MoV or SA-387 Gr.11 Cl.2 base metals, which require temperatures above 720 °C. This creates a conflict: higher PWHT temperatures increase sensitization risk in the 309L overlay. The resolution, as discussed in the literature, is to use E309L (low carbon) rather than E309 (standard carbon) to maintain resistance to sensitization at elevated PWHT temperatures, and to limit the time at temperature to minimize chromium carbide precipitation.

Inspection and Quality Assurance

Non-Destructive Testing Requirements

Delamination detection requires a multi-method NDE approach because individual methods have inherent limitations:

The literature recommends TOFD as the preferred method for bond line inspection in thick tubesheets, supplemented by MT after grinding the overlay surface flush. A minimum 25% thickness removal of the overlay is recommended before MT inspection to reveal any subsurface HIC or thermal mismatch cracks.

Metallographic Verification

Metallographic examination of coupon welds prepared under identical conditions to the production weld is essential for verifying bond line quality. The examination should include:

• Transverse section through the bond line at 5× and 100× magnification
• Etching with 5% nitric acid in ethanol to reveal grain structure and carbide precipitation
• Assessment of chromium carbide (Cr₂₃C₆) precipitation at grain boundaries
• Measurement of the affected zone width in the base metal

A sound bond line should show no cracks, voids, or unmelted flux inclusions at the interface. The heat-affected zone (HAZ) in the base metal should not exceed 3 mm in width for single-pass deposits.

Engineering Practice Cases and Reflections

In a practical case involving a hydrogenation reactor tubesheet (SA-387 Gr.11 Cl.2 base, 120 mm thickness, E309L overlay, 6 mm thickness), delamination was observed after 18 months of service. The root cause analysis revealed a combination of factors: insufficient preheat (120 °C instead of the required 250 °C), excessive interpass temperature exceeding 350 °C in upper overlay passes, and inadequate PWHT soak time (1.5 hours instead of the required 5 hours for 120 mm thickness). Metallographic examination confirmed HIC at the bond line and chromium carbide sensitization in the overlay.

The corrective measures implemented for the replacement tubesheet included: strict enforcement of preheat at 250 °C with thermocouple monitoring at three locations, interpass temperature control at 250–300 °C, use of E309L-16 low-hydrogen flux with moisture control, and extended PWHT soak time of 5 hours at 750 °C with furnace cooling. Post-fabrication TOFD inspection of 100% of the overlay area confirmed a defect-free bond line, and the replacement tubesheet has been in service for over 36 months without any indication of delamination.

This case underscores a critical insight: delamination prevention is not a single parameter optimization but a system-level engineering challenge requiring coordination between welding procedure specification, heat treatment scheduling, and inspection planning. The traditional approach of treating preheat, welding, and PWHT as independent operations is fundamentally flawed for thick-section overlay applications.

Key Questions and Reflections

Several questions remain open based on the literature review. First, the literature does not adequately address the effect of multi-layer overlay welding on bond line quality in tubesheets with curvature radii less than 500 mm. The geometric complexity of curved tubesheets introduces additional stress concentrations that may not be captured by flat-plate qualification procedures. Second, the interaction between PWHT parameters and overlay sensitization deserves further investigation, particularly for overlay materials with higher chromium content (e.g., 316L, 347) where sensitization kinetics differ from 304/309L.

From a practical standpoint, the most important takeaway is that thick-section overlay welding demands a higher level of procedural discipline than conventional welding. Every parameter — from flux moisture content to torch travel speed to PWHT ramp rate — must be controlled within tight windows, and deviations must be documented and evaluated. The cost of delamination failure in a hydrogenation reactor can exceed several million dollars in lost production, equipment replacement, and safety risk, making prevention through rigorous process control a clear economic imperative.

Summary and Conclusions

Delamination of weld overlay layers on thick tubesheets is a multi-mechanism failure driven by thermal mismatch, hydrogen embrittlement, sensitization, and residual stress. Prevention requires a holistic approach encompassing proper preheat selection based on base metal composition and thickness, use of low-carbon and low-hydrogen welding consumables, strict interpass temperature control, adequate PWHT with temperature and time parameters optimized for both base metal stress relief and overlay sensitization resistance, and comprehensive NDE coverage including TOFD for bond line inspection and metallographic verification on qualification coupons. The engineering community must recognize that thick-section overlay welding is not a scaled-up version of thin-section welding but a fundamentally different challenge requiring dedicated procedure development, qualified welder performance, and rigorous quality assurance systems. Future work should focus on developing predictive models for bond line integrity that integrate thermal, mechanical, and metallurgical parameters to enable real-time monitoring and adaptive process control during production welding.