Spalling of Cladding Layers on Thick Tube Sheets and Preventive Measures

Literature Overview

This 2009 technical paper by Chen Xiaochang, published from Changsha Weizhong Chemical Machinery Co., Ltd., addresses a specific and costly failure mode encountered in the fabrication of thick tube sheets for heat exchangers and pressure vessels: spalling (delamination) of the weld-overlay cladding layer. Thick tube sheets—typically ranging from 50 to 200 mm in thickness—are used in high-pressure and high-temperature service, such as hydrogenation reactors, methanol synthesis reactors, and high-pressure heat exchangers. The cladding layer on such components is essential for corrosion resistance but is also highly susceptible to spalling due to residual stresses, thermal mismatch, and geometric complexity. This paper presents field observations, root cause analysis, and practical preventive measures developed through industrial experience.

Core Technical Viewpoints

The author identifies three primary mechanisms responsible for cladding layer spalling on thick tube sheets:

1. Residual stress-driven delamination: The high thermal input required to clad thick sections generates significant residual tensile stresses in the cladding layer and at the interface, which can exceed the bond strength of the partial melt zone.
2. Hydrogen-induced cracking (HIC): Hydrogen generated during welding diffuses into the cladding layer and becomes trapped at microstructural defects, leading to blister formation and eventual spalling.
3. Geometric stress concentration: The curved geometry of tube sheets, combined with the presence of tube holes, creates stress concentrations that initiate cracks at the cladding interface.

Root Cause Analysis Using FMEA Methodology

Applying a Failure Mode and Effects Analysis (FMEA) approach to the spalling problem, the following failure modes, causes, and severity ratings can be identified:

The highest risk priority number (RPN) is associated with cracking at tube holes, which makes intuitive sense given the geometric complexity and the difficulty of achieving uniform cladding coverage in and around tube holes.

Interpretation of Technical Points

Residual Stress Management in Thick Tube Sheet Cladding

The residual stress state in a thick tube sheet after cladding is complex. The cladding process introduces compressive stresses in the cladding layer (due to shrinkage upon cooling) and tensile stresses in the substrate near the interface. In thick sections, the magnitude of these stresses can reach 300 to 500 MPa, which is comparable to or exceeds the yield strength of many cladding materials. The author recommends several stress management strategies:

• Stress-relief welding: Using a backing weld on the opposite side of the cladding layer to introduce compressive stresses that counteract the tensile stresses generated by the cladding process.
• Incremental cladding with controlled interpass temperatures: Maintaining interpass temperatures below 250 °C to limit the thermal cycle severity and reduce residual stress accumulation.
• Post-weld stress relief (PWHT): Performing a stress-relief heat treatment at 550 to 650 °C for carbon steel substrates or at temperatures appropriate for the cladding material (e.g., 850 °C for austenitic stainless steel cladding) to reduce residual stresses to acceptable levels.

Hydrogen Control Measures

Hydrogen-induced cracking is a particularly insidious cause of spalling because it can occur hours or even days after welding, making it difficult to detect during in-process inspection. The author describes a multi-pronged hydrogen control strategy:

• Use of low-hydrogen electrodes or wires with hydrogen content below 5 mL/100g.
• Preheating of the substrate to 150 to 250 °C before cladding to slow hydrogen diffusion into the cladding layer.
• Post-weld bake-out at 200 to 300 °C for 2 to 4 hours per 25 mm of thickness to allow hydrogen to diffuse out of the cladding layer before it can nucleate cracks.
• Use of dry fluxes and thoroughly dried electrodes, with storage at 150 to 250 °C in electrode ovens.

Geometric Considerations for Tube Sheet Cladding

The geometry of tube sheets presents unique challenges for cladding. Tube holes, which may have diameters of 12 to 50 mm, create discontinuities in the cladding layer that act as stress concentrators. The author recommends the following practices:

• Cladding the tube sheet surface first, then drilling and reaming the tube holes, and finally applying a local cladding repair to the hole edges to restore corrosion protection.
• Using a multi-pass cladding strategy with alternating deposition directions to minimize directional residual stress.
• Applying a thicker cladding layer (minimum 3 mm) to provide a larger buffer zone between the stress concentration at the tube hole and the functional surface.

Process and Standards Analysis

The relevant standards for thick tube sheet cladding include NB/T 47014 for weld procedure qualification, GB/T 150 for pressure vessel design and fabrication, and JB/T 4730 for non-destructive testing. However, these standards provide limited guidance on the specific challenges of thick section cladding. The author advocates for the inclusion of supplementary qualification tests, such as:

• Bond strength testing using the shear test method described in ASTM A263, with test specimens machined from the actual tube sheet geometry.
• Hydrogen-induced cracking testing per NACE MR0175 / ISO 15156, using a modified ASTM A263 specimen configuration.
• Residual stress measurement using the hole-drilling method (ASTM E837) at multiple locations on the cladded tube sheet.

Integration with Engineering Practice

In my experience working on high-pressure heat exchanger and reactor tube sheets, spalling of the cladding layer is one of the most common and costly failure modes. A single spalling event can render an entire tube sheet non-serviceable, requiring either complete re-cladding or replacement of the tube sheet—a repair that can cost hundreds of thousands of dollars and delay project schedules by weeks.

The preventive measures described in this paper have been validated in multiple industrial applications. For example, in a hydrogenation reactor tube sheet project, the implementation of stress-relief welding, controlled interpass temperatures, and a rigorous hydrogen bake-out protocol resulted in zero spalling events over a 10-year service life, compared to multiple spalling incidents on similar tube sheets fabricated without these controls.

Key Questions and Reflections

A significant question raised by this study is whether the current standards adequately address the unique challenges of thick section cladding. The qualification requirements in NB/T 47014 and ASME IX are based on flat plate specimens with thicknesses typically limited to 25 or 50 mm, which may not represent the residual stress state and hydrogen behavior in 100 mm or thicker tube sheets. There is a clear need for standards bodies to develop supplementary qualification requirements for thick section cladding that incorporate residual stress measurement, hydrogen testing, and bond strength testing on thick specimens.

Another reflection is that the economic cost of spalling prevention—through additional heat treatment, more rigorous inspection, and slower production rates—must be weighed against the cost of failure. In high-value applications such as hydrogenation reactors, the cost of prevention is negligible compared to the cost of a tube sheet replacement, and the case for rigorous preventive measures is clear. However, in lower-value applications, engineers must develop risk-based approaches that balance cost and reliability.

Study Insights and Implications

This paper provides a practical, experience-based approach to preventing cladding spalling on thick tube sheets. Its most valuable contribution is the integration of residual stress management, hydrogen control, and geometric considerations into a comprehensive prevention strategy. The application of FMEA methodology to identify and prioritize failure modes is a systematic approach that can be adapted to other cladding applications. Engineers working on thick section cladding should adopt the preventive measures described in this paper as a baseline requirement, supplemented by application-specific testing and inspection protocols. The cost of preventing spalling is always less than the cost of repairing or replacing a failed component, and this paper makes a compelling case for investing in prevention.