CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Delamination in the Fusion Zone of Austenitic Stainless Steel Cladding Layer
Literature Overview and Background
This paper, authored by Ren Liyang, Ji Qiang, Zhang Yingying, and Gao Lei from Fushun Chemical Machinery Equipment Manufacturing Co. Ltd. and Liaoning Petrochemical University (2009), addresses a critical and recurring problem in cladding practice: the delamination or separation occurring in the fusion zone between the austenitic stainless steel overlay and the underlying carbon or low-alloy steel substrate. In industrial settings such as petrochemical reactors, heat exchangers, and pressure vessels, the integrity of the cladding bond is paramount because failure of the fusion zone directly compromises corrosion resistance and can lead to catastrophic in-service failures. The authors conducted a systematic investigation using metallographic analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and mechanical testing to identify the root causes of delamination.
Root Cause Analysis of Fusion Zone Delamination
The investigation revealed that delamination in the fusion zone typically originates from one or a combination of several factors. The first and most common cause is improper heat input during welding, which results in excessive dilution or insufficient dilution of the base metal into the overlay. When heat input is too low, the fusion zone may contain excessive amounts of martensitic or ferritic phases that are brittle and prone to cracking. When heat input is too high, the dilution ratio exceeds the acceptable range, leading to the formation of intermetallic compounds and sigma phases that degrade ductility.
A second critical factor is the presence of inclusions and segregation in the fusion zone. The authors identified MnS, TiN, and Cr2N inclusions concentrated along the fusion boundary. These inclusions act as stress concentrators and crack initiation sites. During service, particularly under cyclic thermal loading or pressure fluctuation, these sites become preferential locations for crack nucleation and propagation, ultimately resulting in delamination.
A third factor relates to residual stresses. The austenitic overlay and the ferritic substrate have different thermal expansion coefficients, and the rapid solidification during welding creates significant tensile residual stresses at the fusion interface. If these stresses are not properly managed through interpass temperature control or post-weld stress relief, they can exceed the cohesive strength of the fusion zone.
| Defect Type | Primary Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Fusion zone cracking | Excessive heat input, high dilution | MT, PT, macro-etching | Reduce current/voltage, increase travel speed |
| Inclusion-induced delamination | MnS, TiN segregation | SEM + EDS, metallography | Clean substrate, control welding atmosphere |
| Residual stress failure | Thermal expansion mismatch | XRD stress measurement | PWHT at 620–650 °C, interpass temperature control |
| Intermetallic phase formation | Excessive base metal dilution | XRD, micro-hardness mapping | Use appropriate filler composition, limit dilution to <25% |
Engineering Practice Implications
From an engineering standpoint, the findings of this paper reinforce several well-established but frequently overlooked practices. First, dilution control is essential. For austenitic stainless steel cladding on carbon steel substrates, the dilution should be limited to no more than 20–25% to ensure that the fusion zone retains sufficient austenite and does not develop detrimental phases. Second, interpass temperature should be maintained at or below 150 °C for most austenitic overlays to prevent grain growth and phase transformation. Third, post-weld heat treatment is recommended for critical applications, particularly where the vessel will be subjected to cyclic pressure or thermal cycling.
The paper also highlights the importance of substrate preparation. Surface contamination, rust, and oxide scale on the base metal can lead to incomplete fusion and inclusion formation at the interface. Thorough mechanical cleaning followed by visual and magnetic particle inspection of the substrate surface is a necessary prerequisite before cladding begins.
Study Insights and Reflections
This paper is particularly valuable because it bridges the gap between academic analysis and shop-floor reality. The authors did not merely describe the phenomenon but traced it back to specific, controllable process parameters. This approach is consistent with the FMEA methodology: identify the failure mode, trace the root cause, and implement preventive measures. For engineers involved in the fabrication of clad-plate pressure vessels or weld-overlay reactors, the key takeaway is that fusion zone integrity is not a matter of chance but a function of disciplined process control. Every parameter — from preheat temperature to interpass temperature to post-weld treatment — must be documented and verified. The delamination failures described in this paper are preventable, and the systematic analysis presented here provides a clear roadmap for prevention.