Relationship between Cladding Interface Characteristics and Crack Formation

Literature Overview

Published in 2001 in "Ordnance Materials and Engineering" (now known as Materials Science and Engineering of Ordnance), this study by researchers from Nanchang Hangkong University and Yichun First Machinery Factory investigates the fundamental relationship between the metallurgical interface characteristics of weld overlay cladding and the formation of cracks at the overlay-base metal junction. The work is particularly significant because interface cracking represents one of the most challenging and costly defects in bimetal product manufacturing, affecting everything from repair welding of worn components to the fabrication of clad plate pressure vessels.

Interface Metallurgy and Crack Mechanisms

The interface between the overlay layer and the base metal in cladding operations is a region of extreme metallurgical complexity. During welding, the base metal is partially melted and mixed with the filler metal, creating a transition zone with composition and microstructure that differ from both the overlay and the base metal. This transition zone is characterized by:

• Variable dilution rates that create compositional gradients across the interface
• Rapid solidification that promotes the formation of brittle intermetallic phases
• High residual stresses resulting from thermal gradients and restrained cooling
• Possible formation of martensitic or bainitic structures in the transition zone when dissimilar metals are involved

The study identified several crack initiation mechanisms at the cladding interface:

Dilution and Phase Formation at the Interface

A central finding of the research is that the dilution rate at the interface directly governs the phase composition and crack susceptibility of the transition zone. When overlaying hardfacing alloys (such as Cr-C, Co-Cr, or Ni-based alloys) onto carbon or low-alloy steel substrates, high dilution can lead to the formation of hard, brittle phases such as:

• Chromium carbides (Cr₇C₃, Cr₃C) in Cr-C overlay systems
• Martensitic structures in Ni-Cr-C overlays on steel
• Sigma phase (Cr₂₃C₆) in high-chromium overlays
• Laves phase in Ni-Mo-Cr systems

These phases create localized regions of high hardness (often exceeding 800 HV) and low ductility, which act as crack initiation sites under residual stress. The study demonstrated that reducing the dilution rate below 20% through process optimization significantly reduces the volume fraction of brittle phases and improves the crack resistance of the interface.

Process Parameters Affecting Interface Integrity

The research examined several process variables that influence interface characteristics and crack formation:

1. Base metal preheating: Preheating the base metal to 200-400 °C (depending on carbon equivalent) reduces the thermal gradient at the interface, lowers residual stresses, and decreases the cooling rate, thereby minimizing martensitic transformation in the heat-affected zone. For high-carbon steels, preheating above 300 °C is typically required.
2. Heat input control: Low heat input processes (such as GTAW overlay) produce narrow transition zones with high cooling rates, which can promote brittle phases. Moderate heat input (as in SAW or FCAW overlay) provides a wider transition zone with more gradual compositional gradients, reducing stress concentrations.
3. Filler metal selection: Using filler metals with compositions that minimize dilution effects—such as Ni-based fillers for steel substrates or Cr-rich fillers matched to the base metal chemistry—reduces the formation of brittle intermetallics at the interface.
4. Layer thickness and number: Thin, multiple overlay layers produce a more gradual transition from base metal to overlay composition, reducing the severity of compositional and microstructural discontinuities at the interface.

Engineering Practice and Defect Prevention

In practical cladding operations, interface cracking is often encountered during the fabrication of bimetallic pressure vessels, particularly when overlaying stainless steel or nickel-based alloys onto carbon steel shells. The following engineering practices, supported by the findings of this study, are recommended for preventing interface defects:

• Conduct a thorough metallurgical compatibility assessment before selecting the overlay system, considering the base metal chemistry, the overlay composition, and the expected dilution rate.
• Use low-dilution processes such as GTAW or plasma transferred arc (PTA) for the first overlay layer, transitioning to higher-deposition-rate processes (SAW, FCAW) for subsequent layers.
• Apply appropriate preheating based on the base metal carbon equivalent (CE), typically maintaining the preheat temperature above the threshold required to prevent martensitic transformation in the HAZ.
• Perform post-weld heat treatment (PWHT) to relieve residual stresses and transform any martensitic structures in the transition zone to more ductile tempered structures.
• Implement non-destructive testing (NDT) of the overlay interface using ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) to detect subsurface cracks that may not be visible on the surface.

Key Questions and Reflections

The study raises important questions about the role of microstructural modeling in predicting interface crack susceptibility. While the research provides empirical data on the relationship between dilution, phase formation, and cracking, the development of quantitative models that can predict crack initiation based on process parameters and material properties would greatly enhance the ability of engineers to design crack-free overlay systems. Computational tools such as finite element thermal-mechanical analysis can simulate the residual stress fields in overlay welds, but integrating metallurgical transformation models remains a challenge.

Another reflection concerns the inspection and acceptance criteria for cladding interfaces. Current standards such as ASME IX and NB/T 47014 provide guidance on overlay welding qualification but do not specifically address interface metallurgy. Engineers must exercise judgment in specifying acceptance criteria for interface defects, considering the severity of the service environment and the consequences of interface cracking. For pressure vessels operating under internal pressure, even small interface cracks can propagate under cyclic loading, making thorough NDT inspection essential.

Study Insights and Reference Value

This research provides fundamental insights into the metallurgical mechanisms governing interface cracking in weld overlay cladding, which are applicable across a wide range of bimetal product manufacturing applications. The findings emphasize that interface quality is not merely a function of the welding process but is intimately connected to the metallurgical compatibility of the overlay and base metal systems. For engineers involved in the design and fabrication of clad plate pressure vessels, overlay-repaired components, and bimetallic products, understanding the dilution-phase-crack relationship is essential for specifying appropriate welding procedures, selecting compatible materials, and implementing effective quality control measures.