Effect of Rare Earth Oxides on Crack Resistance of Overlay Metal

Literature Overview

This 1999 study published in the Chinese Journal of Rare Earths by researchers from Yanshan University's School of Materials Science and Chemical Engineering investigates the influence of rare earth oxide additions on the crack resistance of overlay weld metals. The research was supported by the State Key Laboratory of Modern Welding Production Technology, indicating its significance within China's welding research community. The work addresses a well-recognized challenge in overlay welding: the susceptibility of high-alloy deposits to both solidification cracking and solid-state cracking, particularly in applications involving nickel-based alloys and high-chromium stainless steels.

Metallurgical Background and Crack Mechanisms

Overlay welding deposits are inherently prone to cracking due to the combination of high alloy content, steep thermal gradients, and the constraints imposed by the base metal. In nickel-based alloy overlays such as Inconel 625 or Stellite-type alloys, the solidification cracking susceptibility is primarily governed by the mushy zone temperature range and the presence of low-melting-point phases. The solidification cracking susceptibility index (SCSI) for these alloys can reach values as high as 80–90, indicating extreme sensitivity to cracking.

The two primary cracking mechanisms in overlay welds are solidification cracking and solid-state cracking. Solidification cracking occurs during the final stages of solidification when liquid films at the interdendritic boundaries are subjected to tensile stresses that exceed their cohesive strength. Solid-state cracking, including hot short cracking and reheat cracking, occurs after solidification is complete and is associated with the precipitation of brittle phases at grain boundaries during cooling through specific temperature ranges.

Effect of Rare Earth Oxides on Solidification Behavior

The addition of rare earth oxides to overlay weld metals influences the solidification behavior through several mechanisms. First, rare earth elements act as heterogeneous nucleation sites, promoting a finer grain structure that reduces the effective crack length and improves the fracture toughness of the deposit. The nucleation efficiency of CeO₂ and La₂O₃ particles is particularly high because of their low surface energy and chemical compatibility with the molten metal.

Second, rare earth additions modify the segregation behavior of sulfur, phosphorus, and other impurity elements. In conventional overlay welds, these impurities segregate to the interdendritic regions and form low-melting-point phases such as FeS, Fe₂P, and Fe₃P. These phases are responsible for hot short cracking and are particularly detrimental in nickel-based alloys where the sulfur solubility is limited. Rare earth elements form stable compounds with sulfur (such as CeS and La₂S₃) that have higher melting points than FeS and are more ductile, thereby reducing the hot cracking susceptibility.

Third, rare earth oxides influence the grain boundary character distribution. The addition of 0.05–0.15 wt% rare earth elements has been shown to increase the fraction of high-angle grain boundaries with low-energy character, which reduces the grain boundary sliding tendency and improves hot short cracking resistance. This effect is particularly important in overlay welds where the thermal cycling during multi-pass welding creates complex stress states at grain boundaries.

The grain refinement effect of rare earth additions is quantifiable. In a typical nickel-based overlay alloy, the addition of 0.10 wt% CeO₂ reduces the average grain size from approximately 120 μm to 60 μm. This refinement is achieved through both heterogeneous nucleation and the restriction of grain growth during solidification. The finer grain structure not only improves crack resistance but also enhances the mechanical properties of the deposit, including yield strength and impact toughness.

Experimental Results and Microstructural Analysis

The study employed a combination of macroscopic crack evaluation, metallographic examination, and fractographic analysis to assess the effect of rare earth additions on crack resistance. The macroscopic crack evaluation followed standard procedures, measuring the total crack length as a percentage of the weld length. The metallographic examination revealed significant changes in the microstructural morphology with rare earth additions.

In the baseline deposit without rare earth additions, the microstructure consisted of columnar dendrites with coarse interdendritic segregation. The interdendritic regions contained a network of brittle carbides and sulfides that served as crack initiation sites. With the addition of 0.10 wt% CeO₂, the microstructure transitioned to a more equiaxed dendritic morphology with finer interdendritic spacing. The segregation intensity was reduced, and the distribution of brittle phases was more uniform, reducing the stress concentration at individual phases.

Fractographic analysis using scanning electron microscopy (SEM) provided valuable insights into the crack initiation and propagation mechanisms. In the baseline deposit, cracks initiated at interdendritic boundaries and propagated along the columnar dendrite axes, following the path of least resistance through the segregated regions. In the rare earth-modified deposit, crack initiation occurred at a higher stress level and the crack propagation was more tortuous, requiring more energy to propagate through the refined grain structure.

The quantitative crack resistance data showed a consistent trend: increasing rare earth addition up to 0.15 wt% improved crack resistance, but further increases beyond this level showed diminishing returns and, in some cases, slight degradation. This behavior is attributed to the formation of coarse rare earth oxide particles at higher addition levels, which can act as crack initiation sites rather than nucleation sites. The optimal addition level of 0.10–0.15 wt% represents a balance between grain refinement benefit and particle coarsening risk.

Engineering Practice Implications

For practical overlay welding applications, the rare earth modification approach offers several advantages. First, it enables the use of higher-alloy consumables that would otherwise be too crack-sensitive for practical welding. This is particularly relevant for welding nickel-based alloys onto carbon steel substrates, where the dilution effects can create crack-prone compositions in the transition zone. Second, the improved crack resistance allows for more flexible welding procedures, including the possibility of welding without preheating in some cases. Third, the refined microstructure improves the overall mechanical properties of the deposit, including fatigue resistance and stress corrosion cracking resistance.

The implementation of rare earth-modified consumables requires careful control of the rare earth oxide addition level. In electrode coating formulations, the rare earth oxide is typically added as a discrete component to the coating mixture, and the addition level is controlled during the coating application process. For wire consumables used in gas metal arc welding (GMAW) or flux-cored arc welding (FCAW), the rare earth oxide is incorporated into the flux core or the wire composition. The processing route must ensure uniform distribution of the rare earth oxide particles to achieve consistent crack resistance improvement.

Key Technical Challenges and Limitations

Despite the demonstrated benefits, several technical challenges must be addressed before rare earth modification can be widely adopted in industrial practice. First, the cost of rare earth oxides, while not prohibitively high, adds to the consumable cost. The cost increase is typically 5–15% for electrode coatings and 3–8% for wire consumables, which may be acceptable for critical applications but not for routine maintenance welding. Second, the effect of rare earth additions on the wear resistance and corrosion resistance of the deposit has not been fully characterized. In some cases, the grain refinement may slightly reduce the hardness due to the reduction in carbide size, which could affect wear life in abrasive applications. Third, the long-term stability of the crack resistance improvement under thermal cycling conditions requires further investigation.

The study's findings also raise questions about the interaction between rare earth additions and other alloying elements. In complex multi-element alloys, the rare earth elements may form compounds with other elements (such as rare earth carbides or rare earth nitrides) that alter the expected behavior. The formation of rare earth carbides (such as Ce₄C₃ or La₂C₃) can influence the hardness distribution and the wear resistance of the deposit. These interactions require systematic investigation for each specific alloy system before rare earth modification can be reliably applied.

Summary and Practical Recommendations

This research provides compelling evidence that rare earth oxide additions are an effective metallurgical tool for improving the crack resistance of overlay weld metals. The optimal addition level of 0.10–0.15 wt% CeO₂, La₂O₃, or Y₂O₃ produces significant crack resistance improvements through grain refinement, impurity modification, and grain boundary strengthening. Engineers working with crack-sensitive overlay alloys should consider rare earth modification as a viable approach to improving weldability, particularly in applications where preheating is impractical or where the dilution effects create crack-prone compositions. The approach is most beneficial for nickel-based alloy overlays, high-chromium stainless steel overlays, and other high-alloy deposits where the solidification cracking susceptibility is inherently high. Future research should focus on optimizing the rare earth addition level for specific alloy systems and characterizing the long-term performance of rare earth-modified deposits under service conditions.