Effect of Rare Earth Oxides on Crack Resistance of Weld Overlay Metal

Literature Overview and Research Background

The study under review investigates the influence of rare earth oxides (primarily La2O3, CeO2, and Y2O3) on the hot cracking and cold cracking susceptibility of weld overlay deposits. Rare earth elements are well recognized for their unique metallurgical effects including grain refinement, inclusion modification, and impurity scavenging. In the context of weld overlay cladding, where the deposit often experiences high thermal gradients and restricted plastic deformation due to the dilution from the base metal, crack resistance remains a persistent engineering challenge. The literature systematically examines how varying concentrations of rare earth oxides (0%, 0.05%, 0.1%, 0.15%, and 0.2 wt%) alter the solidification behavior, microstructure, and crack formation tendency of the overlay layer.

Core Technical Findings

Effect on Microstructure and Grain Morphology

The addition of rare earth oxides significantly modifies the solidification microstructure of the weld overlay deposit. The rare earth oxides act as heterogeneous nucleation sites during solidification, promoting a more equiaxed and refined grain structure. At an optimal addition level of 0.1 wt% La2O3, the grain size reduced by approximately 35% compared to the baseline condition without rare earth addition. The dendrite arm spacing also decreased markedly, which is beneficial because coarser dendritic structures tend to concentrate hot cracks at interdendritic regions.

Mechanism of Crack Resistance Improvement

The crack resistance enhancement operates through multiple synergistic mechanisms. First, rare earth oxides refine the grain structure, reducing the constraint on plastic flow during solidification and thereby lowering the residual tensile stress. Second, they modify the morphology and distribution of sulfide and oxide inclusions, transforming elongated MnS inclusions into more spherical and dispersed shapes that do not act as crack initiation sites. Third, rare earth elements segregate preferentially to interdendritic regions, where they reduce the sulfur and oxygen content of the last-liquid-phase, thereby narrowing the temperature range of the mushy zone and reducing the hot cracking susceptibility.

Effect on Cold Cracking Resistance

For cold cracking resistance, rare earth oxides demonstrate a beneficial effect on hydrogen diffusion behavior. The refined microstructure and modified inclusion distribution reduce the number of preferential hydrogen trapping sites at grain boundaries. Hydrogen trapping studies indicated that the total hydrogen concentration in the weld metal decreased by approximately 20% when 0.1 wt% La2O3 was added, suggesting that rare earth oxides promote hydrogen escape during the cooling stage. This is particularly significant for overlay welding operations performed in high-humidity environments where hydrogen-induced cracking is a major concern.

Engineering Practice Implications

From an engineering standpoint, the findings carry several practical implications for cladding operations. The optimal rare earth oxide addition level of approximately 0.1 wt% provides the best balance between crack resistance and cost. Excessive addition beyond 0.15 wt% leads to diminishing returns and may introduce brittleness due to excessive rare earth segregation at grain boundaries. For field applications involving nickel-based alloy cladding on carbon steel substrates, where dilution can be as high as 20-30%, incorporating rare earth oxides into the consumable formulation can significantly reduce the cracking risk associated with high sulfur and phosphorus impurities in the base metal.

The study also highlights the importance of preheating and post-weld heat treatment in conjunction with rare earth oxide addition. While rare earth oxides improve the inherent crack resistance of the weld metal, they do not eliminate the need for proper thermal management of the base metal. A preheat temperature of 150-200°C for low-alloy steel substrates, combined with 0.1 wt% rare earth oxide addition in the overlay consumable, provides a robust strategy for minimizing both hot and cold cracking.

Key Questions and Reflections

A critical question that arises from this study is whether the benefits of rare earth oxide addition can be fully attributed to the oxide particles themselves or whether the dissolved rare earth elements in the solid solution also play a significant role. The literature suggests that both mechanisms contribute, but the relative importance varies with the welding process parameters. In processes with high cooling rates such as plasma transferred arc welding, the dissolved rare earth effect may be more pronounced because there is less time for oxide particles to exert their nucleation effect.

Another reflection concerns the scalability of these findings to industrial-scale cladding operations. Laboratory studies typically use small coupon sizes and controlled conditions, whereas industrial cladding involves large plate surfaces, variable thicknesses, and complex geometries. The thermal gradients in industrial applications are substantially higher, which may amplify or attenuate the effects of rare earth oxide addition. Future research should focus on full-scale validation under realistic production conditions.

Summary

The incorporation of rare earth oxides at an optimal level of approximately 0.1 wt% into weld overlay consumables offers a practical and effective approach to improving the crack resistance of cladding deposits through grain refinement, inclusion modification, and hydrogen control. This finding provides a valuable tool for engineers dealing with challenging overlay welding applications on impure or high-strength base metals, and it underscores the continuing importance of microalloying strategies in advancing cladding technology.