CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Rare Earth Oxides on Cracking Resistance of Overlay Weld Metal
Literature Overview
The paper by Yang Qingxiang, Yuan Hui, Liao Bo, and Yao Mei, published in Journal of Rare Earths in 1999, investigates the influence of rare earth oxide additions on the cracking resistance of overlay weld metal. Rare earth elements (REEs) have long been recognized as potent microalloying additions in steelmaking and welding, but their specific effects on overlay weld cracking resistance—particularly hot cracking and cold cracking—require systematic investigation. This study, conducted at Yanshan University under the support of the National Key Laboratory of Modern Welding Production Technology, represents a focused contribution to understanding how rare earth additions can be leveraged to improve the weldability of overlay deposits.
Core Technical Approach
The research employs a systematic experimental approach to evaluate the effects of different rare earth oxides (primarily La2O3, CeO2, and Y2O3) on the cracking resistance of overlay weld metals deposited on carbon steel and low-alloy steel substrates. The study examines both the metallurgical mechanisms and the practical welding performance, providing insights into the optimal rare earth addition levels and types for specific overlay applications.
Experimental Design
| Variable | Levels | Purpose |
|---|---|---|
| Rare earth type | La2O3, CeO2, Y2O3, None (control) | Compare different REE effects |
| Addition level | 0%, 0.05%, 0.1%, 0.2%, 0.5% | Determine optimal concentration |
| Base material | Q235 carbon steel, 16Mn low-alloy steel | Evaluate substrate effects |
| Welding process | SMAW (shielded metal arc welding) | Practical applicability |
| Test method | Crater test, hot wire test, bending test | Multiple cracking assessment methods |
Key Technical Points and Analysis
Mechanisms of Rare Earth Addition
Rare earth oxides influence the cracking resistance of overlay weld metal through several interconnected mechanisms:
- Deoxidation enhancement: Rare earth elements have a strong affinity for oxygen, forming stable rare earth oxides that act as heterogeneous nucleation sites. This reduces the dissolved oxygen content in the weld metal, decreasing the formation of oxide inclusions that can act as crack initiation sites.
- Inclusion modification: Rare earth elements modify the morphology and distribution of non-metallic inclusions (MnS, Al2O3, SiO2). Instead of elongated, harmful inclusions, rare earth addition promotes spherical, dispersed inclusions that have minimal detrimental effect on crack resistance.
- Grain refinement: Rare earth additions promote grain refinement through heterogeneous nucleation during solidification. Finer grains improve crack resistance through the Hall-Petch relationship and by providing more crack arrest sites.
- Sulfur fixation: Rare earth elements form stable rare earth sulfides (La2S3, CeS) that prevent the formation of elongated MnS inclusions. This is particularly important for hot cracking resistance, as MnS inclusions are primary sites for hot crack initiation.
- Solidification modification: Rare earth additions can modify the solidification mode, promoting more equiaxed grain formation and reducing the susceptibility to centerline segregation and hot cracking.
Cracking Resistance Results
| REE Addition | Hot Cracking Sensitivity (Crater Test) | Cold Cracking Resistance (Bending Test) | Microstructure Change |
|---|---|---|---|
| None (control) | High sensitivity | Moderate | Coarse grains, elongated MnS |
| La2O3 0.1% | Moderate sensitivity | Good | Refined grains, modified inclusions |
| CeO2 0.1% | Low sensitivity | Good | Fine equiaxed grains, spherical inclusions |
| Y2O3 0.1% | Low sensitivity | Excellent | Very fine grains, minimal inclusions |
| La2O3 0.5% | Moderate sensitivity | Fair | Some grain coarsening at high addition |
The results demonstrate that CeO2 and Y2O3 are particularly effective in improving both hot and cold cracking resistance, with optimal addition levels in the range of 0.05% to 0.2%. Excessive addition (above 0.5%) can lead to diminishing returns or even detrimental effects due to the formation of brittle rare earth phases.
Metallurgical Analysis
The microstructural evolution in rare earth-modified overlay weld metals follows a characteristic pattern:
- Without REE: Coarse columnar dendrites with inter-dendritic MnS and Al2O3 inclusions. The interdendritic regions are susceptible to solidification cracking.
- With 0.1% CeO2: Fine equiaxed grains with dispersed spherical Ce2O2S inclusions. The equiaxed structure eliminates the preferential crack path along columnar grain boundaries.
- With 0.1% Y2O3: Very fine grains with minimal visible inclusions. The Y atoms effectively fix both oxygen and sulfur, resulting in a clean weld metal with excellent crack resistance.
The grain refinement achieved through rare earth addition can be quantified using the ASTM grain size number, with typical improvements from ASTM 3–4 (without REE) to ASTM 6–8 (with 0.1% REE addition).
Engineering Practice Implications
Application to Overlay Welding Practice
The findings of this research have direct implications for the formulation of welding consumables used in overlay welding applications:
- Electrode and wire development: Rare earth additions of 0.05–0.2% (as REO equivalent) should be incorporated into the electrode or wire composition to improve crack resistance, particularly for overlay welding on thick sections or high-carbon base materials.
- Flux formulation: Rare earth oxides can be added to the flux composition as an alternative to metal addition. This approach provides the metallurgical benefits of REE addition without directly increasing the REE content in the weld metal, which is advantageous for controlling the final composition.
- Process optimization: The presence of rare earth additions in the consumable may allow for more aggressive welding parameters (higher cooling rates, lower preheat temperatures) without compromising crack resistance.
Practical Considerations
| Consideration | Impact | Mitigation |
|---|---|---|
| Cost of rare earth oxides | Increased consumable cost (5–15%) | Justified for critical applications |
| Consistency of REE addition | Variability in rare earth concentrate composition | Use purified rare earth oxides |
| Environmental regulations | Rare earth mining and processing concerns | Source from certified suppliers |
| Interaction with other alloying elements | Complex metallurgical interactions | Comprehensive qualification testing |
Qualification Testing Protocol
For the qualification of rare earth-modified overlay welding consumables, the following testing protocol is recommended:
- Hot cracking test: Crater test or hot wire test to assess solidification cracking susceptibility.
- Cold cracking test: Delayed cracking test at room temperature and elevated temperatures to assess hydrogen-induced cracking resistance.
- Mechanical property test: Tensile, hardness, and impact tests to ensure adequate mechanical performance.
- Metallurgical examination: Microstructural analysis at multiple magnifications to evaluate grain structure and inclusion morphology.
- Bond strength test: Peel or shear test to evaluate the bond strength between the overlay layer and the base metal.
Study Insights and Reflections
The research by Yang Qingxiang and colleagues represents a significant contribution to the understanding of rare earth metallurgy in welding applications. The systematic approach to evaluating different rare earth types and addition levels provides a solid foundation for the development of improved welding consumables. The emphasis on both hot and cold cracking resistance is particularly valuable, as many practical overlay welding applications require resistance to both cracking modes.
One important insight from this research is the recognition that different rare earth elements exhibit different effectiveness in improving cracking resistance. Y2O3 appears to be the most effective single rare earth oxide, likely due to the small ionic radius of Y3+ which provides strong lattice distortion and effective inclusion modification. CeO2 is also highly effective and is more commonly available at lower cost. La2O3, while effective, shows somewhat less improvement than CeO2 or Y2O3 at equivalent addition levels.
The research also highlights the importance of controlling the addition level within a narrow window. The optimal addition level of 0.05–0.2% represents a balance between sufficient metallurgical effect and avoidance of detrimental phase formation. This narrow window poses a practical challenge for consumable manufacturers, requiring precise control of the rare earth addition during production.
From a broader perspective, the findings of this research support the ongoing trend toward microalloying in welding consumable development. The concept of using small additions of strategically selected elements to achieve significant improvements in weldability and performance is a powerful approach that continues to drive innovation in welding materials science. The specific case of rare earth addition for cracking resistance improvement is particularly relevant to modern overlay welding applications where the demands for high-performance, crack-resistant deposits continue to increase.