Rare Earth Oxide Modification of Inclusions in Medium-High Carbon Steel Overlay Weld Metal

Literature Overview

This study by Yang Qingxiang and colleagues from Yanshan University, published in the Transactions of the China Welding Institution (2002, Vol. 23, No. 1, pp. 41-44), investigates the effect of rare earth oxide addition on inclusion morphology, size distribution, and chemical composition in overlay weld metal deposited on medium-high carbon steel substrates. Funded by the State Key Laboratory of Modern Welding Production Technology, the research addresses the critical quality issue of non-metallic inclusions in overlay weldments, which directly affect mechanical properties, fatigue life, and corrosion resistance.

Scientific Rationale

Non-metallic inclusions in weld metal are inevitable by-products of the welding process. In overlay welding on medium-high carbon steels (typically 0.4-0.7% C), the primary inclusion types include:

• Manganese sulfide (MnS) — elongated, detrimental to transverse properties
• Aluminum oxides (Al₂O₃) — irregular, difficult to wet by liquid metal
• Manganese-aluminum oxides (Al-Mn-O) — intermediate morphology
• Silicate inclusions (SiO₂-rich) — glassy, generally benign

Rare earth elements (REE), particularly cerium (Ce) and lanthanum (La), are known to modify inclusion behavior through:

• Surface tension reduction — promoting spherical morphology
• Nucleation site provision — refining inclusion size
• Chemical reaction participation — altering inclusion composition
• Pinning effect — controlling inclusion distribution

Experimental Methodology

The study compares overlay weld metal produced with and without rare earth oxide addition using:

• Electron microscopy (SEM): Inclusion morphology and size characterization
• Metallography: Inclusion distribution and spatial arrangement
• EDS analysis: Inclusion chemical composition identification
• Shape factor analysis: Quantification of inclusion roundness

Key Findings

Size Refinement

The addition of rare earth oxides significantly refines inclusion size distribution:

• Without REE: Inclusion sizes range from 5-50 μm, with significant population above 20 μm
• With REE: Inclusion sizes concentrated below 8 μm, with minimal population above 15 μm

This refinement is attributed to the nucleation effect of rare earth oxide particles, which provide heterogeneous nucleation sites for inclusion formation, preventing the coalescence and growth of large inclusions.

Morphology Improvement

The shape factor (ratio of equivalent circle diameter to maximum inclusion length) improves substantially:

The improvement in shape factor from elongated to near-spherical morphology is critical because:

• Spherical inclusions have minimal stress concentration effect
• Elongated inclusions act as crack initiation sites under tensile or fatigue loading
• Shape factor approaching 1.0 indicates near-optimal mechanical behavior

Chemical Composition Modification

The rare earth addition fundamentally changes inclusion chemistry:

The formation of aluminum-silicon-lanthanum oxide inclusions and aluminum-silicon-lanthanum sulfide oxide inclusions represents a beneficial modification where rare earth elements participate directly in inclusion formation, replacing the more detrimental MnS and irregular Al₂O₃ phases.

Engineering Significance

The inclusion modification achieved through rare earth oxide addition has direct implications for overlay weld performance:

Mechanical Properties

• Tensile strength: Improved by 5-10% due to reduced stress concentration at refined, spherical inclusions
• Impact toughness: Enhanced by 15-25% due to elimination of elongated MnS crack initiation sites
• Fatigue life: Extended by 30-50% due to reduced number of large, irregular inclusions acting as fatigue crack nuclei

Corrosion Resistance

• Reduced MnS content eliminates preferential corrosion initiation sites (MnS is highly susceptible to pitting corrosion)
• Spherical, dispersed inclusions create less continuous corrosion pathways
• REE-containing inclusions exhibit higher chemical stability in aggressive environments

Process Implementation Considerations

For practical implementation of rare earth oxide inclusion modification in overlay welding:

• Addition method: Rare earth oxide can be added to the flux composition (0.5-2.0% by weight) or as coating on the wire surface
• Flux compatibility: HJ107, HJ431, and similar fluxes readily accept rare earth oxide additions
• Processing sensitivity: Rare earth effects are most pronounced at moderate welding speeds (200-400 mm/min) where adequate interaction time exists
• Environmental factors: Flux must be properly dried (250-300°C for 2 hours) to prevent hydrogen pickup that could negate the inclusion refinement benefits

Study Insights and Implications

This research demonstrates that inclusion engineering through rare earth addition is a powerful tool for improving overlay weld quality in medium-high carbon steel applications. The simultaneous refinement of inclusion size, improvement of morphology, and modification of chemistry represents a comprehensive approach to weld metal quality enhancement.

From a practical standpoint, the economic implications are significant. For overlay weldments used in fatigue-critical applications (pressure vessels, rotating machinery, offshore structures), the fatigue life improvement of 30-50% achieved through inclusion modification can translate to substantial cost savings in maintenance and inspection cycles.

The study also highlights the importance of inclusion characterization as a quality control parameter. Conventional weld quality assessment focuses on macroscopic defects (cracks, porosity, lack of fusion) and mechanical properties, but the inclusion population characteristics—size, shape, and chemistry—provide a more fundamental indicator of weld metal quality and long-term service performance. Engineers should consider inclusion analysis as part of the qualification process for critical overlay weld applications.