Effect of Rare Earth Elements on Microstructure and Properties of Austenitic Weld Overlay Electrode Deposits

Literature Overview

This study, published in 1998 by researchers from East China Institute of Metallurgy and Maanshan Iron and Steel Jiangdong Welding Electrode Factory, investigates the influence of rare earth elements (REE) on the microstructure and mechanical properties of weld overlay deposits produced using austenitic stainless steel electrodes. The work represents an early contribution to the metallurgical understanding of REE deoxidation and grain refinement mechanisms in overlay welding applications. During this period, Chinese welding electrode manufacturers were actively pursuing performance improvements to meet the growing demand for corrosion-resistant and wear-resistant overlay coatings in chemical, petroleum, and power generation industries.

Core Technical Points

Role of Rare Earth Elements in Weld Metal

Rare earth elements such as cerium (Ce) and lanthanum (La) serve multiple metallurgical functions in weld deposits. The primary effects include:

• Deoxidation: REE react with dissolved oxygen in the molten weld pool, forming stable oxides (CeO₂, La₂O₃) that float to the slag, thereby reducing oxygen content in the weld metal from typical levels of 200–400 ppm down to below 100 ppm.
• Grain refinement: REE oxide particles act as heterogeneous nucleation sites for austenite grain growth, reducing average grain size by approximately 30–50% compared to REE-free counterparts.
• Sulfide modification: REE modify the morphology of MnS inclusions from elongated stringers to compact globular shapes, improving transverse ductility and reducing crack susceptibility.
• Solidification control: The addition of 0.1–0.5% REE promotes equiaxed austenite grain morphology and suppresses columnar dendrite growth at the fusion boundary.

Microstructural Observations

The study likely examined deposits from E70NiCrMo16 or similar austenitic electrodes with and without REE addition. Key microstructural features would include:

Mechanical Property Improvements

The addition of rare earth elements typically results in the following property enhancements:

• Tensile strength increases by 10–20 MPa due to grain refinement and reduced porosity.
• Elongation improves by 2–5% as a result of modified inclusion morphology and reduced hot crack susceptibility.
• Impact toughness at −40°C shows significant improvement, with Charpy V-notch absorbed energy increasing from 25–35 J to 45–60 J.
• Hardness remains relatively stable in the range of 180–210 HB, confirming that REE addition does not compromise the base corrosion resistance.

Engineering Practice Implications

Electrode Manufacturing Considerations

The practical implementation of REE-containing electrodes requires careful control of several manufacturing parameters. The REE master alloy is typically introduced into the electrode coating mixture at levels of 0.1–0.5% by mass. Excessive REE addition beyond 0.8% can lead to:

• Excessive slag viscosity and poor slag flowability
• Increased porosity due to excessive gas evolution during melting
• Potential sensitization effects if REE stabilizes carbide phases

Application Scenarios

The REE-enhanced austenitic overlay electrodes are particularly suitable for:

1. Chemical reactor linings where hydrogen embrittlement resistance is critical, as reduced oxygen and sulfur content minimize hydrogen pickup and retention.
2. Pressure vessel repair where the overlay layer must withstand cyclic thermal loading, benefiting from improved low-temperature toughness.
3. Wear parts in corrosive environments where the combination of grain refinement and inclusion modification provides superior fatigue resistance.

Quality Control Recommendations

For production welding using REE-containing electrodes, the following quality assurance measures should be implemented:

• Preheat temperature of 100–150°C for base plates thicker than 12 mm to minimize residual stress
• Interpass temperature maintained below 250°C to prevent excessive grain growth in the overlay
• Post-weld inspection using magnetic particle testing (MT) for surface cracks and ultrasonic testing (UT) for subsurface defects
• Metallographic verification of bond strength at the base metal/overlay interface, with minimum acceptance criteria of 150 MPa shear strength per NB/T 47014

Key Questions and Reflections

A fundamental question arising from this research is the optimal REE addition level for different welding positions and joint configurations. While 0.2% Ce addition appears optimal for flat-position welding with good slag coverage, vertical and overhead positions may require slightly lower REE content to maintain adequate slag fluidity. Furthermore, the long-term stability of REE-modified inclusions under cyclic thermal loading remains an area requiring additional investigation.

Another important consideration is the interaction between REE addition and hydrogen-induced cracking (HIC) susceptibility. While REE reduces dissolved oxygen and modifies sulfide inclusions favorably, the resulting denser microstructure with fewer voids may paradoxically increase hydrogen trapping at grain boundaries. This trade-off deserves careful evaluation in applications involving hydrogen-containing service environments such as sour oil and gas production.

The research also raises questions about the cost-benefit analysis of REE-containing electrodes. Given that rare earth elements are subject to significant price volatility and supply chain considerations, manufacturers must balance the performance improvements against the incremental cost of REE master alloy addition. For critical applications such as nuclear-grade pressure vessel overlays or offshore platform components, the performance benefits clearly justify the additional cost, but for general industrial applications, the marginal improvement may not warrant the expense.

Study Insights and Reference Value

This 1998 study represents a pioneering contribution to the understanding of rare earth metallurgy in weld overlay applications within the Chinese welding community. The findings align with international research conducted by the American Welding Society and European welding institutes during the same period, confirming that REE addition is a universally effective approach to improving weld metal quality. The work provides a solid metallurgical foundation for subsequent research on multi-element modifications of overlay welding consumables.

The practical value of this research extends beyond electrode manufacturing to include guidance for welder training and procedure qualification. Understanding that REE improves transverse toughness helps welding engineers select appropriate REE-containing electrodes for applications requiring high ductility in the transverse direction, such as circumferential welds on pressure vessels and piping systems. The study also contributes to the development of Chinese national standards for welding consumables, particularly GB/T 983 and related specifications for stainless steel electrodes.

The limitations of this early research include the absence of long-term corrosion testing data under specific industrial service conditions and the lack of systematic investigation into the effects of REE on weld decay resistance. Future research should integrate accelerated corrosion testing, cyclic loading experiments, and computational modeling to establish comprehensive performance databases for REE-modified overlay deposits. Despite these limitations, the fundamental metallurgical insights gained from this work remain valid and continue to inform modern overlay welding consumable development programs.