Development of Rare Earth Modified Surfacing Electrodes for Hot Shear Blades

Literature Overview

The research by Yang Qingxiang et al. (Yanshan University, 1997) presents a practical solution for extending the service life of hot shear blades used in continuous casting machines. Hot shear blades made from 3Cr2W8V high-speed steel experience severe wear during operation due to the combination of high temperature, abrasive contact with hot steel slabs, and thermal fatigue. The authors developed a surfacing electrode incorporating rare earth oxides to significantly improve the wear resistance and service life of these critical components.

Core Technical Approach

The innovation centers on the addition of rare earth oxides (primarily La2O3, CeO2, and Y2O3) to the flux coating of the surfacing electrode. Rare earth elements are well-known micro-alloying additions that influence solidification behavior, inclusion morphology, and mechanical properties through several mechanisms.

Rare Earth Functions in Surfacing Deposits

Function	Mechanism	Effect on Performance
Inclusion modification	React with S, O to form rare earth sulfides/oxides	Reduce hot shortness, improve hot workability
Grain refinement	Adsorb on growing grain boundaries	Fine grain structure, improved toughness
Thermoplasticity enhancement	Modify intergranular phase composition	Improved resistance to thermal fatigue cracking
Carbide modification	Influence carbide morphology and distribution	More uniform wear resistance
Deoxidation	Strong affinity for oxygen	Cleaner metal, reduced porosity

Microstructural Effects

The addition of rare earth oxides produces several beneficial microstructural changes in the deposited layer:

Inclusion modification: Rare earth elements react with sulfur and oxygen impurities to form La2O3·Al2O3 and La2O2S inclusions, which are more spherical and less detrimental than elongated MnS inclusions. This improves hot ductility and reduces hot cracking susceptibility during deposition.
Grain refinement: Rare earth oxides act as heterogeneous nucleation sites during solidification, reducing the primary dendrite arm spacing by 30-50%. The refined microstructure provides more uniform mechanical properties and better resistance to crack propagation.
Thermoplasticity improvement: The modified inclusion morphology and refined grain structure significantly improve the material's ability to deform at elevated temperatures without cracking. This is critical for hot shear blades that experience repeated thermal cycling.

Performance Results

Service Life Comparison

Component	Material	Service Life (Cycles)	Relative Improvement
Original hot shear blade	3Cr2W8V (bulk)	500-1000	Baseline
Surfaced blade	3Cr2W8V + rare earth electrode	1500-8000	3-8 times
Surfaced blade (optimized)	3Cr2W8V + rare earth electrode	3000-8000	3-8 times

Mechanical Property Comparison

Property	3Cr2W8V Base Metal	Surfaced Layer (with RE)	Improvement
Hardness (HV30)	850-900	900-950	5-10%
Hot hardness (500°C, HV)	750-800	800-850	6-7%
Impact toughness (J/cm²)	35-45	50-70	30-50%
Thermal fatigue life (cycles to crack)	500-800	1500-3000	2-4 times
Abrasive wear rate (mg/1000m)	15-25	8-15	30-40% reduction

Process Parameters and Application Considerations

Recommended Surfacing Parameters

Parameter	Specification	Notes
Electrode type	SMAW, rare earth modified flux	Low hydrogen type preferred
Welding current	100-140 A	Adjust for layer thickness
Arc voltage	22-28 V	Maintain stable arc
Travel speed	150-250 mm/min	Control dilution
Layer thickness	2-4 mm	Balance protection with stress
Number of passes	1-3	Depends on blade condition
Preheat	100-200°C	Reduce thermal stress
Interpass temperature	<300°C	Prevent excessive grain growth

Application Procedure

The surfacing repair procedure for hot shear blades follows a systematic approach:

Surface preparation: Remove existing wear surface by grinding or machining to expose sound base metal. Ensure the surface is clean and free of scale, rust, and contaminants.
Preheating: Apply uniform preheat to reduce thermal stress during welding. For thick blades, consider stepped preheating to manage temperature gradients.
Surfacing: Apply 1-3 passes of the rare earth modified electrode, maintaining consistent bead overlap (50-70%) and uniform layer thickness.
Post-weld treatment: Allow controlled cooling in air or apply mild post-weld heat treatment if required by the application.
Machining: Machine the surfaced layer to final blade geometry and surface finish requirements.

Key Questions and Reflections

The 3-8 times improvement in service life is remarkable, but engineers should consider several practical questions. How does the performance vary with different rare earth oxide compositions and ratios? What is the optimal La/Ce/Y ratio for specific service conditions? How does the surfaced layer perform after extended thermal cycling beyond the initial test period? Additionally, the cost-benefit analysis should consider not only material costs but also downtime reduction and maintenance frequency.

The rare earth modification approach is particularly attractive because rare earth oxides are relatively inexpensive compared to cobalt or nickel additions, yet they provide substantial performance improvements. This makes the technology accessible to a wide range of industrial users.

Study Insights and Implications

This research demonstrates the significant potential of rare earth micro-alloying in improving surfacing performance for high-temperature wear applications. The mechanism of action—improving thermoplasticity and refining microstructure through inclusion modification—is particularly elegant because it addresses the fundamental failure mechanism (thermal fatigue cracking) rather than simply increasing hardness. For continuous casting operations where hot shear blade replacement represents significant downtime and cost, this technology offers a compelling solution. The approach could be extended to other hot-work tool applications including roll surfaces, mold components, and furnace fixtures.