CO2 Arc Overlay of Iron-Based Alloy on 45# Steel

Literature Overview

This paper published in Ordnance Materials Science and Engineering (2011, Vol. 34, No. 6, pp. 64-66) by Shi Haifang, Zhang Bo, Hu Shiju, and Jiang Xintong investigates the microstructure and properties of CO₂ arc overlay deposits produced using Fe55 self-fluxing alloy powder and modified Fe55 powder blends on 45# steel substrates. The research aimed to improve the performance of Fe55 self-fluxing alloy powder through the addition of tungsten carbide (WC) and silicon carbide (SiC) particles.

Technical Approach and Materials

Overlay Materials Studied

1. Base material: Fe55 self-fluxing alloy powder
2. Modified material: Fe55 + 15% W (WC) + 6% SiC mixed powder
3. Substrate: 45# steel (medium carbon steel, approximately 0.45% C)

Characterization Methods

• Metallographic examination using XJL-02A optical microscope
• Phase analysis using 2500PC X-ray diffractometer
• Microhardness testing
• Abrasive wear testing

Key Technical Findings

Metallurgical Bonding

The Fe55 + 15% W + 6% SiC overlay achieved metallurgical bonding with the 45# steel substrate. This is a critical requirement for any overlay application, as poor bonding leads to delamination under service loading. The CO₂ arc welding process provides sufficient heat input to achieve complete fusion at the overlay-substrate interface, while the self-fluxing nature of the Fe55 powder provides slag protection and promotes wetting of the substrate.

Microstructure of Modified Overlay

The overlay microstructure consisted of:

• Matrix: Martensite with retained austenite
• Reinforcement phases: W₃Cr₁₂Si₅ (tungsten chromium silicide) and Cr₃Si (chromium silicide)

The presence of retained austenite in the overlay is beneficial for impact resistance and provides transformation toughening under stress. The martensitic matrix provides the base hardness, while the hard silicide phases provide additional wear resistance through dispersion strengthening.

Performance Results

The achievement of 913 HV microhardness in the modified overlay is remarkable. This value indicates an extremely hard microstructure, likely resulting from the combined effects of the martensitic matrix, fine hard silicide particles, and possible carbon enrichment at particle-matrix interfaces. The doubling of wear resistance compared to the unmodified Fe55 overlay demonstrates the effectiveness of the WC and SiC additions.

Interpretation of Technical Points

Role of WC Addition

The addition of 15% tungsten (as WC particles) serves multiple purposes:

1. Direct reinforcement: WC particles themselves are extremely hard (approximately 2400 HV) and provide direct resistance to abrasive wear.
2. Alloying effect: Tungsten dissolves in the austenite during melting and promotes the formation of tungsten-rich carbides and silicides upon solidification.
3. Matrix modification: Tungsten stabilizes austenite and modifies the transformation behavior, potentially increasing retained austenite content.

Role of SiC Addition

The addition of 6% SiC serves to:

1. Provide silicon for silicide formation: Silicon reacts with chromium and tungsten to form W₃Cr₁₂Si₅ and Cr₃Si phases.
2. Direct reinforcement: Unreacted SiC particles provide additional hard phase content.
3. Modification of solidification behavior: Silicon influences the solidification path and phase selection during cooling.

CO₂ Shielding Gas Effects

The use of CO₂ as the shielding gas introduces carbon into the weld pool through the dissociation of CO₂ at high temperatures. This additional carbon:

• Increases the carbon content of the overlay weld metal
• Promotes martensite formation upon solidification
• Contributes to the hardness of the overlay

The combination of CO₂ shielding and Fe55 powder composition results in a carbon-rich weld metal that transforms to martensite with retained austenite upon cooling.

Engineering Practice Integration

The CO₂ arc overlay technique offers several practical advantages:

• Equipment simplicity: CO₂ arc welding equipment is widely available and relatively inexpensive
• Powder feed flexibility: Self-fluxing powders can be fed through standard powder feeding systems
• Good deposition rates: CO₂ arc welding provides high deposition rates compared to GTAW or plasma arc methods
• No preheating required: The self-fluxing powder provides adequate slag protection without substrate preheating

The application of this technology is relevant to:

• Pipeline components subject to erosion-corrosion
• Pump impellers and valve seats in mining applications
• Mining equipment components (shovel teeth, dragline buckets)
• Industrial grinding and crushing equipment

Key Questions and Reflections

1. Particle size effects: The paper does not specify the particle size distribution of the WC and SiC additions. Particle size significantly affects the wear resistance of composite overlays—finer particles generally provide better wear resistance through more uniform distribution, while coarser particles may provide better resistance to severe abrasion.
2. Optimization of W and SiC ratios: The 15% W and 6% SiC composition was tested, but whether this represents the optimal ratio is unclear. Systematic variation of both parameters would be needed to identify the true optimum.
3. Residual stress: The microhardness of 913 HV suggests a highly stressed microstructure. The residual stress state in the overlay and at the overlay-substrate interface is not addressed, which is critical for fatigue and fracture behavior.
4. Impact toughness: The paper focuses on hardness and wear resistance but does not address impact toughness. For many engineering applications, the overlay must also resist impact damage, and the extremely hard microstructure may have limited toughness.
5. Thermal cycling stability: The W₃Cr₁₂Si₅ and Cr₃Si phases must maintain their stability during thermal cycling in service. The paper does not address long-term thermal stability of the overlay.

Study Insights and Implications

This research demonstrates that simple modifications to existing Fe55 self-fluxing alloy powder—specifically the addition of WC and SiC particles—can significantly enhance overlay performance. The doubling of wear resistance and achievement of 913 HV microhardness represent substantial improvements over the base material. For engineers involved in surface engineering and overlay welding, this work highlights the potential of composite powder overlays to achieve performance levels that would otherwise require more expensive alloy systems or more complex processing. The CO₂ arc welding process provides an economical and widely accessible method for applying these composite overlays in industrial settings.