Wear Resistance of CO2 Welding with Sprayed High Carbon Chromium Iron Composite Surfacing Layer

Literature Overview

This study, published in 2011 in the Welding Journal (焊接学报), authored by Yuan Kaifeng from the Ministry of Education Metal Wear-Resistant Materials and Surface Technology Research Center, Jiamusi University, and colleagues, investigates a novel composite surfacing technique that combines CO2 gas shielded arc welding (GMAW) with external powder spraying of high carbon chromium iron alloy. The research is directly relevant to engineers seeking cost-effective wear protection solutions for piping components, mining equipment, and material handling systems.

Core Technical Methodology

The study employed a modified CO2 welding setup with an external powder spraying nozzle to deposit a composite surfacing layer. The base wire was a standard H08Mn2Si flux-cored or solid wire, and high carbon chromium iron alloy powder was sprayed into the arc zone to modify the weld metal composition. The key process parameters investigated were:

• Powder feed rate (varied across a range)
• Powder carrier gas flow rate (varied across a range)

The optimal conditions were identified as a powder feed rate of 800 g/h and a carrier gas flow rate of 5 L/min.

Key Results

Microstructural Analysis

The study revealed several important microstructural features of the composite surfacing layer:

1. Dislocation tangles: The high carbon chromium iron powder accelerated the cooling rate of the molten pool, promoting the formation of non-equilibrium microstructures characterized by dense dislocation tangles. These dislocation structures contribute to work hardening and increased hardness.
2. Lath martensite distortion: The accelerated cooling produced lath martensite with fractured and distorted morphology. The distorted lath structure creates additional barriers to dislocation motion, enhancing strength and hardness.
3. Twinned martensite irregularity: The twinned martensite exhibited irregular arrangement, which is indicative of rapid solidification conditions. This irregular microstructure contributes to improved wear resistance by impeding crack propagation.
4. Grain refinement: The sprayed high carbon chromium iron powder particles acted as heterogeneous nucleation sites during solidification, resulting in finer grain sizes compared to conventional CO2 welding without powder addition. Finer grains improve both strength and toughness according to the Hall-Petch relationship.
5. Cr carbide formation: The high carbon and chromium content from the sprayed powder promoted the formation of chromium carbides (Cr7C3, Cr23C6) during solidification. These hard carbide phases provide excellent resistance to abrasive wear.

Engineering Practice Considerations

For piping and equipment engineers, this composite surfacing technique offers several practical advantages:

• Equipment compatibility: The process uses standard CO2 welding equipment with only a simple powder spraying attachment, making it accessible to workshops without specialized surfacing equipment.
• Cost-effectiveness: The base wire is a standard low-cost consumable (H08Mn2Si), and the high carbon chromium iron powder is relatively inexpensive compared to specialized hardfacing wires.
• Wear resistance improvement: The 4× improvement in wear resistance compared to conventional CO2 welding is substantial and could significantly extend the service life of wear-prone components.
• Process flexibility: The powder feed rate and gas flow rate can be adjusted to control the alloy composition and microstructure of the surfacing layer, allowing optimization for specific service conditions.

However, several practical challenges must be addressed:

• Powder utilization efficiency: Not all sprayed powder is deposited in the weld; some is lost to the atmosphere or deposited on surrounding surfaces. The powder utilization rate should be measured and optimized to minimize waste.
• Deposition rate: The effective deposition rate of the composite layer may be lower than conventional surfacing due to the need for multiple passes and the time required for powder feeding.
• Process consistency: Maintaining consistent powder feed rate and gas flow rate during automated or semi-automated welding is critical for uniform coating properties. Process monitoring and control systems should be implemented.

Key Questions and Reflections

The study does not address the long-term wear behavior of the composite coating under realistic service conditions, including thermal cycling, corrosion-wear interaction, and impact loading. In my experience, the wear resistance of surfacing layers is often significantly affected by the operating environment, and laboratory wear test results may not directly correlate with field performance.

Another important consideration is the effect of the composite coating on the fatigue properties of the base component. The residual stresses introduced during surfacing, combined with the hard and potentially brittle microstructure of the coating, could affect the fatigue life of the component. Fatigue testing should be conducted to establish the allowable stress range for components with this type of coating.

The study also does not discuss the dilution ratio between the base metal and the composite overlay. The dilution ratio directly affects the final alloy composition and microstructure of the coating, and should be controlled and reported in any qualification data. For the 800 g/h powder feed rate and 5 L/min gas flow rate identified as optimal, the dilution ratio should be measured to establish the relationship between process parameters and coating composition.

Study Insights and Implications

This research demonstrates that a simple modification to conventional CO2 welding—adding external powder spraying—can significantly enhance the wear resistance of the deposited layer. The 4× improvement in wear resistance at 55 HRC hardness is achieved through a combination of microstructural refinement, increased hard phase content, and enhanced dislocation density. The approach is particularly attractive for applications where the base component is made of carbon steel and the wear protection requirement is moderate to high, such as pipe elbows in slurry lines, valve bodies in mineral processing, and conveyor components in coal handling systems.

The microstructural analysis reveals that the accelerated cooling rate induced by the powder addition is the primary driver of the wear resistance improvement. This insight suggests that other methods of increasing cooling rate, such as chill plates or flux-assisted cooling, could potentially achieve similar results. The study also highlights the importance of understanding the relationship between solidification conditions, microstructure, and wear behavior in designing effective surfacing solutions.

For engineers developing welding procedure specifications for wear-resistant overlays, this study provides valuable guidance on the importance of process parameter control and microstructural characterization. The combination of XRD and TEM analysis used in this study is essential for understanding the phase composition and microstructural features that contribute to wear resistance. Engineers should ensure that their qualification procedures include comprehensive microstructural analysis and correlate the findings with wear test results to establish reliable property-structure relationships.