CO2 Shielded Hardfacing of Iron-Based Alloy Powders A Study Note

Literature Overview

This paper by Shi Haifang, Zhang Bo, Hu Shiju, and Jiang Xintong from Liaoning Technical University, published in Ordnance Materials Science and Engineering (2011, Vol. 34, No. 6, pp. 64–66), investigates the microstructure and properties of CO2-shielded hardfacing deposits produced using Fe55 self-fluxing alloy powder and composite powders incorporating tungsten carbide (WC) and silicon carbide (SiC). The research aims to enhance the performance of the well-known Fe55 alloy powder by incorporating refractory carbide additions, and the results demonstrate a significant improvement in both hardness and wear resistance.

Background and Motivation

The Fe55 self-fluxing alloy powder is one of the most widely used hardfacing powders in industrial applications. It is characterized by a high chromium content (approximately 25–30%) that promotes the formation of hard chromium carbide phases (Cr₇C₃, Cr₃C) in a martensitic matrix, providing excellent abrasive wear resistance. However, the as-welded hardness of Fe55 is typically in the range of 50–60 HRC, which may be insufficient for applications requiring extreme wear resistance.

The incorporation of refractory carbide additions such as WC and SiC is a well-established strategy for enhancing the hardness and wear resistance of iron-based hardfacing alloys. WC (tungsten carbide) has a hardness of approximately 2400 HV and excellent thermal stability, while SiC (silicon carbide) has a hardness of approximately 2700 HV and exceptional chemical inertness. However, the effective incorporation of these hard particles into the hardfacing matrix requires careful control of the welding process parameters and powder composition.

Core Technical Findings

Powder Composition Optimization

The study investigated three powder compositions:

1. Fe55 base powder: The standard self-fluxing alloy powder without refractory carbide additions.
2. Fe55 + 15% WC: The Fe55 powder blended with 15 wt% tungsten carbide particles.
3. Fe55 + 15% WC + 6% SiC: The Fe55 powder blended with 15 wt% WC and 6 wt% SiC particles.

The optimal composition was identified as Fe55 + 15% WC + 6% SiC, which achieved the highest hardness and wear resistance.

Microstructural Analysis

The microstructure of the Fe55 + 15% WC + 6% SiC hardfacing deposit was characterized by metallographic examination and X-ray diffraction analysis. The key findings include:

1. Metallurgical bonding: The hardfacing deposit achieved metallurgical bonding with the 45# steel base metal, indicating good wetting and adhesion. This is critical for preventing spalling and delamination during service.
2. Matrix structure: The deposit microstructure consists of martensite and retained austenite, which is characteristic of high-chromium iron-based hardfacing alloys. The martensitic matrix provides the base hardness and toughness, while the retained austenite contributes to toughness and crack resistance.
3. Reinforcement phases: The XRD analysis identified the presence of W₃Cr₁₂Si₅ and Cr₃Si intermetallic compounds in addition to the expected Cr₇C₃ and Cr₃C carbide phases. These intermetallic compounds form through the reaction between the Fe55 matrix and the WC and SiC additions during the welding process. The formation of these complex intermetallics is a significant finding, as it indicates that the refractory carbide additions do not simply remain as inert particles but participate in the alloy chemistry of the deposit.
4. Phase distribution: The hard phases are distributed throughout the martensitic matrix, providing a composite-like microstructure that combines the toughness of the metallic matrix with the hardness of the refractory carbide and intermetallic phases.

Hardness and Wear Resistance

The Fe55 + 15% WC + 6% SiC hardfacing deposit achieved a microhardness of 913 HV, which represents a substantial improvement over the base Fe55 powder. The wear resistance was quantified as approximately twice that of the unmodified Fe55 deposit. This two-fold improvement in wear resistance, combined with the high hardness, makes this composite powder formulation suitable for the most demanding abrasive wear applications.

Technical Parameters Summary

Engineering Practice Implications

CO2 Shielded Arc Hardfacing Process Considerations

The CO2 shielded arc welding process is a widely available and cost-effective method for hardfacing powder application. However, several process-specific considerations must be addressed:

1. Powder feeding rate: The powder feeding rate must be precisely controlled to ensure uniform deposit composition and thickness. The addition of WC and SiC particles increases the powder density, which may require adjustment of the powder feeder settings.
2. Arc stability: CO2 shielding can produce arc instability at low currents, which may affect powder transfer efficiency and deposit quality. The use of a pulsing current or a mixed gas shielding (such as CO2 + Ar) may be beneficial.
3. Porosity control: CO2 is an active gas that can promote nitrogen and hydrogen porosity if the shielding coverage is inadequate. Proper gas flow rates (typically 15–25 L/min) and proper torch-to-workpiece distance are critical.
4. Dilution management: The dilution from the 45# steel base metal is typically in the range of 10–30% for single-pass hardfacing. The high alloy content of the Fe55 + WC + SiC composite powder provides sufficient alloy reserve to maintain the desired deposit composition even with moderate dilution.

Application Scenarios

The Fe55 + 15% WC + 6% SiC composite powder hardfacing system is particularly suitable for the following applications:

Comparison with Other Hardfacing Systems

Key Questions and Reflections

The study raises several important questions regarding the long-term performance of the composite powder hardfacing system. First, the effect of thermal cycling on the stability of the W₃Cr₁₂Si₅ and Cr₃Si intermetallic phases is not investigated. These phases may be susceptible to decomposition at elevated temperatures, which could lead to a reduction in hardness and wear resistance during service.

Second, the particle size distribution of the WC and SiC additions is not specified. The particle size has a significant influence on the hardness and wear resistance of the deposit, as larger particles provide higher local hardness but may create stress concentrations that promote crack initiation. A bimodal particle size distribution may be optimal for achieving both high hardness and good toughness.

Third, the study does not address the effect of the composite powder on the toughness of the hardfacing deposit. While the wear resistance is doubled, the impact resistance may be compromised by the presence of hard, brittle intermetallic phases. A systematic evaluation of the fracture toughness and impact resistance of the composite deposit would be valuable for applications involving impact loading.

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