Zhuojin Pipe Fitting Co., LtdPreparation and Research of Graphite Self-Lubricating Iron-Based Alloy Surfacing Layer
Literature Overview
This paper, authored by Wang Songge, Zhuang Minghui, Ma Zhen, and Li Muqin from the College of Materials Science and Engineering at Jiamusi University, was published in the Journal of Jiamusi University (Natural Science Edition) (Vol. 37, No. 5, 2019, pp. 790-793). Supported by the National Natural Science Foundation of China (Grant No. 31370979) and a Jiamusi University Innovation Project (XZYE2018-18), the study investigates the preparation of a graphite self-lubricating iron-based alloy surfacing layer on Q235 steel using CO2 gas shielded welding (GMAW). The key innovation is the use of SiC addition to the flux-cored wire to generate in-situ graphite lubricating phases through high-temperature reduction reactions during welding.
Core Technical Approach
The research focuses on the concept of in-situ self-generation of graphite lubricating phases within the surfacing layer. Rather than adding pre-formed graphite particles (which tend to oxidize and decompose during the high-temperature welding process), SiC is added to the flux-cored wire. During welding, the SiC reacts with oxygen in the arc atmosphere and the molten pool, releasing carbon that precipitates as graphite during solidification. This approach ensures that the graphite phases are thermodynamically stable and uniformly distributed within the surfacing matrix.
Optimal Process Parameters
The authors systematically varied welding parameters to identify the conditions most favorable for graphite phase formation:
| Parameter | Optimal Value | Range Studied |
|---|---|---|
| Welding current | 200 A | 150-250 A |
| Welding voltage | 20-22 V | 18-24 V |
| Welding speed | 1.5 mm/s | 1.0-2.5 mm/s |
| Single pass width | 20 mm | 15-25 mm |
| SiC addition (wt.%) | 25% | 10-35% |
Under these optimal conditions, the surfacing layer exhibits minimal welding spatter, good surface formation, and uniform distribution of in-situ generated graphite phases. The graphite particles appear as spherical morphology with sizes ranging from 8 to 15 micrometers, reaching a maximum volume fraction of approximately 8%.
Microstructural Evolution with Alloying Additions
The study systematically investigates how additional alloying elements affect the graphite phase characteristics and overall performance:
| Formulation | Graphite Volume Fraction | Graphite Size (μm) | Macro Hardness (HRC) | Wear Performance |
|---|---|---|---|---|
| Base (25% SiC) | ~8% | 8-15 | 38.9 | Good self-lubrication |
| +4% Ni | ~4% | Refined | 42.7 | Best wear resistance |
| +4% Ni +40% FeB +15% B4C | ~2% | Further refined | 53.3 | High hardness, reduced lubrication |
Metallurgical Mechanism
The addition of 4 wt.% Ni powder to the flux-cored wire refines the graphite phase and increases the volume fraction stability during welding. Nickel acts as a grain refiner and promotes the nucleation of graphite particles, resulting in smaller and more uniformly distributed graphite phases. The wear resistance is optimized in this formulation because the balance between lubrication (from graphite) and hardness (from the Ni-modified matrix) is ideal for reducing friction and wear simultaneously.
When 40 wt.% boron iron powder and 15 wt.% boron carbide powder are additionally incorporated, the hard boride phases significantly increase the macro hardness to 53.3 HRC. However, the graphite volume fraction decreases to approximately 2%, which reduces the self-lubricating capability. This trade-off between hardness and lubrication is a fundamental design consideration for self-lubricating surfacing materials.
Performance Characterization
The surface morphology of the surfacing layer is characterized by smooth, continuous weld beads with no visible porosity or cracking under the optimal process parameters. The macro hardness of 38.9 HRC for the base formulation is adequate for general wear applications, while the Ni-modified formulation at 42.7 HRC provides enhanced load-bearing capacity without sacrificing self-lubrication properties.
The wear testing (typically pin-on-disk or block-on-ring configurations) demonstrates that the self-lubricating surfacing layer significantly reduces the coefficient of friction compared to the base Q235 steel. The graphite phases act as solid lubricants, forming a transfer film on the counterface during sliding contact, which reduces direct metal-to-metal contact and consequently reduces wear rates.
Engineering Application and Limitations
The self-lubricating surfacing technology has direct applications in components subject to boundary or mixed lubrication conditions, such as:
- Hydraulic cylinder barrels: Where oil supply is limited or intermittent
- Guide rails and slides: In machine tools and automation equipment
- Valve stems and shafts: In high-pressure or high-temperature environments
- Pump shafts and bearings: Where conventional lubrication is impractical
However, several limitations must be acknowledged:
- Load capacity: The relatively low hardness (38.9-42.7 HRC for the best self-lubricating formulations) limits the maximum contact stress the surfacing layer can withstand.
- Temperature sensitivity: Graphite can oxidize above 500°C in air, limiting the service temperature range.
- Wear mode dependency: The self-lubricating effect is most effective under sliding wear conditions; under severe impact or rolling contact, the graphite phases may be crushed or extruded, reducing lubrication effectiveness.
Study Insights and Implications
This research demonstrates a practical and cost-effective approach to developing self-lubricating surfacing materials through in-situ graphite generation. The use of SiC as a carbon source, rather than pre-formed graphite, is a significant advancement that addresses the historical challenge of graphite loss during welding. The systematic investigation of Ni addition as a graphite refiner and the subsequent hardening with boride phases provides a clear design roadmap for tailoring the wear-lubrication balance according to specific application requirements. For engineers designing tribological surfacing solutions, this work highlights the importance of understanding the fundamental mechanisms of phase formation during welding and the ability to manipulate these mechanisms through alloy composition and process parameter optimization.