Zhuojin Pipe Fitting Co., LtdMicrostructure Hardness and Wear Resistance of Carbon Arc Surfacing In-Situ Synthesized Particle-Reinforced Fe-05 Alloy Coating
Literature Overview
This paper, published by researchers from Liaoning University of Technology in the journal "Hot Working Technology" in 2020, investigates the microstructure, hardness, and wear resistance of an Fe-05 alloy coating prepared by carbon arc surfacing with in-situ synthesized reinforcing particles on a 16Mn steel substrate. The study systematically examines how the surfacing current affects the coating's microstructural evolution, hardness distribution, and wear performance. The Fe-05 alloy powder block is melted during the surfacing process and reacts with the base metal to form hard carbide and boride phases in-situ, creating a composite coating with enhanced wear resistance.
The work is significant because carbon arc surfacing is a cost-effective and widely used technique for surface hardening and repair of steel components, and the in-situ synthesis approach offers a way to enhance the coating's performance without requiring expensive alloying elements or post-treatment processes.
Coating Composition and Microstructural Analysis
The Fe-05 alloy coating is composed of a base matrix of alpha-(Fe,Cr) solid solution reinforced with in-situ synthesized hard phases including (Fe,Cr)7C3, Cr23C6, Cr7C3, Fe2B, Fe3B, and CrFeB. These hard phases form during the rapid solidification of the molten pool through eutectic reactions between the alloying elements and carbon or boron. The microstructure is characterized by a dendritic solid solution matrix with interdendritic hard phase networks.
| Surfacing Current | Average Hardness (HV) | Primary Hard Phases | Wear Volume (mm3) |
|---|---|---|---|
| 230 A | 650–750 | Cr23C6, (Fe,Cr)7C3 | 8.5–9.2 |
| 250 A | 800–950 | Cr23C6, Cr7C3, Fe2B | 5.2–5.8 |
| 270 A | 700–850 | Cr23C6, Cr7C3 | 7.8–8.5 |
The microhardness distribution shows a general decreasing trend from the coating surface toward the substrate, which is attributed to the dilution of alloying elements by the base metal as the heat input increases deeper into the coating. The highest hardness is achieved at the coating surface where the alloying element concentration is highest and the cooling rate is fastest.
Effect of Surfacing Current on Coating Properties
The surfacing current is the primary process parameter that controls the heat input and, consequently, the coating's microstructure and properties. At lower currents (230 A), the heat input is insufficient to fully melt the alloy powder, resulting in incomplete in-situ synthesis and a lower volume fraction of hard phases. At intermediate currents (250 A), the heat input is optimal for complete melting and reaction, producing a high volume fraction of fine, uniformly distributed hard phases. At higher currents (270 A), the excessive heat input causes over-melting of the base metal, increased dilution, and coarsening of the hard phases, which reduces the coating's hardness and wear resistance.
The wear resistance follows a non-monotonic trend with surfacing current, peaking at 250 A. This is because the wear resistance depends on both the hardness of the coating and the integrity of the hard phase network. At 250 A, the optimal combination of high hardness and fine, interconnected hard phases provides the best wear resistance. At 270 A, although the hardness is still relatively high, the coarsening of the hard phases and the increased dilution weaken the coating's resistance to abrasive wear.
Process Optimization and Engineering Considerations
The optimization of the carbon arc surfacing process for this coating system requires careful control of several parameters:
- Surfacing current: 250 A provides the optimal balance of hardness and wear resistance for the Fe-05 alloy on 16Mn steel.
- Surfacing speed: 150–200 mm/min is recommended to ensure adequate melting without excessive dilution.
- Electrode composition: the carbon arc electrode should have a consistent composition to ensure reproducible coating properties.
- Layer thickness: 3–5 mm per layer is optimal for achieving a dense, crack-free coating.
- Interpass temperature: should be maintained below 250 degrees Celsius to prevent excessive grain growth in the previous layer.
The FMEA analysis of the surfacing process identifies the following critical failure modes:
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Cracking | 9 | 4 | 6 | 216 | Control interpass temperature, use low-hydrogen electrode |
| Porosity | 7 | 5 | 7 | 245 | Thorough surface cleaning, controlled shielding gas flow |
| Excessive dilution | 8 | 6 | 5 | 240 | Reduce current, increase surfacing speed |
| Delamination | 9 | 3 | 4 | 108 | Improve substrate preparation, optimize heat input |
Study Insights and Implications
The research on carbon arc surfacing in-situ synthesized Fe-05 alloy coatings provides practical guidance for the surface hardening of steel components in wear-critical applications. The key finding is that the surfacing current of 250 A produces the optimal coating with a hardness exceeding 900 HV and a wear resistance that is 40–60% better than the untreated 16Mn substrate. The in-situ synthesis approach is particularly attractive because it eliminates the need for expensive alloying additions and post-heat treatment, making it a cost-effective solution for industrial applications. Engineers should note that the coating's performance is sensitive to process parameters, and strict process control is essential to ensure consistent quality. The study also highlights the importance of understanding the relationship between microstructure and wear behavior, as the type, size, and distribution of hard phases are the primary factors governing the coating's wear resistance.