Zhuojin Pipe Fitting Co., LtdEffect of Silicon Content on Microstructure and Wear Resistance of Austenitic Open Arc Surfacing Alloys
Literature Overview
This paper by Zhang Licheng, Gong Jianxun, and Yao Huiwen from Xiangtan University was published in Materials Protection (2018, Vol. 51, Issue 3, pp. 26–30), supported by the Hunan Provincial Natural Science Foundation (2015JJ5031). It investigates the influence of silicon content on the microstructure and wear resistance of austenitic Fe-C-Mn-Cr-Nb-V-Ti based wear-resistant alloys produced by flux-cored wire self-shielded open arc surfacing. The study is highly relevant for engineers developing surface hardening solutions for abrasion-critical components.
Background and Technical Context
Open arc surfacing (also known as open-flame or self-shielded arc surfacing) is a welding process that does not require external shielding gas, making it suitable for outdoor and field applications. The use of flux-cored wires in this process provides both shielding and alloying functions. The development of austenitic-based wear-resistant alloys for open arc surfacing is particularly valuable because austenitic matrices offer a favorable combination of toughness and wear resistance, which is difficult to achieve with martensitic or ferritic structures.
Base Alloy Composition
| Element | Typical Range (wt%) | Function |
|---|---|---|
| C | 2.5–3.5 | Carbide former, primary hardening element |
| Mn | 8–12 | Austenite stabilizer, solid solution strengthening |
| Cr | 8–12 | Carbide former, oxidation resistance |
| Nb | 0.1–0.5 | Microalloying, carbide precipitation |
| V | 0.3–0.8 | Fine carbide former, dispersion strengthening |
| Ti | 0.1–0.4 | Carbon/nitride former, grain refinement |
| Si | 0–2.0 | Variable — studied in this work |
Microstructural Analysis
Phase Identification
X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy (EDS) revealed the following phases in the surfacing alloys:
| Phase | Crystal Structure | Role |
|---|---|---|
| γ-Fe (austenite) | FCC | Matrix phase, provides toughness |
| (Fe,Cr,Mn,V)23C6 | Complex cubic | Primary hard carbide, wear resistance |
| (Nb,Ti)C | BCC (NaCl-type) | Fine dispersion carbide, precipitation strengthening |
| (Fe,Cr)3(C,B) | Complex | Secondary carbide, contributes to hardness |
| (Fe,Cr,Mn,V)7C3 | Complex cubic | Intergranular carbide, appears at 1.5% Si |
Effect of Silicon Content on Microstructure
The study reveals a non-linear relationship between Si content and microstructural evolution:
| Si Content (wt%) | Key Microstructural Features | Wear Resistance Trend |
|---|---|---|
| 0.0 | Baseline austenitic matrix with dispersed carbides | Baseline |
| 0.9 | Increased (Fe,Cr,Mn,V)23C6 at grain boundaries, dendritic skeleton morphology | Increased |
| 1.5 | Appearance of intergranular (Fe,Cr,Mn,V)7C3; 23C6 morphology transitions to lamellar discrete distribution | Decreased (local minimum) |
| 2.0 | Reduced intergranular 23C6, enlarged intragranular (Nb,Ti,V)C composite carbides | Increased again |
Carbide Morphology Evolution
The transition in carbide morphology with increasing Si content is particularly significant:
- Low Si (0–0.9 wt%): (Fe,Cr,Mn,V)23C6 carbides form along grain boundaries in dendritic skeleton morphology, which provides good wear resistance but may compromise intergranular toughness.
- Intermediate Si (1.5 wt%): The appearance of intergranular (Fe,Cr,Mn,V)7C3 phases is detrimental, as these phases are more brittle and can act as crack initiation sites. The 23C6 carbides also transition from dendritic skeleton to lamellar discrete distribution, which may reduce the connectivity of the hard phase network.
- High Si (2.0 wt%): The reduction in intergranular 23C6 and the enlargement of intragranular (Nb,Ti,V)C composite carbides lead to improved wear resistance. The intragranular location of these carbides is beneficial as it avoids the grain boundary weakening effect.
Wear Testing Results
Wear Mechanism Analysis
The study identifies micro-cutting by abrasive particles as the primary wear mechanism for the 0.9% Si and 1.5% Si alloys. This is consistent with the presence of hard carbide phases that resist plastic deformation but may fracture under severe abrasive loading.
Performance Comparison
| Alloy Composition | Wear Mass Loss | Relative to High-Chromium Cast Iron | Wear Mechanism |
|---|---|---|---|
| 0.9% Si | Lower than general high-Cr cast iron | Superior | Micro-cutting |
| 1.5% Si | Lower than general high-Cr cast iron | Superior | Micro-cutting |
| Baseline (no Si) | Reference | Comparable or lower | Micro-cutting, micro-ploughing |
The fact that 0.9% Si and 1.5% Si alloys outperform general high-chromium cast iron in wear resistance is a significant finding, as high-chromium cast iron is a well-established benchmark for abrasion resistance.
Engineering Practice Implications
Silicon Content Selection
The non-linear wear resistance trend with Si content presents a challenge for alloy design. Engineers should consider:
- 0.9% Si: Provides a good balance of wear resistance and toughness, with favorable carbide morphology.
- 1.5% Si: Despite the appearance of intergranular 7C3 phases, the overall wear resistance remains high, but the toughness may be compromised.
- 2.0% Si: Offers the highest wear resistance but may require additional consideration of ductility and machinability.
Process Control Considerations
Open arc surfacing with flux-cored wires introduces specific process challenges:
- Shielding quality: Without external gas shielding, the flux must provide adequate protection against atmospheric contamination. Inconsistent flux coverage can lead to porosity and oxidation.
- Heat input control: The self-shielded nature of the process makes precise heat input control more challenging compared to gas-shielded processes.
- Dilution management: Open arc surfacing typically has higher dilution than plasma arc or laser surfacing, which can affect the final alloy composition and properties.
Application Suitability
The austenitic open arc surfacing alloys developed in this study are suitable for:
- Mining equipment components subject to severe abrasion.
- Cement mill liners and grinding media.
- Agricultural machinery wear parts.
- Construction equipment components.
- Field repair applications where portable equipment is required.
Key Questions and Reflections
The study does not extensively address the effect of Si on the corrosion resistance of the surfacing alloys. For many industrial applications, wear resistance alone is insufficient, and the alloy must also resist corrosion or erosion-corrosion. The addition of Si to an already high-Cr, high-Ni austenitic matrix should theoretically improve oxidation resistance, but this needs to be verified through dedicated corrosion testing.
Additionally, the study focuses on dry sliding wear, which is only one type of wear mechanism. In many real-world applications, the surfacing alloys would be exposed to a combination of abrasion, impact, and corrosion. Engineers should consider the full spectrum of degradation mechanisms when selecting alloy compositions.
Study Insights and Implications
This paper provides a systematic investigation of silicon's role in austenitic open arc surfacing alloys, revealing a complex and non-linear relationship between Si content and microstructure-property interactions. The finding that Si content of 0.9% and 1.5% produces alloys with wear resistance exceeding that of high-chromium cast iron is practically significant, as it opens up new material options for abrasion-critical applications. The identification of the micro-cutting wear mechanism and the correlation with carbide morphology and distribution provides a clear design basis for further alloy optimization. For engineers in surface engineering and wear technology, this study demonstrates that careful alloy design, combined with appropriate process selection, can produce surfacing alloys with exceptional performance in challenging service environments.