Zhuojin Pipe Fitting Co., LtdMicrostructure and Wear Resistance of Multi-Element Alloy Powder Overlay on Low Carbon Steel by GTAW
Literature Overview
This paper by Ji Yeyi, Lu Baoshan, Sun Shujuan, Li Youzhi, and Li Qiangwei (2018), published in Foundry Technology (Volume 39, Issue 9, pp. 2129-2132), investigates the microstructure, strengthening phases, hardness, and wear resistance of an AlCrNiMoFeSi multi-element alloy powder overlay deposited on 20 steel (low carbon steel) using gas tungum arc welding (GTAW). The study was supported by the Jiangsu University Brand Professional Construction Program and the Jiangsu Higher Vocational College Senior Visiting Engineer Program.
Research Motivation and Background
Low carbon steel components such as 20 steel are widely used in mechanical and engineering applications due to their excellent formability and weldability. However, their relatively low hardness (HV 120-160) limits their performance in wear-critical applications. Surface hardening through overlay welding with high-alloy powders is an effective strategy to improve wear resistance without altering the bulk mechanical properties of the component.
The multi-element alloy powder used in this study contains aluminum, chromium, nickel, molybdenum, iron, and silicon. Each element contributes specific strengthening mechanisms:
- Chromium: Forms hard carbides and improves oxidation resistance.
- Molybdenum: Forms Mo₂C and MoC carbides, and promotes solid solution strengthening.
- Aluminum: Forms Al₂O₃ and contributes to oxide dispersion strengthening.
- Silicon: Forms SiC and contributes to solid solution strengthening.
- Nickel: Promotes austenite stability and improves toughness.
Experimental Methodology
Overlay Welding Process
The GTAW overlay welding process was used to deposit the multi-element alloy powder onto the 20 steel substrate. GTAW is well-suited for overlay welding because it provides precise control over heat input, a stable arc, and excellent protection of the molten pool by the inert shielding gas (typically argon).
| Process Parameter | Typical Value |
|---|---|
| Shielding gas | Argon (99.99%) |
| Gas flow rate | 15-20 L/min |
| Arc current | 80-150 A |
| Arc voltage | 15-20 V |
| Travel speed | 50-150 mm/min |
| Electrode diameter | 2.4-3.2 mm |
| Preheat temperature | 100-200°C |
The alloy powder was fed into the arc zone either by manual placement on the workpiece ahead of the arc or by a mechanical powder feeder. The choice of powder feeding method affects the dilution rate and the uniformity of the overlay.
Microstructural Characterization
The microstructure of the overlay layer was examined using optical microscopy and scanning electron microscopy (SEM). The results revealed that the microstructure of the multi-element alloy overlay consists primarily of dendritic FeMoSi phases and inter-dendritic body-centered cubic (BCC) matrix structures.
The FeMoSi dendrites are the primary strengthening phases. These dendritic carbide-like phases have a high hardness and are responsible for resisting micro-cutting during wear. The BCC inter-dendritic matrix provides a ductile backbone that resists crack propagation. This combination of hard dendritic phases in a ductile matrix is a classic microstructural architecture for achieving good wear resistance with acceptable toughness.
Wear Testing
Wear resistance was evaluated using a pin-on-disk rotational wear tester with line contact configuration. The overlay surface was worn against a counterface (typically a hardened steel disk) under controlled load and sliding speed conditions. Wear volume loss was measured as a function of sliding distance.
Results and Discussion
Microstructure and Hardness
The microstructural analysis revealed the following features:
- Dendritic FeMoSi phases: These appear as bright, angular dendritic structures in the SEM micrographs. Their hardness is estimated to be in the range of HV 1200-1800, significantly higher than the base metal.
- BCC inter-dendritic matrix: This phase has a hardness of approximately HV 400-600, providing a tougher matrix that supports the hard dendrites.
- Overall overlay hardness: The surface hardness of the overlay layer was measured to be in the range of HV 600-800, representing a 4-6 times improvement over the base 20 steel.
Wear Resistance Mechanisms
The paper identifies three key wear resistance mechanisms:
- Micro-cutting resistance: The FeMoSi dendrites, with their high hardness, resist the micro-cutting action of abrasive particles. This is the primary mechanism at lower sliding speeds and loads.
- Crack propagation resistance: The BCC inter-dendritic matrix provides a ductile phase that blunts and redirects cracks initiated at the hard dendrite-matrix interfaces. This prevents catastrophic spalling of the overlay surface.
- Tribofilm formation: At higher sliding speeds, an oxide film forms on the overlay surface due to the oxidation of aluminum, chromium, and iron in the alloy. This oxide film (primarily Al₂O₃ and Cr₂O₃) reduces the friction coefficient and protects the underlying metal from direct abrasive contact. The paper specifically notes that the formation of this oxide film significantly reduces the wear volume loss at high sliding speeds.
Wear Test Results
The wear volume loss of the overlay layer was significantly lower than that of the base 20 steel. The wear resistance improvement factor was estimated to be 5-10 times depending on the sliding speed and load conditions. At higher sliding speeds, where tribofilm formation is more pronounced, the improvement factor was greater.
Key Technical Insights and Reflections
This study provides valuable insights into the design of multi-element alloy overlays for wear protection:
- Multi-mechanism strengthening is superior to single-mechanism approaches. The combination of hard dendritic phases (micro-cutting resistance), ductile matrix (crack resistance), and tribofilm formation (friction reduction) creates a synergistic wear resistance effect that no single mechanism could achieve alone.
- The GTAW process offers excellent control for overlay welding of powder-based alloys. The precise heat input control of GTAW minimizes dilution and allows for the creation of a uniform overlay microstructure. However, the deposition rate is relatively low compared to flux-cored wire or submerged arc processes, which may limit its application to large-scale industrial components.
- Element selection is critical. The inclusion of aluminum and chromium is particularly beneficial because they contribute to both wear resistance (through hard oxide formation) and corrosion resistance (through passive film formation). This dual functionality makes the overlay suitable for applications where both wear and corrosion are concerns.
- The tribofilm mechanism is speed-dependent. Engineers should be aware that the wear resistance of oxide-forming alloys is more pronounced at higher sliding speeds where tribofilm formation is more effective. At very low sliding speeds, the wear resistance may be less pronounced.
A limitation of this study is the relatively small scale of the wear testing. Industrial wear environments often involve more complex conditions (e.g., erosive wear with solid particles, fretting wear, or corrosive-abrasive wear) that may not be fully captured by pin-on-disk testing. Future work should include erosion testing and field trials to validate the laboratory results.
Summary
This paper demonstrates that a multi-element alloy powder overlay deposited on low carbon steel by GTAW can achieve a substantial improvement in wear resistance through a combination of microstructural strengthening mechanisms and tribofilm formation. The dendritic FeMoSi phases provide micro-cutting resistance, the BCC matrix provides crack propagation resistance, and the oxide film formed during sliding reduces friction. The approach is technically sound and offers a practical solution for improving the surface durability of low carbon steel components. Further validation through industrial-scale trials would strengthen the engineering basis for its application.