CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of Multi-Component Alloy Powder Coatings on Low Carbon Steel by GTAW
Literature Overview
This 2018 study by Ji Yeyi, Lu Baoshan, Sun Shujuan, Li Youzhi, and Li Qiangwei, published in "Foundry Technology," investigates the microstructure and wear resistance of multi-component alloy powder coatings deposited on low carbon steel using gas tungsten arc welding (GTAW). The research was supported by the Jiangsu Provincial University Brand Specialty Construction Project (PPZY2015B186) and the Jiangsu Provincial Higher Vocational College Senior Visiting Engineer Program (2015FG038). The work represents a collaborative effort between Suzhou Industrial Vocational and Technical College, Jiangsu University, Suzhou Construction and Transportation Higher Vocational Technical School, and Suzhou University Engineering Training Center. The study addresses the growing demand for wear-resistant surface coatings in industrial applications where low carbon steel substrates are used but enhanced surface properties are required.
Powder Composition and Deposition Parameters
The multi-component alloy powder used in this study is designed to provide a combination of hardness, toughness, and wear resistance through the synergistic effect of multiple alloying elements. The powder composition typically includes iron-based binder with additions of chromium, tungsten, cobalt, nickel, and carbon to form a complex microstructure with hard phases dispersed in a ductile matrix.
| Powder Component | Typical Composition (wt%) | Function |
|---|---|---|
| Iron (Fe) | Balance | Binder phase |
| Chromium (Cr) | 15–25 | Forms Cr7C3 hard carbides |
| Tungsten (W) | 5–15 | Forms WC hard carbides |
| Cobalt (Co) | 5–10 | Promotes solid solution strengthening |
| Nickel (Ni) | 3–8 | Improves toughness and ductility |
| Carbon (C) | 3–6 | Forms carbides and increases hardness |
The GTAW deposition parameters are optimized to achieve a dense, defect-free coating with minimal dilution from the low carbon steel substrate:
| Parameter | Value |
|---|---|
| Welding current | 120–180 A |
| Arc voltage | 12–18 V |
| Travel speed | 3–8 cm/min |
| Shielding gas | Argon, 15–20 L/min |
| Tungsten electrode diameter | 2.4–3.2 mm |
| Powder feed rate | 0.5–1.5 g/min |
| Powder nozzle to arc distance | 5–10 mm |
| Number of deposition passes | 1–3 |
Microstructural Characterization and Wear Mechanism
Metallographic analysis of the deposited coatings reveals a complex microstructure consisting of a dendritic matrix with dispersed hard carbide phases. The matrix is primarily composed of austenite and martensite, with the specific phases depending on the cooling rate and alloy composition. The hard carbide phases include Cr7C3, WC, and M7C3 (where M represents Cr, W, and Fe), which are distributed throughout the matrix and provide the primary wear resistance.
The hardness of the coating surface typically ranges from 700 to 900 HV, which is significantly higher than the substrate hardness of 150–200 HV. The hardness profile across the coating thickness shows a gradient from the highest hardness at the surface to lower hardness near the coating-substrate interface, reflecting the dilution effect and the changing phase composition.
Wear testing is conducted using a pin-on-disk tribometer with alumina (Al2O3) balls as the counterface. The wear rate is measured under different normal loads and sliding distances. The results show that the multi-component alloy coating exhibits a wear rate that is 3–5 times lower than the uncoated low carbon steel substrate. The wear mechanism transitions from abrasive wear on the uncoated substrate to a mixed abrasive and adhesive wear mechanism on the coated surface, with the hard carbide phases providing resistance to abrasive wear and the ductile matrix accommodating plastic deformation.
Defect Analysis and Process Optimization
Common defects in GTAW-deposited powder coatings include:
- Porosity: Gas porosity can occur if the shielding gas coverage is inadequate or if the powder contains moisture. Dense porosity near the coating-substrate interface can significantly reduce the bond strength and wear resistance. Maintaining a consistent gas flow rate and using dry, pre-heated powder can minimize this defect.
- Cracking: The high carbon content of the powder can promote martensite formation and cracking during cooling. Adding nickel to the powder composition improves the toughness of the matrix and reduces cracking susceptibility. Preheating the substrate to 150–200 °C and applying interpass temperature control can also reduce residual stresses and prevent cracking.
- Unmelting and lack of fusion: Insufficient heat input can result in incomplete melting of the powder particles and poor bonding between passes. Optimizing the welding current, travel speed, and powder feed rate is essential to ensure complete melting and good fusion.
Engineering Applications and Performance Comparison
The multi-component alloy powder coating technology is applicable to a wide range of industrial components where wear resistance is critical. Typical applications include:
- Mining equipment: Excavator buckets, crusher jaws, and conveyor rollers
- Cement industry: Mill liners, grinding balls, and kiln wear plates
- Power generation: Boiler burners, fan blades, and dust collector components
- Agricultural machinery: Plowshares, harvester components, and tillage tools
A comparison of the coating performance with alternative surface treatment methods shows the following:
| Treatment Method | Hardness (HV) | Wear Rate (mm3/N·m) | Cost | Applicability |
|---|---|---|---|---|
| Uncoated low carbon steel | 150–200 | 80–120 | Baseline | General purpose |
| GTAW multi-component powder coating | 700–900 | 20–35 | Moderate | Large components, in-situ repair |
| HVOF Cr3C2-NiCr coating | 1000–1200 | 10–20 | High | Small components, high precision |
| Thermal spray WC-Co coating | 1200–1500 | 5–15 | High | High wear severity applications |
| Case hardening (carburizing) | 800–1000 (surface) | 15–30 | Low | Through-hardening required |
The GTAW multi-component powder coating offers a favorable balance of wear resistance, cost, and applicability, making it an attractive option for many industrial applications.
Study Insights and Reflections
This study provides valuable insights into the design and optimization of multi-component alloy powder coatings for wear resistance applications. The key finding is that the synergistic effect of multiple hard carbide phases dispersed in a ductile matrix provides superior wear resistance compared to single-phase hard coatings. The GTAW deposition process offers excellent flexibility for coating large components and for in-situ repair of worn parts, which is a significant advantage over thermal spray methods that require component removal and often have limitations on component size.
The study also highlights the importance of understanding the wear mechanism in selecting the appropriate coating composition. For applications dominated by abrasive wear, a higher carbide content is beneficial, while for applications involving adhesive wear or fretting, a more ductile matrix with moderate carbide content may be more appropriate. Engineers must carefully evaluate the wear conditions in the specific application to select the optimal coating composition and deposition parameters.
Summary
The GTAW deposition of multi-component alloy powder coatings on low carbon steel provides an effective and economical solution for enhancing wear resistance in industrial applications. The combination of hard carbide phases and a ductile matrix offers excellent wear resistance, while the GTAW process provides flexibility for coating large components and performing in-situ repairs. Engineers should carefully optimize the powder composition and deposition parameters to achieve the desired balance of hardness, toughness, and wear resistance for the specific application requirements.