CLADDING TECHNOLOGY SHANXI CO., LTDEffect of WC Addition on Microstructure and Properties of FeCrNiSiB Overlay Layer Prepared by Plasma Cladding
Literature Overview and Research Context
This 2024 publication by Bing Shaowang, Tan Hua, Wang Xinning, and colleagues from Qingdao Guoshi Technology Group, Qingdao Shuangrui Marine Environment Engineering Co., Ltd., and the National Marine Equipment Quality Inspection Center represents a cutting-edge investigation into marine-grade overlay coatings. The research is supported by the Qingdao Pilot National Laboratory for Marine Science and Technology "14th Five-Year Plan" major project (20220NLM030001-4), underscoring its strategic importance for China's marine equipment industry. The study systematically examines how varying WC content affects the microstructure, hardness, corrosion resistance, and wear performance of FeCrNiSiB-based plasma cladding overlays.
Core Technical Content
Research Motivation and Material System
The FeCrNiSiB system is a promising base alloy for marine applications due to its inherent corrosion resistance derived from chromium and nickel content, combined with the ability to form hard boride and carbide phases that provide wear resistance. The addition of WC serves to further enhance wear performance while potentially influencing the phase composition and corrosion behavior.
| Component | Role in Coating System | Typical Content Range |
|---|---|---|
| Fe | Base element; provides ductility | Balance |
| Cr | Corrosion resistance; carbide former | 20-30 wt% |
| Ni | Stabilizes austenite; enhances corrosion resistance | 10-20 wt% |
| Si | Deoxidizer; modifies phase formation | 2-5 wt% |
| B | Forms hard borides; enhances wear resistance | 3-8 wt% |
| WC | Reinforcement; additional carbon source | 0-30 wt% |
Microstructural Response to WC Addition
The study reveals that WC content has a profound influence on the coating microstructure:
- Low WC content (0-10 wt%): The microstructure is dominated by the FeCrNiSiB matrix with dispersed boride and carbide phases. The WC particles largely dissolve, contributing carbon to form chromium carbides and tungsten to modify the matrix composition.
- Medium WC content (10-20 wt%): A transition regime where both dissolved and undissolved WC particles coexist. The undissolved WC particles act as nucleation sites for the solidification of the matrix, potentially refining the grain structure.
- High WC content (20-30 wt%): Predominantly undissolved WC particles with a reduced volume fraction of matrix. The microstructure becomes increasingly composite-like, with WC particles serving as the primary wear-resistant phase.
Phase Evolution and XRD Analysis
The X-ray diffraction analysis reveals the following phase evolution with increasing WC content:
| Phase | Low WC | Medium WC | High WC |
|---|---|---|---|
| FeCrNiSiB matrix (austenite/ferrite) | Dominant | Present | Reduced |
| Cr7C3 | Significant | Moderate | Reduced |
| Cr23C6 | Present | Moderate | Reduced |
| FeB / Fe2B | Significant | Significant | Reduced |
| CrB / Cr2B | Present | Present | Reduced |
| WC (undissolved) | Minimal | Moderate | Dominant |
| W2C | Trace | Trace | Present |
The formation of W2C at high WC content indicates that some tungsten carbide transformation occurs during the thermal cycle, likely due to the high temperature of the plasma arc causing partial decomposition and recombination of the WC phase.
Mechanical Properties
| Property | 0 wt% WC | 10 wt% WC | 20 wt% WC | 30 wt% WC |
|---|---|---|---|---|
| Vickers hardness (HV) | 800-900 | 1000-1100 | 1200-1300 | 1300-1450 |
| Wear resistance (relative) | 1.0 | 2.5-3.0 | 4.0-5.0 | 5.0-6.0 |
| Bond strength (MPa) | 250-300 | 240-280 | 220-260 | 200-240 |
| Corrosion potential (mV vs. SCE) | -200 to -100 | -250 to -150 | -300 to -200 | -350 to -250 |
The data indicates a clear trade-off: increasing WC content improves hardness and wear resistance but may slightly reduce bond strength and corrosion resistance. This trade-off is critical for marine applications where both wear and corrosion resistance are essential.
Engineering Practice and Application Considerations
Marine Environment Challenges
Marine equipment operates in a highly corrosive environment characterized by:
- High chloride ion concentration (3.5% NaCl in seawater)
- Biofouling organisms that create differential aeration cells
- Abrasive wear from sand and particulate matter in seawater
- Cyclic thermal loading in some applications
- Hydrogen-induced cracking susceptibility in high-strength steels
Process Optimization for Marine Applications
| Process Parameter | Recommended Value | Rationale |
|---|---|---|
| Plasma current | 250-350 A | Balances melting rate and dilution |
| Powder feed rate | 250-400 g/min | Ensures stable deposition |
| Travel speed | 300-500 mm/min | Controls cooling rate for optimal microstructure |
| Torch-to-substrate distance | 10-15 mm | Maintains arc stability and powder delivery |
| Preheat temperature | 100-200 °C | Reduces thermal stress and cracking risk |
| Interpass temperature | <200 °C | Prevents excessive grain growth |
| Post-weld heat treatment | Solution treatment at 1050-1150 °C for 1-2 h | Dissolves intermetallic phases; enhances corrosion resistance |
Defect Analysis and Quality Control
| Defect Type | Detection Method | Prevention Strategy |
|---|---|---|
| Cracking | MT / PT | Optimize thermal input; control interpass temperature |
| Pores | UT / RT | Ensure powder quality; maintain stable feed rate |
| Incomplete fusion | UT / TOFD | Increase plasma power; ensure proper torch alignment |
| Excessive dilution | Hardness traverse | Control plasma parameters; use appropriate powder composition |
| Spallation | Visual / UT | Optimize residual stress; consider post-weld stress relief |
Study Insights and Strategic Implications
This research provides a systematic framework for understanding the WC content-structure-property relationships in FeCrNiSiB-based plasma cladding coatings, which is directly applicable to engineers designing marine equipment coatings. The study demonstrates that the optimal WC content is application-specific: for applications where corrosion resistance is paramount, lower WC content (0-10 wt%) is preferred, while for applications dominated by abrasive wear, higher WC content (20-30 wt%) provides superior performance.
The research also highlights the importance of post-weld heat treatment in optimizing coating properties. Solution treatment can dissolve detrimental intermetallic phases that form during the rapid solidification of PTA cladding, thereby improving both corrosion resistance and toughness without significantly compromising hardness.
For engineering practice, the key recommendation is to perform a thorough application analysis to determine the relative importance of wear resistance versus corrosion resistance, and then select the WC content and process parameters accordingly. The study provides a valuable baseline dataset that can be used for coating selection and process specification in marine equipment fabrication.