CLADDING TECHNOLOGY SHANXI CO., LTDEffect of WC Addition on Microstructure and Properties of FeCrNiSiB Overlay Layers Prepared by Plasma Surfacing
Literature Overview
This study examines the influence of tungsten carbide (WC) addition on the microstructure and mechanical properties of FeCrNiSiB overlay layers fabricated by plasma transferred arc (PTA) surfacing. The FeCrNiSiB alloy system is a well-established base composition for wear-resistant coatings, combining the benefits of iron as the matrix element with chromium for carbide formation, nickel for austenite stabilization and toughness improvement, silicon for oxidation resistance, and boron for hard boride formation. The addition of WC particles introduces an additional hard phase that can significantly enhance wear resistance, but the interaction between WC and the FeCrNiSiB matrix during PTA processing requires careful analysis.
Core Technical Findings
The study systematically varies the WC content in the powder feed and examines the resulting microstructure, hardness, wear resistance, and bond strength of the overlay layers. The findings demonstrate that WC addition has a profound effect on the coating microstructure, with the degree of WC dissolution and the resulting phase assemblage being the primary determinants of coating performance.
Experimental Matrix
| Sample | WC Content (wt%) | Binder Composition | PTA Power (kW) | Travel Speed (mm/min) |
|---|---|---|---|---|
| S0 | 0 | FeCrNiSiB | 5 | 200 |
| S1 | 10 | FeCrNiSiB | 5 | 200 |
| S2 | 20 | FeCrNiSiB | 5 | 200 |
| S3 | 30 | FeCrNiSiB | 5 | 200 |
| S4 | 40 | FeCrNiSiB | 5 | 200 |
| S5 | 50 | FeCrNiSiB | 5 | 200 |
Microstructural Analysis
The microstructural evolution with increasing WC content follows a clear pattern. At 0 wt% WC (S0), the overlay layer consists of a martensitic matrix with dispersed Cr carbides and Fe borides. The microstructure is relatively uniform, with hardness values in the range of 550–620 HV.
As WC content increases to 10–20 wt%, the WC particles begin to dissolve partially during PTA processing. The dissolved W and C atoms interact with the FeCrNiSiB matrix, forming additional carbides and modifying the matrix composition. The microstructure shows a mixture of martensite, retained austenite, and various carbide phases. Hardness increases to 650–750 HV, and the wear resistance improves significantly.
At 30–40 wt% WC, a transition occurs where a significant fraction of WC particles survive the PTA process as residual phases. The coating microstructure becomes a composite-like structure with a FeCrNiSiB matrix reinforced by hard WC and W2C particles. The hardness reaches a maximum of 750–850 HV in this range. However, the increased brittleness of the matrix and the presence of large residual WC particles begin to affect the coating toughness.
At 50 wt% WC, the coating becomes increasingly brittle. The high volume fraction of hard particles creates a discontinuous matrix, which reduces the coating's ability to accommodate deformation. Cracking becomes more prevalent, and the bond strength to the substrate decreases due to the increased dilution of the binder phase.
Property Trends with WC Content
| WC Content (wt%) | Hardness (HV10) | Wear Volume Loss (mm³) | Bond Strength (MPa) | Crack Density |
|---|---|---|---|---|
| 0 | 580 ± 30 | 45 | 55 | Very low |
| 10 | 680 ± 25 | 28 | 52 | Low |
| 20 | 740 ± 30 | 18 | 50 | Low |
| 30 | 810 ± 35 | 12 | 48 | Moderate |
| 40 | 830 ± 40 | 10 | 45 | Moderate |
| 50 | 720 ± 50 | 22 | 38 | High |
The data clearly shows that the optimal WC content for balancing hardness, wear resistance, bond strength, and crack resistance is in the range of 20–40 wt%, with 30 wt% representing the best overall performance.
Phase Transformation Mechanisms
The study provides detailed insights into the phase transformation mechanisms during PTA processing of WC-containing powders. The key mechanisms include:
- WC dissolution: Fine WC particles dissolve completely into the molten pool, contributing W and C atoms to the matrix.
- WC decomposition: Partially melted WC particles decompose into W2C and free carbon, which then form new carbide phases.
- Residual WC survival: Large WC particles survive the PTA process intact, acting as hard reinforcement phases.
- Carbide precipitation: During solidification, Cr, W, and C atoms precipitate as various carbide phases (Cr7C3, Cr23C6, WC, W2C).
- Boride formation: B atoms combine with Fe and Cr to form Fe2B and CrB phases, contributing to hardness.
The interplay between these mechanisms determines the final phase assemblage and, consequently, the coating properties. The study emphasizes that the PTA parameters (particularly power and travel speed) significantly influence the degree of WC dissolution and the resulting microstructure.
Engineering Implications and Recommendations
For engineers applying PTA surfacing with WC-containing powders, the following recommendations emerge from the study:
- Powder preparation: Ensure uniform mixing of WC particles with the FeCrNiSiB binder powder. Particle size distribution should be controlled to optimize the balance between dissolved and residual WC phases.
- Process parameters: Use moderate PTA power (4–6 kW) and travel speed (150–250 mm/min) to achieve partial WC dissolution. Avoid excessive power that leads to complete WC melting and decomposition.
- WC content: Select WC content in the 20–40 wt% range for optimal performance. For applications requiring maximum wear resistance, 30–40 wt% is recommended. For applications requiring good bond strength and low crack susceptibility, 10–20 wt% is more appropriate.
- Post-weld treatment: Stress relief at 550–600 °C for 1–2 hours can significantly reduce residual stresses and improve coating toughness without significantly reducing hardness.
- Quality control: Implement comprehensive inspection including hardness profiling, XRD phase analysis, and bond strength testing to ensure coating quality meets specifications.
Application-Specific Recommendations
| Application | Recommended WC Content | Key Property | PTA Parameters |
|---|---|---|---|
| Mining equipment (high abrasion) | 30–40 wt% | Maximum wear resistance | Higher power, moderate speed |
| Cement industry (moderate wear) | 20–30 wt% | Balanced properties | Moderate power and speed |
| Power plant components | 10–20 wt% | Corrosion + wear resistance | Lower power, higher speed |
| Food processing equipment | 10–15 wt% | Hygienic + wear resistant | Low power, high speed |
Study Insights and Concluding Reflections
This study provides a systematic and comprehensive understanding of how WC addition affects the microstructure and properties of FeCrNiSiB PTA overlay layers. The identification of the optimal WC content range and the detailed phase transformation analysis are particularly valuable for process development and optimization. The study demonstrates that the performance of WC-containing PTA coatings is governed by a complex interplay between powder composition, processing parameters, and phase evolution. Engineers must carefully balance the competing requirements of hardness, wear resistance, bond strength, and crack resistance when designing PTA surfacing processes for specific applications.
A significant insight from this study is the importance of considering the coating as a composite material rather than a homogeneous alloy. The residual WC particles act as reinforcement phases within the FeCrNiSiB matrix, and the interface between these phases is critical for crack propagation and wear mechanisms. Future work should focus on optimizing the WC particle size distribution and the interfacial bonding between WC particles and the matrix to further improve coating performance. Additionally, the study highlights the need for in-situ monitoring of PTA processes to control the degree of WC dissolution in real time, which could lead to more consistent and predictable coating quality. Overall, this study serves as an essential reference for engineers developing and optimizing PTA surfacing processes for wear-resistant applications across various industries.