Zhuojin Pipe Fitting Co., LtdGTAW Remelting Treatment of Fe-Cr-B Arc Spray Coatings
Literature Overview
This paper by Zong Xuemei and colleagues from XCMG Group's Jiangsu Xuzhou Engineering Machinery Research Institute, published in China Surface Engineering (中国表面工程), Vol. 29, Issue 5, 2016, pages 102-108, investigates the effect of gas tungsten arc welding (GTAW) remelting on the properties of Fe-Cr-B arc spray coatings. The research was conducted at the National Key Laboratory of Intelligent Manufacturing of High-End Engineering Machinery, reflecting the practical engineering focus of the study.
Background and Motivation
Fe-Cr-B (iron-chromium-boron) coatings are widely used for wear protection in engineering machinery, particularly in applications involving abrasive wear and high-temperature oxidation. High-velocity arc spraying (HVAF) is a common method for depositing these coatings, but the as-sprayed coatings often suffer from poor bonding strength with the substrate, porosity, and residual stresses that limit their service life.
The GTAW remelting treatment addresses these limitations by partially melting the sprayed coating, transforming the mechanical bond between the coating and substrate into a metallurgical bond. This process also modifies the coating microstructure, improving hardness, wear resistance, and overall durability. The study systematically compares the properties of as-sprayed and remelted Fe-Cr-B coatings using comprehensive characterization techniques.
Coating Preparation and Characterization Methods
The authors prepared Fe-Cr-B coatings using high-velocity arc spraying technology and then subjected selected samples to GTAW remelting treatment. The characterization methods included:
- X-ray diffraction (XRD) for phase structure analysis
- Metallographic microscopy for microstructural examination
- X-ray residual stress testing for stress measurement
- Microhardness testing for hardness profiling
- Dynamic load abrasion testing for wear resistance evaluation
- Scanning electron microscopy (SEM) for wear surface morphology analysis
Phase Structure and Microstructural Changes
The remelting treatment induces significant changes in the coating phase structure and microstructure. The following table summarizes the key differences.
| Property | As-Sprayed Coating | Remelted Coating |
|---|---|---|
| Primary phases | Fe-based amorphous, borides | Cr₂B, (Cr,Fe)₂B, α-Fe |
| Bonding type | Mechanical | Metallurgical |
| Surface hardness | 689 HV0.1 | 960 HV0.1 |
| Wear loss rate | 0.088 g/(cm²·min⁻¹) | 0.0046 g/(cm²·min⁻¹) |
| Wear mechanism | Micro-fracture | Deformation wear, micro-cutting |
The transformation from amorphous to crystalline phases is a critical change that enhances the coating's mechanical properties. The formation of Cr₂B and (Cr,Fe)₂B phases provides significant hardening, while the α-Fe matrix ensures adequate toughness. The metallurgical bonding achieved through remelting dramatically improves the coating's adhesion to the substrate, reducing the risk of spallation during service.
Microstructural Zoning in Remelted Coating
The remelted coating exhibits distinct microstructural zones from the surface to the substrate:
- Surface layer: Primary borides and eutectic microstructure, providing maximum hardness and wear resistance.
- Transition zone: Eutectic borides + martensite + austenite, offering a combination of hardness and toughness.
- Sub-transition zone: Primary austenite and eutectic microstructure, ensuring adequate ductility.
- Heat-affected zone: Modified base metal microstructure, indicating thermal influence on the substrate.
This graded microstructure is advantageous for wear-resistant coatings, as it provides a hard surface for wear resistance while maintaining adequate toughness in the underlying layers to prevent catastrophic failure.
Wear Performance Analysis
The wear performance improvement is dramatic, with the wear loss rate reduced by approximately 95% after remelting. The as-sprayed coating exhibits micro-fracture as the primary wear mechanism, which is characteristic of brittle coatings with poor bonding. The remelted coating shows deformation wear and micro-cutting as the dominant mechanisms, indicating that the coating deforms plastically under wear conditions rather than fracturing.
The improved wear resistance is attributed to several factors: the increased hardness of the remelted coating, the metallurgical bonding that prevents coating spallation, and the refined microstructure that resists crack initiation and propagation. The dynamic load abrasion test simulates realistic service conditions, providing reliable data for engineering design.
Residual Stress Considerations
The remelting process also affects the residual stress state of the coating. As-sprayed coatings typically contain high tensile residual stresses due to the rapid cooling of individual splats during deposition. The remelting process partially relieves these stresses through thermal cycling, although new stresses may be introduced during the remelting and subsequent cooling. The X-ray residual stress testing provides quantitative data on this important parameter, which influences the coating's fatigue resistance and dimensional stability.
Engineering Practice Implications
The findings of this study have direct applications in the surface engineering of engineering machinery components. Fe-Cr-B coatings are commonly used on components such as hydraulic cylinders, wear plates, and cutting tools in construction and mining equipment. The remelting treatment significantly extends the service life of these components by improving coating adhesion and wear resistance.
From a manufacturing perspective, the GTAW remelting process is relatively simple and can be integrated into existing production lines with minimal modification. The process parameters, including welding current, voltage, travel speed, and shielding gas flow rate, must be optimized to achieve the desired remelting depth and microstructure. Excessive remelting may dilute the coating composition and reduce its wear resistance, while insufficient remelting may not achieve complete metallurgical bonding.
In the context of steel pipe manufacturing, similar remelting techniques could be applied to sprayed coatings on pipe surfaces for corrosion and wear protection. The principles of achieving metallurgical bonding through partial melting are directly transferable to other coating systems, including thermal spray coatings on pipe fittings and flanges.
Study Insights and Reflections
This research provides valuable insights into the improvement of thermal spray coating properties through remelting treatment. The dramatic improvement in wear resistance and bonding strength demonstrates the effectiveness of the GTAW remelting approach. The detailed microstructural analysis reveals the mechanisms responsible for the property enhancements, providing a scientific basis for process optimization. For future work, I would recommend investigating the effect of remelting parameters on the coating properties, conducting long-term wear testing under realistic service conditions, and extending the research to other coating compositions and substrate materials. The technology has significant potential for extending the service life of engineering components and reducing maintenance costs in heavy machinery applications.