Zhuojin Pipe Fitting Co., LtdTIG Welding of FeAlNbB Hardfacing Overlay Microstructure and Properties
Literature Overview
This paper, published in Hot Working Technology (2020, Vol. 49, No. 5, pp. 53–56) by Zhao Fei, Zhou Yong, Dang Mohe, and Xing Xiaofang from Xi'an Shiyou University, presents the results of TIG hardfacing of an FeAlNbB alloy system on 20 steel using a flux-cored wire. The research is supported by the Xi'an Shiyou University Graduate Innovation and Practical Ability Training Project (YCS172 11038) and the provincial key discipline program (YS37020203). The FeAlNbB system is an advanced hardfacing alloy designed for high-temperature oxidation resistance and wear resistance, combining the protective aluminum oxide layer with niobium carbide and boride strengthening phases.
Core Technical Findings
Microstructure and Phase Analysis
The hardfacing overlay exhibits a uniform and dense structure with no porosity or cracking defects, indicating good weldability and process control. The overlay achieves sound metallurgical bonding with the 20 steel substrate. The microstructure consists of a ferritic matrix with dispersed intermetallic compounds throughout the overlay. Near the fusion line, a small amount of Widmanstätten ferrite is observed, while the heat-affected zone (HAZ) contains a limited amount of bainitic structure.
The XRD analysis identifies the following phases in the overlay:
| Phase | Description | Significance |
|---|---|---|
| α-Fe | Ferritic matrix | Base structure providing ductility |
| Fe₃Al | Iron-aluminum intermetallic | Strengthening; oxidation resistance |
| FeAl | Iron-aluminum intermetallic | High-temperature strength |
| Al₂O₃ | Aluminum oxide | Minor; surface oxidation product |
The average hardness of the overlay is approximately 736 HV0.1, representing a dramatic improvement over the 20 steel substrate (typically 120–150 HV0.1). This represents a hardness increase of roughly fivefold, which is exceptional for a single-pass hardfacing operation.
Interface and HAZ Analysis
The presence of Widmanstätten ferrite near the fusion line is noteworthy. This microstructure typically forms under conditions of rapid cooling from the austenite region, which is consistent with the relatively low heat input of TIG welding compared to processes like submerged arc welding. The bainitic structure in the HAZ indicates that the cooling rate in the substrate was sufficient to suppress pearlite formation and promote bainite, which provides improved toughness compared to coarse pearlite.
Process Analysis and Technical Discussion
The use of a flux-cored wire for TIG hardfacing is an interesting process choice. In conventional TIG welding, solid wire is typically used, but the flux-cored wire introduces additional alloying elements and can modify the arc characteristics. The flux in the wire core likely serves multiple purposes:
- Arc stabilization: The flux components can improve arc stability and penetration characteristics.
- Alloying: The core flux may contain additional alloying elements that contribute to the final composition.
- Slag formation: The flux can form a protective slag layer that shields the weld from atmospheric contamination.
The TIG process provides excellent control over heat input, which is critical for hardfacing applications where dilution control is paramount. The low dilution achieved with TIG welding ensures that the overlay composition remains close to the intended FeAlNbB alloy design, preserving the high hardness and oxidation resistance properties.
The absence of porosity and cracking is particularly significant for hardfacing alloys containing aluminum, which is prone to forming oxide inclusions and hydrogen porosity. The careful control of TIG parameters, combined with the flux-cored wire design, appears to have effectively managed these challenges.
Engineering Practice Considerations
The FeAlNbB hardfacing system is designed for applications requiring simultaneous wear resistance and high-temperature oxidation resistance. Potential applications include:
- High-temperature wear parts: Sliding surfaces in furnace equipment, kiln components, and hot gas ducts where temperatures exceed 600°C.
- Chemical processing equipment: Parts exposed to corrosive and abrasive environments at elevated temperatures.
- Aerospace components: Turbine-related components requiring oxidation resistance and thermal stability.
The hardness of 736 HV0.1 places this overlay in the category of high-hardness hardfacing alloys, comparable to some ceramic-reinforced systems. However, the brittleness associated with such high hardness must be carefully managed in design. The overlay should not be used in applications subject to impact loading or cyclic fatigue without further evaluation of fracture toughness.
Key Questions and Reflections
Several aspects of this study merit further consideration. The wear testing results are not explicitly reported in the abstract, which limits the ability to correlate the measured hardness with actual wear performance. In hardfacing applications, wear resistance depends not only on hardness but also on the toughness of the matrix, the size and distribution of hard particles, and the oxidation resistance under service conditions.
The Al₂O₃ phase identified in the XRD analysis, while minor, is worth noting. In high-temperature applications, the formation of a protective Al₂O₃ scale is beneficial, but in the as-welded condition, it may represent oxide inclusions that could act as crack initiation sites. The distribution and morphology of these oxide particles would be important to characterize.
Additionally, the study does not address the thermal cycling performance of the overlay. In real service conditions, repeated heating and cooling cycles can cause thermal fatigue cracking, particularly at the fusion line where the thermal expansion mismatch between the overlay and substrate is greatest. The FeAlNbB system's coefficient of thermal expansion relative to 20 steel is a critical design parameter that should be evaluated.
Study Insights and Conclusions
This study demonstrates that TIG hardfacing with flux-cored wire is an effective method for producing high-hardness FeAlNbB overlays on carbon steel substrates. The achieved hardness of 736 HV0.1, combined with a defect-free microstructure and sound metallurgical bonding, indicates good process control and alloy design. The ferritic matrix with dispersed intermetallic compounds provides a good balance between hardness and some degree of toughness. For engineers selecting hardfacing solutions for high-temperature wear applications, the FeAlNbB system represents a promising candidate, though comprehensive evaluation of thermal fatigue resistance, fracture toughness, and long-term oxidation behavior under actual service conditions is essential before production deployment.