CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of FeAlNbB Overlay Layer Fabricated by TIG Welding
Literature Overview and Research Background
This study investigates the microstructure evolution and mechanical properties of a FeAlNbB-based overlay layer deposited on a carbon steel substrate using gas tungsten arc welding (GTAW/TIG). FeAlNbB belongs to the class of high-entropy alloys (HEAs) or multi-principal-element alloys, which have attracted significant attention in recent years for their exceptional combination of high temperature strength, oxidation resistance, and wear resistance. The selection of TIG as the overlay process is deliberate—TIG provides precise thermal input control, low dilution rates, and the ability to achieve sound single-pass or multi-pass deposits with minimal spatter. The base material is typically a low-carbon or low-alloy steel (such as Q235 or 16Mn), while the consumable is a FeAlNbB wire or rod designed to produce an overlay with enhanced hardness and thermal stability.
The engineering motivation behind this research is clear: in applications such as furnace components, hot ducts, and wear-corrosion parts operating above 800°C, conventional overlay materials (e.g., 310SS, Inconel 625) often suffer from insufficient hardness retention at elevated temperatures or excessive cost. FeAlNbB alloys offer a cost-effective alternative with promising high-temperature mechanical behavior, making them attractive for industrial cladding applications where both economic viability and performance are critical.
Core Technical Points and Microstructure Analysis
The microstructure of the FeAlNbB overlay deposited by TIG typically exhibits a complex multiphase morphology consisting of B2 (CsCl-type ordered) phase, BCC (body-centered cubic) phase, and intermetallic phases such as NbB and Fe₂B. The specific phase distribution and volume fraction are strongly influenced by the cooling rate, which in turn is governed by the welding parameters.
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Welding current | 120–200 A | Higher current → coarser grains, more equilibrium phases |
| Travel speed | 3–8 cm/min | Lower speed → slower cooling, larger dendrite spacing |
| Arc voltage | 16–22 V | Determines heat input and penetration depth |
| Shielding gas flow | 8–12 L/min (Ar) | Prevents oxidation; insufficient flow causes nitride inclusions |
| Wire diameter | 1.6–2.4 mm | Affects deposition rate and bead geometry |
| Number of passes | 1–5 | Multi-pass causes partial remelting, grain refinement |
The dilution rate between the FeAlNbB overlay and the carbon steel substrate is a critical parameter. In TIG overlay, dilution rates of 15–30% are typical for single-pass deposits, which can be managed through proper joint preparation and stringer bead techniques. The interfacial region between the overlay and substrate often shows a transition zone with mixed phases and possible microcracking due to thermal strain mismatch.
A key finding from this type of research is that the NbB intermetallic phase, while contributing significantly to hardness (often exceeding 800 HV in localized regions), also introduces brittleness. The B2 phase provides ductility and acts as the matrix in which the harder intermetallics are dispersed. The overall hardness of the overlay typically ranges from 550 to 750 HV, depending on the phase composition and cooling rate.
Mechanical Properties and Performance Evaluation
The mechanical properties of the FeAlNbB overlay include hardness, wear resistance, oxidation resistance, and bond strength with the substrate. Hardness measurements along the cross-section reveal a gradient from the high-hardness overlay (600–750 HV) to the substrate (150–200 HV for Q235 steel), with a transition zone where hardness changes most rapidly.
Wear resistance tests (typically pin-on-disk or block-on-ring configurations) demonstrate that the FeAlNbB overlay exhibits 3–5 times the wear resistance of the base carbon steel under dry sliding conditions. The wear mechanism transitions from abrasive wear in the overlay to adhesive wear in the substrate, confirming the effectiveness of the cladding.
Oxidation resistance is evaluated through isothermal oxidation tests at 800–1000°C in air. The formation of a protective oxide scale containing Fe₂O₃, Al₂O₃, and Nb₂O₅ provides a synergistic protective effect. Weight gain per unit area after 24-hour exposure at 900°C is typically in the range of 1.5–3.0 mg/cm², which is comparable to or better than 310SS under similar conditions.
Bond strength between the overlay and substrate is assessed through bend tests or tensile shear tests. Acceptable bond strength (no delamination at 180° bend) requires careful control of welding parameters to avoid excessive dilution and thermal cracking at the interface.
Engineering Practice Integration and Defect Analysis
From an engineering practice perspective, several defects are commonly encountered when fabricating FeAlNbB overlays by TIG:
- Hot cracking: Caused by the low ductility of NbB intermetallic phases during solidification. Countermeasures include preheating the substrate to 200–300°C, reducing cooling rate, and using a slightly higher carbon content in the consumable to form carbides that reduce the liquid range.
- Porosity: Argon shielding contamination or insufficient gas flow leads to nitrogen and oxygen pickup. The porosity rate should be controlled below 1% by volume according to JB/T 4730 standards.
- Delamination: Thermal strain mismatch between the overlay and substrate can cause interfacial cracking, particularly in multi-pass deposits. Using a transition layer (such as a dilution-resistant nickel-based filler) can mitigate this issue.
- Excessive dilution: When the dilution rate exceeds 35%, the overlay properties degrade significantly. Stringer bead techniques with reduced wire feed and higher travel speed help maintain dilution below acceptable limits.
Study Insights and Implications
This research underscores the potential of multi-principal-element alloys as next-generation overlay materials for high-temperature applications. The combination of TIG welding with FeAlNbB consumables offers a practical route to achieve cost-effective cladding solutions that rival conventional nickel-based overlays in oxidation resistance while providing superior hardness. However, the engineering challenge lies in balancing the competing requirements of hardness (driven by intermetallic phases) and toughness (limited by the same phases). Future work should focus on optimizing the consumable composition to reduce the volume fraction of brittle NbB while maintaining adequate hardness, and on developing multi-pass welding strategies that minimize dilution while ensuring sound bonding. For pressure vessel applications governed by GB/T 150 or ASME VIII Div.1, the qualification of such novel overlay materials would require comprehensive weld procedure qualification (WPQ) per NB/T 47014, including mechanical testing, intergranular corrosion testing, and hydrostatic pressure testing of production units.