CLADDING TECHNOLOGY SHANXI CO., LTDBoride-Strengthened Iron-Based Cladding Alloy Microstructure and Properties
Literature Overview and Research Context
This 2024 publication in the Journal of Thermal Processing Technology by researchers from Shenyang University of Chemical Technology and China Construction Installation Group Co., Ltd., supported by the National Natural Science Foundation of China (Youth Fund Project 51901141) and the Liaoning Provincial Department of Education Scientific Research Project (LJ2020034), presents a comprehensive investigation of boride-strengthened iron-based cladding alloys. The study addresses the growing industrial demand for wear-resistant overlay materials that combine high hardness with acceptable toughness for applications in mining, construction, and material handling equipment.
Boron is a well-known hardening element in iron-based alloys due to its ability to form hard boride phases (Fe2B, FeB, and Fe2-3B) that provide excellent wear resistance. However, the high brittleness of these boride phases presents a significant challenge in terms of toughness and crack resistance. The research focuses on optimizing the boride phase morphology, distribution, and volume fraction through careful alloy design and welding process parameter control to achieve an optimal balance between hardness and toughness.
Core Technical Points and Microstructure-Property Relationships
The study systematically investigates the effect of boron content (ranging from 0.5 to 2.5 wt%) on the microstructure and mechanical properties of the cladding alloy. The researchers found that the boride phase morphology undergoes a significant transformation as the boron content increases. At low boron levels (0.5 to 1.0 wt%), the boride phases form as isolated, fine particles dispersed in a pearlite-ferrite matrix, providing moderate hardness enhancement with minimal toughness degradation. At intermediate boron levels (1.0 to 1.8 wt%), the boride phases begin to coalesce into network-like structures along the prior austenite grain boundaries, which provides higher hardness but introduces crack initiation sites. At high boron levels (above 1.8 wt%), the boride phases form continuous, interconnected networks that severely compromise the toughness of the overlay.
The optimal boron content for achieving the best balance between hardness and toughness was identified as approximately 1.2 to 1.5 wt%, which corresponds to a boride volume fraction of approximately 15 to 25 percent. At this composition, the boride phases form as discrete, rounded particles with an average size of 2 to 5 micrometers, providing a hardness of approximately 700 to 800 HV while maintaining an acceptable impact toughness.
| Boron Content (wt%) | Hardness (HV) | Impact Toughness (J) | Boride Volume Fraction (%) | Microstructure Description |
|---|---|---|---|---|
| 0.5 | 450 to 500 | 45 to 55 | 5 to 8 | Fine isolated boride particles in pearlite-ferrite matrix |
| 1.0 | 580 to 650 | 30 to 40 | 10 to 15 | Moderately distributed boride particles |
| 1.5 | 700 to 780 | 20 to 28 | 18 to 25 | Discrete rounded boride particles (optimal) |
| 2.0 | 820 to 880 | 10 to 18 | 30 to 38 | Partially coalesced boride networks |
| 2.5 | 900 to 950 | 5 to 10 | 40 to 50 | Continuous interconnected boride networks |
The study also investigates the effect of additional alloying elements such as chromium, manganese, molybdenum, and vanadium on the boride phase formation and the overall mechanical properties. Chromium addition (2 to 4 wt%) promotes the formation of chromium-rich boride phases (CrB, Cr2B) that are harder and more wear-resistant than iron borides, but it also increases the brittleness of the overlay. Molybdenum addition (0.5 to 1.5 wt%) improves the high-temperature hardness retention and the resistance to thermal softening during service. Vanadium addition (0.5 to 1.0 wt%) promotes the formation of fine vanadium carbide and vanadium boride particles that further refine the microstructure and improve the wear resistance.
Welding Process Parameters and Defect Control
The welding process parameters play a critical role in determining the boride phase morphology and distribution in the cladding layer. The study evaluates several welding processes including submerged arc welding (SAW), flux-cored arc welding (FCAW), and gas metal arc welding (GMAW), and identifies the optimal parameter windows for each process.
For SAW processes, the key parameters are the welding current (300 to 500 A), voltage (28 to 38 V), welding speed (150 to 350 mm/min), and preheat temperature (150 to 250 degrees Celsius). The higher thermal input of SAW promotes the coarsening of boride phases, which must be compensated by reducing the boron content or increasing the welding speed.
For FCAW processes, the key parameters are the welding current (150 to 300 A), voltage (22 to 32 V), wire feed speed (5 to 10 m/min), and shielding gas composition (Ar with 2 to 5 percent CO2). The lower thermal input of FCAW compared to SAW results in finer boride phases, but it also increases the risk of porosity and incomplete fusion.
Common defects in boride-strengthened cladding layers include:
- Cracking due to the high brittleness of the boride phases, particularly at the overlay/substrate interface.
- Porosity caused by hydrogen absorption and gas evolution from the boron-containing filler material.
- Incomplete fusion at the overlay/substrate interface due to the high melting point of the boride phases.
- Dilution from the steel substrate leading to a reduction in boron content and a degradation of the wear resistance.
To mitigate these defects, the following countermeasures are recommended: preheating the substrate to 200 to 300 degrees Celsius to reduce thermal gradients and residual stresses, using low-dilution welding processes with careful wire feed control, applying multiple thin passes instead of a single thick pass to control the thermal cycle, and using a transition layer of lower boron content between the substrate and the high-boron overlay to accommodate the thermal expansion mismatch.
Study Insights and Implications
The research by the Shenyang University of Chemical Technology team provides valuable guidance for the design and fabrication of boride-strengthened iron-based cladding alloys. The key insight is that the boron content must be carefully optimized to balance hardness and toughness, and that the boride phase morphology is as important as the boride volume fraction in determining the overall wear resistance of the overlay. The study also demonstrates that the combination of boron with other alloying elements such as chromium, molybdenum, and vanadium can produce synergistic improvements in wear resistance that exceed the sum of the individual contributions.
From an engineering practice perspective, the study highlights the importance of welding process selection and parameter optimization in achieving the desired boride phase morphology. The lower thermal input processes such as FCAW and GMAW are generally preferred for boride-strengthened overlays because they produce finer boride phases, but they require more careful control of shielding gas composition and wire feed parameters to avoid porosity and incomplete fusion. The study also emphasizes the importance of post-weld heat treatment in relieving residual stresses and improving the toughness of the overlay, particularly for high-boron compositions where the residual stress levels can be critical.