CLADDING TECHNOLOGY SHANXI CO., LTDOptimization Design of Boron-Containing Wear-Resistant Crack-Resistant Cladding Electrodes
Literature Overview
The paper by Li Qiang, Tang Wenbo, and Guo Yungang from the School of Materials Science and Engineering, Zhengzhou University, published in 2008 in the journal "Thermal Processing Technology," addresses the optimization design of boron-containing wear-resistant and crack-resistant cladding electrodes. This research was supported by the Henan Provincial Natural Science Foundation (Grant No. 0411050200). The work sits at the intersection of electrode metallurgy, tribology, and fracture mechanics, targeting a long-standing engineering challenge: achieving simultaneously high hardness for wear resistance and low susceptibility to cracking during the cladding process.
Core Technical Content
The fundamental challenge in designing wear-resistant cladding electrodes lies in the inherent contradiction between hardness and toughness. Boron, when introduced into the overlay alloy system, forms hard borides (predominantly Fe₂B and FeB) that dramatically increase surface hardness but simultaneously increase the carbon equivalent and promote martensitic transformation during cooling, which elevates the risk of cold cracking. The authors addressed this by systematically optimizing the chemical composition, coating formulation, and welding process parameters to balance these competing requirements.
The optimization strategy involved several key variables:
| Parameter | Typical Range | Role in Performance |
|---|---|---|
| Boron content (wt%) | 0.3 – 1.2 | Controls boride volume fraction and hardness |
| Carbon equivalent | < 0.55% | Limits cold cracking susceptibility |
| Coating flux SiO₂ content | 15 – 30% | Deoxidation and slag viscosity control |
| Coating flux CaF₂ content | 5 – 15% | Arc stability and hydrogen control |
| Preheating temperature | 150 – 250 °C | Mitigates residual stress and HIC risk |
| Interpass temperature | ≤ 250 °C | Controls cooling rate and microstructure |
The authors demonstrated that boron content in the range of 0.5–0.8 wt% yielded an optimal balance, producing overlay layers with surface hardness exceeding 58 HRC while maintaining acceptable crack resistance. The microstructure consisted of a tempered martensite matrix with dispersed boride particles, providing both wear resistance through abrasive-hardening and crack resistance through the toughened matrix.
Process and Metallurgical Analysis
The welding process employed was SMAW (shielded metal arc welding) with coated electrodes, which is the most widely used method for field cladding applications. The electrode coating served dual purposes: it provided arc stability through fluxing and alloying elements, and it acted as a source of boron and other alloying elements that transferred to the overlay layer.
The key metallurgical considerations included:
- Dilution control: The dilution rate from the base metal (typically carbon steel or low-alloy steel) was managed by controlling the number of passes, the current density, and the electrode travel speed. Higher dilution reduces boride concentration and hardness but improves toughness.
- Cooling rate management: The cooling rate from the solidus to 500 °C (C500) was a critical parameter governing martensite formation. Slower cooling rates promoted tempering and reduced residual stress.
- Hydrogen control: The coating flux formulation was designed to minimize hydrogen pickup, which is particularly critical for boron-containing electrodes due to the high carbon equivalent.
The authors also discussed the effect of heat treatment on the overlay layer. Post-weld heat treatment (PWHT) at 550–650 °C for 2 hours per 25 mm thickness was recommended to relieve residual stresses and temper the martensite, improving both toughness and dimensional stability.
Engineering Practice Implications
In practical engineering applications, boron-containing wear-resistant cladding electrodes are commonly used for repairing and hardening components such as:
- Excavator bucket teeth and cutting edges
- Crusher jaws and roll surfaces
- Coal handling equipment components
- Cement mill liners
- Agricultural machinery implements
The optimization results from this study have direct applicability to field repair operations where SMAW is the only feasible method. The recommended parameter windows provide a practical starting point for welding procedure qualification under NB/T 47014 or ASME IX.
Key Reflections and Study Insights
The most valuable insight from this paper is the systematic approach to balancing competing requirements through composition design rather than relying solely on process parameter adjustment. The authors demonstrated that boron content is the single most influential variable, and that the optimal window is narrow enough to require careful control of both the electrode manufacturing process and the field welding conditions.
A critical observation is that the crack resistance improvement achieved through composition optimization complements but does not replace the need for proper welding procedure control. Preheating, interpass temperature control, and post-weld heat treatment remain essential for reliable field application. The study also highlights the importance of understanding the boride morphology—plate-like FeB is harder but more brittle than granular Fe₂B, and the ratio depends on cooling rate and composition.
For engineers designing welding procedures for boron-containing overlay applications, the key takeaway is that the electrode design and the welding procedure must be developed as an integrated system. Changes in welding parameters (current, travel speed, number of passes) directly affect the effective boron content and dilution in the overlay, which in turn affects both hardness and crack resistance. This interdependence necessitates a holistic approach to procedure qualification.