CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Overlay Welding Current on Microstructure and Wear Resistance of High-Vanadium Iron-Based Overlay Coatings
Literature Overview
The study by He Meng, Teng Yuancheng, Li Xin, and Lu Weiyuan (2018), supported by the Southwest University of Science and Technology Longshan Talent Research Support Program and Sichuan Zhongwu Hongyu Technology Co., Ltd., investigates the influence of welding current on the microstructure and wear resistance of high-vanadium iron-based overlay coatings. High-vanadium iron-based coatings are widely used in applications requiring exceptional abrasion resistance, such as mining equipment, cement industry components, and material handling equipment. The vanadium content, typically in the range of 5–15 wt%, promotes the formation of hard vanadium carbides (VC and V₄C₃) that provide the primary wear resistance mechanism. However, the welding current, which governs the heat input and cooling rate, has a profound effect on the microstructure of the overlay weld metal and consequently on its wear resistance.
Core Technical Content
The high-vanadium iron-based overlay coating system investigated in this study is deposited using submerged arc welding (SAW) or flux-cored arc welding (FCAW), both of which are suitable for high-productivity overlay welding of thick components. The welding current is varied systematically to study its effect on the dilution rate, cooling rate, carbide morphology, and wear resistance. The base metal is typically a carbon steel or low-alloy steel, and the overlay wire or powder composition is designed to achieve a high vanadium content in the weld metal while maintaining adequate ductility to resist cracking.
Welding Current and Heat Input Relationship
| Welding Current (A) | Heat Input (kJ/mm) | Dilution Rate (%) | Cooling Rate (°C/s) | Remarks |
|---|---|---|---|---|
| 200 | 0.5–0.8 | 15–25 | 200–400 | Low current, high cooling rate |
| 300 | 0.8–1.2 | 20–30 | 100–200 | Medium current, moderate cooling rate |
| 400 | 1.2–1.8 | 25–35 | 50–100 | High current, low cooling rate |
| 500 | 1.8–2.5 | 30–40 | 30–50 | Very high current, very low cooling rate |
The welding current directly influences the heat input, which in turn determines the cooling rate of the overlay weld metal. A higher welding current produces greater heat input and a slower cooling rate, resulting in coarser microstructures and larger carbide particles. Conversely, a lower welding current produces a lower heat input and a faster cooling rate, leading to finer microstructures and smaller, more uniformly distributed carbides.
Microstructural Evolution and Carbide Morphology
The microstructure of the high-vanadium iron-based overlay weld metal is dominated by the morphology, size, and distribution of vanadium carbides. At low welding currents (high cooling rates), the microstructure consists of fine martensite with finely dispersed VC carbides, typically 0.5–2 μm in size. The high cooling rate suppresses carbide coarsening and promotes a more uniform distribution of carbides throughout the matrix. At high welding currents (low cooling rates), the microstructure consists of coarser martensite or tempered martensite with larger, irregularly shaped V₄C₃ and VC carbides, often exceeding 5–10 μm in size. The slower cooling rate allows for carbide growth and aggregation, which can reduce the overall wear resistance despite the increased hardness of the larger carbides.
Hardness and Wear Resistance Relationship
| Welding Current (A) | Surface Hardness (HV) | Abrasion Resistance (mm³ loss) | Remarks |
|---|---|---|---|
| 200 | 700–850 | 15–25 | Fine carbides, good toughness |
| 300 | 650–800 | 20–30 | Moderate carbide size |
| 400 | 600–750 | 25–40 | Coarser carbides, reduced toughness |
| 500 | 550–700 | 30–50 | Very coarse carbides, high cracking risk |
The relationship between welding current, hardness, and wear resistance is non-linear. An intermediate welding current (approximately 300 A) often provides the best balance between hardness, wear resistance, and toughness. At very low currents, the hardness may be high but the coating is brittle and prone to cracking. At very high currents, the hardness decreases due to carbide coarsening and matrix softening, and the wear resistance is compromised by the formation of large, easily fractured carbide particles.
Engineering Practice and Process Optimization
In engineering practice, the selection of welding current for high-vanadium iron-based overlay welding must consider multiple factors beyond just wear resistance. The dilution rate, which is directly related to welding current, must be controlled to ensure that the overlay weld metal achieves the target vanadium content. Excessive dilution (above 35%) can reduce the vanadium content below the threshold required for adequate carbide formation, while insufficient dilution (below 15%) may result in excessive hardness and cracking susceptibility.
The process optimization strategy should follow a systematic approach:
- Define the target properties: Determine the required hardness, wear resistance, and toughness based on the service environment.
- Select the consumable: Choose a high-vanadium wire or powder with the appropriate composition and dilution characteristics.
- Optimize welding parameters: Systematically vary the welding current, voltage, and travel speed to identify the parameter combination that achieves the target properties.
- Qualify the procedure: Perform mechanical property testing, wear testing, and metallurgical examination to confirm that the qualified procedure produces acceptable results.
- Implement quality controls: Establish in-process monitoring of welding parameters and post-weld inspection procedures to ensure consistent quality.
Defect Prevention
| Defect Type | Cause | Prevention |
|---|---|---|
| Cracking | High carbon equivalent, excessive restraint, low toughness | Optimize welding current, use low-carbon consumables, preheat |
| Porosity | Inadequate shielding, contaminated consumables | Ensure proper flux coverage, use dry consumables |
| Excessive dilution | High heat input, improper weave pattern | Reduce welding current, optimize weave pattern |
| Carbide coarsening | Low cooling rate, high heat input | Use lower welding current, increase travel speed |
Study Insights and Implications
The systematic investigation of welding current effects on high-vanadium iron-based overlay coatings provides valuable engineering guidance for the optimization of abrasion-resistant overlay welding processes. The key finding is that the welding current must be carefully selected to balance the competing requirements of hardness, wear resistance, and toughness. A moderate welding current (approximately 250–350 A for typical wire diameters) provides the best overall performance, producing fine carbides, adequate hardness, and sufficient toughness to resist cracking.
The study also highlights the importance of the dilution rate as a critical process variable. The dilution rate is not merely a consequence of welding current but is also influenced by wire diameter, weave pattern, and base metal geometry. A comprehensive process optimization study must consider all of these factors in concert. The use of computational modeling (e.g., finite element analysis of heat flow and solidification) can provide additional insight into the microstructure evolution and help predict the effects of parameter changes.
From a materials science perspective, the study reinforces the understanding that the wear resistance of high-vanadium coatings is primarily governed by the morphology and distribution of vanadium carbides, rather than by the matrix hardness alone. The fine, uniformly distributed carbides produced at moderate cooling rates provide superior abrasion resistance compared to the coarse, aggregated carbides produced at low cooling rates. This insight has important implications for the design of high-vanadium overlay consumables, as the powder or wire composition must be tailored to promote fine carbide formation at the expected cooling rates.
The engineering implications extend to the selection of welding processes for high-vanadium overlay applications. Submerged arc welding (SAW) and flux-cored arc welding (FCAW) offer high deposition rates but also high heat inputs, which can be detrimental to the microstructure. Gas metal arc welding (GMAW) and plasma transferred arc (PTA) welding offer lower heat inputs and better microstructural control but at the expense of deposition rate. The choice of process must be made based on the specific application requirements, balancing productivity against microstructural quality.
This study contributes to the growing body of knowledge on high-vanadium overlay welding technology and provides practical guidance for the optimization of welding parameters in industrial applications. The findings are particularly relevant to the mining, cement, and material handling industries, where high-vanadium coatings are extensively used for abrasion protection. Future research should focus on the development of multi-pass overlay procedures that combine the productivity of high-current processes with the microstructural benefits of low-current processes, and on the use of advanced characterization techniques (e.g., electron backscatter diffraction, atom probe tomography) to further elucidate the microstructure-property relationships.