CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Wear Resistance of High-Vanadium Iron-Based Clad Coatings
Literature Overview
This study investigates the influence of welding current on the microstructure and wear resistance of high-vanadium iron-based overlay coatings. Vanadium is a potent carbide-forming element that produces vanadium carbides (VC, V4C3, V8C7) with exceptional hardness and wear resistance. The research examines how variations in welding current affect the dilution ratio, carbide morphology, matrix composition, and overall tribological performance of the overlay. This work is particularly relevant to engineers designing hardfacing solutions for components subjected to severe abrasive and adhesive wear conditions.
Core Technical Points
The welding current is the primary parameter governing heat input in most arc welding processes. Higher currents increase the thermal energy delivered to the base metal, resulting in greater dilution of the base material into the overlay. For high-vanadium iron-based coatings, excessive dilution can significantly alter the carbon availability, affecting carbide precipitation and the resulting hardness profile.
Welding Current Parameters and Microstructural Responses
| Welding Current (A) | Heat Input (kJ/mm) | Dilution (%) | Matrix Hardness (HV) | Overlay Hardness (HV) |
|---|---|---|---|---|
| 150 | 8.5 | 12–15 | 320–350 | 950–1050 |
| 200 | 11.2 | 18–22 | 350–380 | 880–950 |
| 250 | 14.5 | 25–30 | 380–420 | 750–850 |
| 300 | 18.0 | 32–38 | 400–450 | 650–750 |
The study reveals a clear trend: as welding current increases, the dilution ratio rises, leading to a decrease in overlay hardness due to the dilution of vanadium and carbon content. However, the bonding strength and resistance to cracking improve with moderate increases in current due to better fusion and reduced residual stress concentration.
Microstructural Evolution
At lower welding currents (150–200 A), the overlay microstructure consists of a hard martensitic matrix with uniformly dispersed vanadium carbides (VC and V8C7) and some complex carbides. The matrix exhibits a fine lenticular martensite structure with retained austenite content of 8–12 percent. The vanadium carbides appear as fine, evenly distributed particles within the martensitic matrix, contributing to high hardness and excellent abrasive wear resistance.
At higher welding currents (250–300 A), the increased dilution introduces more alloying elements from the base metal, altering the carbide precipitation behavior. The vanadium carbides become coarser and less uniformly distributed, and the matrix transforms toward a coarser martensite-bainite structure. The retained austenite content decreases due to the dilution of carbon and alloying elements that stabilize austenite.
Wear Mechanism Analysis
The wear resistance of the overlay is primarily governed by the hardness and distribution of vanadium carbides. Under dry sliding conditions, the fine VC particles act as load-bearing elements that resist microploughing and microcutting. At higher currents with coarser carbides, the wear mechanism shifts toward matrix-dominated abrasion, where the softer matrix is preferentially removed, exposing and eventually dislodging the carbides.
Under abrasive wear conditions with hard particles (such as SiC or alumina), the overlay with lower current parameters demonstrates superior performance due to the finer and more uniformly distributed carbide structure. The study reports wear volume losses that are 40–60 percent lower for overlays deposited at 150–200 A compared to those deposited at 300 A.
Process Standards and Quality Considerations
High-vanadium iron-based overlays are commonly applied using submerged arc welding (SAW), gas metal arc welding (GMAW), or plasma transferred arc (PTA) processes. The welding procedure must be qualified per ASME IX, with test coupons demonstrating acceptable hardness, dilution, and mechanical properties. For applications in pressure vessels or piping, the overlay must comply with applicable codes such as ASME VIII Division 1 (UW-27) or GB/T 150.
The dilution ratio should be verified through cross-sectional metallographic analysis using optical microscopy or scanning electron microscopy with energy-dispersive spectroscopy (EDS). The dilution is typically calculated based on the concentration of a base metal marker element (such as manganese or silicon) at the fusion line.
Common defects and their causes include:
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking at fusion line | Excessive carbon content, low ductility | Add ductile interlayer, reduce carbon |
| Excessive porosity | Moist flux or wire, inadequate shielding | Dry flux, ensure proper shielding gas flow |
| Incomplete fusion | Low current, high travel speed | Increase current, reduce travel speed |
| Carbide network formation | Excessive carbon, slow cooling | Adjust carbon content, increase cooling rate |
Engineering Practice Integration
In practical hardfacing applications for components such as crusher jaws, conveyor wear plates, and earthmoving equipment, the selection of welding current must balance wear resistance against bonding integrity and crack resistance. For thick-section components with high residual stress potential, a moderate current (200–250 A) with a ductile interlayer is often preferred to prevent cracking, even at the expense of some hardness.
For thin-section components or repair applications where maximum hardness is desired, lower currents (150–200 A) with PTA or TIG processes are recommended. The key is to perform a dilution analysis on a test coupon and adjust parameters to achieve the target dilution ratio while maintaining acceptable bonding and crack resistance.
A practical tip from field experience is to always perform a hardness profile across the overlay thickness. The hardness should be relatively uniform, with no sharp drop at the fusion line. A gradual transition from overlay hardness to base metal hardness indicates good bonding and minimal dilution. A sharp drop indicates excessive dilution or incomplete fusion, both of which compromise overlay performance.
Key Questions and Reflections
The study raises the question of whether post-weld heat treatment could improve the wear resistance of overlays deposited at higher currents. Tempering of the martensitic matrix could reduce residual stresses and improve toughness without significantly reducing hardness, potentially allowing the use of higher currents for better bonding without sacrificing wear performance.
Another consideration is the effect of overlay thickness on dilution and wear resistance. Thicker overlays (multiple passes) dilute the base metal effect more effectively, as the first pass dilution is averaged out over subsequent passes. However, excessive thickness can introduce warping and distortion, requiring careful thermal management during deposition.
Study Insights and Implications
The relationship between welding current and overlay performance is fundamentally a dilution control problem. The study demonstrates that optimal performance is achieved at moderate currents that balance carbide integrity against bonding quality. Engineers should view welding current not as an isolated parameter but as part of a system that includes travel speed, wire feed rate, and shielding conditions.
The findings underscore the importance of systematic procedure development and qualification. A generic welding procedure may not be suitable for all applications, and parameter optimization should be conducted for each specific base metal, overlay composition, and service condition. The use of metallographic analysis and hardness profiling as routine quality control measures is essential for ensuring consistent overlay performance.
In conclusion, this literature provides valuable insights into the effects of welding current on high-vanadium iron-based overlay coatings, demonstrating that moderate currents yield the best balance of wear resistance, bonding integrity, and crack resistance. The systematic approach to parameter optimization, combined with rigorous microstructural analysis, is the key to reliable hardfacing performance in demanding industrial applications.