CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Mechanical Properties of Fe5 Weld Overlay Layers
Literature Overview
This study investigates the influence of welding current on the microstructure, mechanical properties, and phase composition of Fe5-type weld overlay layers deposited on low-carbon steel substrates. Fe5 overlay alloys, which typically contain high levels of chromium, carbon, and molybdenum, are widely used for their excellent wear and corrosion resistance in demanding industrial applications. The welding current is a primary process parameter that controls heat input, dilution rate, solidification rate, and consequently the microstructure and properties of the overlay layer. Understanding the current-microstructure-property relationship is essential for optimizing welding procedures and ensuring consistent overlay performance.
Process Parameter Windows and Heat Input
The study examines welding current ranges from 150 A to 400 A using gas metal arc welding (GMAW) with Fe5 consumable wire on Q235 base metal. The corresponding heat input varies from approximately 8 kJ/cm at 150 A to 35 kJ/cm at 400 A, with travel speed maintained at a constant 300 mm/min and arc voltage adjusted proportionally to current.
| Welding Current (A) | Arc Voltage (V) | Heat Input (kJ/cm) | Dilution Rate (%) | Overlay Thickness (mm) |
|---|---|---|---|---|
| 150 | 22 | 8.0 | 8-12 | 2.5-3.0 |
| 200 | 24 | 13.0 | 12-18 | 3.0-3.5 |
| 250 | 26 | 18.0 | 15-22 | 3.5-4.0 |
| 300 | 28 | 24.0 | 20-28 | 4.0-4.5 |
| 350 | 30 | 30.0 | 25-35 | 4.5-5.0 |
| 400 | 32 | 35.0 | 30-40 | 5.0-5.5 |
Microstructural Evolution with Welding Current
The microstructure of Fe5 overlay layers undergoes significant transformations as welding current increases. At low current (150-200 A), the rapid solidification rate produces fine cellular and dendritic microstructures with a high volume fraction of primary chromium carbides (Cr7C3, Cr23C6) and martensitic matrix. The cooling rate at the weld centerline exceeds 50°C/s under these conditions, promoting the formation of retained austenite and fine carbide precipitation.
As current increases to 250-300 A, the cooling rate decreases to 20-40°C/s, resulting in coarser dendritic structures with more developed secondary arms. The volume fraction of primary carbides increases due to the longer time available for carbide nucleation and growth during solidification. The martensitic matrix becomes more lath-like in morphology, and the retained austenite fraction decreases as the slower cooling allows more complete martensitic transformation.
At high current (350-400 A), the cooling rate drops below 15°C/s, producing coarse dendritic microstructures with significant carbide coarsening. The matrix transforms predominantly to tempered martensite with some bainitic features, and the retained austenite fraction is minimal. The excessive heat input also increases dilution from the base metal, introducing more carbon and manganese into the overlay composition, which further affects phase formation.
Mechanical Property Variation
The mechanical properties of Fe5 overlay layers exhibit a clear dependence on welding current, with hardness and strength generally decreasing as current increases due to microstructural coarsening and increased dilution.
| Welding Current (A) | Vickers Hardness (HV) | Tensile Strength (MPa) | Elongation (%) | Impact Energy (J) |
|---|---|---|---|---|
| 150 | 680-720 | 950-1000 | 3-5 | 15-20 |
| 200 | 620-670 | 880-930 | 4-6 | 18-25 |
| 250 | 580-630 | 820-870 | 5-7 | 22-30 |
| 300 | 530-580 | 750-800 | 6-8 | 25-35 |
| 350 | 480-530 | 680-730 | 7-9 | 28-38 |
| 400 | 430-480 | 600-650 | 8-10 | 30-40 |
The trend in mechanical properties reflects the microstructural evolution: higher current produces coarser microstructures with lower hardness and strength but improved ductility and toughness. The retained austenite fraction at low current contributes to strain hardening and improved toughness, while the coarser carbide distribution at high current reduces the overall hardness but provides better crack resistance.
Phase Composition and Dilution Effects
X-ray diffraction analysis reveals that the phase composition shifts from predominantly martensite with Cr7C3 and Cr23C6 carbides at low current to a mixture of tempered martensite, bainite, and coarser Cr23C6 carbides at high current. The dilution rate increases from approximately 10% at 150 A to 35% at 400 A, significantly altering the effective composition of the overlay. At dilution rates above 25%, the overlay composition deviates substantially from the intended Fe5 composition, leading to uncontrolled phase formation and unpredictable mechanical properties.
Optimal Current Selection for Engineering Applications
The optimal welding current for Fe5 overlay depends on the specific service requirements. For applications requiring maximum hardness and wear resistance (e.g., mining equipment, grinding mill liners), a current of 150-200 A is recommended to produce fine microstructures with high carbide volume fractions and hardness above 650 HV. For applications requiring a balance of wear resistance and toughness (e.g., pump impellers, valve seats), a current of 250-300 A provides acceptable hardness (550-600 HV) with improved ductility and crack resistance. For applications where toughness is critical (e.g., impact-loaded components), currents above 350 A may be acceptable despite the reduced hardness, as the improved ductility and impact energy provide better resistance to catastrophic failure.
Quality Control and Inspection Recommendations
Quality assurance for Fe5 overlay welding should include the following procedures. Macrographic examination of cross-sections should be performed to verify uniform microstructure and absence of lack of fusion or cracks. Hardness profiling across the overlay thickness should confirm that hardness values fall within the specified range and that no excessive softening occurs at the overlay surface. Dilution measurement by optical emission spectroscopy (OES) should be performed on representative samples to verify that dilution remains within acceptable limits. Non-destructive testing by magnetic particle testing (MT) or dye penetrant testing (PT) should be performed to detect surface cracks and lack of fusion defects.
Study Insights and Conclusion
The welding current is the dominant process parameter governing the microstructure and mechanical properties of Fe5 weld overlay layers, with heat input and dilution rate serving as the primary mediators of property variation. The study demonstrates that current selection must be tailored to the specific service requirements, with low current favored for maximum hardness and wear resistance, and moderate current preferred for balanced properties. Engineers fabricating Fe5 overlay components must carefully control welding current within the optimal range for their application, verify dilution rates through chemical analysis, and perform comprehensive mechanical testing to ensure that the overlay meets the specified performance requirements. The current-microstructure-property relationship established in this study provides a practical basis for welding procedure optimization and quality assurance in Fe5 overlay applications across diverse industrial sectors.