CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Properties of Fe90 Plasma Cladding Deposits
Literature Overview
This technical study investigates the influence of plasma transfer arc (PTA) welding current on the microstructural characteristics and mechanical properties of Fe90 hardfacing alloy deposits. Fe90 is a widely used iron-based hardfacing alloy characterized by high chromium and carbon content, designed for severe abrasive wear applications. The systematic variation of welding current provides critical insights into process parameter sensitivity, which is essential for achieving reproducible and high-quality cladding layers in industrial production.
Core Technical Points and Process Parameters
The PTA cladding process is inherently sensitive to electrical current because it directly governs the arc power density, melt pool geometry, and cooling rate. In this study, the welding current was varied across a range of 150 A to 350 A while maintaining other parameters such as travel speed, powder feed rate, and shielding gas flow at constant values. The resulting deposits were examined for dilution ratio, microstructure, hardness, and wear resistance.
| Current (A) | Dilution Ratio (%) | Avg. Hardness (HV) | Retained Austenite (%) | Grain Size (m) |
|---|---|---|---|---|
| 150 | 8-12 | 580-620 | 5-8 | 25-35 |
| 200 | 10-15 | 600-650 | 4-7 | 30-40 |
| 250 | 12-18 | 620-680 | 3-6 | 35-50 |
| 300 | 15-22 | 640-700 | 2-5 | 40-55 |
| 350 | 18-25 | 650-720 | 1-4 | 45-60 |
Microstructural Evolution with Current Variation
At lower current values (150 A), the heat input is relatively low, resulting in rapid solidification and fine-grained microstructures. The dilution ratio is minimized, ensuring that the deposited layer composition closely matches the nominal alloy composition. The microstructure consists of a martensitic matrix with fine M7C3 carbides dispersed throughout. However, the low current may lead to incomplete melting of the substrate surface, potentially compromising the metallurgical bond.
As the current increases to the intermediate range (200-250 A), the heat input rises, promoting greater substrate melting and a higher dilution ratio. The carbide morphology begins to change, with some coarsening observed due to increased thermal exposure. The retained austenite fraction decreases slightly as the cooling rate moderates, shifting the transformation kinetics toward complete martensitic transformation.
At higher current values (300-350 A), the excessive heat input leads to significant dilution, carbide coarsening, and potential grain growth in the matrix. While the overall hardness may increase marginally due to solid solution strengthening effects, the microstructure becomes more susceptible to cracking under thermal cycling. The risk of hot cracking increases because the wider melt pool allows more time for low-melting-point intermetallic compounds to segregate at grain boundaries.
Mechanical Properties and Wear Performance
The hardness of Fe90 deposits generally increases with current up to approximately 300 A, after which the rate of increase diminishes. The peak hardness is attributed to the optimal balance between carbide volume fraction and matrix strengthening. However, the wear resistance does not follow the same trend monotonically. Abrasive wear tests indicate that the intermediate current range (200-250 A) provides the best wear resistance because the fine and uniformly distributed carbides offer the most effective resistance to abrasive material removal.
The tensile bond strength between the cladding layer and the substrate is also affected by current. Lower currents yield higher bond strength because the dilution ratio is lower and the metallurgical interface is cleaner. Excessive current can lead to intermetallic formation at the interface, which may reduce ductility and increase the risk of delamination under cyclic loading.
Defect Analysis and Countermeasures
The study identifies several defect types associated with current variation:
| Defect Type | Current Range | Root Cause | Countermeasure |
|---|---|---|---|
| Porosity | < 180 A | Incomplete arc penetration | Increase current or reduce travel speed |
| Cracking | > 300 A | Excessive residual stress and dilution | Reduce current, apply preheating |
| Excessive dilution | > 280 A | High heat input | Use lower current, increase travel speed |
| Poor bonding | < 160 A | Inadequate substrate melting | Increase current or add a surfacing pass |
| Carbide coarsening | > 280 A | Prolonged high-temperature exposure | Optimize current and interpass temperature |
Engineering Practice Integration
In production environments, the selection of welding current for Fe90 PTA cladding requires a careful balance between dilution control, microstructural refinement, and deposition efficiency. For thin-walled components or applications requiring minimal heat input, currents in the 180-220 A range are recommended. For thicker sections or when higher deposition rates are needed, currents up to 280 A can be employed, provided that interpass temperature is controlled below 150 degrees Celsius to prevent carbide coarsening.
A practical approach adopted in many fabrication shops is to perform a parameter qualification trial on a coupon representative of the production geometry. The coupon is sectioned, mounted, and examined metallographically to confirm dilution ratio, carbide distribution, and absence of defects before proceeding with full-scale cladding.
Key Reflections and Study Insights
The study highlights a fundamental principle in cladding engineering: process parameters do not act in isolation but interact synergistically to determine the final microstructure and performance. Current is not merely a variable to maximize deposition rate; it is a critical lever for controlling the metallurgical quality of the overlay. The engineer must understand the underlying metallurgical mechanisms to make informed parameter selections rather than relying on generic parameter charts.
Furthermore, the study underscores the importance of characterization techniques such as X-ray diffraction for quantifying retained austenite, electron probe microanalysis for mapping compositional segregation, and nanoindentation for measuring the intrinsic hardness of individual phases. These techniques provide the data foundation for process optimization and quality assurance.
Conclusion
The systematic investigation of welding current effects on Fe90 PTA cladding deposits demonstrates that an intermediate current range of 200-250 A offers the optimal combination of fine microstructure, controlled dilution, and superior wear resistance. Engineers must calibrate their process parameters based on the specific substrate geometry, thermal mass, and service conditions rather than applying a one-size-fits-all approach. This study provides a solid technical foundation for parameter selection in industrial Fe90 cladding applications.