CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Electromagnetic Stirring on Microstructure and Properties of Iron-Based Wear-Resistant Hardfacing Alloys
Literature Overview
This 2018 publication by Jia Hua and Li Meng from Dalian Ocean University's School of Applied Technology investigates the influence of electromagnetic stirring (EMS) on the microstructure and mechanical properties of iron-based wear-resistant hardfacing alloys. Funded by the Liaoning Provincial Department of Education Science Research Program (L2015075), this work addresses a relatively underexplored area in hardfacing technology where electromagnetic field application during solidification can be used to control microstructure evolution.
Core Technical Content
Electromagnetic stirring involves applying an alternating or pulsed magnetic field to the molten weld pool during hardfacing deposition. The induced Lorentz forces create convective flow within the melt, which fundamentally alters the solidification pattern compared to conventional unstirred conditions. For iron-based wear-resistant hardfacing alloys, which typically contain high levels of carbon, chromium, molybdenum, and vanadium, the solidification microstructure is critical to achieving the desired balance of hardness, wear resistance, and impact toughness.
The electromagnetic stirring parameters and their effects can be summarized as follows:
| EMS Parameter | Range Investigated | Effect on Microstructure |
|---|---|---|
| Frequency | 50-500 Hz | Controls stirring intensity and penetration depth |
| Current density | 5-50 A/cm² | Determines Lorentz force magnitude |
| Application timing | During solidification | Affects dendrite fragmentation and grain refinement |
| Cooling rate interaction | Coupled with thermal cycle | Modifies partition coefficient and segregation |
Microstructural Analysis
Without electromagnetic stirring, iron-based hardfacing alloys typically exhibit a coarse dendritic microstructure with significant inter-dendritic segregation of alloying elements. The primary phases include austenite or martensite matrix with dispersed carbides of various types:
- M₇C₃ type carbides (Cr₇C₃, Fe₇C₃): Provide moderate wear resistance with good toughness
- M₂C type carbides (Mo₂C, W₂C): Provide high hardness and thermal stability
- MC type carbides (VC, Cr₃C): Provide extreme hardness but may reduce toughness
Electromagnetic stirring promotes several beneficial microstructural modifications:
- Grain refinement: The mechanical stirring action fragments dendrite arms, increasing the nucleation site density and reducing grain size by 30-60%.
- Reduced macrosegregation: Enhanced convective mixing homogenizes the composition within the weld pool, reducing the concentration gradient between dendrite cores and interdendritic regions.
- Carbide redistribution: The stirring action breaks up coarse carbide networks and promotes a more uniform dispersion of carbide particles.
- Phase fraction modification: EMS can shift the equilibrium between ferrite, austenite, and martensite phases by affecting the cooling rate distribution within the deposit.
Mechanical Property Improvements
The mechanical property improvements achieved through electromagnetic stirring are quantified as follows:
| Property | Without EMS | With EMS | Improvement |
|---|---|---|---|
| Hardness (HV) | 750-850 | 800-920 | 7-10% |
| Wear resistance (relative) | 1.0 | 1.3-1.5 | 30-50% |
| Impact toughness (J/cm²) | 8-12 | 10-15 | 25-30% |
| Compressive strength (GPa) | 3.5-4.0 | 3.8-4.5 | 8-12% |
The simultaneous improvement in both hardness and toughness through EMS represents a significant advancement over conventional hardfacing approaches, where these properties are typically inversely related.
Engineering Practice and Implementation Challenges
Implementing electromagnetic stirring in production hardfacing operations presents several practical challenges. The electromagnetic coil system must be integrated with the welding torch without interfering with the arc stability or wire feeding mechanism. For submerged arc welding (SAW) overlay, which is commonly used for thick hardfacing deposits, the EMS coil can be positioned around the flux-covered weld zone. For gas metal arc welding (GMAW) overlay, the integration is more complex due to the compact geometry of the welding torch.
The power requirements for effective EMS in hardfacing applications are moderate (typically 500-2000 W), but the system must be precisely synchronized with the welding parameters to ensure optimal stirring during the critical solidification period.
Key Reflections
This research demonstrates that electromagnetic stirring offers a non-invasive method to improve hardfacing deposit quality without modifying the base material or filler metal chemistry. The approach is particularly attractive for repair applications where the base material cannot be altered. However, the industrial adoption of EMS hardfacing requires addressing equipment portability, operator training, and cost-benefit analysis for production environments. The findings suggest that EMS is most beneficial for thick deposits (>5 mm) where segregation and coarse microstructure are most pronounced, making it particularly relevant for heavy-duty wear parts in mining, cement, and power generation industries.