CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of TH-950HN Electrode Overlay Welding
Literature Overview
This 2013 study from the Guangxi Institute of Technology investigates the microstructure and mechanical properties of overlay weld deposits produced using the TH-950HN electrode. The TH-950HN electrode is a specialized overlay welding consumable designed for depositing high-strength, wear-resistant layers on industrial components subject to severe abrasion and impact. The research, supported by doctoral fund and natural science foundation grants, provides a comprehensive metallurgical characterization of the overlay deposits, including microstructure, hardness distribution, wear resistance, and fracture behavior.
Core Technical Analysis
Electrode Composition and Classification
The TH-950HN electrode is classified as a high-nickel, high-chromium alloy overlay consumable. The "950" designation indicates a minimum tensile strength of approximately 950 MPa, while the "HN" suffix denotes high nickel content. The typical composition of the deposited metal includes:
| Element | Content (wt%) | Function |
|---|---|---|
| C | 0.3-0.6 | Carbide formation, hardness |
| Cr | 12-18 | Corrosion resistance, carbide stability |
| Ni | 8-15 | Solid solution strengthening, ductility |
| Mo | 2-5 | Secondary hardening, creep resistance |
| Mn | 1.0-2.0 | Deoxidization, grain boundary strengthening |
| Si | 0.5-1.5 | Deoxidization, slag formation |
| Fe | Balance | Base matrix |
Microstructural Evolution
The overlay weld deposit of TH-950HN exhibits a complex microstructure resulting from the rapid solidification and cooling conditions inherent to manual arc welding:
- Base matrix: A martensitic or bainitic ferrite matrix, depending on the cooling rate and interpass temperature. The high carbon and alloy content promotes martensite formation upon air cooling.
- Carbide phases: Chromium-rich carbides (Cr7C3, Cr23C6) and complex carbides (M7C3, M23C6) form at grain boundaries and within the matrix. These carbides are responsible for the primary wear resistance of the deposit.
- Grain structure: The weld deposit typically exhibits a columnar grain structure near the fusion boundary, transitioning to equiaxed grains in the center of the deposit. The grain size is influenced by the welding parameters and interpass temperature.
- Phase distribution: The spatial distribution of carbide phases varies across the weld cross-section, with higher carbide volume fractions near the surface where cooling rates are higher.
Mechanical Properties
The mechanical properties of the TH-950HN overlay deposits were characterized through hardness testing, tensile testing, and wear testing:
| Property | Value | Test Method |
|---|---|---|
| Hardness (HRC) | 45-55 | Rockwell C |
| Tensile strength (MPa) | ≥950 | Tensile test |
| Elongation (%) | 15-25 | Tensile test |
| Impact energy (J) | 30-60 | Charpy V-notch |
| Wear rate (mm³/N·m) | 0.5-1.5 | Pin-on-disk |
| Dilution rate (%) | 5-15 | Metallographic |
Effect of Welding Parameters on Microstructure and Properties
The study systematically examines the influence of key welding parameters on the deposit microstructure and properties:
| Parameter | Effect on Microstructure | Effect on Hardness | Effect on Toughness |
|---|---|---|---|
| Current increase | Coarser grains, more carbides | Increases | Decreases |
| Travel speed increase | Finer grains, less carbides | Decreases | Increases |
| Interpass temperature increase | Coarser grains, softer matrix | Decreases | Increases |
| Arc length increase | More porosity, coarser grains | Variable | Decreases |
Process Interpretation and Engineering Practice
Application Scenarios
The TH-950HN electrode overlay is particularly suited for:
- Mining equipment: Crusher jaws, conveyor rollers, and bucket teeth subject to severe abrasive wear.
- Construction machinery: Excavator buckets, bulldozer blades, and hydraulic cylinder rods.
- Power generation: Coal handling equipment, feedwater pumps, and turbine components.
- Cement industry: Mill liners, kiln wear plates, and fan blades.
Quality Control Considerations
The following quality control measures are essential for TH-950HN overlay welding:
- Preheating: Preheat the base material to 150-250°C to reduce residual stresses and prevent cold cracking.
- Interpass temperature control: Maintain interpass temperatures below 250°C to preserve the hardness of the deposit.
- Post-weld heat treatment: Optional stress relief at 550-600°C for 2 hours can reduce residual stresses without significantly reducing hardness.
- Non-destructive testing: Magnetic particle testing (MT) or ultrasonic testing (UT) should be performed on each pass to detect surface and subsurface defects.
- Hardness verification: Perform hardness testing on each layer to verify the required hardness profile and detect areas of excessive dilution.
Defect Analysis
Common defects in TH-950HN overlay welding include:
- Cracking: Both hot cracking in the deposit and cold cracking in the heat affected zone can occur. Hot cracking is associated with high sulfur and phosphorus content in the consumable, while cold cracking results from hydrogen embrittlement and high residual stresses.
- Porosity: Gas porosity from moisture contamination of the electrode coating or insufficient shielding. Countermeasure: Thorough electrode drying and proper shielding gas flow.
- Inclusion: Slag inclusions from incomplete slag removal between passes. Countermeasure: Careful slag removal and adequate arc voltage.
- Dilution: Excessive base metal dilution reduces the hardness and wear resistance of the deposit. Countermeasure: Reduce welding current or increase travel speed.
Study Insights and Reflections
The TH-950HN electrode represents a practical solution for overlay welding applications requiring high strength and wear resistance. The study's comprehensive characterization of microstructure and properties provides a solid foundation for process optimization and quality control. The key engineering insight is that the balance between hardness and toughness in overlay deposits is governed by the interplay of composition, cooling rate, and welding parameters. In practice, the selection of welding parameters must be tailored to the specific application, considering the trade-off between wear resistance and impact resistance. The research methodology, combining metallographic analysis with mechanical testing and wear testing, provides a replicable framework for evaluating overlay welding consumables and processes. The practical implication is that TH-950HN electrode overlay welding, when properly executed with attention to parameter control and quality verification, offers a reliable and cost-effective solution for extending the service life of wear-critical components in heavy industry.