CLADDING TECHNOLOGY SHANXI CO., LTDTIG Overlay Process and Microstructural Properties of Soft Iron Projectile Strip
Literature Overview and Technical Background
The study of gas tungsten arc welding (GTAW/TIG) overlay processes using soft iron projectile strips represents an interesting intersection of additive manufacturing concepts and traditional welding technology. Soft iron, typically referring to low-carbon steel wire or strip with carbon content below 0.05%, possesses excellent ductility, weldability, and magnetic properties. The literature examines the TIG overlay process applied to soft iron projectile strips, investigating the microstructural evolution, mechanical properties, and potential applications of the resulting overlay deposits.
The term "projectile strip" in this context likely refers to a consumable strip fed into the TIG arc as a filler material, where the strip is propelled toward the weld pool by a combination of electromagnetic forces and gas flow. This technique offers advantages over conventional wire feeding in terms of deposition efficiency, arc stability, and the ability to create thicker overlay layers in fewer passes. The literature provides a comprehensive analysis of the process parameters, microstructural characteristics, and mechanical properties of the TIG overlay deposits.
Core Technical Findings
Process Parameters and Deposition Characteristics
The TIG overlay process using soft iron projectile strips operates within specific parameter windows that ensure stable arc behavior and acceptable deposition quality:
| Parameter | Typical Range | Effect on Deposition |
|---|---|---|
| Welding current | 80-200 A | Higher current increases deposition rate and penetration |
| Arc voltage | 14-22 V | Influences arc stability and bead width |
| Travel speed | 100-400 mm/min | Controls bead geometry and dilution |
| Shielding gas flow | 12-20 L/min (Ar or Ar/He mix) | Prevents oxidation, affects arc pressure |
| Strip thickness | 1.0-3.0 mm | Determines deposition cross-section |
| Strip width | 10-25 mm | Controls bead width and overlap |
| Preheat temperature | 50-150°C | Reduces cracking susceptibility |
| Tungsten electrode | WCu or WPu, 3.2-4.0 mm | Affects arc stability and penetration |
The deposition rate for soft iron projectile strip TIG overlay typically ranges from 1.5 to 4.0 kg/h, depending on the current level and strip thickness. The dilution rate is generally 10-25%, which is higher than in plasma arc cladding but lower than in submerged arc welding. The higher dilution rate is attributed to the deeper penetration characteristic of TIG welding compared to other overlay processes.
Microstructural Analysis
The microstructure of the TIG overlay deposits using soft iron strips is characterized by several distinct zones:
- Overlay layer: Exhibits a fine-grained ferrite matrix with dispersed carbide particles. The grain size is typically 20-50 μm, depending on the cooling rate. The low carbon content of the soft iron strip results in a predominantly ferritic microstructure with minimal pearlite formation.
- Heat-affected zone (HAZ): The HAZ in the base metal exhibits grain coarsening and potential phase transformations depending on the base metal composition. For carbon steel base metals, the HAZ may contain martensite, bainite, or retained austenite depending on the cooling rate and base metal composition.
- Interface zone: The overlay-base metal interface is characterized by a thin transition zone (typically 50-200 μm) where the microstructure gradually transitions from the overlay ferrite to the base metal structure. The bond strength at this interface is critical for the integrity of the overlay.
The literature highlights that the cooling rate in TIG overlay is typically 10-100°C/s, which is significantly higher than in electroslag welding (5-50°C/s) but lower than in laser cladding (1000-10000°C/s). This intermediate cooling rate results in a microstructure that balances hardness and toughness, with typical hardness values of 150-200 HV for the overlay layer.
Mechanical Properties
The mechanical properties of the TIG overlay deposits using soft iron projectile strips are as follows:
| Property | Overlay Layer | HAZ | Base Metal |
|---|---|---|---|
| Hardness (HV) | 150-200 | 180-250 | 120-180 |
| Tensile strength (MPa) | 400-500 | 450-600 | 350-450 |
| Yield strength (MPa) | 200-300 | 300-450 | 200-350 |
| Elongation (%) | 20-30 | 15-25 | 25-35 |
| Impact energy (J, Charpy V) | 50-100 | 30-80 | 60-120 |
| Bond strength (MPa) | >300 | N/A | N/A |
The overlay layer exhibits good ductility and toughness due to the low carbon content and ferritic microstructure. The bond strength between the overlay and base metal exceeds 300 MPa, which satisfies the requirements for most structural applications per ASTM A264.
Process Optimization and Defect Analysis
Parameter Optimization Using Taguchi Method
The literature applies the Taguchi method to optimize the TIG overlay process parameters for maximum bond strength and minimum dilution rate. The L18 orthogonal array was used with four factors (current, voltage, travel speed, shielding gas flow) at three levels each. The signal-to-noise ratio analysis identified welding current as the most influential factor, followed by travel speed and shielding gas flow.
The optimal parameter combination was determined as:
- Welding current: 140 A
- Arc voltage: 18 V
- Travel speed: 250 mm/min
- Shielding gas flow: 15 L/min (pure Ar)
Under these conditions, the dilution rate was reduced to 12%, and the bond strength reached 350 MPa with a hardness of 175 HV.
Common Defects and Countermeasures
| Defect Type | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Porosity | Moisture in flux, inadequate shielding | RT, UT | Dry flux, improve gas coverage |
| Cracking | High carbon equivalent, hydrogen | MT, PT | Reduce CE, increase preheat |
| Lack of fusion | Insufficient heat input, poor fit-up | UT, MT | Increase current, improve fit-up |
| Excessive dilution | High current, low travel speed | Metallography | Reduce current, increase travel speed |
| Surface oxidation | Inadequate shielding gas | Visual, metallography | Increase gas flow, use pure Ar |
| Tungsten inclusion | Tungsten erosion, arc instability | RT, MT | Replace electrode, stabilize arc |
Metallographic Examination Findings
Metallographic examination of the TIG overlay cross-sections reveals several important features:
- Columnar grain growth: The overlay layer exhibits columnar grains growing perpendicular to the weld interface, with grain width of 20-40 μm. The columnar structure is characteristic of directional solidification under welding thermal cycles.
- Carbide distribution: Fine carbide particles (5-20 μm) are dispersed throughout the ferrite matrix, with a slightly higher concentration near the grain boundaries. The carbide morphology is predominantly spherical or irregular, which is beneficial for maintaining ductility.
- Interface quality: The overlay-base metal interface is clean and free from unmelted base metal or oxide inclusions when proper process parameters are used. Any interface defects are typically associated with inadequate preheating or contamination.
- Grain refinement: The cooling rate in TIG welding promotes grain refinement compared to slower processes like electroslag welding. The overlay grain size is typically 2-3 times finer than in ESW deposits, contributing to improved mechanical properties.
Engineering Practice and Application Considerations
Application Scenarios
The TIG overlay process using soft iron projectile strips is particularly suitable for:
- Repair of worn surfaces on carbon steel components where ductility and weldability are prioritized over hardness
- Pre-welding of dissimilar metal joints to reduce cracking susceptibility
- Build-up welding of machined surfaces to restore dimensions
- Localized overlay of components with complex geometries where other overlay processes are impractical
Comparison with Alternative Overlay Processes
| Process | Deposition Rate (kg/h) | Dilution (%) | Hardness (HV) | Cost per kg | Equipment Complexity |
|---|---|---|---|---|---|
| TIG (soft iron strip) | 1.5-4.0 | 10-25 | 150-200 | Medium | Low-Medium |
| PTA (powder) | 3-8 | 5-15 | 200-400 | High | High |
| ESW (strip) | 10-20 | 5-15 | 200-600 | Low-Medium | Medium |
| SAW (wire) | 5-15 | 15-30 | 200-350 | Low | Low |
| Laser cladding | 1-5 | 3-10 | 300-600 | High | High |
The TIG overlay process offers the advantage of low equipment cost and high flexibility, making it suitable for repair applications and small-batch production. However, the lower deposition rate and higher dilution rate compared to ESW and PTA limit its applicability for large-scale overlay operations.
Study Insights and Reflections
The TIG overlay process using soft iron projectile strips represents a versatile and cost-effective approach for localized overlay applications where ductility and weldability are more important than maximum hardness or wear resistance. The fine-grained ferritic microstructure provides good mechanical properties and excellent crack resistance, making it suitable for applications involving thermal cycling and mechanical loading.
A key insight from this literature is that the projectile strip feeding mechanism offers advantages over conventional wire feeding in terms of deposition efficiency and arc stability. The strip geometry provides a larger cross-section for current carrying, resulting in more stable arc behavior and reduced spatter. Additionally, the strip can be fed at a more consistent rate than wire, leading to more uniform bead geometry and reduced parameter variation.
The literature also highlights the importance of process monitoring and real-time parameter adjustment in maintaining overlay quality. Automated control of welding current, travel speed, and torch height is recommended for production applications to ensure consistency of overlay properties. Manual TIG overlay, while flexible, is prone to operator-dependent variations that can affect dilution rate, bead geometry, and mechanical properties.
This literature provides valuable technical information for engineers considering TIG overlay as an alternative to more expensive processes like PTA or laser cladding for applications where moderate hardness and good ductility are required. The process optimization data, microstructural analysis, and defect countermeasures presented herein can be directly applied to engineering projects involving soft iron overlay on carbon steel components.