Effect of Electric Current on Microstructure and Properties of Plasma Arc Cladding Fe90 Hardfacing Alloy
1. Definition and Fundamental Principles
1.1 Plasma Arc Cladding (PAC) Overview
Plasma Arc Cladding (PAC) is an advanced thermal spray and weld overlay process that utilizes a high-velocity, high-temperature plasma jet generated by a constricted arc to melt a powder feedstock or consumable electrode onto a base substrate. Unlike conventional TIG or MIG weld overlay, plasma arc cladding employs a transfer arc or non-transferred arc configuration where the plasma arc is generated between a cathode and an anode (or workpiece), producing a highly concentrated heat source with energy densities significantly exceeding those of conventional arc processes. This results in deeper melt penetration, improved metallurgical bonding, and reduced dilution ratios when properly controlled.
1.2 Fe90 Hardfacing Alloy Characterization
Fe90 is an iron-based carbide hardfacing alloy, typically containing chromium, molybdenum, tungsten, and carbon in proportions designed to produce a microstructure rich in hard carbide phases (Cr₇C₃, Mo₂C, WC, or mixed carbides) dispersed within a martensitic or austenitic matrix. The "90" designation in the Fe90 nomenclature refers to the alloy's classification within the iron-based hardfacing system, indicating a specific hardness level and carbide morphology targeted for severe wear and abrasion resistance. Fe90 alloys typically achieve as-cast hardness in the range of 60-70 HRC, with enhanced wear resistance derived from the uniform distribution and size of the hard carbide phase.
1.3 Electric Current as the Dominant Process Variable
In plasma arc cladding, the electric current (I) is the single most influential process parameter governing heat input, arc stability, powder melting efficiency, dilution rate, and ultimately the microstructure and mechanical properties of the deposited cladding layer. The current directly determines:
- Arc power: P = I × V (arc voltage), which governs total energy input
- Melt pool geometry: Higher currents produce deeper, wider melt pools
- Dilution rate: Greater heat input increases base metal dilution into the cladding
- Cooling rate: Affects solidification microstructure and phase transformation
- Carbide morphology: Determines carbide size, shape, and distribution
2. Technical Purpose and Industrial Value
2.1 Rationale for Current Parameter Optimization
The study of electric current effects on Fe90 plasma arc cladding is fundamentally aimed at establishing a quantitative relationship between process parameters and output quality characteristics. This knowledge is critical for:
- Process window definition: Identifying the current range that produces optimal hardness, wear resistance, and crack-free deposition
- Dilution control: Minimizing base metal dilution to preserve the designed carbide chemistry of Fe90
- Microstructure engineering: Achieving the desired balance between matrix toughness and carbide hardness
- Reproducibility: Ensuring consistent quality across production batches and multiple operators
- WPS development: Providing the technical basis for Welding Procedure Specification qualification
2.2 Business Positioning
This technical knowledge falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd., specifically in the advanced plasma arc cladding sub-category. It represents a bridge between laboratory research and production-scale qualification, enabling the company to offer customers optimized overlay solutions for severe wear applications where standard hardfacing alloys require precise microstructural control.
3. Key Process Parameters and Their Interrelationships
3.1 Electric Current Range and Effects
| Current Range (A) | Heat Input Level | Dilution Rate (%) | Carbide Morphology | Hardness (HRC) | Crack Susceptibility |
|---|---|---|---|---|---|
| Low: 80-120 | Low | 5-10 | Fine, dispersed; possible incomplete melting | 55-60 | Low |
| Medium: 120-180 | Moderate | 10-18 | Well-distributed; optimal size range | 60-68 | Low-Moderate |
| High: 180-250 | High | 18-30+ | Coarse, agglomerated; possible matrix over-tempering | 58-65 | Moderate-High |
| Excessive: >250 | Excessive | >30 | Over-coarse; loss of alloy identity | <55 | High |
3.2 Correlated Process Parameters
While electric current is the dominant variable, the following parameters must be coordinated to achieve optimal results:
| Parameter | Typical Range for Fe90 PAC | Relationship to Current |
|---|---|---|
| Plasma gas flow (Ar or Ar+H₂) | 5-25 L/min | Higher current requires higher gas flow for arc stability |
| Shielding gas flow | 10-20 L/min | Independent but must match heat input |
| Travel speed | 200-600 mm/min | Higher current requires higher travel speed to maintain dilution |
| Powder feed rate | 0.5-3.0 kg/h | Must match melting capacity of arc power |
| Nozzle-to-workpiece distance | 3-8 mm | Critical for arc stability at given current |
| Interpass temperature | <250°C (typical) | Must be controlled regardless of current level |
3.3 Microstructure Evolution with Current
4. Microstructural Analysis
4.1 Low Current Regime (80-120 A)
At lower current levels, the plasma arc produces a confined melt pool with limited heat input. The resulting microstructure characteristics include:
- Incomplete powder melting: Unmelted or partially melted powder particles may be present, reducing metallurgical bond quality
- Fine carbide precipitation: Rapid solidification produces fine carbide particles (1-5 μm) uniformly distributed
- Retained austenite: Higher cooling rates may retain metastable austenite in the matrix
- Lower hardness: Due to incomplete melting and dilution effects, hardness may be sub-optimal
- Poor interlayer bonding: Insufficient heat may result in cold shuts between passes
4.2 Optimal Current Regime (120-180 A)
The medium current range typically produces the best balance of processability and microstructure quality:
- Complete powder melting: All feedstock is fully molten before deposition
- Optimal carbide morphology: Carbides in the 3-8 μm range, well-dispersed without agglomeration
- Controlled dilution: Base metal dilution remains within 10-18%, preserving alloy chemistry
- Matrix transformation: Appropriate cooling rate produces tempered martensite or bainite matrix
- Peak hardness: Typically 60-68 HRC achieved with optimal carbide distribution
- Good interpass bonding: Sufficient heat ensures metallurgical continuity between layers
4.3 High Current Regime (180-250+ A)
Excessive current levels introduce several detrimental effects:
- Excessive dilution: Base metal contamination exceeds 30%, altering the designed alloy chemistry
- Carbide coarsening: Higher temperatures promote carbide growth and agglomeration (10-30 μm)
- Matrix softening: Over-tempering of the martensitic matrix reduces hardness
- Hot cracking susceptibility: Wider melt pools with higher thermal gradients increase cracking risk
- Porosity: Excessive gas entrapment from over-melted zones
- Warping and distortion: Excessive heat input causes base metal deformation
5. Mechanical Properties and Performance Correlation
5.1 Hardness vs. Current Relationship
The hardness of Fe90 plasma arc cladding exhibits a non-monotonic relationship with electric current, following a peak-shaped curve. Below the optimal range, hardness is limited by incomplete melting and dilution. Above the optimal range, hardness decreases due to carbide coarsening and matrix softening. The peak hardness region corresponds to the optimal current window where carbide distribution and matrix microstructure are both maximized for wear resistance.
5.2 Wear Resistance Assessment
Abrasion resistance (ASTM G65 or equivalent) is not solely determined by hardness but by the synergistic interaction of:
- Carbide hardness and volume fraction
- Matrix toughness (ability to support carbides without cracking)
- Carbide size and distribution uniformity
- Coating thickness and integrity
5.3 Bond Strength
The metallurgical bond strength between the Fe90 cladding and base material is directly affected by current level through its influence on dilution and interface chemistry. Typical minimum bond strength requirements for production applications range from 200-300 MPa (depending on base material and application), which must be verified through ASTM B671 or equivalent bond strength testing.
6. Applicable Standards and Acceptance Criteria
6.1 Weld Overlay Standards
- ASTM A388: Standard Specification for Hard Facing Alloys (general hardfacing requirements)
- ASTM A291: Standard Specification for Hard Facing Alloys (castings)
- ASME Section IX: Qualification of Welding Procedures, Welders, and Welding Operators (WPS/PQR requirements)
- ASME Section II Part D: Chemical composition limits for weld overlay materials
- GB/T 13814: Welding consumables — Classification of welding consumables for hardfacing
- NB/T 47014: Qualification test methods for welding procedure of pressure equipment (China)
- API 944: Welding and Welding Inspection Qualification Requirements (for API equipment)
- ISO 14176: Welding — Weld overlay procedures (qualification and qualification testing)
6.2 NDT and Acceptance Standards
- ASTM E1417: Standard Practice for Magnetic Particle Examination
- ASTM E164: Standard Practice for Liquid Penetrant Inspection
- ASTM E165: Standard Practice for Eddy Current Examination
- GB/T 11345: Non-destructive testing of welds — Ultrasonic testing
- NB/T 47013: Non-destructive testing methods for pressure equipment (China)
- ISO 17638: Non-destructive testing of welds — Magnetic particle testing
6.3 Acceptance Criteria Summary
| Test Parameter | Acceptance Criterion | Test Method |
|---|---|---|
| Hardness | 60-70 HRC (as-specified per application) | ASTM E18 / GB/T 230.1 |
| Bond strength | ≥200 MPa (minimum) | ASTM B671 |
| Dilution rate | ≤20% (typical maximum) | Optical emission spectroscopy (OES) |
| Surface cracks | No cracks permitted (MT/PT) | ASTM E1417 / E165 |
| Porosity | Per acceptance level (typically Level 1-2) | Visual / Radiographic |
| Overlay thickness | Per WPS specification ± tolerance | Ultrasonic / Visual |
| Carbide distribution | Uniform, no agglomeration | Metallurgical examination |
7. Common Risks and Controls
7.1 Process Risks
- Risk: Arc instability at high current
Control: Ensure adequate plasma gas flow, proper nozzle condition, and stable power supply with current regulation - Risk: Excessive dilution from over-current
Control: Implement OES dilution monitoring; establish current-travel speed correlation curves; use backing materials where needed - Risk: Hot cracking in high-carbon Fe90 deposits
Control: Optimize current to minimize thermal gradient; implement appropriate preheat and interpass temperature control; consider multi-pass strategies - Risk: Carbide agglomeration at high current
Control: Maintain current within optimal window; control travel speed to ensure rapid solidification; consider powder feed rate adjustments - Risk: Incomplete melting at low current
Control: Establish minimum current thresholds through qualification testing; verify powder feed rate compatibility with arc power
7.2 Quality Assurance Controls
- Pre-qualification testing: Conduct systematic current sweep experiments (typically 100-250 A in 20 A increments) to establish the process window
- WPS documentation: Record all qualified parameter combinations with corresponding test results
- Production monitoring: Implement real-time current monitoring with alarm limits
- Periodic verification: Perform hardness and dilution checks at defined intervals during production runs
- NDT coverage: Apply 100% MT/PT inspection for surface cracks; UT for volumetric defects in thick overlays
8. Application Across Technology Routes
8.1 TIG/MIG Weld Overlay (Primary Route)
Plasma arc cladding of Fe90 is a specialized variant of the TIG/MIG weld overlay technology route. The current optimization knowledge directly translates to:
- Process transfer: Current-heat input relationships established for plasma arc can inform conventional TIG overlay parameter selection
- Multi-layer strategy: Understanding how current affects each pass enables optimal multi-pass build-up strategies for thick Fe90 overlays
- Transition layer design: Current control knowledge supports the design of graded transition layers between base material and Fe90 overlay
- Equipment qualification: Establishes the operational envelope for plasma arc cladding equipment within the company's TIG/MIG overlay capability
8.2 Hydraulic Explosive Bonding (Complementary Route)
While plasma arc cladding is a thermal process, the knowledge of Fe90 microstructure and properties is directly applicable to hydraulic explosive bonding (HEB) applications:
- Material selection: Understanding Fe90's response to thermal input helps evaluate its compatibility with HEB-formed clad structures
- Post-bonding treatment: If HEB-formed Fe90 cladding requires post-weld heat treatment, current/heat input knowledge informs PWHT parameter selection
- Repair overlay: HEB-produced clad components may require localized TIG/PAC repair of damaged areas, where Fe90 current optimization is directly applicable
- Hybrid solutions: Combination of HEB for base cladding and PAC overlay for surface hardening in severe wear applications
8.3 Explosion Welding (Complementary Route)
Explosion welding of Fe90 or Fe90-compatible materials benefits from plasma arc cladding research in the following ways:
- Microstructure comparison: Understanding thermal process effects on Fe90 enables comparison with explosion-welded joint microstructures
- Surface preparation: Post-explosion grinding and surface conditioning may require overlay repair, where PAC parameters are critical
- Hybrid manufacturing: Explosion-welded components with Fe90 overlay surfaces represent a high-value combined solution for extreme wear environments
- Qualification support: Current optimization data supports the technical documentation required for combined process qualification
9. Contribution to Qualification Building and Customer Value
9.1 WPS Qualification Support
The systematic study of electric current effects provides the technical foundation for developing qualified Welding Procedure Specifications (WPS) for Fe90 plasma arc cladding. Key qualification elements supported include:
- Essential variables definition: Establishes current as a primary essential variable with defined limits
- Non-essential variables: Identifies travel speed, gas flow, and other parameters that can be varied within limits without requalification
- Performance qualification record (PQR): Provides test data (hardness, dilution, bond strength, NDT) for each qualified parameter combination
- Procedure transfer: Enables qualification transfer between similar processes and equipment configurations
9.2 Product Delivery Enhancement
For production delivery, current optimization knowledge directly translates to:
- Reduced rework rates: Operating within qualified current windows minimizes defects and rework
- Consistent quality: Documented parameter ranges ensure batch-to-batch consistency
- Operator training: Provides clear parameter guidelines for operator training and certification
- Efficiency improvement: Optimal current selection maximizes deposition rate while maintaining quality
- Thick overlay capability: Multi-pass strategies with controlled current per pass enable thick, defect-free Fe90 overlays
9.3 Customer Value Proposition
"The systematic understanding of electric current effects on Fe90 plasma arc cladding microstructure and properties enables Cladding Technology Shanxi Co., Ltd. to deliver engineering-optimized overlay solutions with guaranteed performance. Customers benefit from documented process qualifications, consistent hardness and wear resistance across all delivered components, and the confidence that each production batch operates within a scientifically validated parameter envelope."
9.4 Technical Differentiation
This research capability positions the company as a technically differentiated provider rather than a commodity overlay contractor. The ability to:
- Demonstrate quantitative understanding of process-structure-property relationships
- Customize current parameters for specific customer application requirements
- Provide detailed metallurgical documentation with each delivery
- Support customer's own qualification programs with technical data
- Rapidly develop new WPS for novel Fe90 applications
10. Implementation Recommendations
10.1 Process Development Protocol
- Phase 1 — Parameter Screening: Conduct current sweep from 80 A to 250 A at fixed travel speed and gas flow to identify the general optimal range
- Phase 2 — Fine Optimization: Within the identified range, vary current in 10 A increments while adjusting travel speed to maintain constant heat input per unit length
- Phase 3 — Multi-pass Validation: Apply optimized single-pass parameters to multi-pass builds; verify interpass bonding and cumulative dilution
- Phase 4 — Qualification Testing: Perform full NDT, hardness, dilution, bond strength, and wear testing per applicable standards
- Phase 5 — WPS Documentation: Formalize qualified parameters, limits, and acceptance criteria into a production-ready WPS
- Phase 6 — Production Transfer: Train operators, establish monitoring procedures, and implement quality control checkpoints
10.2 Key Performance Indicators for Production
| KPI | Target Value | Monitoring Frequency | Corrective Action Trigger |
|---|---|---|---|
| Current stability | ±2% of setpoint | Continuous | Immediate parameter adjustment |
| Hardness (per batch) | 62-68 HRC | Every 2 hours / per component | Recalibrate and retest |
| Dilution rate | ≤15% (target); ≤20% (max) | Per shift / per component | Adjust current or travel speed |
| Surface defect rate (MT/PT) | <2% of inspected area | 100% inspection | Reprocess affected area |
| Bond strength | ≥250 MPa | Per qualification lot | Process investigation required |
11. Conclusion
The systematic investigation of electric current effects on Fe90 plasma arc cladding microstructure and properties represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. This research directly enables:
- Development of qualified, reproducible WPS for Fe90 overlay applications
- Optimization of wear resistance through controlled carbide morphology
- Minimization of dilution and preservation of designed alloy chemistry
- Reduction of production defects through parameter window control
- Technical credibility and differentiation in the competitive overlay market
- Support for customer qualification programs and regulatory compliance
By integrating this knowledge into the company's TIG/MIG weld overlay technology route and cross-referencing with hydraulic explosive bonding and explosion welding capabilities, the organization can offer comprehensive, scientifically-backed cladding solutions for the most demanding wear-resistant applications across oil and gas, mining, power generation, and heavy industrial sectors.