Effect of Welding Current on Microstructure and Wear Resistance of Open-Arc High-Boron Overlay Alloys

1. Definition and Technical Principles

Open-arc welding of high-boron overlay alloys refers to the deposition of boron-rich hardfacing materials—typically containing 10–25 wt% boron by weight—onto a base substrate using conventional arc processes such as MIG (GMAW) or TIG (GTAW) in a non-shielded or minimally shielded configuration. The term "明弧" (open/bright arc) specifically denotes a welding mode in which the arc is exposed to ambient atmosphere without full gas shielding, often employed for field repair, large-scale equipment hardfacing, or preliminary metallurgical studies where process economics and accessibility are prioritized over maximum metallurgical refinement.

High-boron overlay alloys derive their exceptional hardness (typically 70–90 HRC in the as-deposited condition) from the formation of hard boride phases, predominantly FeB and Fe₂B. The microstructure of these deposits is governed by competitive solidification kinetics between the austenite/ferrite matrix and the boride phases. Welding current is the primary thermal input variable that controls:

The fundamental metallurgical principle underlying this study is that boride phase formation is highly sensitive to the local boron activity at the solidification front. At higher currents, increased dilution reduces the effective boron concentration in the melt, potentially suppressing fine boride formation and favoring coarser, thermodynamically stable phases. Conversely, lower currents maintain higher boron supersaturation, promoting fine, evenly distributed boride networks but risking incomplete fusion and porosity.

2. Category and Business Positioning

This technical study falls within the Weld Overlay (Hardfacing) Process Development category of Cladding Technology Shanxi Co., Ltd's capability portfolio. Specifically, it contributes to the company's TIG/MIG weld overlay technology route, which is one of three principal technology platforms:

The business positioning of this open-arc high-boron alloy study is threefold:

  1. Process Qualification Foundation: Establishing the relationship between current parameters and metallurgical outcomes provides the empirical basis for Welding Procedure Specifications (WPS) qualification under applicable codes.
  2. Product Performance Optimization: Enabling the company to deliver high-boron overlay products with guaranteed minimum hardness, wear life, and microstructural integrity for demanding applications in mining, cement, power, and oil & gas industries.
  3. Technical Knowledge Asset: Building proprietary process knowledge that differentiates the company in competitive bidding and supports engineering consultations with end-users.

3. Technical Purpose and Value

3.1 Purpose of the Study

The primary purpose of investigating the effect of welding current on high-boron overlay alloy microstructure and wear resistance is to establish quantitative process windows that ensure:

3.2 Technical Value to Product Delivery

High-boron overlay alloys are among the most challenging hardfacing materials to deposit consistently due to their high thermal cracking susceptibility, sensitivity to dilution, and the critical dependence of wear performance on boride morphology. Without a thorough understanding of how current variations affect these properties, the company faces risks of:

3.3 Contribution to Qualification Building

This study directly supports qualification activities under the following frameworks:

4. Key Process and Implementation Points

4.1 Current Range Investigation

The study systematically evaluates welding current across a range typically spanning 100–350 A for open-arc high-boron overlay deposits, depending on the specific alloy system and wire diameter. The following table summarizes the expected metallurgical and performance outcomes across current ranges:

Current Range (A) Heat Input (kJ/mm) Dilution (%) Hardness (HRC) Boride Morphology Wear Resistance Index Cracking Tendency
100–150 0.5–1.5 10–18 78–88 Fine, dispersed Fe₂B particles High Low (thin deposits)
150–220 1.5–3.0 18–25 75–82 Mixed FeB/Fe₂B, moderate network Very High (optimal) Moderate
220–280 3.0–5.5 25–35 70–78 Coarse FeB dendrites, reduced fraction Moderate High
280–350 5.5–9.0 35–45 65–72 Coarse, segregated borides; matrix-dominated Low Very High

4.2 Critical Process Parameters

The following parameters must be controlled in conjunction with welding current to achieve optimal results:

Parameter Recommended Range Rationale
Travel Speed (mm/s) 2.5–8.0 Controls heat input in combination with current; too slow increases dilution and coarsening
Arc Voltage (V) 18–28 Determines arc length and spray transfer stability; affects pool geometry
Wire Diameter (mm) 1.2–1.6 Larger wire supports higher current but increases spatter and dilution
Preheat Temperature (°C) 100–250 Reduces cracking susceptibility; must not exceed 300°C to avoid boride spheroidization
Interpass Temperature (°C) ≤200 Controls cooling rate between passes; critical for multi-pass deposits
Deposition Rate (g/s) 1.5–4.0 Affects dilution and layer thickness; influences final property profile
Number of Passes 1–3 Multi-pass increases total dilution; first pass dilution is highest

4.3 Microstructural Analysis Methodology

The metallurgical characterization program for this study includes:

  1. Optical Microscopy (OM): Examination at 100×–1000× magnification to identify boride phase morphology, distribution, and network connectivity. Etchants include Nital (2–5%) and Keller's reagent for phase contrast.
  2. Scanning Electron Microscopy (SEM) with EDS: Detailed characterization of boride phases, identification of FeB vs. Fe₂B stoichiometry, mapping of boron distribution across the deposit cross-section, and quantification of dilution gradient.
  3. X-Ray Diffraction (XRD): Phase identification and quantification of boride fraction (typically 15–40 vol% in optimized deposits).
  4. Hardness Profiling: Vickers or Rockwell C hardness traversals across the weld cross-section at 50 µm intervals to map the hardness gradient from substrate through dilution zone to deposit centerline.
  5. Dilution Analysis: Spark OES or ICP-OES analysis of the dilution zone to quantify base metal contribution and resulting composition shift.

4.4 Wear Testing Protocols

Wear resistance is validated through standardized testing methods:

5. Applicable Standards and Acceptance Criteria

5.1 Material Standards

5.2 Process Standards

5.3 Acceptance Criteria

Criterion Acceptance Limit Test Method
Deposit Hardness ≥75 HRC (or per customer specification) ASTM E18 / Rockwell C
Maximum Dilution ≤25% (first pass), ≤15% (subsequent passes) Spark OES / ICP-OES
Crack Length No cracks >5 mm; total crack length <10% of weld length Visual / MT (ASTM E709)
Porosity No individual pore >1 mm; no cluster porosity Visual / RT (ASTM E94)
Wear Rate (ASTM G65) ≤ specified value (typically <5 mg/1000 cycles for high-boron) ASTM G65
Impact Toughness (if required) ≥5 J at -20°C (for low-temperature service) ASTM E23 / Charpy V-Notch

5.4 NDT Requirements

Non-destructive examination of high-boron overlay deposits follows:

6. Common Risks and Controls

6.1 Metallurgical Risks

Risk Cause Mitigation Strategy
Hot Cracking High current → excessive dilution → reduced sulfur/phosphorus tolerance; low current → thick deposits with high boride volume fraction creating restraint Optimize current to maintain dilution 15–25%; use low-sulfur consumables (S <0.02%); control preheat and interpass temperature; limit single-pass thickness to ≤3 mm
Excessive Dilution High current with low travel speed; excessive wire stickout; poor torch positioning Use multi-pass technique with controlled overlap (30–50%); maintain consistent travel speed; use stringer bead technique for first pass
Boride Coarsening High heat input → slow cooling → thermally activated boride growth Limit heat input to <3 kJ/mm; use rapid travel speeds; consider water-cooled backing plate for thick substrates
Porosity Open-arc conditions → nitrogen and hydrogen pickup; wet flux or contaminated base metal Ensure thorough surface preparation (grind to bright metal); use dry wire electrodes; minimize arc length; consider partial shielding even for "open-arc" processes
Insufficient Fusion Low current → inadequate heat input for base metal melting Ensure minimum current sufficient for fusion (typically >150 A for steel substrates); verify by sectioning and macrograph examination

6.2 Process Control Measures

  1. Parameter Monitoring: Implement in-process current and voltage monitoring with data logging to ensure parameters remain within qualified WPS limits throughout production.
  2. Welder Qualification: Welders must be qualified per ASME Section IX (QW-300) or ISO 9606-1 for the specific hardfacing process and material.
  3. Consumable Control: Maintain wire electrode inventory under controlled conditions (dry storage at 100–150°C for coated wires); implement lot traceability and incoming inspection per ISO 16839.
  4. Substrate Preparation: Mandatory surface preparation to remove coatings, rust, and contaminants; minimum 5 mm wide ground preparation zone on each side of the weld line.
  5. Thermal Management: Implement preheat and interpass temperature monitoring using calibrated thermocouples; document temperatures at defined intervals.
  6. Post-Weld Treatment: For applications requiring reduced residual stress, apply controlled post-weld stress relief (PWHT) at 550–650°C for 1 hour per 25 mm thickness (consult alloy supplier for maximum allowable temperature).

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay Route

The primary application of this current-microstructure-wear relationship knowledge is within the TIG/MIG weld overlay technology route. Specific applications include:

For TIG/MIG overlay, the current optimization data enables the company to:

7.2 Hydraulic Explosive Bonding (HEB) Route

While hydraulic explosive bonding produces clad plates through kinetic energy-driven solid-state bonding rather than fusion welding, the knowledge of high-boron alloy microstructure and wear behavior remains relevant in the following ways:

7.3 Explosion Welding (EW) Route

Explosion welding produces clad plates through high-velocity impact bonding. The relevance of this welding current study to the EW route includes:

8. Qualification Building and Certification Support

8.1 WPS Development Framework

This study provides the technical foundation for developing Welding Procedure Specifications that meet the following qualification requirements:

  1. Essential Variables Identification: Based on ASME Section IX QW-451 and ISO 14732, the study identifies welding current as a Group 1 essential variable for hardfacing procedures. The qualified current range establishes the limits within which the procedure remains valid without requalification.
  2. Supplementary Essential Variables: Preheat temperature, interpass temperature, and post-weld heat treatment are identified as supplementary essential variables that must be qualified for specific service conditions (low-temperature, sour service, fatigue-critical).
  3. Performance Qualification: The hardness and wear testing data supports performance qualification per customer specifications, demonstrating that the qualified procedure produces deposits meeting or exceeding specified performance criteria.

8.2 Certification System Integration

The technical knowledge gained from this study integrates into the company's quality management system (QMS) as follows:

8.3 Customer Value Proposition

The technical depth of this current-microstructure-wear study translates directly into customer value:

  1. Guaranteed Performance: Customers receive high-boron overlay products with documented, reproducible hardness and wear-life performance, reducing the risk of premature component failure in service.
  2. Reduced Lifecycle Cost: Optimized current parameters produce deposits with maximum wear life, reducing replacement frequency and total cost of ownership for end-users.
  3. Technical Confidence: Detailed metallurgical documentation (micrographs, hardness maps, wear test reports) provides customers with the technical confidence required for specification approval and procurement decisions.
  4. Customization Capability: The understanding of how current affects properties enables the company to tailor deposit properties to specific customer requirements (e.g., higher hardness for abrasive service, slightly lower hardness for improved toughness in impact service).
  5. Regulatory Compliance: Fully qualified WPS documentation supports customer regulatory submissions and code stamping requirements for pressure-containing or safety-critical applications.

9. Implementation Recommendations

9.1 Process Optimization Protocol

  1. Baseline Testing: Establish baseline properties at a reference current (typically mid-range, e.g., 200 A) to serve as the control condition for all subsequent comparisons.
  2. Systematic Variation: Vary current in increments of 25–50 A while holding all other parameters constant (travel speed, voltage, wire feed rate, torch angle, gas flow). Test minimum 3 specimens per current level.
  3. Microstructural Mapping: For each current level, perform full cross-sectional metallurgical examination including OM, SEM, EDS, and XRD analysis.
  4. Property Correlation: Correlate microstructural observations with measured hardness profiles and wear test results to establish quantitative property-prediction models.
  5. Optimal Window Selection: Identify the current range that provides the best combination of hardness, wear resistance, and cracking resistance for the target application.
  6. WPS Documentation: Document the optimal current range, all associated parameters, and acceptance criteria in a formal WPS suitable for qualification testing.
  7. Procedure Qualification: Execute PQR testing per applicable code requirements, including mechanical testing, NDT, and hardness verification.

9.2 Production Implementation

9.3 Continuous Improvement

  1. Service Feedback Loop: Collect field performance data from customer installations to validate predicted wear life and identify opportunities for process optimization.
  2. Periodic Requalification: Schedule periodic requalification testing (typically every 2 years or per customer requirements) to verify that qualified procedures continue to produce conforming results.
  3. Technology Transfer: Document and disseminate process knowledge across the company's three technology routes to ensure consistent quality standards regardless of the production method employed.
  4. Research and Development: Extend the current study to investigate additional variables (travel speed, pulse parameters, multi-pass sequences) and new alloy compositions to expand the company's technical capability envelope.

10. Conclusion

The systematic investigation of welding current effects on high-boron overlay alloy microstructure and wear resistance represents a fundamental process development activity that underpins the company's ability to deliver reliable, high-performance hardfacing products across all three technology routes. By establishing quantitative relationships between current parameters and metallurgical outcomes, the company gains the technical authority to:

This technical knowledge, when properly documented, qualified, and implemented within the company's quality management system, forms a critical component of the intellectual property and competitive advantage that Cladding Technology Shanxi Co., Ltd brings to the bimetallic cladding and weld overlay market.