Plasma Arc Weld Overlay of Ni-Cr-B-Si System Alloys: Microstructure, Microhardness, and Performance Characterization
1. Definition and Fundamental Principles
Plasma arc weld overlay (PAWO) of Ni-Cr-B-Si system alloys represents an advanced surface engineering technique in which a consumable wire or electrode composed of nickel-based alloys strengthened by chromium, boron, and silicon is deposited onto a base substrate using a high-density, constricted plasma arc as the heat source. The resulting overlay deposit forms a metallurgically bonded functional surface layer that imparts superior resistance to oxidation, corrosion, abrasion, and high-temperature wear compared to the base material.
The Ni-Cr-B-Si alloy system belongs to the broader family of nickel-base hardfacing and weld overlay alloys, with boron and silicon serving as potent microstructural modifiers. Boron promotes the formation of hard, metastable intermetallic phases (such as NiB, Ni₂B, and borides) and refines the dendritic grain structure. Silicon enhances the stability of chromium-rich carbide and boride phases while contributing to solid solution strengthening and improving oxidation resistance through the formation of protective SiO₂ and Cr₂O₃ scales at elevated temperatures. The synergistic interaction between these alloying elements creates a composite microstructure comprising a ductile austenitic or martensitic matrix dispersed with hard secondary phases, yielding an optimal balance between hardness, toughness, and thermal stability.
The plasma arc heat source provides a highly concentrated energy density (typically 10–30 kW/cm²), enabling deep yet controlled penetration, minimal dilution of the base material into the overlay, and precise thermal cycle management. Compared to conventional arc welding processes, plasma arc welding produces a narrower weld bead, lower heat input, and reduced thermal distortion, making it particularly suitable for overlay applications where dilution control and microstructural integrity of the deposit are critical.
2. Category and Business Positioning
Within Cladding Technology Shanxi Co., Ltd.'s technology portfolio, plasma arc weld overlay of Ni-Cr-B-Si system alloys falls squarely within the TIG/MIG Weld Overlay Technology Route, which encompasses all arc-based cladding and overlay processes including submerged arc welding (SAW), gas metal arc welding (GMAW/MIG), gas tungsten arc welding (GTAW/TIG), and plasma arc welding (PAW). This entry specifically addresses the metallurgical research and process qualification dimension of the weld overlay route, focusing on the fundamental understanding of deposit microstructure and hardness that underpins WPS (Welding Procedure Specification) development, process optimization, and product certification.
The study of two distinct Ni-Cr-B-Si alloy compositions—likely differing in boron and silicon content ratios—positions this work at the intersection of materials science research and production engineering. It serves as a knowledge asset that directly informs:
- WPS Qualification Development: Providing the metallurgical justification for process parameter selection during welding procedure qualification testing.
- Product Specification Support: Enabling the company to specify guaranteed microhardness values and microstructural characteristics for customer deliverables.
- Technical Differentiation: Demonstrating deep metallurgical expertise that distinguishes the company from competitors who rely solely on empirical process trial-and-error.
3. Technical Purpose and Value
3.1 Research Objectives
The primary technical purpose of studying two Ni-Cr-B-Si system alloy plasma weld overlay deposits is to establish a quantitative relationship between alloy composition, plasma arc process parameters, resulting microstructure, and microhardness distribution. This relationship forms the scientific foundation for:
- Composition Optimization: Determining the optimal B/Si content ratio that maximizes hardness while maintaining adequate toughness and resistance to hot cracking.
- Process Parameter Correlation: Identifying how plasma arc current, arc voltage, travel speed, and gas flow rates influence deposit dilution, grain morphology, and phase distribution.
- Performance Prediction: Developing predictive models that allow engineers to estimate overlay hardness and microstructural characteristics prior to production welding.
- Heat Treatment Protocol Development: Establishing post-weld heat treatment (PWHT) schedules that optimize the balance between hardness and residual stress relief.
3.2 Value to Product Delivery
The microstructural and microhardness data obtained from this research directly translates into verifiable product specifications. When a customer requires a Ni-Cr-B-Si overlay with a specified hardness range (e.g., HV 600–800), the company can confidently deliver the product with documented microstructural evidence, reducing qualification risk and accelerating acceptance. This is particularly valuable in industries such as power generation, petrochemical, and mining, where overlay performance is directly linked to asset life and safety.
4. Key Process and Implementation Points
4.1 Alloy System Characterization
The two Ni-Cr-B-Si alloys studied typically differ in their boron and silicon concentrations. The following table summarizes the typical compositional ranges and resulting microstructural features:
| Parameter | Alloy Variant A (Higher B) | Alloy Variant B (Higher Si) |
|---|---|---|
| Ni (balance) | Balance | Balance |
| Cr (%) | 20–28 | 18–26 |
| B (%) | 1.5–3.0 | 0.5–1.5 |
| Si (%) | 1.0–2.0 | 2.5–4.5 |
| Primary Hard Phases | NiB, Ni₃B, CrB, M₇C₃ carbides | SiC, NiSi, Cr₇C₃, Cr₂O₃ (oxidized) |
| Matrix Structure | Austenitic with martensitic transformation | Austenitic-ferritic |
| Typical Microhardness (HV) | 650–850 | 550–750 |
| Key Advantage | Higher hardness, superior abrasive wear resistance | Better oxidation resistance, improved thermal cycling stability |
4.2 Plasma Arc Weld Overlay Process Parameters
| Process Parameter | Typical Range | Effect on Microstructure | Effect on Microhardness |
|---|---|---|---|
| Plasma Arc Current | 80–180 A | Higher current → deeper penetration, increased dilution, coarser grains | Higher current → lower hardness due to dilution |
| Arc Voltage | 18–28 V | Higher voltage → wider bead, increased dilution | Higher voltage → reduced hardness |
| Travel Speed | 200–500 mm/min | Faster speed → lower heat input, finer grains, less dilution | Faster speed → higher hardness (less dilution) |
| Plasma Gas (Ar) Flow | 2–5 L/min | Higher flow → more stable arc, better shielding | Indirect effect via arc stability |
| Shielding Gas (Ar) Flow | 8–15 L/min | Higher flow → better oxide prevention, cleaner deposit | Indirect effect via reduced oxide inclusions |
| Wire Feed Speed | Correlated with current | Higher feed → thicker bead, potential for incomplete fusion | Thicker bead → more uniform hardness profile |
| Number of Passes | 2–6 | Multiple passes → homogenized microstructure through interpass reheating | Multiple passes → more uniform hardness distribution |
4.3 Microstructural Analysis Methodology
The study employs a comprehensive metallurgical characterization approach:
- Optical Microscopy (OM): Examination of etched cross-sections to identify dendritic morphology, grain boundaries, and phase distribution. Standard etchants include Kalling's reagent (10 g NaOH, 10 g KCN, 100 mL H₂O, 100 mL HNO₃) or picric acid solution for nickel-base alloys.
- Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS): High-magnification imaging of hard phase morphology (carbides, borides, silicides) and elemental mapping to identify phase chemistry and distribution uniformity.
- X-Ray Diffraction (XRD): Phase identification to quantify the relative fractions of austenite (γ), martensite (α′), and intermetallic phases (M₇C₃, M₃B, NiSi, etc.).
- Vickers Microhardness Testing: Hardness profiling across the overlay thickness and from the weld interface to the free surface using HV0.2 or HV0.5 loads, in accordance with ASTM E92 or ISO 6507.
- Transmission Electron Microscopy (TEM) (if applicable): Nanoscale characterization of precipitate morphology and coherency with the matrix.
4.4 Typical Microhardness Distribution
A critical finding in Ni-Cr-B-Si plasma overlay deposits is the hardness gradient from the base metal interface to the free surface. Dilution at the interface reduces hardness to values approaching the base material, while the upper layers exhibit peak hardness. The following represents a typical profile:
| Depth from Free Surface (mm) | Alloy A Hardness (HV) | Alloy B Hardness (HV) | Dilution Estimate (%) |
|---|---|---|---|
| 0.0 (surface) | 780–850 | 680–750 | < 5 |
| 0.5 | 700–780 | 600–680 | 5–10 |
| 1.0 | 650–720 | 550–620 | 10–15 |
| 1.5 | 580–650 | 480–550 | 15–25 |
| 2.0 (interface) | 400–500 | 350–450 | 25–40 |
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure and Qualification Standards
- ASME Section IX: Governs the qualification of welding procedures, welders, and welding operators for pressure vessel and piping applications. WPS development for plasma arc weld overlay must comply with QW-250 (Plasma Arc Welding) and applicable qualification requirements.
- NB/T 47014 (GB/T 19866): Chinese national standard for qualification testing of welding procedures for pressure vessels, applicable to plasma arc overlay qualification in Chinese manufacturing environments.
- ASME B31.3 / B31.1: Piping codes that reference overlay requirements for corrosion-resistant weld overlays in process and power piping.
- API 579 / API 570: Fitness-for-service and piping inspection codes that reference overlay thickness and quality requirements for remaining life assessment.
5.2 Material and Performance Standards
- ASTM A213 / A269: Specifications for austenitic and ferritic alloy tubing that may serve as base materials for Ni-Cr-B-Si overlay applications.
- ASTM E92 / ISO 6507: Standard test methods for Vickers hardness testing, governing microhardness measurement and reporting.
- ASTM E4 / E10: Rockwell hardness testing standards for macro-hardness verification of overlay deposits.
- ASTM A388 / A396: Specifications for chromium-nickel castings and weld overlay materials.
- ISO 3677 / ISO 18275: Classification of weld metals for fusion welding, applicable to nickel-base weld overlay consumables.
- NACE MR0175 / ISO 15156: Materials for use in H₂S-containing environments in oil and gas production, relevant when Ni-Cr-B-Si overlays are applied in sour service.
5.3 Non-Destructive Testing and Acceptance
- ASME Section V / ASTM E709: Magnetic particle testing (MT) for surface and near-surface defect detection in overlay deposits.
- ASME Section V / ASTM E164: Liquid penetrant testing (PT) for surface defect detection.
- ASME Section V / ASTM E2318: Ultrasonic testing of weld overlay cladding for thickness measurement and defect detection.
- ASME Section V / ASTM E1444: Eddy current testing for overlay thickness and defect detection on non-ferromagnetic overlays.
5.4 Acceptance Criteria
| Acceptance Parameter | Typical Requirement | Test Method |
|---|---|---|
| Overlay Microhardness | HV 600–850 (as-deposited, upper 75% of overlay thickness) | ASTM E92 / ISO 6507 |
| Dilution | ≤ 20% at interface (for hardfacing applications) | Optical emission spectroscopy (OES) or SEM-EDS line scan |
| Overlay Thickness | As specified (typically 1.5–6 mm) | Ultrasonic thickness measurement (ASTM E797) |
| Surface Defects | No cracks, porosity > 0.5 mm, or undercut | MT (ASTM E709) + PT (ASTM E165) |
| Adhesion / Bond Strength | No delamination at specified load | Tensile shear test (ASTM E227) or bend test |
| Macrostructure | Uniform, no incomplete fusion, no cold cracks | Macrograph examination after etching |
6. Common Risks and Controls
6.1 Hot Cracking
Risk: Ni-Cr-B-Si alloys with high boron content are susceptible to solidification cracking due to the formation of low-melting-point eutectics (Ni-B, Ni-Cr eutectics) that segregate to dendrite boundaries during solidification.
Controls:
- Reduce boron content to ≤ 2.5% for critical applications.
- Apply appropriate interpass temperature control (typically 150–250°C for multi-pass overlays).
- Use a backing plate or backing material to ensure full penetration and reduce dilution.
- Optimize travel speed to achieve a lower heat input and faster solidification rate, reducing time in the cracking-susceptible temperature range.
- Employ a single-pass strategy where feasible to minimize the number of thermal cycles.
6.2 Excessive Dilution
Risk: High dilution from the base material into the overlay reduces hardness below specification and compromises corrosion/wear resistance. Carbon steel base materials introduce high dilution due to the large difference in thermal conductivity.
Controls:
- Use a low-heat-input plasma arc process with reduced current and increased travel speed.
- Apply a transition layer (e.g., 309L stainless steel) between carbon steel base and Ni-Cr-B-Si overlay.
- Use a backing plate of low-carbon steel to prevent burn-through and reduce dilution from the back side.
- Limit the number of passes to minimize cumulative dilution.
6.3 Residual Stress and Distortion
Risk: The thermal gradient between the overlay and base material generates significant residual tensile stresses, which can lead to overlay spallation, especially under thermal cycling or mechanical loading.
Controls:
- Implement controlled multi-pass welding with interpass temperature management.
- Apply post-weld stress relief annealing at 550–650°C for 1–2 hours (depending on alloy variant and base material).
- Use a weld sequence that minimizes thermal distortion (e.g., step-welding, back-step welding).
- Consider preheating the base material to reduce thermal gradient.
6.4 Phase Instability and Softening
Risk: The metastable hard phases (martensite, certain borides) may transform or coarsen during post-weld heat treatment or in-service thermal cycling, leading to hardness loss.
Controls:
- Select the alloy variant (higher Si) for applications requiring thermal cycling stability.
- Limit PWHT temperature and duration to avoid excessive softening.
- Document the as-deposited vs. heat-treated hardness for each batch.
7. Application Scenarios Across the Company's Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
Plasma arc weld overlay of Ni-Cr-B-Si alloys is a core application within the company's TIG/MIG weld overlay technology route. The research findings on microstructure and microhardness directly support:
- WPS Development: The dilution-hardness relationship established through this study enables systematic WPS parameter selection. Engineers can predict the achievable hardness for a given process parameter set, reducing the number of qualification trials required.
- Multi-Layer Overlay Design: Understanding the hardness gradient across overlay thickness allows the design of multi-layer overlay systems where the lower layers provide adhesion and the upper layers provide wear resistance.
- Consumable Selection: The comparative study of two alloy variants enables the company to recommend the optimal consumable for each application—Alloy A for high-abrasion environments, Alloy B for high-temperature oxidation environments.
- Process Optimization: Data on grain morphology and phase distribution guides the selection of plasma arc current, travel speed, and gas flow rates for production welding.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding (hydroforming-based explosive cladding) is primarily used for producing clad plates and pipes with uniform thickness and large-area coverage, the Ni-Cr-B-Si overlay knowledge contributes in the following ways:
- Hybrid Cladding Systems: For components requiring both a thick corrosion-resistant base cladding (produced by explosive bonding) and a thin, ultra-hard wear-resistant surface layer (produced by plasma arc weld overlay), the company can offer a combined solution. The Ni-Cr-B-Si plasma overlay is applied as a top layer on an explosively bonded clad plate.
- Repair and Retrofit: When explosively bonded clad components suffer localized wear damage, the Ni-Cr-B-Si plasma overlay provides a targeted repair capability that does not compromise the underlying explosive bond interface.
- Material Compatibility Knowledge: Understanding the microstructure of Ni-Cr-B-Si deposits on various base materials (including clad plates) informs the selection of appropriate explosive bonding parameters for the underlying layers.
7.3 Explosion Welding Route (Complementary Application)
Explosion welding produces clad plates and pipes through high-velocity impact bonding, creating a mechanically interlocked interface with excellent metallurgical compatibility. The Ni-Cr-B-Si overlay research supports this route through:
- Post-Weld Overlay Enhancement: Explo sively clad plates with Ni-Cr-B-Si base layers can be further enhanced with a plasma arc weld overlay top layer for applications requiring both corrosion resistance and wear resistance.
- Edge Cladding: For explosively clad pipes, the cut edges expose the base material. Ni-Cr-B-Si plasma arc weld overlay can be applied to the cut edges to restore corrosion and wear protection.
- Welding Qualification Support: When welding explosively clad plates, the weld metal composition and microstructure must be compatible with the clad layer. The Ni-Cr-B-Si microstructure knowledge informs the selection of filler metals for welding clad components.
- Quality Verification: The microhardness testing methodology developed for plasma overlay deposits is directly applicable to verifying the quality of explosion weld interfaces and the integrity of the clad layer.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This metallurgical research directly supports the company's qualification and certification programs:
- WPS Qualification: The dilution-hardness correlation data provides the technical basis for establishing qualification ranges in welding procedure specifications. This reduces the number of trial coupons required for ASME Section IX or NB/T 47014 qualification testing.
- PQR Documentation: Performance qualification records (PQRs) benefit from documented microstructural and microhardness data, providing objective evidence of weld overlay performance.
- Third-Party Certification: For customers requiring third-party inspection or certification (e.g., TUV, Lloyd's, DNV), the microstructural characterization data provides the technical substantiation required for approval of welding procedures and overlay specifications.
- Material Certification: The hardness and microstructure data supports the issuance of material test reports (MTRs) with verified overlay performance characteristics.
8.2 Product Delivery
The research findings enable more predictable and consistent product delivery:
- Reduced Rework: By understanding the process-microstructure-hardness relationship, the company can set optimal process parameters on the first production run, minimizing the need for rework due to hardness out-of-specification.
- Faster Delivery: Reduced qualification cycles and fewer trial runs translate into shorter project timelines and faster delivery to customers.
- Consistent Quality: The documented microstructural characteristics serve as a reference standard for in-process quality monitoring, ensuring batch-to-batch consistency.
- Customized Solutions: The comparative data on two alloy variants enables the company to offer customized overlay solutions tailored to specific customer requirements (e.g., higher hardness vs. better thermal stability).
8.3 Customer Value
The technical knowledge gained from this study creates tangible value for the company's customers:
- Extended Asset Life: Ni-Cr-B-Si overlays with verified microhardness of HV 600–850 provide significantly extended service life in abrasive and corrosive environments, reducing unplanned shutdowns and maintenance costs.
- Technical Confidence: Customers receive products backed by rigorous metallurgical characterization, providing confidence in overlay performance and reducing qualification risk on their end.
- Optimized Cost-Benefit: By selecting the appropriate alloy variant based on the specific service environment, customers achieve optimal performance at the lowest total cost of ownership.
- Compliance Assurance: The documented microstructural and hardness data supports compliance with industry codes and standards (ASME, API, NACE), facilitating regulatory approval and insurance coverage.
- Technical Partnership: The company's metallurgical expertise positions it as a technical partner rather than a simple fabrication supplier, strengthening long-term customer relationships and enabling collaborative product development.
9. Conclusion and Forward-Looking Recommendations
The study of microstructure and microhardness in Ni-Cr-B-Si system plasma arc weld overlay deposits represents a foundational metallurgical knowledge asset for Cladding Technology Shanxi Co., Ltd. It bridges the gap between empirical process development and scientifically grounded engineering, enabling the company to deliver higher-quality overlay products with greater consistency and predictability.
To maximize the value of this research, the following actions are recommended:
- Expand the alloy matrix: Investigate additional Ni-Cr-B-Si compositions with varying Cr content and minor additions of Mo, W, or Ti to broaden the performance envelope.
- Conduct accelerated wear and corrosion testing: Complement microhardness data with quantitative tribological (ASTM G99) and electrochemical corrosion testing (ASTM G5, ASTM G102) to establish performance correlations.
- Develop a digital process database: Systematically record all process parameters, microstructural observations, and hardness measurements in a searchable database to support rapid WPS development for future projects.
- Pursue joint research partnerships: Collaborate with universities and research institutes to explore advanced characterization techniques (e.g., atom probe tomography, in-situ high-temperature SEM) for deeper microstructural understanding.
- Integrate with all three technology routes: Develop hybrid processing protocols that combine explosive bonding for bulk cladding with plasma arc weld overlay for surface enhancement, creating differentiated product offerings that no single-technology competitor can match.
Key Takeaway: The microstructure and microhardness of Ni-Cr-B-Si plasma weld overlay deposits are not merely academic observations—they are the technical foundation upon which welding procedure specifications, product quality assurance, and customer trust are built. Every HV measurement and every micrograph documented in this research directly contributes to the company's ability to deliver certified, high-performance cladding solutions that extend asset life and reduce total cost of ownership for end users across power, petrochemical, mining, and heavy industry sectors.