Solution Treatment and Aging Effects on Plasma-Welded WCp/18Ni300 Composite Coating Microstructure and Performance
1. Definition and Technical Principles
Solution treatment and aging heat treatment of plasma-welded WCp/18Ni300 steel composite coatings is a post-weld thermal processing technology designed to optimize the microstructure, phase distribution, hardness, and wear resistance of tungsten carbide (WCp) particle-reinforced 18Ni300 (CoCr alloy, equivalent to Stellite 6) overlay coatings applied via plasma arc welding (PAW). The underlying principle involves two distinct thermal stages:
- Solution Treatment (Solid Solution Dissolution): Heating the composite coating to a high temperature (typically 1050–1150°C) to dissolve carbides, intermetallic phases, and precipitates formed during the rapid solidification of plasma welding back into the austenitic matrix. This homogenizes the matrix and eliminates microsegregation of carbon, chromium, and nickel.
- Aging (Precipitation Hardening): Controlled cooling followed by holding at an intermediate temperature (typically 750–900°C) to precipitate fine, uniformly distributed carbides (Cr₇C₃, Cr₂₃C₆) and intermetallic phases that provide additional hardening while maintaining the integrity of WCp reinforcement particles.
The 18Ni300 base alloy (Co-18Cr-6W-6Ni system) inherently possesses excellent high-temperature strength and corrosion resistance. The addition of WCp (typically 20–40 vol%) provides substantial hardness enhancement. However, the as-welded state often exhibits coarse dendritic structures, carbide network at grain boundaries, and uneven WCp distribution—all of which are addressed through solution treatment and aging.
2. Category and Business Positioning
This technology falls within the TIG/MIG Weld Overlay route of Cladding Technology Shanxi Co., Ltd.'s three principal technology platforms:
- TIG/MIG Weld Overlay: Primary category. Plasma arc welding (a variant of TIG welding) is used to deposit WCp-reinforced 18Ni300 composite coatings onto steel substrates, followed by post-weld heat treatment.
- Hydraulic Explosive Bonding: Indirect relevance. Provides base clad plate substrates onto which WCp/18Ni300 coatings may subsequently be applied.
- Explosion Welding: Indirect relevance. Similar to hydraulic explosive bonding, serves as a substrate preparation route for subsequent overlay welding.
The business positioning of this capability is as a high-value-added surface engineering solution for components requiring extreme wear resistance combined with corrosion and thermal stability. It differentiates the company in specialized markets including mining equipment, cement industry rollers, power generation turbine components, and petrochemical valves.
3. Technical Purpose and Value
3.1 Engineering Purpose
The solution treatment and aging process serves the following critical engineering objectives:
- Microstructural Homogenization: Eliminates dendritic segregation and carbide networks formed during the rapid cooling of plasma welding (cooling rates typically 100–500°C/s).
- WCp Particle Integrity Preservation: Optimizes the thermal cycle to avoid excessive WC decomposition (WC → W₂C → Fe₃W₃C) while achieving full solution of matrix carbides.
- Hardness Enhancement: Achieves coating hardness of 700–900 HV₀.₃ through precipitation hardening of the 18Ni300 matrix, complementing the intrinsic 2200–2400 HV hardness of WCp particles.
- Toughness Improvement: Reduces brittle carbide networks at grain boundaries, improving coating fracture toughness and resistance to spalling.
- Residual Stress Relief: Mitigates tensile residual stresses (typically 200–400 MPa in as-welded state) that can cause coating cracking and delamination.
3.2 Quantitative Value Demonstration
| Parameter | As-Welded State | After Solution + Aging | Improvement |
|---|---|---|---|
| Coating Hardness (HV₀.₃) | 600–750 HV | 750–900 HV | +20–25% |
| WCp Integrity | Partial decomposition (~15–25%) | Minimal decomposition (<5%) | Significant |
| Carbide Network | Continuous, coarse | Discrete, fine precipitates | Major |
| Residual Stress | 200–400 MPa (tensile) | 50–100 MPa (tensile or compressive) | 75–90% reduction |
| Wear Life (ASTM G99) | Baseline (1.0×) | 2.5–4.0× baseline | +150–300% |
| Crack Sensitivity | High (intergranular) | Low (transgranular) | Major |
4. Key Process and Implementation Points
4.1 Plasma Welding Parameters for WCp/18Ni300 Deposition
| Parameter | Typical Range | Optimization Notes |
|---|---|---|
| Plasma Arc Current | 80–160 A | Higher current increases dilution; keep <120 A for WCp integrity |
| Arc Voltage | 18–25 V | Controls penetration depth; lower voltage preferred |
| Travel Speed | 150–400 mm/min | Faster speed reduces thermal input and WC decomposition |
| WCp Particle Size | 5–45 μm | Finer particles improve distribution; coarser particles provide higher hardness |
| WCp Volume Fraction | 20–40 vol% | 30 vol% is optimal balance of hardness and toughness |
| Shielding Gas | Argon (99.99%) | Optional 2% H₂ addition for arc stability |
| Coating Thickness per Pass | 0.3–0.8 mm | Multiple passes (2–4) for total thickness 1.0–3.0 mm |
| Interpass Temperature | <200°C | Critical for maintaining WCp integrity between passes |
4.2 Solution Treatment Parameters
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Solution Temperature | 1050–1120°C | Above Acm line for 18Ni300; below WC decomposition onset (~1200°C) |
| Soak Time | 1.0–2.0 hours | Complete dissolution of matrix carbides; longer times risk WC decomposition |
| Heating Rate | 100–150°C/hour | Controlled to avoid thermal shock and coating cracking |
| Cooling Method | Air cooling or controlled furnace cooling | Avoid water quench (causes cracking); air cool for fine grain |
| Atmosphere | Argon or vacuum (≤10⁻³ Pa) | Prevent oxidation of Co-Cr surface |
4.3 Aging Treatment Parameters
| Aging Temperature | Soak Time | Resulting Microstructure | Hardness (HV₀.₃) |
|---|---|---|---|
| 750°C | 2–4 hours | Fine Cr₇C₃ precipitates; minimal WC change | 700–780 HV |
| 800°C | 2–4 hours | Optimal Cr₇C₃ + Cr₂₃C₆ distribution | 780–850 HV |
| 850°C | 2–4 hours | Coarser precipitates; slight WC decomposition onset | 800–880 HV |
| 900°C | 1–2 hours | Coarse carbides; measurable WC → W₂C conversion | 850–900 HV |
4.4 Critical Implementation Sequence
- Surface Preparation: Grind substrate to expose sound metal; clean to SA 2.5 (SS-PC 2) per ISO 8501-1.
- Plasma Welding: Apply WCp/18Ni300 composite coating in 2–4 passes with controlled interpass temperature.
- Post-Weld Inspection: Visual and magnetic particle inspection (MT) per ASTM E709 for surface cracks.
- Solution Treatment: Heat to 1050–1120°C, hold 1.0–2.0 hours in argon atmosphere, air cool.
- Aging Treatment: Reheat to 800°C (optimal), hold 2–4 hours, furnace cool to below 300°C.
- Final Inspection: Hardness testing (ASTM E384), metallographic examination, wear testing (ASTM G99), and NDT verification.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Standards
- GB/T 12469-2017 — Steel and iron — Welding consumables — Classification
- GB/T 19804-2017 — Welding — Welding procedure qualification test methods
- GB/T 3375-2017 — Welding — Terms and definitions
- ASME Section IX — Qualification rules for welding procedures, welders, and welding operators
- ASTM A213/A312 — For substrate pipe specifications
- NB/T 47014-2011 — Qualification test procedures for welding procedures of pressure vessels
5.2 Heat Treatment Standards
- GB/T 16923-2008 — Metallic materials — Heat treatment of steel — General technical delivery conditions
- ASTM A397 — Standard specification for castings, cobalt-chromium-iron, for elevated temperature service
- ASTM A276 — Standard specification for austenitic chromium-nickromium stainless steel bar and shapes for general application
- ISO 10447-1 — Heat treatment of steel — General technical delivery conditions
5.3 Inspection and Acceptance Standards
- ASTM E384-2016 — Standard test method for Knoop microhardness
- ASTM E10-2016 — Standard test method for Vickers hardness of metallic materials
- ASTM G99-16 — Standard test methods for wear testing with a reciprocating pin-on-disk apparatus
- ASTM E709-2015 — Standard practice for magnetic particle testing
- GB/T 19866-2005 — Nondestructive testing of welds — Magnetic particle testing
- GB/T 11345-2013 — Nondestructive testing of welds — Ultrasonic testing
- ISO 17638 — Non-destructive testing — Magnetic particle testing
- ISO 17640 — Non-destructive testing — Ultrasonic testing
5.4 Acceptance Criteria
| Acceptance Parameter | Critical Threshold | Test Method |
|---|---|---|
| Coating Hardness (after aging) | ≥ 750 HV₀.₃ (average of 5 measurements) | ASTM E384 / ASTM E10 |
| WCp Integrity | ≥ 85% of original WCp particles retained (optical microscopy) | GB/T 13298 metallographic analysis |
| Surface Cracks | No cracks > 0.5 mm length or > 0.1 mm width | ASTM E709 / GB/T 19866 |
| Coating Adhesion | No delamination at interface after 1120°C solution treatment | Sectioning and metallographic examination |
| Coating Thickness Uniformity | ± 0.2 mm deviation from nominal | Ultrasonic thickness measurement (GB/T 11345) |
| Wear Resistance | ≥ 2.5× improvement over uncoated substrate (ASTM G99) | ASTM G99 pin-on-disk |
6. Common Risks and Controls
6.1 During Plasma Welding
| Risk | Consequence | Control Measure |
|---|---|---|
| Excessive thermal input | WC decomposition to W₂C and Fe₃W₃C; loss of hardness | Limit current to ≤120 A; maintain travel speed ≥ 200 mm/min | High interpass temperature | Cumulative thermal damage; carbide coarsening | Monitor with infrared thermometer; enforce ≤ 200°C interpass limit |
| WCp agglomeration | Uneven hardness distribution; localized soft spots | Pre-mix WCp thoroughly; use powder feeder with controlled feed rate |
| Substrate dilution | Carbon content reduction in coating; decreased hardness | Use consumable nozzle; minimize arc penetration; limit pass thickness |
| Hydrogen-induced cracking | Delayed cracking in HAZ and coating | Dry flux/powder; preheat to 100–150°C; post-weld baking at 200°C for 1 hour |
6.2 During Solution Treatment
| Risk | Consequence | Control Measure |
|---|---|---|
| Temperature overshoot (>1150°C) | Significant WC decomposition; coating softening | Use calibrated pyrometers; limit furnace zone differential to ±15°C |
| Excessive soak time | Grain growth; WC decomposition; reduced toughness | Strict time control; maximum 2.0 hours at 1120°C |
| Oxidation during treatment | Surface scale; reduced corrosion resistance | Inert atmosphere (Ar) or vacuum (≤10⁻³ Pa); monitor dew point |
| Thermal shock cracking | Coating delamination from substrate | Controlled heating rate ≤ 150°C/hour; avoid water quench |
6.3 During Aging Treatment
| Risk | Consequence | Control Measure |
|---|---|---|
| Over-aging (>900°C or >8 hours) | Precipitate coarsening; reduced hardness gain | Strict temperature and time control; document furnace calibration |
| Under-aging | Incomplete precipitation; suboptimal hardness | Validate with hardness coupon testing before full production |
| Uneven temperature distribution | Non-uniform hardness across component | Use thermocouples at multiple positions; verify furnace uniformity per ASTM E2383 |
7. Application Scenarios Across Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Application)
This is the direct and primary application of solution treatment and aging for WCp/18Ni300 composite coatings. Typical applications include:
- Mining Equipment: Excavator bucket teeth, conveyor rollers, and crusher hammers receiving WCp/18Ni300 overlay with post-heat treatment for extreme abrasion resistance.
- Cement Industry: Mill liners, roller presses, and kiln wear plates where both abrasion and thermal cycling are present.
- Power Generation: Steam turbine blade tips, boiler tube wear plates, and fan blades requiring high-temperature wear resistance.
- Petrochemical: Valve seats, pump impellers, and heat exchanger tubes where corrosion-wear synergy is critical.
- Oil and Gas Drilling: Drill collars, stabilizers, and downhole tools requiring combined hardness and thermal stability.
The qualification process for this route involves:
- WPS (Welding Procedure Specification) development per ASME Section IX or NB/T 47014-2011.
- PQR (Procedure Qualification Record) with full metallographic, hardness, and wear testing.
- Qualification of the combined welding + heat treatment cycle as a single process.
- Welder/operator certification per ASME Section IX or GB/T 15169.
7.2 Hydraulic Explosive Bonding (Substrate Preparation)
In the hydraulic explosive bonding route, WCp/18Ni300 plasma-welded coatings serve as functional surface layers on explosively bonded substrates. The application scenario involves:
- Creating a multi-layer composite structure: Steel base → Explosively bonded 18Ni300 clad layer → Plasma-welded WCp/18Ni300 surface coating → Solution + aging treatment.
- The explosive bonding provides a metallurgically sound, defect-free interface between dissimilar materials, while the plasma-welded WCp layer provides the wear-resistant surface.
- Post-weld heat treatment must be carefully designed to be compatible with the explosive bond interface (typically limited to ≤1100°C to avoid bond degradation).
7.3 Explosion Welding (Substrate Preparation)
Similar to hydraulic explosive bonding, explosion welding provides clad plate substrates that can be further enhanced with WCp/18Ni300 overlay coatings. Key considerations:
- Explosion-welded clad plates (per ASTM A420 or GB/T 21490) provide the base substrate.
- WCp/18Ni300 overlay is applied to the exposed cladding surface for additional wear protection.
- The combined heat treatment must respect both the explosion weld interface integrity and the overlay coating requirements.
- Acceptance per ASTM A420 requires verification of bond quality (macroetch, bend test) after all heat treatments.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
This technical capability directly contributes to the company's qualification portfolio in the following ways:
- WPS/PQR Development: Establishes qualified welding procedure specifications for WCp-reinforced overlay coatings with post-weld heat treatment, expanding the company's certified capability matrix.
- Process Innovation Documentation: The systematic study of solution treatment and aging parameters creates a proprietary process knowledge base that supports patent applications and technical publications.
- Cross-Route Integration: Demonstrates the ability to integrate weld overlay with heat treatment, creating a higher-value process chain that distinguishes the company from competitors offering only welding or only heat treatment services.
- Standards Compliance: Ensures all processes meet GB, NB, ASTM, ASME, and ISO requirements, facilitating acceptance in regulated industries (pressure vessels, power generation, petrochemical).
8.2 Product Delivery Enhancement
- Extended Service Life: The 2.5–4× wear life improvement translates directly to reduced maintenance intervals and lower total cost of ownership for customers.
- Customizable Performance: The ability to tune aging parameters allows customization of hardness/toughness balance for specific service conditions.
- Quality Assurance: Documented heat treatment procedures with full traceability (furnace records, thermocouple charts, hardness maps) provide customers with complete quality documentation.
- Reduced Rejection Rates: Systematic control of thermal cycles minimizes cracking, delamination, and non-conformance, improving first-pass yield.
8.3 Customer Value Proposition
"The solution treatment and aging treatment of WCp/18Ni300 plasma-welded coatings transforms a standard weld overlay into a precision-engineered surface solution. By controlling the microstructural evolution through optimized thermal cycles, we deliver coatings with predictable, repeatable performance that exceeds the capabilities of as-welded overlays by 150–300% in wear resistance. This translates to extended equipment uptime, reduced unplanned maintenance, and significant cost savings for our customers across mining, cement, power, and petrochemical industries."
9. Conclusion and Forward Outlook
The mastery of solution treatment and aging for WCp/18Ni300 plasma-welded composite coatings represents a critical differentiator in the surface engineering industry. It bridges the gap between conventional weld overlay (limited by as-welded microstructure) and advanced ceramic coatings (limited by brittleness and substrate compatibility). The technology enables:
- Precise control of the hardness-toughness balance through aging parameter optimization.
- Integration with all three of the company's technology routes for maximum versatility.
- Full compliance with international standards (ASTM, ASME, ISO, GB, NB) for regulated applications.
- Quantifiable performance improvements (2.5–4× wear life, 75–90% residual stress reduction) that provide clear ROI for customers.
Future development should focus on automated thermal cycle control, real-time monitoring of WCp integrity during heat treatment, and expansion of the qualified parameter envelope to accommodate larger components and more demanding service conditions.