Effects of Aging Treatment on Microstructure and Properties of Co-Based Alloy Weld Overlay Remelt Layers

1. Definition and Fundamental Principles

Co-based alloy weld overlay remelt layers are engineered thermal-spray or arc-welded coatings deposited on base substrates and subsequently remelted (typically via TIG or plasma arc) to achieve full metallurgical bonding, elimination of porosity, and homogenization of the microstructure. Aging treatment—a controlled heat treatment process involving solution treatment followed by one or more precipitation hardening cycles—is applied to these remelted Co-based overlay layers to precipitate fine, coherent secondary phases (such as M₆C, M₂₃C₆, and intermetallic compounds like Ni₃Al and NiAl) that significantly enhance hardness, wear resistance, and high-temperature strength.

The fundamental metallurgical principle governing aging in Co-based alloys is precipitation hardening. During solution treatment (typically at 1050–1150°C), the alloy is heated to dissolve carbides and intermetallics into a single-phase solid solution. Upon controlled cooling and subsequent aging (at 750–950°C for 2–8 hours), supersaturated solid solutions decompose, forming nanoscale precipitates that impede dislocation motion. The resulting microstructure comprises a matrix of γ-Co (FCC) and/or γ′-Ni (ordered FCC) with dispersed carbide particles (WC, Cr₇C₃, Cr₂₃C₆) and intermetallic phases that provide exceptional resistance to abrasive, adhesive, and erosive wear.

2. Category and Business Positioning

This technical competency falls squarely within the TIG/MIG Weld Overlay Technology route of Cladding Technology Shanxi Co., Ltd. It represents an advanced post-weld heat treatment (PWHT) capability that elevates the performance of Co-based overlay cladding systems beyond the as-welded condition. In the company's product portfolio, this capability positions Co-based alloy overlays—particularly Stellite-type alloys (Stellite 6, Stellite 21, Stellite 31), Haynes alloys, and proprietary Co-Cr-W compositions—as premium solutions for extreme-duty applications where hardness, corrosion resistance, and thermal stability must be simultaneously maximized.

The business value lies in the ability to deliver overlay systems with verified, repeatable mechanical properties traceable to specific WPS/PQR qualifications. By mastering the aging response of Co-based remelt layers, the company can guarantee minimum hardness levels (typically HV 400–600 depending on alloy composition and aging schedule), reduce coating failure rates in service, and qualify for demanding specifications in power generation, oil and gas, aerospace, and mining industries.

3. Technical Purpose and Value

The primary technical objectives of aging treatment on Co-based weld overlay remelt layers are:

• Hardness Enhancement: Achieving peak hardness through optimal precipitate density, typically 20–40% improvement over the as-remelted condition
• Microstructural Stabilization: Preventing coarse carbide network formation at grain boundaries that would otherwise reduce fatigue life and intergranular corrosion resistance
• Tensile Strength and Fatigue Performance: Improving the overlay's ability to withstand cyclic loading and thermal cycling without cracking
• Corrosion Resistance Optimization: Controlling the morphology and distribution of carbide phases to minimize galvanic coupling and intergranular attack
• Property Uniformity: Ensuring consistent mechanical performance across the entire overlay thickness and surface area

The value proposition extends to customer confidence: aging-treated Co-based overlays deliver predictable, long-term performance in high-temperature erosive-corrosive environments, reducing unplanned maintenance intervals and extending component service life by factors of 2–5x compared to untreated overlays.

4. Key Process and Implementation Points

4.1 Typical Co-Based Alloy Systems and Aging Parameters

Alloy System	Typical Composition	Solution Temp (°C)	Solution Time (h)	Aging Temp (°C)	Aging Time (h)	Expected Hardness (HV)
Stellite 6 (Co-Cr-W)	Co-28Cr-5W-5Mo-3Fe	1050–1100	2–4	750–800	4–8	380–450
Stellite 21 (Co-Cr-Mo)	Co-25Cr-13Mo-5Ni	1100–1150	2–4	800–850	4–8	400–480
Stellite 31 (Co-Cr-Ni)	Co-30Ni-25Cr-6Mo	1050–1100	2–4	700–750	4–8	350–420
Haynes 25 (Ni-Co-Cr)	Ni-21Co-18Cr-2Mo-1Ti-1Al	1100–1150	2–4	750–800	8–16	350–430
Proprietary Co-W-Cr	Co-30Cr-8W-3Mo-2Ti	1080–1120	2–3	780–820	6–10	450–580

4.2 Critical Implementation Parameters

• Heating Rate: Controlled at 100–200°C/h to prevent thermal shock and differential expansion cracking at the overlay/base interface. For thick overlays (>6 mm), lower heating rates (50–100°C/h) are recommended.
• Temperature Uniformity: Maximum allowable gradient of ±20°C across the treated area. Achieved through proper furnace loading, thermocouple placement at multiple depths, and insulation of non-critical areas.
• Atmosphere Control: Inert gas (Ar) or vacuum (≤10⁻² Pa) atmosphere to prevent oxidation and decarburization. Protective coatings or packing materials may be used for complex geometries.
• Cooling Rate: Furnace cool for single-stage aging; air cool or controlled water quench for two-stage aging to maximize precipitation density. Cooling rate from solution temperature should not exceed 150°C/h for thick sections.
• Overlay Thickness Considerations: Minimum effective thickness for aging treatment is 2 mm; optimal range is 3–8 mm. Thicker overlays require extended heating times for thermal equilibrium.

4.3 Microstructural Evolution During Aging

The aging response of Co-based remelt layers follows a characteristic progression:

1. As-Remelted Condition: Widmanstätten dendritic structure with interdendritic carbide networks (Cr₇C₃, Cr₂₃C₆), coarse grain size (typically 50–200 μm), and moderate hardness (HV 280–380)
2. After Solution Treatment: Homogenized single-phase (or near single-phase) solid solution, carbide dissolution, possible grain coarsening (100–400 μm), reduced hardness (HV 200–280)
3. After Aging (Under-Aged): Fine, coherent precipitates (γ′, M₆C) nucleating throughout the matrix, moderate hardness increase (HV 320–400)
4. After Aging (Peak-Aged): Optimal precipitate density and size (5–50 nm), maximum hardness (HV 400–580), fine coherent precipitate distribution
5. After Aging (Over-Aged): Coarsened precipitates, loss of coherency, reduced hardness (HV 300–380), potential for intergranular carbide re-precipitation

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards

Standard	Scope	Relevant Requirements
ASTM B1020	Co-base cast alloys for weld overlay	Chemical composition, heat treatment specifications, mechanical properties
ASTM A213/A213M	Co-base alloy tubing for wear parts	Hardness, tensile properties, aging response characterization
ASME BPV Section II, Part D	Welding procedures for pressure vessels	WPS qualification, PWHT requirements for overlay welds
ASME Section IX	Qualification of welding procedures	PQR requirements, essential variables for PWHT
NB/T 47014	Welding procedure qualification for pressure equipment	Procedure qualification, impact testing, hardness verification
GB/T 3375	Welding consumables - Co-based alloys	Classification, composition, mechanical properties
GB/T 13916	Welding procedure qualification rules	Qualification requirements, essential variables
NACE MR0175/ISO 15156	Materials for H2S environments	Hardness limits, PWHT requirements for sour service
ASTM E92/E92M	Vickers hardness testing	Test method, calibration, acceptance protocols
API 579/ASME FFS-1	Fitness-for-service assessment	Overlay life prediction, damage assessment

5.2 Acceptance Criteria

• Hardness: Minimum hardness per alloy specification (typically HV 350–550 depending on alloy and service requirement); maximum hardness per NACE MR0175 (≤250 HBW for carbon/low-alloy steel base; Co-based overlays exempt if properly qualified)
• Microstructure: No continuous intergranular carbide network; grain boundary carbide continuity ≤10% by linear intercept method; no coarse (>50 μm) primary carbide particles at grain boundaries
• Tensile Properties: Minimum tensile strength per ASTM B1020 (typically ≥620 MPa for Stellite-type alloys); elongation ≥15% (transverse direction)
• Impact Properties: Charpy V-notch impact energy ≥27 J at 25°C (if required by specification); no cleavage fracture surfaces
• Hardness Gradient: Maximum hardness gradient at overlay/base interface ≤50 HV/mm to ensure ductility transition
• NDT: No cracks, lack of fusion, or porosity exceeding 1% area fraction (per ASTM E1647 or equivalent); magnetic particle or liquid penetrant inspection of surface

6. Common Risks and Controls

Risk	Cause	Consequence	Control Measure
Overlay cracking during solution treatment	Thermal shock from rapid heating; high residual stress from welding; low ductility of solution-treated condition	Service failure, overlay spallation	Controlled heating rate (≤150°C/h); preheating to 200°C before furnace loading; stress-relief treatment before solution heat
Interface cracking (overlay/base)	Differential thermal expansion; brittle intermetallic formation at interface; inadequate dilution control	Loss of metallurgical bond; delamination in service	Transition layer welding (309L or Ni-base); controlled dilution ratio; interfacial hardness gradient monitoring
Over-aging (embrittlement)	Excessive aging temperature or time; inaccurate thermocouple placement; furnace calibration drift	Reduced hardness, grain boundary embrittlement, reduced fatigue life	Redundant thermocouples; furnace calibration per ASTM E220; aging curve development and documentation; witness coupon testing
Under-aging (insufficient hardening)	Inadequate aging temperature or time; thermal lag in thick sections; poor furnace uniformity	Insufficient hardness; premature wear failure	Minimum aging time verification; center-core temperature monitoring; hardness verification at multiple depths
Oxidation and decarburization	Inadequate atmosphere control; furnace leakage; improper packing	Reduced corrosion resistance; surface cracking; loss of alloying elements	Continuous atmosphere monitoring (dew point control); vacuum leak testing; protective coatings; post-treatment inspection
Distortion	Non-uniform heating/cooling; asymmetric geometry; high residual stress	Dimensional non-conformance; assembly difficulties	Fixture design for uniform heat distribution; controlled cooling rates; post-treatment dimensional verification per drawing tolerances
Carbon segregation and grain boundary carbide precipitation	Slow cooling from solution temperature; high carbon content; inadequate homogenization	Reduced toughness; intergranular corrosion susceptibility	Rapid quench from solution temperature (if compatible with base material); controlled cooling schedules; grain boundary carbide continuity testing

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay Route (Primary Application)

Aging treatment is most directly applicable to the TIG/MIG weld overlay route, where Co-based alloys are deposited via tungsten inert gas or metal inert gas arc welding and subsequently remelted and aged. Key applications include:

• Steam turbine blades and casings: Co-based overlays on blade tips and guide vanes, followed by solution + aging treatment to achieve peak hardness for resistance to hot gas erosion and oxidation
• Oil and gas valve trim: Stellite 6 or proprietary Co-W-Cr overlays on ball valve seats, gate valve wedge faces, and plug valve surfaces, aged to HV 450–550 for resistance to sand-laden fluid erosion
• Mining and cement industry components: Heavy-duty overlays on crusher jaws, grinding mill liners, and kiln seals, aged for maximum wear life in abrasive environments
• Power generation components: Boiler tube overlays, superheater tube cladding, and HRSG tube protection, aged for resistance to high-temperature corrosion and oxidation
• Hydrogen service equipment: Co-based overlays on reactor internals and piping in sour service, with aging treatment controlled to meet NACE MR0175/ISO 15156 hardness requirements

7.2 Hydraulic Explosive Bonding Route

In hydraulic explosive bonding, aging treatment is applied to the base material or to hybrid clad products where a Co-based layer has been explosively bonded and subsequently welded or machined. The aging process in this context serves to:

• Stabilize the microstructure of the base material (typically low-alloy or stainless steel) after the severe plastic deformation of explosive bonding
• Homogenize the microstructure of Co-based alloys that have been explosively bonded to a base and subsequently locally remelted at the bonding interface
• Optimize the mechanical properties of hybrid clad plates where Co-based surface layers are bonded to structural steel substrates for subsequent service in high-wear applications

7.3 Explosion Welding Route

For explosion-welded Co-based clad products, aging treatment addresses the unique microstructural conditions created by the explosive welding process:

• Relief of residual stresses (which can exceed yield strength in explosion-welded interfaces) through controlled thermal cycling
• Refinement of the dynamically recrystallized grain structure at the bonding interface
• Achievement of target hardness levels in the Co-based layer that may differ from the as-welded condition due to the extreme deformation and rapid solidification inherent in explosion welding
• Elimination of any metastable phases formed during the high-strain-rate bonding process

8. Contribution to Qualification Building, Product Delivery, and Customer Value

8.1 Qualification Building

Mastery of aging treatment for Co-based overlay remelt layers directly contributes to the company's WPS/PQR qualification portfolio. Each aging schedule developed and validated constitutes a qualified essential variable set that can be applied to production welding procedures. Key qualification deliverables include:

• Procedure qualification records (PQR) demonstrating hardness, tensile, impact, and microstructural acceptance for each alloy-aging combination
• Welding procedure specifications (WPS) incorporating aging parameters as essential variables per ASME Section IX and NB/T 47014
• Process capability studies demonstrating statistical control of hardness and microstructure across production batches
• Third-party laboratory verification of mechanical properties per ASTM B1020 and applicable customer specifications

8.2 Product Delivery

The aging treatment capability enables the company to deliver products with verified, repeatable performance characteristics:

• Traceability: Each production lot receives a heat treatment certificate documenting furnace ID, thermocouple readings, time-temperature profile, and post-treatment hardness verification
• Consistency: Statistical process control (SPC) of hardness at multiple overlay depths ensures uniform performance across production batches
• Compliance: Documentation package meets customer and regulatory requirements including NACE, ASME, and industry-specific qualification standards
• Performance Guarantee: Aging-treated overlays deliver predictable service life with documented hardness, wear rate, and corrosion resistance data

8.3 Customer Value

The technical capability translates directly to customer value through:

• Extended Service Life: Aging-optimized Co-based overlays deliver 2–5x the wear life of untreated overlays, reducing maintenance frequency and unplanned shutdowns
• Risk Reduction: Verified mechanical properties and microstructural quality reduce the probability of in-service failure, protecting against costly production losses
• Specification Compliance: Ability to meet stringent industry standards (NACE MR0175, ASME BPV, API specifications) opens access to premium market segments
• Engineering Support: Aging response data enables the company to provide customers with detailed technical documentation, failure analysis support, and life prediction models
• Competitive Differentiation: The combination of Co-based overlay expertise with controlled aging treatment positions the company as a premium provider capable of delivering maximum-performance cladding solutions

9. Conclusions and Recommendations

The systematic study and application of aging treatment on Co-based alloy weld overlay remelt layers represents a critical technical competency for Cladding Technology Shanxi Co., Ltd. This capability bridges the gap between overlay deposition and optimal in-service performance, ensuring that Co-based cladding systems deliver their full potential in demanding industrial applications.

Key recommendations for continued capability development include:

1. Expand the aging database to include additional Co-based alloy systems and proprietary compositions, with documented TTT (Time-Temperature-Transformation) diagrams for each alloy
2. Develop advanced monitoring systems including real-time thermocouple data acquisition, automated furnace control, and digital heat treatment certificates
3. Establish multi-scale characterization protocols combining optical microscopy, SEM/EDS, XRD, and nanoindentation to fully document microstructural evolution
4. Pursue third-party certifications including ASME "Q" stamp for welding procedure qualification and NACE MR0175 compliance certification
5. Develop aging simulation software to predict microstructure and property outcomes for novel alloy compositions and processing conditions
6. Establish customer-specific aging qualification programs to support individual project requirements and accelerate approval timelines

Through rigorous application of aging treatment principles, Cladding Technology Shanxi Co., Ltd. can deliver Co-based overlay solutions that consistently meet or exceed the most demanding performance specifications, securing long-term competitive advantage in the industrial cladding market.