Temperature Effects on Stellite 6 and Stellite 21 Weld Overlay Microstructure and Performance

1. Definition and Fundamental Principles

Stellite 6 and Stellite 21 are cobalt-based hardfacing alloys widely employed in weld overlay applications where exceptional resistance to abrasive wear, thermal fatigue, and high-temperature oxidation is required. The study of temperature effects on their microstructure and performance addresses a critical knowledge domain that governs coating integrity from the moment of deposition through the entire service life.

Stellite 6 (UNS C30000) is a castable and weldable cobalt-chromium-tungsten alloy containing approximately 59% Co, 21% Cr, 7% W, and 5% Mo. Its microstructure consists of a γ (FCC) cobalt-chromium solid solution matrix with dispersed carbides—primarily MC-type (W, Ta, Nb) and M₇C₃-type carbides. The hardening mechanism at elevated temperatures relies on solid solution strengthening from W and Mo, combined with precipitation hardening of fine carbides and γ' (Ni₃Sn) intermetallic phases that form during controlled thermal exposure.

Stellite 21 (UNS C33500) is a cobalt-chromium alloy with reduced tungsten content (~2% W) and elevated chromium content (~28% Cr), supplemented with 6% Mo and 5% Ni. Its microstructure is dominated by a γ matrix with predominantly M₇C₃ carbides (Cr-rich). Stellite 21 offers superior corrosion resistance and thermal stability compared to Stellite 6, making it preferred for high-temperature oxidation environments, though at a slight trade-off in room-temperature hardness.

The temperature effects studied encompass two critical domains:

• Process temperature (deposition and interpass temperature): Governs solidification mode, grain morphology, carbide precipitation kinetics, dilution level, and residual stress development during weld overlay fabrication.
• Service temperature (operating and thermal cycling): Determines phase stability, carbide coarsening (Ostwald ripening), oxidation kinetics, thermal fatigue crack initiation, and long-term hardness retention.

2. Category and Business Positioning

This technical knowledge entry falls under the company's Weld Overlay Coating technology route—specifically the TIG (GTAW) and MIG (GMAW) weld overlay processes. It represents an advanced metallurgical understanding capability that differentiates the company from purely execution-focused overlay contractors. The entry positions the company as a technically qualified partner capable of:

• Selecting the optimal Stellite grade (6 vs. 21) for specific thermal service conditions
• Designing WPS (Welding Procedure Specifications) with validated interpass temperature windows
• Predicting coating life under thermal cycling and high-temperature oxidation
• Providing engineering justification for coating selection to OEMs and end-users

In the company's three-route technology portfolio (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding), this entry directly supports the weld overlay route where Stellite coatings are deposited on carbon steel, low-alloy steel, or stainless steel substrates for tribological and corrosion protection. It does not directly apply to explosive bonding or hydraulic explosive bonding routes, which rely on kinetic energy bonding rather than metallurgical melting.

3. Technical Purpose and Value

3.1 Engineering Selection Guidance

The primary value of understanding temperature-microstructure-property relationships is enabling informed alloy selection. The decision between Stellite 6 and Stellite 21 for a given application is not arbitrary—it must be grounded in quantitative understanding of how each alloy responds to the thermal environment:

• Below 400°C service: Both alloys perform well; Stellite 6 offers higher hardness (HRC 40–48 as-cast) and superior abrasive wear resistance.
• 400–600°C service: Stellite 21's higher Cr content provides better oxidation resistance; Stellite 6 begins to experience carbide coarsening and slight softening.
• 600–800°C service: Stellite 21 maintains structural integrity with better thermal stability; Stellite 6 requires careful heat treatment to restore hardness after prolonged exposure.
• Above 800°C service: Both alloys experience significant carbide dissolution; alternative solutions (thermal spray coatings, ceramic overlays) may be required.

3.2 Process Optimization

Understanding the effect of interpass temperature on microstructure enables optimization of the welding procedure. Excessive interpass temperature leads to:

• Coarse grain growth at the weld root
• Incomplete fusion between successive passes
• Increased dilution from the substrate
• Reduced hardness due to carbide coarsening in previously deposited layers

3.3 Life Prediction and Reliability

Thermal exposure data allows the company to provide customers with coating life predictions under specific thermal cycling conditions, reducing unplanned shutdowns and extending component service intervals.

4. Key Process and Implementation Points

4.1 Interpass Temperature Control

Parameter	Stellite 6 Recommendation	Stellite 21 Recommendation	Rationale
Interpass Temperature (Max)	≤150°C (GTAW); ≤200°C (GMAW)	≤150°C (GTAW); ≤200°C (GMAW)	Prevent grain coarsening and carbide dissolution in prior passes
Base Metal Preheat	100–150°C for thick sections	100–150°C for thick sections	Reduce thermal gradient and residual stress; prevent cracking in substrate
Heat Input (GTAW)	0.5–1.5 kJ/mm	0.5–1.5 kJ/mm	Control dilution; maintain carbide dispersion
Heat Input (GMAW)	1.5–3.0 kJ/mm	1.5–3.0 kJ/mm	Balance productivity with microstructural quality
Post-Weld Heat Treatment	800–850°C / 1–2 hr + air cool (for hardness restoration)	Generally not required; 900°C / 1 hr if needed	Re-distribute carbides; relieve residual stress

4.2 Microstructural Evolution with Temperature

Temperature Range	Stellite 6 Microstructural Change	Stellite 21 Microstructural Change	Performance Impact
Room Temperature (As-deposited)	γ matrix + fine MC + M7C3 carbides	γ matrix + predominantly M7C3 (Cr-rich)	Peak hardness; optimal wear resistance
200–400°C	Minor carbide coarsening; γ' precipitation begins	Stable; minimal change	Hardness retention >90%; oxidation resistance improves
400–600°C	Significant MC coarsening; γ' thickening; hardness drops 5–15%	M7C3 coarsening begins; Cr-rich oxide layer forms	Stellite 21 outperforms Stellite 6 in oxidation; both retain wear capability
600–800°C	Extensive carbide dissolution; grain boundary migration; hardness drops 20–35%	Partial M7C3 dissolution; matrix Cr depletion; hardness drops 10–20%	Stellite 21 preferred; Stellite 6 requires re-hardening or replacement
>800°C	Major phase transformation; loss of wear resistance	Matrix softening; Cr depletion at surface	Both alloys unsuitable; alternative protection required

4.3 Deposition Strategy for Multi-Pass Overlay

For multi-pass weld overlay applications (common in thick coatings or large surface areas), the temperature management strategy must account for cumulative thermal input:

1. Pass 1 (Bond Coat): Typically deposited with a transition alloy (e.g., ENi-CrFe or 309L) to reduce dilution and improve metallurgical bonding to the base material. Interpass temperature must be controlled to prevent substrate cracking.
2. Passes 2–N (Build-up): Stellite 6 or 21 deposited with strict interpass temperature monitoring. For GTAW, active cooling (compressed air or water spray at edges) may be employed for large surface areas.
3. Final Pass: Orientation optimized for maximum surface quality. Temperature at this stage determines the final microstructure of the functional surface layer.

4.4 Post-Weld Heat Treatment Considerations

For applications requiring hardness restoration after welding or for thick coatings where residual stress relief is critical:

• Stellite 6: Solution treatment at 800–850°C for 1–2 hours followed by air cooling. This dissolves coarsened carbides and allows re-precipitation of fine MC carbides during cooling, restoring hardness to HRC 45–50.
• Stellite 21: Generally does not require post-weld heat treatment due to its inherent thermal stability. If required, 900°C for 1 hour in air provides stress relief without significant microstructural degradation.

5. Applicable Standards and Acceptance Criteria

5.1 Material Standards

• ASTM B881: Standard Specification for Cobalt-Chromium-Tungsten Alloy (Stellite 6) Welding Electrodes and Rods
• ASTM A396: Standard Specification for Cobalt-Chromium-Tungsten Alloy (Stellite 6) Castings
• UNS C30000: Stellite 6 alloy designation
• UNS C33500: Stellite 21 alloy designation
• ISO 3677: Welding consumables—Welding rod electrodes for hardfacing
• NB/T 47010: Chinese standard for weld overlay consumables (where applicable)
• GB/T 34035: Chinese standard for cobalt-based hardfacing alloys (where applicable)

5.2 Welding Procedure Standards

• ASME Section IX: Welding and Brazing Qualifications (QW-11 through QW-15 for GTAW/GMAW)
• EN ISO 15614-1: Qualification testing of welding procedures for metallic materials
• NB/T 47014: Qualification testing of welding procedures for pressure vessels
• GB/T 985: Chinese welding procedure qualification standard
• API 16C: Specification for overlay welding of carbon and alloy steels (for pipeline applications)

5.3 Acceptance Criteria

Test Parameter	Stellite 6 Acceptance	Stellite 21 Acceptance	Test Method
Hardness (as-deposited)	≥ HRC 40 (HV 400+)	≥ HRC 38 (HV 380+)	ASTM E18 / E92
Hardness (after 600°C / 100 hr)	≥ HRC 32	≥ HRC 35	ASTM E18
Dilution (max)	≤ 30% substrate content in surface layer	≤ 30% substrate content in surface layer	Optical emission spectroscopy (OES)
Coating Thickness	As specified (typically 1.0–6.0 mm)	As specified (typically 1.0–6.0 mm)	Ultrasonic thickness measurement
Adhesion (pull-off)	≥ 60 MPa	≥ 60 MPa	ASTM D4541 (adapted)
Crack Free (surface)	No cracks > 0.1 mm	No cracks > 0.1 mm	Visual + PT (ASTM E709)
UT Bonding (if applicable)	≥ 95% sound area	≥ 95% sound area	ASTM E164 / E269

5.4 Non-Destructive Testing Standards

• ASTM E709: Magnetic particle testing (for surface crack detection)
• ASTM E164: Ultrasonic examination of weld overlay coatings
• ASTM E269: Ultrasonic thickness measurement
• ASTM E309: Radiographic examination (for subsurface defects)
• NB/T 47013: Chinese standard for NDT of pressure vessel welds

6. Common Risks and Controls

6.1 High Interpass Temperature

Risk: Excessive interpass temperature (>250°C) leads to coarse grain structure, reduced hardness, increased porosity from trapped gases, and potential cracking due to thermal cycling stresses in the substrate.

Controls:

• Mandatory temperature gun readings between each pass, documented in weld logs
• Active cooling (compressed air) for large surface areas
• WPS-specified maximum interpass temperature with ±25°C tolerance
• Welder training emphasizing thermal management discipline

6.2 Excessive Dilution

Risk: High substrate dilution (>35%) reduces hardness, introduces carbon depletion zones, and may cause cracking in the dilution zone due to carbon migration from substrate to overlay.

Controls:

• Use of transition/bond coat (ENi-CrFe, 309L) as first pass
• Optimized GTAW parameters (lower amperage, higher travel speed) to minimize heat input
• Bevel preparation on base material to increase overlay cross-section
• OES verification of surface composition after welding

6.3 Thermal Cracking in Substrate

Risk: Cobalt-based alloys have high thermal expansion mismatch with carbon steels. Without proper preheat and post-weld cooling control, cracking can initiate at the weld root or substrate surface.

Controls:

• Preheat base material to 100–150°C (higher for thick or high-carbon substrates)
• Controlled cooling rate (≤100°C/hr for sections >50 mm)
• Use of low-stress welding sequences (symmetrical, step-back patterns)
• Post-weld stress relief at 600°C / 2 hr for critical applications

6.4 Carbide Coarsening During Service

Risk: Prolonged exposure to elevated temperatures causes Ostwald ripening of carbides, leading to progressive softening and premature coating failure.

Controls:

• Correct alloy selection based on maximum service temperature (Stellite 21 for >400°C)
• Post-weld heat treatment to establish optimal carbide distribution before service
• Periodic hardness monitoring during service intervals
• Design coating thickness with margin for expected wear/softening rate

6.5 Oxidation and Scale Formation

Risk: At temperatures above 500°C, oxidation of the coating surface can lead to spalling, reducing effective coating thickness and exposing the substrate.

Controls:

• Stellite 21 preferred for oxidation-critical applications (higher Cr content)
• Post-weld annealing to promote Cr-rich oxide layer formation
• Avoiding sulfur-containing environments that promote intergranular oxidation
• Protective coatings or thermal barriers for extreme oxidation environments

7. Application Scenarios Across Company Technology Routes

7.1 TIG/MIG Weld Overlay Route (Primary Application)

This is the primary technology route where Stellite 6 and Stellite 21 overlay coatings are applied. Typical applications include:

• Power generation: Steam turbine blade tips, boiler tube plugging patches, superheater tube spools (Stellite 21 for >500°C service)
• Oil and gas: Valve seats, pump shafts, API 6D valve trim (Stellite 6 for room temperature wear; Stellite 21 for hot well applications)
• Mineral processing: Mill liners, crusher jaws, ball mill raceways (Stellite 6 for abrasive wear at ambient temperature)
• Cement industry: Kiln seals, rotary kiln wear rings, preheater tower components (Stellite 21 for 400–600°C service)
• Chemical processing: Agitator shafts, pump impellers in hot corrosive media (Stellite 21 for combined wear + oxidation resistance)

7.2 Hydraulic Explosive Bonding Route (Indirect Application)

While hydraulic explosive bonding produces cladding through mechanical interlocking rather than metallurgical melting, the temperature knowledge from this study contributes to:

• Post-bonding heat treatment design: Understanding how Stellite responds to elevated temperatures informs the selection of post-bonding thermal treatments that enhance bonding quality without degrading the Stellite layer.
• Service life prediction for hybrid components: When explosive-bonded cladding is subsequently weld-overlaid with Stellite (hybrid approach), thermal compatibility between the bonded interface and weld overlay must be evaluated.
• Customer advisory: Providing guidance on maximum service temperatures for explosively bonded components where Stellite is the cladding material.

7.3 Explosion Welding Route (Indirect Application)

Similar to hydraulic explosive bonding, the temperature effects knowledge supports:

• Thermal cycling qualification: Validating that explosively bonded Stellite cladding maintains integrity through thermal cycling to specified service temperatures.
• WPS development for post-explosion welding repairs: When explosion-welded components require local repair welding with Stellite, the thermal management knowledge ensures repair welds are compatible with the existing cladding.
• Material specification writing: Defining temperature limitations in product specifications for explosion-welded Stellite-clad components.

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

8.1 Qualification Building

This technical knowledge directly supports the company's qualification portfolio:

• WPS Development: Enables creation of qualified welding procedure specifications with scientifically justified parameters (interpass temperature, heat input, PWHT) rather than trial-and-error approaches.
• Material Qualification: Provides metallurgical justification for Stellite alloy selection, supporting customer engineering reviews and third-party inspection requirements.
• ISO 9001 / ISO 3834 Compliance: Demonstrates documented technical competence in process parameter justification, a key requirement for welding quality management system certification.
• ASME Section IX Compliance: Supports procedure qualification testing by providing theoretical basis for parameter ranges, reducing the number of qualification trials required.
• NB/T 47014 Compliance: Enables qualification of welding procedures for pressure vessel overlay applications with documented temperature control procedures.

8.2 Product Delivery Excellence

• Reduced Rework Rates: Temperature-controlled welding procedures minimize defects (cracking, porosity, inadequate hardness), reducing non-conformance and rework costs.
• Consistent Quality: Documented interpass temperature protocols ensure repeatable microstructure and performance across production batches.
• First-Pass Yield Improvement: Scientific understanding of thermal effects enables parameter optimization that maximizes first-pass acceptance rates.
• Traceability: Temperature monitoring records provide full traceability from raw material to finished product, supporting customer audit requirements.

8.3 Customer Value Creation

• Extended Component Life: Properly selected and deposited Stellite coatings (based on temperature knowledge) deliver 3–10× life extension over bare substrate, reducing customer maintenance costs.
• Reduced Unplanned Downtime: Accurate prediction of coating performance under thermal cycling prevents premature failures and production interruptions.
• Engineering Partnership: The company's demonstrated metallurgical competence positions it as a design partner rather than a simple fabrication contractor, enabling value-added engineering services.
• Specification Compliance: Ability to meet stringent customer specifications (hardness after thermal exposure, oxidation resistance at specific temperatures) that less technically capable competitors cannot satisfy.
• Cost Optimization: Correct alloy selection (Stellite 6 vs. 21) based on actual thermal service conditions avoids over-specification while ensuring adequate performance, optimizing total cost of ownership.

9. Implementation Recommendations

1. Integrate temperature monitoring into all Stellite overlay WPS: Include maximum interpass temperature, preheat requirements, and cooling rate limits as mandatory parameters with documented verification.
2. Establish a thermal database: Collect hardness data from production welds at various service temperatures to build a proprietary database that validates and supplements published literature.
3. Develop temperature-specific qualification coupons: Qualify WPS not only for as-deposited properties but also for properties after defined thermal exposures matching target service conditions.
4. Train welding personnel on thermal management: Ensure all welders performing Stellite overlay understand the metallurgical consequences of temperature deviations and the importance of strict interpass temperature control.
5. Create customer-facing technical bulletins: Publish simplified versions of this analysis as customer guidance documents, demonstrating technical competence and supporting specification writing.
6. Implement post-weld inspection protocols: Include hardness testing at multiple locations, with particular attention to areas where interpass temperature may have been elevated (trailing edges, multi-pass overlaps).

10. Conclusion

The understanding of temperature effects on Stellite 6 and Stellite 21 weld overlay microstructure and performance represents a fundamental metallurgical competency that underpins the entire weld overlay business. This knowledge transforms the company from a reactive fabrication service into a proactive engineering partner capable of predicting coating behavior, optimizing process parameters, and delivering solutions with quantified performance guarantees. By integrating this understanding into WPS development, qualification testing, production monitoring, and customer advisory services, the company builds a defensible technical moat that supports premium positioning, long-term customer relationships, and sustained competitive advantage in the weld overlay market.