Plasma Arc Weld Overlay of Stellite Alloys: High-Temperature Friction and Wear Characteristics

1. Definition and Fundamental Principles

Plasma arc weld overlay (PAWO) is a precision thermal spray-adjacent process that uses a high-velocity, high-temperature plasma arc to melt a consumable electrode or wire of a specified alloy composition and deposit it onto a substrate surface in successive, controlled layers. When applied to Stellite alloys—primarily cobalt-chromium-tungsten-based superalloys such as Stellite 6, Stellite 21, Stellite 6B, and Stellite 25—the process produces a metallurgically bonded overlay layer with exceptional resistance to abrasive wear, adhesive wear, erosive wear, and thermal fatigue, particularly under elevated-temperature service conditions.

The plasma arc is generated by ionizing a noble gas (typically argon or argon-hydrogen mixture) through a constricted nozzle, creating a stable, high-temperature (10,000–30,000 K) plasma jet. This jet melts the Stellite wire or rod at its tip, and the molten droplets are transferred to the substrate in a directed stream. The narrow heat-affected zone (HAZ) and controlled dilution (typically 2–10% substrate dilution with proper preheating and process control) preserve the alloying integrity of the cobalt-based matrix, which is critical for maintaining the desired microstructural features: carbide precipitates (MC and M₆C types), solid solution strengthening from W, Mo, and Cr, and a fine grain structure.

1.1 Microstructural Basis of High-Temperature Wear Resistance

The superior high-temperature friction and wear characteristics of plasma-deposited Stellite overlays stem from several microstructural mechanisms:

• Carbide precipitation: WC and Cr₇C₃ carbides provide hard, thermally stable particles that resist ploughing and microcutting by abrasive counterparts. At temperatures up to 650°C, these carbides remain coherent and effective.
• Work hardening capacity: The FCC cobalt matrix exhibits high strain-hardening rates even at elevated temperatures, generating a hardened subsurface layer during sliding contact.
• Oxide film formation: Chromium-rich oxide layers (Cr₂O₃) form spontaneously at temperatures above 400°C, reducing adhesive wear by acting as a protective lubricating film between sliding surfaces.
• Thermal stability: The solid solution of W and Mo in the Co matrix provides minimal softening below 700°C, maintaining hardness levels that exceed most steels at equivalent temperatures.

2. Category and Business Positioning

This research entry falls within the company's TIG/MIG weld overlay technology route, specifically under the advanced plasma arc weld overlay (PAWO) sub-category. It represents a knowledge-intensification activity that bridges fundamental materials research with applied manufacturing capability. The study of high-temperature tribological behavior directly supports the company's positioning as a technical specialist in corrosion-wear-resistant overlay solutions for demanding industrial environments.

Within the broader business framework of Cladding Technology Shanxi Co., Ltd., this entry contributes to:

• Technical differentiation: Demonstrating deep understanding of overlay performance under extreme conditions distinguishes the company from competitors who may only offer generic welding services.
• WPS qualification depth: Process knowledge gained from tribological research directly informs the development of more sophisticated Welding Procedure Specifications (WPS) with optimized parameters for specific service conditions.
• Customer advisory capability: Enabling the company to recommend optimal Stellite alloy selection and overlay design based on quantified performance data rather than empirical experience alone.

3. Technical Purpose and Value

3.1 Primary Research Objectives

The study of high-temperature friction and wear characteristics of plasma-deposited Stellite alloys serves several critical engineering purposes:

1. Temperature-dependent wear mapping: Establishing quantitative relationships between sliding temperature (20°C to 700°C), wear rate, friction coefficient, and overlay microstructure.
2. Wear mechanism identification: Determining the dominant wear mechanisms (abrasive, adhesive, oxidative, erosive, thermal fatigue) at different temperature regimes and sliding conditions.
3. Alloy selection optimization: Comparing the tribological performance of different Stellite grades (Stellite 6 vs. 21 vs. 6B vs. 25) under identical high-temperature sliding conditions to enable informed material selection.
4. Process parameter correlation: Linking plasma arc parameters (current, travel speed, wire feed rate, shielding gas composition) to resulting overlay microstructure and consequent wear performance.

3.2 Engineering Value

The practical value of this research is substantial. In industries such as power generation, oil and gas, mining, and cement manufacturing, components subject to combined thermal and mechanical loading experience accelerated degradation. Conventional hardfacing deposits may perform acceptably at ambient temperature but fail prematurely under thermal cycling. By understanding the high-temperature tribological behavior of Stellite overlays, the company can:

• Specify overlay systems that maintain protective performance at operating temperatures up to 650°C
• Predict service life with greater accuracy for customer asset management programs
• Reduce unplanned maintenance events by selecting overlays with proven high-temperature durability
• Develop proprietary process windows that outperform generic industry practice

4. Key Process Parameters and Implementation Points

4.1 Plasma Arc Weld Overlay Process Parameters

Parameter	Typical Range	Effect on Overlay Quality
Plasma Arc Current	150–350 A	Controls melt pool size and dilution; higher current increases dilution
Travel Speed	200–600 mm/min	Affects bead geometry and layer uniformity; too fast causes incomplete fusion
Wire Feed Rate	1.5–4.0 m/min	Controls deposit thickness per pass; must be synchronized with travel speed
Shielding Gas	Ar (95%) + H2 (5%) or Ar + He	H2 increases arc temperature and penetration; He improves thermal efficiency
Gas Flow Rate	5–15 L/min	Insufficient flow causes oxidation; excessive flow disturbs arc stability
Substrate Preheat	100–250°C	Reduces thermal shock and cracking risk; critical for high-carbon substrates
Interpass Temperature	150–300°C	Controls grain growth and residual stress; must not exceed recommended maximum
Number of Passes	2–6 passes	Determines final overlay thickness (typically 1.5–6.0 mm total)
Electrode/Wire Diameter	1.6–3.2 mm	Larger diameters allow thicker deposits but increase dilution

4.2 Critical Implementation Considerations

4.2.1 Substrate Preparation

Substrate surface preparation is the single most critical factor in achieving a sound metallurgical bond. The base metal surface must be:

• Ground to a uniform matte finish (Ra ≤ 6.3 μm) to remove scale, rust, and existing coatings
• Chemically cleaned to remove oils, grease, and contaminants (solvent degreasing followed by acid pickling if required)
• Preheated uniformly to the specified temperature to minimize thermal gradients
• Fitted with proper backing bars or chill plates where thermal management is critical

4.2.2 Dilution Control

Substrate dilution is the primary mechanism by which Stellite overlay performance degrades. In plasma arc weld overlay, dilution is typically controlled to 2–8% (compared to 15–30% in arc-stripping or surfacing processes). Key dilution control strategies include:

• Using a transition layer of lower-alloy material (e.g., 309L or 310L stainless steel) before applying the Stellite overlay
• Employing a "pile-up" or "drip-feed" technique where the arc is focused on the wire tip rather than the substrate
• Using multiple thin passes rather than fewer thick passes to reduce per-pass dilution
• Applying a high-velocity air or gas cooling technique at the leading edge of the bead

4.2.3 Layer Build Strategy

For overlays exceeding 2 mm in thickness, a multi-layer build strategy is essential. The recommended approach follows a "stair-step" or "wave" pattern where each subsequent pass overlaps the previous by 50–70%. This ensures uniform coverage and minimizes the risk of lack of fusion between layers. The interpass temperature must be monitored with a calibrated pyrometer or thermocouple and must not exceed the maximum specified in the WPS (typically 300°C for Stellite 6 on carbon steel substrates).

4.3 High-Temperature Tribological Testing Methodology

The research component of this entry involves systematic tribological testing under controlled high-temperature conditions. The standard testing methodology includes:

Test Parameter	Typical Conditions	Measurement Objective
Test Temperature	20°C, 200°C, 400°C, 600°C, 700°C	Temperature-dependent wear behavior
Counterface Material	SiC pin, Al2O3 ball, 45# steel, Stellite 6	Simulate realistic contact pairs
Normal Load	5–50 N	Contact stress variation
Sliding Speed	0.1–1.0 m/s	Velocity-dependent wear mechanisms
Test Duration	10–120 minutes per run	Wear rate stabilization
Ambient Atmosphere	Air, N2, or inert gas	Oxidative vs. non-oxidative wear

Wear rate is quantified as mass loss per unit sliding distance (mg/km) or as volumetric wear rate (mm³/Nm). The friction coefficient is recorded continuously and analyzed for stability, stick-slip behavior, and temperature-induced transitions.

5. Applicable Standards and Acceptance Criteria

5.1 Process and Material Standards

• ASTM A213/A335: For Stellite alloy bar and wire specifications (Stellite 6, 21, 6B, 25)
• ASTM A555: Welding consumables for Stellite alloy surfacing
• ASME Section IX: Qualification of welding procedures and welders for overlay welding
• ASME B31.3: Piping code requirements for overlay welds on process piping
• API 570/578: Inspection code requirements applicable to overlay welds on pressure equipment
• ISO 13919-1: Plasma arc welding process specification
• ISO 18275: Welding consumables for plasma arc welding
• GB/T 8110: Chinese national standard for welding consumables
• NACE MR0175: Materials for H₂S-containing environments (relevant for overlay selection in oil and gas)

5.2 NDT and Quality Standards

• ASTM E709: Magnetic particle testing for surface defects in overlay welds
• ASTM E164: Visual examination of welds
• ASTM E2318: Eddy current testing for overlay thickness and bond quality
• ASTM E1417: Penetrant testing for surface-breaking defects
• ASME V Article 4/5/6: UT, MT, PT acceptance criteria for overlay welds
• ISO 17637: UT examination of welds - acceptance levels
• NB/T 47013: Chinese national standard for NDT of pressure equipment welds

5.3 Tribological Test Standards

• ASTM G99: Standard test method for wear testing with a pin-on-disk apparatus
• ASTM G129: Standard practice for wear testing with a reciprocating ball-on-plate apparatus
• ISO 20808: Wear testing - ball-on-disc testing
• ASTM G166: Abrasive wear testing with dry particles

5.4 Acceptance Criteria for Plasma Stellite Overlays

Acceptance Criterion	Requirement	Verification Method
Overlay Thickness	≥ 90% of specified nominal thickness (e.g., 3.0 mm ± 0.3 mm)	Ultrasonic thickness measurement (ASTM E797)
Surface Roughness	Ra ≤ 12.5 μm (as-deposited); Ra ≤ 3.2 μm (after machining)	Surface profilometer
Surface Defects	No cracks, porosity > 0.5 mm, or lack of fusion (per ASME V)	MT (ASTM E709) or PT (ASTM E1417)
Bond Strength	≥ 200 MPa shear strength (per ASTM G106 or equivalent)	Shear test on coupon specimens
Hardness	35–45 HRC (Stellite 6); 38–48 HRC (Stellite 21)	HRC hardness testing at multiple points
Chemical Composition	Within 2.0% of nominal for Co, Cr, W, Mo (per ASTM A213)	OES or XRF spectroscopy
Dilution	≤ 8% substrate dilution (for Stellite 6 on carbon steel)	Microstructural analysis and OES at overlay/substrate interface

6. Common Risks and Controls

6.1 Process Risks

Risk	Cause	Control Measure
Cracking in overlay	Excessive dilution, high carbon content from substrate, rapid cooling	Use transition layer; control interpass temperature; select low-carbon Stellite variants
Cracking at overlay/substrate interface	Inadequate substrate preparation, thermal stress concentration	Thorough surface preparation; proper preheating; use of compatible transition alloys
Excessive porosity	Contaminated surface, improper gas shielding, excessive travel speed	Strict cleaning protocols; verify gas flow rates; optimize travel speed
Uneven overlay thickness	Inconsistent wire feed, operator error, substrate geometry	Use automated PAWO systems; establish consistent travel patterns; verify with UT
Overheating and grain coarsening	Too many passes, excessive interpass temperature	Monitor interpass temperature; limit number of passes; use thinner individual passes
Spalling or delamination in service	Poor bond strength, thermal fatigue, residual stress	Post-weld stress relief; optimize overlay/substrate CTE match; verify bond strength

6.2 Tribological Performance Risks

• Thermal softening: At temperatures above 650°C, even Stellite alloys experience progressive softening. Control: select higher-alloy grades (Stellite 25 with higher Mo content) or limit operating temperature.
• Thermal fatigue cracking: Repeated thermal cycling generates fatigue cracks at the overlay/substrate interface. Control: incorporate flexible transition layers; apply post-weld stress relief; design for thermal expansion accommodation.
• Oxidative wear acceleration: At high temperatures in oxidizing atmospheres, rapid oxide scale formation can lead to spallation. Control: ensure adequate Cr content (≥ 25%); consider post-weld annealing to promote Cr enrichment at the surface.
• Adhesive seizure: At elevated temperatures with high contact pressure, adhesive wear can dominate. Control: optimize surface finish; consider lubrication strategies; select Stellite variants with lower adhesion tendency.

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay Route (Primary Application)

Plasma arc weld overlay of Stellite alloys is the flagship application within the TIG/MIG weld overlay technology route. Key application scenarios include:

• Power generation: Steam turbine blade tips, hot gas path components, boiler tube wear plates, and flue gas duct linings exposed to temperatures of 400–650°C with erosive fly ash.
• Oil and gas: Downhole tool components, pump impellers, valve seats, and wellhead components subject to abrasive formation particles at elevated temperatures. NACE MR0175 compliance may be required for sour service.
• Mining and mineral processing: Crusher jaws, ball mill liners, conveyor roller surfaces, and slurry pump components operating at temperatures up to 300°C with severe abrasive and erosive wear.
• Cement and construction: Kiln wear plates, preheater cyclone liners, and grinding mill components exposed to hot particulate streams at 200–500°C.
• Marine and offshore: Propeller shaft seals, rudder stock bearings, and subsea equipment components subject to combined corrosion and wear at elevated temperatures.

7.2 Hydraulic Explosive Bonding Route

While plasma arc weld overlay is the primary delivery method for Stellite alloys, the hydraulic explosive bonding route complements it in scenarios where:

• Large-area clad plate production: Hydraulic explosive bonding can produce large-format Stellite-clad plates (e.g., 6000 mm × 2400 mm) for subsequent machining into wear parts. The plasma overlay research informs the selection of appropriate Stellite alloy grades for the cladding layer.
• Composite component fabrication: For components requiring both a corrosion-resistant base and a wear-resistant surface, hydraulic explosive bonding can create a carbon steel/Stellite clad plate, which is then further refined with a plasma overlay finish coat.
• Thick overlay requirements: When overlay thickness exceeds 6 mm, hydraulic explosive bonding provides a more economical base layer, with plasma overlay applied as a final functional surface layer.

7.3 Explosion Welding Route

In explosion welding applications, the high-temperature tribological research on Stellite overlays informs:

• Clad plate alloy selection: Understanding which Stellite grades maintain wear resistance at elevated temperatures guides the selection of cladding alloys for explosion-welded clad plates intended for high-temperature service.
• Post-explosion surface treatment: Explosion-welded clad plates may require a plasma overlay finish coat to achieve the desired surface hardness and wear resistance, particularly when the explosion process produces a wavy interface that needs to be machined flat and then re-clad.
• Interface characterization: The microstructural knowledge gained from plasma overlay research aids in interpreting the metallurgical bond quality at explosion weld interfaces, ensuring the cladding layer remains intact under thermal cycling.

8. Contribution to Qualification Building and Customer Value

8.1 Qualification Building

This research entry directly contributes to the company's qualification portfolio in several ways:

1. WPS Development: The tribological data enables the development of qualified WPS documents that specify not only process parameters but also expected performance outcomes. This transforms the company from a process executor into a performance guarantor.
2. ASME Section IX Qualification: Understanding the relationship between process parameters and overlay microstructure supports the qualification of welding procedures under ASME Section IX, QW-460 (overlay welding) requirements. The company can demonstrate that its procedures produce overlays meeting specified performance criteria.
3. Customer-Specific Qualifications: Many OEM customers (particularly in power generation and oil and gas) require supplier qualification based on demonstrated technical competence. The ability to present quantitative tribological data positions the company favorably in these qualification processes.
4. ISO 9001 Quality Management: The research and testing activities demonstrate a commitment to continual improvement and data-driven decision-making, which are core ISO 9001 principles.

8.2 Product Delivery Enhancement

• Performance-based specification: Rather than specifying only overlay thickness and alloy grade, the company can now specify minimum wear life, maximum friction coefficient, and temperature-dependent performance limits.
• Accelerated qualification testing: High-temperature tribological test data allows the company to predict field performance from laboratory results, reducing the need for extended field trials.
• Warranty confidence: Quantified performance data provides the basis for offering performance warranties, which is a significant competitive advantage in capital equipment procurement.

8.3 Customer Value Creation

The ultimate value delivered to customers through this research is reduced total cost of ownership for wear-critical components. By selecting and applying the optimal Stellite overlay system based on verified high-temperature tribological performance, customers experience:

• Extended component service life (typically 3–10× improvement over unprotected or inadequately protected components)
• Reduced unplanned downtime (fewer emergency repairs and component replacements)
• Lower maintenance labor costs (longer intervals between scheduled maintenance)
• Improved safety (reduced risk of component failure leading to hazardous releases or accidents)
• Optimized asset utilization (predictable maintenance scheduling enables better production planning)

9. Conclusions and Recommendations

The study of high-temperature friction and wear characteristics of plasma-deposited Stellite alloys represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. It transforms the company's plasma arc weld overlay capability from a manufacturing service into a performance-engineered solution. The key recommendations for leveraging this research include:

1. Establish a permanent high-temperature tribology testing capability to support ongoing product development and customer-specific qualification programs.
2. Develop a Stellite alloy selection database correlating alloy grade, process parameters, and verified performance across temperature ranges.
3. Integrate tribological data into WPS documentation to provide customers with quantified performance expectations alongside process specifications.
4. Expand testing to include cyclic thermal loading in addition to steady-state high-temperature sliding, to better simulate real-world operating conditions.
5. Pursue publication and patent protection of novel findings to further establish the company's technical authority and intellectual property portfolio.

By maintaining this depth of technical research and continuously translating it into qualified processes, verified products, and documented performance data, Cladding Technology Shanxi Co., Ltd. positions itself as the preferred technical partner for demanding overlay welding applications across all three technology routes.