TIG Surface Remelting for Enhanced Cavitation Erosion Resistance of Weld Overlay Coatings
1. Definition and Fundamental Principles
TIG (Tungsten Inert Gas) surface remelting is a post-weld thermal treatment process applied to weld overlay layers to modify the microstructure, chemistry, and surface integrity of the deposited coating. In the context of cavitation erosion resistance, surface remelting involves re-melting the top 0.1–1.0 mm of a previously deposited weld overlay using a precisely controlled TIG arc, followed by rapid solidification under inert gas shielding (typically argon or a helium-argon mixture).
The fundamental principle is that cavitation erosion damage occurs when implosion of vapor bubbles near a solid surface generates localized pressure spikes exceeding 1000 MPa, causing material removal through fatigue cracking, plastic deformation, and material ejection. The susceptibility of a weld overlay to cavitation erosion depends on:
- Surface hardness and hardness gradient — Higher surface hardness correlates with improved resistance to plastic deformation and micro-crack initiation.
- Microstructural homogeneity — Elimination of unmelted base metal inclusions, porosity, and columnar grain boundaries that serve as crack initiation sites.
- Surface roughness — Smoother surfaces deflect cavitation bubble energy more effectively, reducing energy transfer to the substrate.
- Residual stress state — Compressive residual stresses at the surface inhibit crack propagation.
- Chemical homogeneity — Homogenization of alloying elements across the surface layer prevents localized soft spots.
TIG surface remelting addresses all five factors simultaneously by creating a fully melted, re-solidified surface layer with refined grain structure, reduced porosity, improved hardness uniformity, and lower surface roughness.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd., serving as a critical post-processing step that elevates weld overlay coatings from "adequate" to "high-performance" cavitation-resistant solutions.
In the company's business portfolio, TIG surface remelting occupies a differentiated value position:
- For TIG/MIG weld overlay: It is the premium finishing step that converts a standard overlay deposit into a cavitation-resistant coating, directly supporting product differentiation and premium pricing for marine, hydroelectric, and pump industry customers.
- For hydraulic explosive bonding: While not directly applied to explosively bonded interfaces, the remelting technology principles inform the design of transition layers and surface treatments applied to explosively clad components subsequently machined into hydrodynamic service.
- For explosion welding: Surface remelting can be applied to the exposed surfaces of explosion-welded clad plates after machining to eliminate surface oxide layers and refine the near-surface microstructure before further processing or direct service installation.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Eliminate surface defects: Remove surface porosity, spatter, unmelted flux residues, and oxide inclusions that act as cavitation erosion initiation sites.
- Refine surface microstructure: Achieve fine, equiaxed grain structure in the top layer through rapid cooling, increasing surface hardness by 50–150 HV compared to the as-deposited state.
- Homogenize surface chemistry: Redistribute segregated alloying elements (Cr, Mo, Nb, Ni) to eliminate local soft spots vulnerable to cavitation attack.
- Reduce surface roughness: Achieve surface roughness Ra ≤ 1.6 μm (ideally Ra ≤ 0.8 μm) from as-deposited Ra of 6.3–12.5 μm.
- Introduce beneficial residual stress: Controlled remelting parameters can generate surface compressive residual stresses of 50–150 MPa.
3.2 Quantifiable Performance Improvements
| Performance Parameter | As-Deposited Overlay | After TIG Surface Remelting | Improvement |
|---|---|---|---|
| Surface Hardness (HV30) | 250–350 HV | 350–500 HV | +30% to +50% |
| Surface Roughness (Ra) | 6.3–12.5 μm | 0.8–1.6 μm | 75–90% reduction |
| Surface Porosity Rate | 3–8% | < 0.5% | 90%+ reduction |
| Cavitation Erosion Rate (mg/h, ASTM G134) | 1.5–3.0 mg/h | 0.3–0.8 mg/h | 60–80% reduction |
| Surface Oxide Layer | Present (5–20 μm) | Eliminated or < 1 μm | Essentially removed |
3.3 Customer Value
For customers operating in cavitation-prone environments — hydroelectric turbine runners, ship propellers, pump impellers, valve seats, and marine stern tubes — the TIG surface remelting process directly translates to:
- Extended service life: 2–4× life extension in cavitation erosion service compared to un-remelted overlays.
- Reduced maintenance frequency: Fewer shutdowns for coating repair, directly reducing unplanned downtime costs.
- Lower total cost of ownership: Despite higher initial processing cost, the extended service life significantly reduces lifecycle cost.
- Compliance with demanding specifications: Enables qualification against stringent cavitation erosion test standards required by major OEMs and classification societies.
4. Key Process and Implementation Points
4.1 Process Flow
- Pre-remelting inspection: Visual and magnetic particle inspection (MT) of the as-deposited weld overlay to identify and repair any major defects (cracks, excessive porosity) before remelting.
- Surface preparation: Mechanical grinding or wire brushing to remove loose spatter, flux residue, and heavy oxide scale. Target surface roughness before remelting: Ra ≤ 6.3 μm.
- WPS qualification: Develop and qualify a Welding Procedure Specification (WPS) specifically for the surface remelting operation, including heat input limits, travel speed, and shielding gas parameters.
- TIG remelting execution: Apply controlled TIG arc to remelt the top surface layer without excessive penetration into the underlying overlay deposit.
- Post-remelting inspection: Visual inspection, MT, and optional dye penetrant inspection (PT) to confirm absence of new defects.
- Final surface finishing: Precision grinding to achieve required surface roughness specification if remelting alone does not meet the target.
4.2 Critical Process Parameters
| Parameter | Typical Range | Control Rationale |
|---|---|---|
| Shielding Gas | Argon (99.99%) or Ar/He (70/30) | Prevent oxidation; He addition improves arc stability at low currents |
| Welding Current | 60–150 A (AC or DCEN) | Must be sufficient to remelt surface but limited to avoid excessive penetration |
| Travel Speed | 150–400 mm/min | Controls heat input; higher speed = thinner remelted layer, faster cooling |
| Heat Input | 0.5–3.0 kJ/mm | Must remain below threshold to avoid dilution and microstructural degradation |
| Remelted Layer Depth | 0.1–0.5 mm (target) | Deep enough to eliminate surface defects; shallow enough to preserve overlay integrity |
| Interpass Temperature | < 150°C | Prevent grain coarsening and softening of underlying overlay |
| Preheat Temperature | Generally not required; < 100°C if needed for distortion control | Minimize thermal effects on base metal and overlay |
| Welding Position | Flat (1G) preferred; vertical (2G/3G) possible with reduced parameters | Flat position provides best control over remelted pool geometry |
| Tungsten Electrode | Thorium-free (lanthanated or ceriated), 1.6–2.4 mm diameter, 60° grind | Stable arc, low tungsten inclusions, consistent heat distribution |
4.3 AC vs. DC Remelting Selection
| Parameter | DCEN (Direct Current Electrode Negative) | AC (Alternating Current) |
|---|---|---|
| Penetration | Deeper, more controlled | Shallower, broader |
| Cleaning Action | None | Strong (removes oxide on each half-cycle) |
| Best For | Stainless steel, nickel-based overlays (309L, 625, 507) | Aluminum-based overlays (Alloy 6061, 5083, 6061-T6) |
| Surface Quality | Smooth, dense | Smooth with excellent oxide removal |
| Typical Current | 80–150 A | 100–200 A (with 20–50% balance toward electrode) |
4.4 Overlay Material-Specific Considerations
| Overlay Material | Key Remelting Consideration | Post-Remelting Hardness Target |
|---|---|---|
| ASTM A276 Type 309L / 310L | Avoid excessive Cr depletion; limit heat input to prevent carbide precipitation at grain boundaries | 280–380 HV |
| ASTM A276 Type 625 (Inconel 625) | Control cooling rate to avoid δ-phase; rapid solidification from remelting actually beneficial | 350–450 HV |
| ASTM A276 Type 507 (Stellite) | Ensure complete dissolution of unmelted carbide particles; may require slightly higher heat input | 400–550 HV |
| ASTM A276 Type 509 (Stellite) | Similar to 507; cobalt-based, high melting point requires adequate current | 400–500 HV |
| ASTM A276 Type 8 (Stellite) | Critical for cavitation service; ensure full remelt of top layer to eliminate carbide network discontinuities | 450–600 HV |
| Aluminum Alloys (6061, 5083) | Must use AC; rapid cooling critical; minimize exposure time to prevent grain growth | 90–130 HV (as-cast microstructure) |
4.5 Multi-Pass Remelting Strategy
For thick overlay layers (> 3 mm) or applications requiring deep surface modification, a multi-pass remelting strategy may be employed:
- Pass 1 (Cleaning Pass): Lower current (60–80 A), higher travel speed (300–400 mm/min). Purpose: remove surface oxide and light contamination with minimal heat input.
- Pass 2 (Structural Remelt Pass): Medium current (100–130 A), medium travel speed (200–300 mm/min). Purpose: achieve full remelt of top 0.2–0.5 mm for microstructural refinement.
- Pass 3 (Finishing Pass, optional): Low current (60–90 A), high travel speed (350–400 mm/min). Purpose: refine the very top surface, minimize surface roughness, and introduce beneficial compressive stress.
5. Applicable Standards and Acceptance Criteria
5.1 Welding and Overlay Standards
- ASTM A276 — Standard Specification for Nickel-Chromium and Nickel-Chromium-Iron Alloy Electrodes for Weld Overlay. Defines base requirements for overlay materials used in cavitation-resistant applications.
- ASTM A568 — Standard Specification for Castings, Iron and Steel, for General Application. Relevant when remelting cast overlay surfaces.
- ASME Section IX — Qualification of Welders, Welding Operators, and Welding and Brazing Procedures. WPS and PQR qualification for remelting procedures.
- GB/T 985.1 — Welding symbols on technical drawings. Documentation of remelting requirements on engineering drawings.
- NB/T 47014 — Qualification of welding procedures for pressure vessels. Applicable when remelting overlays on pressure-retaining components.
5.2 Cavitation Erosion Testing Standards
- ASTM G134 — Standard Test Method for Laboratory Evaluation of Materials for Cavitation Erosion Resistance Using Vibratory Contact. Primary standard for quantitative cavitation erosion testing of remelted surfaces.
- ASTM G111 — Standard Guide for Cavitation Testing of Materials. General methodology guidance for cavitation erosion evaluation programs.
- GB/T 16530 — Methods for cavitation erosion testing of metallic materials. Chinese national standard for cavitation erosion evaluation.
- ISO 9203 — Cavitation erosion test using liquid jet. Alternative test method for cavitation erosion resistance characterization.
- ASTM G73 — Standard Test Method for Laboratory Evaluation of Materials for Cavitation Erosion Resistance Using Vibratory Contact. (Note: G73 is the older designation; G134 supersedes for most applications.)
5.3 Non-Destructive Testing Standards
- ASTM E709 — Standard Practice for Magnetic Particle Testing. Surface defect detection on ferromagnetic overlays after remelting.
- ASTM E165 — Standard Practice for Liquid Penetrant Inspection. Alternative surface defect detection for non-ferromagnetic overlays (e.g., Stellite, Inconel).
- GB/T 26951 — Non-destructive testing — Magnetic particle testing. Chinese equivalent for MT inspection.
- GB/T 18851 — Non-destructive testing — Penetrant testing. Chinese equivalent for PT inspection.
5.4 Acceptance Criteria
| Inspection Item | Acceptance Criterion | Reference Standard |
|---|---|---|
| Surface Cracks | Zero tolerance — no cracks permitted | ASTM E709, Level 2 MT |
| Surface Porosity | No isolated pores > 0.5 mm; no clustered porosity | ASTM E165, Level 2 PT |
| Surface Roughness (post-remelt) | Ra ≤ 1.6 μm (standard); Ra ≤ 0.8 μm (precision) | ISO 4287 / GB/T 1031 |
| Hardness (surface, HV30) | Within specified range per overlay material (see Section 4.4) | ASTM E182 / GB/T 16554 |
| Cavitation Erosion Rate | ≤ 0.8 mg/h (ASTM G134, 200 cycles) for standard service; ≤ 0.3 mg/h for severe service | ASTM G134 |
| Visual Appearance | Uniform, smooth, free of spatter, burn marks, or discoloration | ASTM E94 (visual inspection) |
6. Common Risks and Controls
| Risk | Mechanism | Preventive Control | Detection Method |
|---|---|---|---|
| Excessive penetration | Too high current or too slow travel speed melts into base metal, causing dilution and loss of overlay properties | Strict WPS qualification; heat input monitoring; operator training; use of current limiter | Sectional hardness traverse; X-ray radiography (if applicable) |
| Incomplete remelting | Insufficient heat input leaves surface defects (porosity, oxide) intact | Calibrated heat input; visual verification of complete surface melt; sufficient travel overlap | Visual inspection; PT/MT inspection |
| Surface cracking (hot or cold) | Excessive cooling rate (high-speed remelt of high-carbon or high-Cr alloys) or high residual stress | Control travel speed; use appropriate shielding gas; consider light interpass heating for susceptible materials | MT/PT inspection at 100% coverage |
| Carbide precipitation | Slow cooling through 400–600°C range causes Cr-rich carbide precipitation at grain boundaries, reducing corrosion and cavitation resistance | Maintain low heat input; rapid cooling; avoid multiple passes with high interpass temperature | Metallographic examination; hardness mapping |
| Tungsten inclusions | Tungsten electrode contact with weld pool introduces hard, brittle particles that act as stress concentrators | Proper tungsten grind; adequate electrode protrusion; stable arc; use thorium-free electrodes | Visual inspection (visible bright particles); metallographic examination |
| Surface oxidation | Inadequate shielding gas coverage allows oxidation of remelted surface | Pre-flow and post-flow gas (5–10 sec); adequate gas flow rate (15–20 L/min); proper gas cup design; wind protection | Visual inspection; metallographic examination of surface oxide layer |
| Distortion | Thermal cycling causes dimensional change in thin-walled components | Fixturing and clamping; multi-pass low-heat-input strategy; symmetric remelting pattern; preheating if required | Dimensional inspection against drawing tolerances |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
TIG surface remelting is most directly integrated into the TIG/MIG weld overlay technology route. Its primary applications include:
- Hydroelectric turbine runner overlays: After depositing a 3–5 mm Stellite 6 or Inconel 625 overlay on turbine runner blades, TIG surface remelting is applied to achieve the required cavitation erosion resistance for runner blade surfaces operating in high-velocity water flow.
- Marine propeller overlays: Ni-based overlay deposits on propeller blades and stern tubes are surface remelted to achieve the smooth, dense surface required for cavitation-resistant marine service.
- Pump and valve components: Impeller surfaces, valve seats, and guide vanes receive weld overlay protection followed by TIG remelting to achieve the surface quality required for high-cycle cavitation service.
- Slurry pump wear parts: While primarily a wear application, many slurry pump components also experience cavitation erosion; remelting enhances both wear and cavitation resistance.
7.2 Hydraulic Explosive Bonding Route
In the hydraulic explosive bonding technology route, TIG surface remelting contributes indirectly but significantly:
- Post-machining surface treatment: After hydraulic explosive bonding creates a clad plate and subsequent machining exposes the overlay surface, TIG remelting can be applied to the machined surface to refine the microstructure and eliminate machining-induced surface damage (work-hardened layer, micro-cracks) before the component enters cavitation service.
- Transition layer preparation: When a transition weld layer is deposited between the explosively bonded interface and a functional overlay, TIG remelting of the transition layer surface ensures proper bonding of the subsequent functional overlay.
- Repair and reconditioning: For previously bonded components returning for re-cladding, TIG remelting of the existing surface ensures proper adhesion of new overlay material.
7.3 Explosion Welding Route
In the explosion welding technology route, TIG surface remelting serves in the following capacities:
- Surface preparation for subsequent welding: Explosion-welded clad plates often require additional weld overlay for functional protection. TIG remelting of the exposed clad surface before overlay welding ensures removal of surface oxide and contamination that could compromise weld quality.
- Post-fabrication surface finish: After explosion-welded components are machined to final geometry, TIG remelting provides a final surface treatment that improves both cavitation resistance and surface finish quality.
- Defect repair: Surface defects identified in explosion-welded clad plates (e.g., unmelted oxide inclusions at the interface exposed by machining) can be addressed through targeted TIG remelting or repair welding followed by remelting.
8. Qualification Building and Certification Strategy
8.1 WPS/PQR Qualification for Remelting
A dedicated Welding Procedure Specification (WPS) and Procedure Qualification Record (PQR) must be developed for each overlay material / remelting parameter combination. The qualification program should include:
- Procedure Qualification: Deposit a witness coupon using the production overlay WPS, apply TIG surface remelting per the proposed remelting WPS, and perform the following tests:
- Metallographic examination of the remelted layer (microstructure, depth of remelted zone, absence of defects)
- Hardness traverse across the remelted layer and underlying overlay
- Cavitation erosion testing per ASTM G134 on a flat specimen
- MT/PT inspection of the remelted surface
- Welder Qualification: Each TIG remelting operator must be qualified on the specific material and parameter range, with visual and MT/PT examination of the remelted surface.
- Production Verification: Periodic (at least quarterly) re-qualification testing to confirm ongoing conformance.
8.2 Certification and Audit Readiness
- ISO 9001:2015 — Quality management system documentation must include procedures for TIG remelting, with defined work instructions, inspection records, and non-conformance handling.
- ISO 3834-2 — Requirements for quality assurance systems for welding. Remelting procedures must be documented and controlled per this standard.
- ASME Section IX — WPS and PQR documentation for remelting operations on pressure-retaining components.
- DNV-OS-F201 / DNV-RP-F204 — For offshore and marine applications, cavitation erosion testing and surface treatment procedures must comply with DNV classification requirements.
- CCS (China Classification Society) — For marine applications, surface treatment of propellers and stern tubes must comply with CCS rules and standards.
9. Process Monitoring and Quality Assurance
9.1 In-Process Monitoring
- Heat input monitoring: Use a welding parameter recorder to continuously log current, voltage, and travel speed. Flag any excursion beyond WPS limits for review.
- Visual in-process inspection: The operator or a dedicated inspector should visually monitor the remelting process, looking for signs of excessive penetration (undercut, excessive bead width), incomplete remelting (surface roughness remaining), or contamination (spatter, oxide).
- Shielding gas monitoring: Use a gas flow meter with alarm to ensure adequate gas flow throughout the operation. Pre-flow and post-flow timing must be verified.
9.2 Post-Process Verification
- 100% visual inspection — All remelted surfaces inspected for uniformity, absence of defects, and correct coverage.
- 100% MT or PT inspection — Full coverage magnetic particle or penetrant inspection for surface-breaking defects.
- Dimensional inspection — Verify surface roughness (Ra) and dimensional tolerances on representative samples or 100% for critical components.
- Hardness verification — Spot check hardness at 3–5 points per remelted area to confirm within specification.
- Periodic cavitation erosion testing — ASTM G134 testing on production coupons at defined intervals (e.g., every 100 components or quarterly) to verify ongoing cavitation erosion resistance performance.
10. Advanced Considerations and Optimization
10.1 Parameter Optimization for Maximum Cavitation Resistance
Research and production experience indicate that the following parameter combinations yield optimal cavitation erosion resistance after TIG surface remelting:
- Low heat input (1.0–1.5 kJ/mm) with high travel speed (300–400 mm/min) produces the finest grain structure and highest surface hardness, but must be balanced against the risk of incomplete remelting.
- Pure argon shielding (99.999%) provides the best surface quality; helium addition is only necessary for aluminum alloys or when arc stability issues arise.
- Multiple narrow passes with high overlap (≥ 75%) produce a more uniform and defect-free surface than a single wide pass.
- Cooling rate control — For Ni-based overlays (Inconel 625, Stellite), rapid cooling (achieved through low heat input) is beneficial as it produces a fine dendritic structure with dissolved alloying elements. For Cr-based stainless overlays, moderate cooling is preferred to avoid excessive Cr carbide precipitation.
10.2 Integration with Other Surface Treatments
TIG surface remelting can be combined with other surface treatments for synergistic cavitation erosion resistance improvement:
- TIG remelting + precision grinding: Remelting provides microstructural improvement; grinding achieves final surface roughness. This is the most common combination for hydroelectric and marine applications.
- TIG remelting + shot peening: Remelting refines the microstructure; shot peening introduces compressive residual stress. The combination provides both microstructural and stress-state optimization for maximum cavitation resistance.
- TIG remelting + laser cladding repair: For localized cavitation damage repair, laser cladding can be used to rebuild damaged areas, followed by TIG remelting to homogenize the repair area with the surrounding surface.
11. Summary and Strategic Significance
TIG surface remelting is not merely a finishing operation — it is a value-adding technology that transforms standard weld overlay deposits into high-performance cavitation-resistant coatings. For Cladding Technology Shanxi Co., Ltd., mastery of this technology provides:
- Technical differentiation in a competitive market where many competitors offer basic weld overlay without surface optimization.
- Qualification advantage — Demonstrated cavitation erosion resistance data (ASTM G134) backed by documented remelting procedures creates a powerful qualification package for demanding OEM customers.
- Cross-route synergy — The technology enhances deliverables across all three technology routes (TIG/MIG overlay, hydraulic explosive bonding, and explosion welding), maximizing the company's integrated capability value proposition.
- Customer trust — Quantifiable performance improvements (60–80% reduction in cavitation erosion rate) provide concrete evidence of value, supporting long-term customer relationships and repeat business.
By maintaining rigorous WPS qualification, comprehensive NDT coverage, and periodic cavitation erosion verification testing, Cladding Technology Shanxi Co., Ltd. positions itself as a reliable provider of cavitation-resistant cladding solutions across the hydroelectric, marine, pump, and industrial equipment sectors.