Weld Overlay Repair of Salt Evaporation Circulation Pump Impellers
1. Definition and Technical Principles
1.1 Scope of Application
The weld overlay repair of salt evaporation circulation pump impellers is a specialized surface engineering technique applied to restore corroded, eroded, or worn impeller components in concentrated brine evaporation systems. This technology falls squarely within the TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) weld overlay capability domain, addressing the extreme material degradation caused by high-temperature, high-concentration chloride environments typical of salt production processes.
1.2 Fundamental Principles
The repair methodology is based on the deposition of corrosion-resistant and wear-resistant alloy layers onto the base material of the impeller through controlled arc welding processes. The key metallurgical principles include:
- Thermal input control: Managing heat-affected zone (HAZ) dilution to prevent sensitization of the base material and ensure adequate alloy content in the overlay weld metal.
- Metallurgical compatibility: Ensuring proper wetting and bonding between the overlay alloy and the cast iron or low-alloy steel substrate through appropriate preheating and interlayer selection.
- Residual stress management: Minimizing thermal distortion and cracking through controlled interpass temperatures and post-weld heat treatment (PWHT) where applicable.
1.3 Corrosion Mechanisms Addressed
Salt evaporation circulation pumps operate under conditions of 100–140°C saturated NaCl brine with dissolved HCl, MgCl₂, CaSO₄, and other aggressive species. The primary degradation mechanisms include:
- Uniform corrosion due to chloride ion attack on passive films
- Pitting and crevice corrosion at stress concentration points
- Erosion-corrosion at high-velocity flow passages near the impeller eye and outlet
- Galvanic corrosion at weld joints between dissimilar materials
- Stress corrosion cracking (SCC) in sensitized austenitic stainless steel regions
2. Category and Business Positioning
2.1 Technology Classification
This repair application belongs to the weld overlay repair and restoration category within the broader cladding technology portfolio. It represents a high-value, technically demanding service that bridges the gap between routine maintenance and full component replacement, delivering significant cost savings and reduced downtime for customers in the chemical, salt, and evaporation industries.
2.2 Strategic Business Positioning
- Market differentiation: Few fabrication companies possess the combined expertise in metallurgy, welding procedure qualification, and corrosion engineering required for impeller overlay repair in aggressive chloride environments.
- Customer lock-in: Once a WPS is qualified for a specific pump model and operating condition, repeat business is virtually guaranteed for the fleet of identical pumps.
- Technical barrier to entry: The requirement for NDT capability, metallurgical laboratory access, and field welding expertise creates a sustainable competitive moat.
- Revenue diversification: Supplementing bulk cladding plate/pipe production with high-margin repair services improves overall revenue quality and cash flow stability.
2.3 Value Chain Integration
This entry demonstrates the company's ability to integrate three core competencies into a single deliverable:
- Metallurgical assessment — determining appropriate overlay alloy selection based on failure analysis
- Welding procedure development and qualification — per applicable codes
- Quality assurance and NDT — ensuring defect-free overlay deposition meeting acceptance criteria
3. Technical Purpose and Value
3.1 Primary Objectives
- Restore dimensional integrity of the impeller to manufacturer specifications (balance, concentricity, and flow passage geometry)
- Extend component service life by 3–5 times compared to uncoated replacement
- Reduce overall lifecycle cost by 40–60% compared to purchasing new impellers in specialty alloys
- Minimize production downtime from weeks (procurement lead time) to days (in-house repair cycle)
3.2 Economic Value Quantification
| Cost Element | New Impeller Replacement | Weld Overlay Repair | Savings |
|---|---|---|---|
| Material cost | ¥80,000–150,000 | ¥8,000–15,000 | 85–90% |
| Manufacturing/procurement lead time | 8–16 weeks | 5–10 working days | 80–90% |
| Service life achieved | 18–24 months | 24–36 months | 50–100% extension |
| Production downtime cost (est.) | ¥500,000–2,000,000 | ¥50,000–150,000 | 75–85% |
3.3 Technical Value to Customer
The weld overlay repair approach delivers measurable improvements in reliability, availability, and maintainability (RAM) metrics. For continuous-process industries such as salt evaporation, where unplanned shutdowns cascade through the entire production chain, the ability to rapidly restore critical rotating equipment represents an existential operational advantage.
4. Key Process and Implementation Points
4.1 Pre-Repair Assessment and Preparation
Failure analysis: The impeller must undergo systematic examination to determine the root cause of degradation. This includes visual inspection, dimensional measurement, and where necessary, metallographic examination of cross-sections to identify cracking modes, corrosion products, and remaining material thickness.
Surface preparation: The corroded or eroded surface must be machined back to sound base material, achieving a minimum remaining thickness that accounts for the planned overlay build-up plus a safety margin. Surface roughness should be controlled to Ra ≤ 6.3 μm to ensure adequate weld metal adhesion.
Base material identification: Positive material identification (PMI) using optical emission spectroscopy (OES) is mandatory to confirm the actual composition of the cast impeller material, as foundry materials may deviate from nominal specifications.
4.2 Alloy Selection Matrix
| Base Material | Operating Condition | Recommended Overlay Alloy | Welding Process | Typical Layers |
|---|---|---|---|---|
| Cast Iron (HT200/QT500) | Hot concentrated brine, erosive | Layer 1: 309L (transition) Layer 2: 316L or 317L Layer 3: Alloy 6 (6% Mo austenitic) |
TIG (GTAW) | 3 layers |
| Low-carbon steel (Q235/Q345) | Hot brine with HCl traces | Layer 1: 309L (transition) Layer 2: 316L Layer 3: Alloy 20 or Hastelloy C-276 |
TIG (GTAW) | 3 layers |
| Stainless steel (304/316) — already sensitized | Severe chloride pitting | Layer 1: 309L (resensitize control) Layer 2: Alloy 6 or C-276 |
TIG (GTAW) | 2 layers |
| Any base — high erosion zones | High-velocity flow passages | Layer 1: 309L Layer 2: 316L Layer 3: Stellite 6 (Co-Cr-W) |
TIG (GTAW) | 3 layers |
4.3 Welding Process Parameters
The following parameter ranges represent typical qualified values for TIG weld overlay on cast iron and low-alloy steel impeller substrates. All values must be confirmed through formal WPS/PQR qualification:
| Parameter | Transition Layer (309L) | Intermediate Layer (316L) | Final Overlay Layer |
|---|---|---|---|
| Welding current (A) | 80–120 | 100–160 | 100–160 |
| Travel speed (mm/min) | 60–100 | 80–140 | 80–140 |
| Interpass temperature (°C) | ≤ 100 (cast iron) / ≤ 150 (steel) | ≤ 150 | ≤ 150 |
| Shielding gas | Ar (99.99%) | Ar (99.99%) | Ar (99.99%) or Ar/He mix |
| Gas flow rate (L/min) | 12–18 | 15–20 | 15–20 |
| Electrode diameter (mm) | 2.4–3.2 | 2.4–3.2 | 2.4–3.2 |
| Deposition thickness per pass (mm) | 1.5–2.5 | 1.5–2.5 | 1.5–2.5 |
| Weld bead width (mm) | 8–12 | 10–15 | 10–15 |
4.4 Critical Implementation Controls
- Preheating: Cast iron impellers require preheating to 250–350°C to prevent cold cracking in the HAZ. Low-carbon steel impellers may require 100–200°C preheat depending on carbon equivalent.
- Back purging: The reverse side of the impeller must be purged with argon or nitrogen during welding to prevent oxidation and ensure full penetration quality.
- Weld sequence optimization: The welding pattern must be designed to minimize residual stress and distortion. A symmetric, balanced sequence radiating from the impeller hub outward is recommended.
- Interlayer inspection: Each layer must be inspected (PT or MT) before proceeding to the next layer to detect and rectify defects early.
- Post-weld treatment: Stress relief annealing at 550–650°C for low-alloy steel, or solution treatment at 1050–1100°C with rapid quench for austenitic overlays (where geometrically feasible).
- Final machining: The overlay surface must be machined to final dimensional tolerances (typically IT7–IT8 for impeller profiles) with a minimum remaining overlay thickness of 2.0 mm verified by ultrasonic measurement.
4.5 Quality Assurance and NDT Requirements
| Inspection Stage | Method | Acceptance Criteria | Reference Standard |
|---|---|---|---|
| Pre-weld surface preparation | Visual + Magnetic Particle (MT) | No cracks, no active corrosion | GB/T 15825 |
| After each overlay layer | Penetrant Testing (PT) | No linear indications | GB/T 18851 / ASTM E1417 |
| Final overlay (full coverage) | Ultrasonic Testing (UT) | No indications > 6 mm equivalent | GB/T 11345 / ASTM E2387 |
| Final overlay (surface) | Penetrant Testing (PT) | Level II acceptance | GB/T 18851 / ISO 3452 |
| Overlay thickness verification | Ultrasonic thickness measurement | ≥ 2.0 mm minimum (or per design) | GB/T 5940 / ASTM E797 |
| Corrosion resistance verification | Salt spray test or field coupon | No pitting within 500 h (ASTM B117) | ASTM B117 |
| Hardness verification | HV10 microhardness | Compatible with design specification | GB/T 6398 / ISO 6507 |
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- GB/T 985.1 — Welding procedures for steel — Part 1: General rules
- GB/T 985.2 — Welding procedures for steel — Part 2: Arc welding
- ASME Section IX — Qualification rules for welding procedures, welders, and welding operators (if applicable for pressure equipment)
- NB/T 47014 — Qualification test procedure for welding procedures of pressure vessels
- ISO 15614-1 — Qualification testing procedures for welding of metallic materials — Part 1: Qualification testing of arc welding processes
- GB/T 19866.1 — Welding consumables for overlay welding
5.2 Material Standards
- GB/T 4237 — Flat products of stainless steel
- GB/T 17748 — Welding consumables for stainless steel
- ASTM A240 — Chromium and chromium-nickel stainless steel plate
- ASTM A582 — Filler metal for welding (stainless steel electrodes)
- GB/T 13810 — Cast iron for engineering purposes
5.3 NDT Standards
- GB/T 18851 — Non-destructive testing — Penetrant testing
- GB/T 11345 — Non-destructive testing of welds — Ultrasonic testing
- GB/T 15825 — Non-destructive testing — Magnetic particle testing
- ASTM E2387 — Standard practice for UT of weld overlay cladding
- ISO 17638 — Non-destructive testing — Penetrant testing
5.4 Acceptance Criteria Summary
The final acceptance of the repaired impeller is governed by a composite criteria set:
- Overlay thickness ≥ 2.0 mm (or design-specified minimum) at all functional surfaces, verified by ultrasonic measurement at ≥ 20 points per impeller
- No cracks, porosity clusters, or lack of fusion detected by PT and UT at Level II or better
- Geometric tolerances: concentricity ≤ 0.05 mm, runout ≤ 0.03 mm, balance quality ≤ G6.3 per ISO 21940-11
- Surface finish: Ra ≤ 3.2 μm on flow passage surfaces
- Corrosion resistance: overlay alloy must demonstrate ≥ 500 hours resistance in ASTM B117 salt spray test at 35°C, 5% NaCl
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Cold cracking in HAZ | Excessive cooling rate on cast iron; high hydrogen content | Impeller failure in service; catastrophic pump seizure | Adequate preheat (250–350°C); low-hydrogen consumables; controlled interpass temperature |
| Hot cracking in overlay weld | High sulfur/phosphorus segregation; restrained solidification | Leakage through crack; progressive overlay spallation | Low S/P filler metal; proper weld geometry; minimize restraint |
| Excessive dilution | Too high heat input; large bead width; insufficient layers | Reduced corrosion resistance; loss of alloy properties | Multiple thin layers; controlled heat input; minimum 3 layers for critical applications |
| Thermal distortion | Asymmetric welding sequence; excessive total heat input | Imbalance; bearing misalignment; premature seal failure | Optimized symmetric welding sequence; clamping fixtures; stress relief |
| Overlay spallation | Poor metallurgical bonding; thermal fatigue cycling | Progressive loss of protective layer; accelerated corrosion | Proper transition layer; compatible thermal expansion coefficients; adequate overlay thickness |
| Porosity in weld | Inadequate shielding; surface contamination; cast iron gas release | Reduced overlay integrity; localized corrosion initiation | Full back purging; thorough surface cleaning; controlled gas flow |
6.2 Quality Risks
- WPS not qualified for actual conditions: Mitigated by performing full WPS/PQR qualification on production-representative coupons before commencing repair work.
- Welder skill degradation: Mitigated by maintaining welder performance records per ASME IX or ISO 9606-1 with periodic requalification.
- Material traceability failure: Mitigated by implementing full material traceability from consumable certification through to final inspection report.
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This impeller repair application is the flagship use case for the TIG/MIG weld overlay route. The technology leverages:
- Precision GTAW (TIG) for thin-section, high-quality overlay deposition on complex geometries
- Multi-layer alloy design (transition + intermediate + final overlay) for maximum corrosion resistance
- Computer-controlled or robotic TIG for repeatable deposition on production impeller batches
- On-site or shop-based execution depending on impeller size and customer logistics
Qualification value: Successfully executing this repair builds a qualified WPS database that can be rapidly adapted to similar pump impellers across the customer's fleet, creating scalable repeat business.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding is primarily applied to large-area cladding of pump casings, volute plates, and heat exchanger tubesheets, it serves a complementary role in the impeller repair ecosystem:
- For pump casings that have experienced severe internal corrosion, hydraulic explosive bonding can be used to clad the entire internal wetted surface with stainless steel or nickel alloy in a single operation
- The bonded cladding provides a corrosion-resistant substrate upon which TIG weld overlay can be applied at localized wear points (impeller seats, bearing journals)
- This hybrid approach (hydraulic bonding for area coverage + TIG overlay for precision) maximizes service life while minimizing material cost
7.3 Explosion Welding Route (Strategic Application)
Explosion welding finds application in the broader pump repair context through:
- Manufacture of explosion-welded clad impeller blanks: a corrosion-resistant alloy (e.g., Alloy 6, Hastelloy C-276, or duplex 2205) is explosion-bonded to a structural steel backing, then machined into the final impeller geometry
- This produces a "permanent" repair solution with overlay thickness of 5–15 mm, suitable for pumps operating in the most aggressive conditions where weld overlay thickness would be consumed within acceptable maintenance intervals
- Explosion-welded clad impellers represent a premium product offering that positions the company as a full-spectrum surface engineering solutions provider
7.4 Integrated Solution Architecture
| Component | Technology Route | Overlay Thickness | Expected Service Life | Relative Cost |
|---|---|---|---|---|
| Impeller (mild wear) | TIG weld overlay (3 layers) | 3–5 mm | 24–36 months | Baseline |
| Impeller (severe corrosion) | TIG weld overlay (4 layers, Ni-based final) | 5–8 mm | 36–48 months | 1.5–2× baseline |
| Pump casing | Hydraulic explosive bonding | 3–6 mm | 48–60 months | 2–3× baseline |
| Impeller (extreme conditions) | Explosion welding (clad blank) | 5–15 mm | 60–84 months | 3–5× baseline |
8. Contribution to Qualification Building and Customer Value
8.1 Qualification and Certification Building
The successful execution of salt evaporation pump impeller repair serves as a critical qualification milestone for the company:
- WPS database expansion: Each unique impeller repair generates qualified welding procedures that extend the company's technical library and reduce future qualification costs.
- Industry-specific credentials: Demonstrated capability in salt/chemical industry pump repair builds credibility for adjacent markets (chemical processing, petrochemical, marine desalination, mining).
- NDT capability validation: The complex geometry of impellers requires advanced UT techniques (curved surface testing, phased array), which simultaneously qualifies the company for similar NDT requirements in pressure vessel and piping inspection.
- Metallurgical laboratory capability: Failure analysis and overlay metallography requirements drive investment in laboratory equipment that serves multiple business lines.
8.2 Product Delivery Excellence
The systematic approach to impeller repair translates into reliable product delivery through:
- Standardized repair procedures reducing cycle time variability
- Documented quality records providing traceability from receipt through to release-for-service
- Performance-guaranteed overlays supported by corrosion coupon data and field performance tracking
- Rapid turnaround enabled by pre-qualified WPS and trained personnel, typically 5–10 working days from receipt to delivery
8.3 Customer Value Proposition
"The weld overlay repair of salt evaporation circulation pump impellers represents the convergence of metallurgical science, welding engineering, and process industry expertise. For the customer, this translates into a single-source solution that eliminates the coordination overhead of managing separate metallurgical consulting, welding repair, machining, and inspection vendors. The company's integrated approach delivers faster turnaround, greater accountability, and measurably extended component life — directly contributing to the customer's operational excellence and total cost of ownership reduction."
9. Conclusion and Forward Path
The technical learning captured in this entry — the systematic application of multi-layer TIG weld overlay to restore salt evaporation pump impellers — represents a mature, repeatable, and economically compelling technology application. Its strategic value extends beyond the immediate repair transaction to encompass qualification building, technical capability demonstration, and customer relationship deepening.
Future development priorities should include:
- Robotization of the overlay process for batch impeller repair operations
- Development of proprietary overlay alloy compositions optimized for specific brine compositions
- Integration of in-situ monitoring (acoustic emission, thermal imaging) for real-time weld quality assessment
- Expansion into adjacent applications: evaporator tubes, heat exchanger plates, pump shafts, and seal faces
- Pursuit of industry-specific certifications (e.g., API 610/682 familiarity, chemical industry safety standards)
This entry validates the company's technical depth in the TIG/MIG weld overlay route and positions it as a credible, high-value service provider in the industrial pump repair and surface engineering market segment.