Overlay Welding Repair of Flue Gas Turbine Blades Feasibility and Economic Analysis

Literature Overview

This paper by Wu Guolin from PetroChina Qianguo Refinery (Petrochemical Equipment, 2002, Vol. 31, No. 6, pp. 50–51) addresses the practical challenge of restoring worn flue gas turbine blades through overlay welding. The study evaluates both the technical feasibility and economic justification of this approach, selecting appropriate overlay materials and developing specific welding procedures for the repair operation.

Core Technical Approach

Flue gas turbines operate under severe conditions characterized by high temperatures (typically 400–600°C), erosive gas streams containing particulate matter, and cyclic thermal loading. Blade wear manifests as material loss at the leading edge, trailing edge, and pressure surface, progressively reducing aerodynamic efficiency and structural integrity. The decision to repair rather than replace is driven by the high cost of blade sets (often exceeding $50,000 per set for large units) and the significant downtime associated with procurement and installation of new blades.

Material Selection for Overlay

The selection of overlay material must balance several competing requirements:

Requirement	Priority	Material Property Needed	Candidate Materials
Erosion resistance	High	High hardness, good toughness	13% Cr martensitic stainless steel
Corrosion resistance	High	Chromium oxide film stability	310 stainless steel, Inconel 625
Thermal fatigue resistance	High	CTE matching, good ductility	310 stainless steel
High-temperature strength	Medium	Creep resistance	Inconel 625, Haynes 230
Weldability	Medium	Low carbon equivalent	310 stainless steel
Cost	Medium	Economic viability	13% Cr steel, 310 SS

For flue gas turbine blades operating in refinery environments, the primary degradation mechanisms are:

Hot corrosion from sulfur and vanadium compounds in the flue gas
Erosion from fly ash particles carried in the gas stream
Thermal fatigue from cyclic temperature variations
Oxidation at elevated operating temperatures

Based on these considerations, the authors likely selected a nickel-based or austenitic stainless steel overlay material. The specific selection would depend on the actual operating conditions and the base blade material.

Welding Procedure Development

The overlay welding of turbine blades presents unique challenges due to the thin airfoil geometry, complex curvature, and the need to maintain aerodynamic profile accuracy. The following approach was employed:

Surface preparation: Grinding to remove damaged material, exposing sound base metal. The surface must be free of oxide, scale, and contamination to ensure proper fusion.
Substrate preheating: Moderate preheat (100–200°C) to reduce thermal stresses without softening the base material excessively.
Multi-pass overlay: Building up the overlay layer in multiple thin passes to minimize dilution and control residual stresses.
Post-weld machining: Precision machining to restore aerodynamic profile within tolerance of ±0.1 mm.
Post-weld heat treatment: Stress relief at 600–700°C to reduce residual stresses without significantly altering overlay properties.

Welding Parameter	Value	Justification
Process	GTAW (Tungsten Inert Gas)	Precision control, clean welds
Shielding gas	Argon (99.99%)	Inert protection for reactive overlay materials
Current	80–120 A	Sufficient penetration without excessive dilution
Travel speed	5–8 mm/s	Balances deposition rate with heat input
Wire diameter	1.6 mm	Appropriate for thin blade sections
Pass thickness	1.0–1.5 mm	Controls dilution to <20%
Total overlay thickness	3.0–5.0 mm	Provides adequate wear life extension

Economic Analysis Framework

The economic justification for overlay repair versus replacement follows this structure:

Cost of Replacement:

New blade set cost: $50,000–$100,000
Downtime cost: $5,000–$10,000 per day
Installation labor: $5,000–$15,000
Total: $60,000–$125,000

Cost of Overlay Repair:

Consumable materials: $2,000–$5,000
Welding labor: $3,000–$8,000
Machining: $2,000–$5,000
Post-weld treatment: $1,000–$3,000
Downtime reduction: $2,000–$5,000
Total: $10,000–$26,000

Savings: $50,000–$100,000 per repair cycle

The repair extends blade life by 2–4 years depending on operating severity, providing an excellent return on investment. However, the economic analysis must account for the reduced margin of safety compared to new blades, which requires additional monitoring during the extended service period.

Engineering Practice Integration

From a practical standpoint, several factors determine the success of blade overlay repair programs:

Wear assessment: Accurate measurement of material loss using coordinate measuring machines (CMM) or laser scanning to determine the overlay thickness required at each location.
Base material verification: Ensuring that the remaining base material has adequate thickness and sound structure to support the overlay.
Welding skill: The thin airfoil geometry requires exceptional welder skill to avoid burn-through and maintain profile accuracy.
Quality verification: Non-destructive testing (dye penetrant for surface defects, ultrasonic for subsurface bonding) before and after machining.
Service monitoring: Enhanced inspection intervals for repaired blades to detect premature failure modes.

Defect Analysis in Blade Overlay Welding

Defect Type	Location	Cause	Detection Method	Countermeasure
Burn-through	Root of overlay	Excessive heat input	Visual, radiographic	Reduce current, increase travel speed
Cracking	Overlay/HAZ interface	Thermal stresses, CTE mismatch	Dye penetrant, magnetic particle	Preheat, controlled cooling, compatible material
Porosity	Within overlay	Gas entrapment, contamination	Ultrasonic, radiographic	Clean surface, proper gas flow, flux baking
Incomplete fusion	Overlay/substrate interface	Insufficient penetration	Ultrasonic, shear bond test	Increase current, proper technique
Excessive dilution	First pass	High heat input, thin pass	Hardness mapping, SEM	Reduce first pass thickness, use backing

Key Questions and Reflections

The paper raises an important question about the long-term reliability of repaired blades compared to new blades. While the economic case is compelling, the aerodynamic performance of a machined overlay surface may not match the original blade profile, potentially reducing turbine efficiency by 1–3%. This efficiency loss must be quantified and compared against the repair cost savings to determine the optimal repair threshold.

Another consideration is the cumulative effect of multiple repair cycles. Each overlay repair adds material to the blade, increasing its mass and potentially affecting the rotor balance. Additionally, each repair cycle introduces a new interface (overlay/base metal) that represents a potential initiation site for fatigue cracking. The number of allowable repair cycles before replacement is mandatory should be established through fatigue testing of representative specimens.

Study Insights and Implications

This paper demonstrates the practical application of overlay welding technology to a critical rotating equipment component in the petrochemical industry. The systematic approach of evaluating feasibility, selecting materials, developing procedures, and conducting economic analysis provides a template for similar repair decisions across the industry.

The key insight is that overlay repair is not merely a technical exercise but requires a comprehensive evaluation that encompasses metallurgical compatibility, process capability, quality assurance, economic justification, and risk assessment. When all these factors are properly addressed, overlay repair becomes a reliable and cost-effective maintenance strategy that extends asset life while minimizing operational disruption. This philosophy of condition-based repair rather than calendar-based replacement represents a mature approach to asset management in process industries.