Flue Gas Turbine Blade Overlay Repair

Literature Overview

This study note examines the overlay repair of flue gas turbine blades, published in 2002 in the journal Petrochemical Equipment. The author, Wu Guolin from PetroChina Qianguo Refinery, conducted a practical investigation into the overlay welding repair of damaged flue gas turbine blades in a refinery power generation system.

This research is particularly relevant to engineers working in power generation, chemical processing, and oil refining industries, where flue gas turbines are used to recover energy from high-temperature exhaust gases. The study addresses a critical maintenance challenge: repairing damaged turbine blades through overlay welding while maintaining the blade's aerodynamic profile and structural integrity.

Core Technical Analysis

Flue gas turbines operate in extremely harsh environments with temperatures ranging from 500°C to 1000°C, exposure to corrosive gases (SO₂, SO₃, H₂S, HCl), and high mechanical stresses from centrifugal forces and vibration. The turbine blades are typically manufactured from nickel-based superalloys such as Inconel 713, Inconel 718, or Hastelloy X, which provide excellent creep resistance, corrosion resistance, and high-temperature strength.

Damage to turbine blades typically occurs through the following mechanisms:

Hot corrosion: Attack by molten salts (Na₂SO₄, K₂SO₄) formed from sulfur compounds in the flue gas, leading to oxidation and material loss at the blade surface.
Thermal fatigue cracking: Cyclic thermal stresses from alternating heating and cooling during turbine operation, leading to crack initiation and propagation at the blade surface.
Erosion: Impact of solid particles (ash, dust) in the flue gas, leading to material loss at the blade leading edge and pressure side.
Creep damage: Time-dependent deformation under sustained high-temperature stress, leading to blade distortion and loss of aerodynamic profile.

The overlay welding repair process involves the following steps:

Inspection and assessment: Non-destructive testing (NDT) to identify the extent and location of damage, including visual inspection, ultrasonic testing, and dye penetrant testing.
Surface preparation: Mechanical grinding and cleaning of the damaged area to remove all damaged material and create a clean, smooth surface for overlay welding.
Overlay welding: Application of overlay weld material to restore the blade geometry and provide a protective barrier against further corrosion and erosion.
Post-weld heat treatment: Solution treatment or stress relief treatment to restore the microstructure and mechanical properties of the overlay layer and the heat-affected zone.
Final inspection: NDT and dimensional inspection to verify the quality and integrity of the repair.

Key Process Parameters and Countermeasures

The study provides detailed recommendations for the overlay welding repair process. The following table summarizes the recommended process parameters:

Process Parameter	Critical Value	Effect on Repair Quality
Weld wire composition	Ni-base superalloy (Inconel 625, 617)	Provides corrosion resistance and high-temperature strength
Heat input	5-10 kJ/mm	Minimizes thermal distortion and HAZ softening
Interpass temperature	150-250°C	Prevents cracking and maintains microstructure
Preheat temperature	150-250°C	Reduces residual stress and cracking susceptibility
Shielding gas	Argon or helium	Provides adequate shielding and arc stability
Post-weld heat treatment	1050-1150°C solution treatment + aging	Restores microstructure and mechanical properties
Overlay layer thickness	0.5-2.0 mm	Provides adequate protection without excessive distortion
Weld sequence	From root to tip	Minimizes thermal distortion and residual stress

The study identifies several critical quality factors that must be controlled:

Microstructure control: The overlay layer microstructure must be controlled to avoid brittle phases such as sigma phase, Laves phase, and carbides that can reduce toughness and corrosion resistance.
Bond quality: The bond between the overlay layer and the base metal must be strong and free of defects such as lack of fusion, porosity, and cracks.
Geometric accuracy: The overlay layer must restore the blade geometry to within acceptable tolerances to maintain aerodynamic performance.

Engineering Practice Integration

In the context of refinery power generation systems, flue gas turbine blade repair is a critical maintenance activity that directly impacts plant availability and efficiency. The overlay welding repair process must be carefully designed and executed to ensure that the repaired blade meets the original design specifications for strength, durability, and aerodynamic performance.

The NDT protocol for flue gas turbine blade repair includes:

Visual inspection (VT): 100 percent coverage for surface defects, geometric irregularities, and weld appearance.
Dye penetrant testing (PT): 100 percent coverage for surface cracks and pores, as nickel-based superalloys are non-magnetic and MT is not applicable.
Ultrasonic testing (UT): Spot check for subsurface defects and bond quality, using a contact probe with frequency of 5-10 MHz for thin overlay layers.
X-ray radiographic testing (RT): Spot check for internal defects such as porosity and lack of fusion, using thin-section radiography for thin blade sections.
Microhardness testing: Hardness profile across the overlay-to-base metal interface to verify the hardness gradient and the absence of brittle phases.
Metallographic examination: Cross-sectional examination of the overlay-to-base metal interface to verify the microstructure and the absence of brittle phases.

The study also discusses the importance of post-weld heat treatment for restoring the microstructure and mechanical properties of the overlay layer and the HAZ. Solution treatment at 1050-1150°C dissolves harmful intermetallic phases and carbides, while aging at 720-750°C precipitates strengthening phases such as gamma-prime (γ') in nickel-based superalloys.

Study Insights and Reflections

This literature provides a practical perspective on the overlay welding repair of flue gas turbine blades in a refinery environment. The key insight is that the repair process must be carefully designed to address the specific damage mechanisms (hot corrosion, thermal fatigue, erosion) that caused the original blade failure.

One particularly valuable aspect of this work is the emphasis on post-weld heat treatment as a critical step in the repair process. The microstructure of the overlay layer and the HAZ must be carefully controlled through heat treatment to ensure that the repaired blade meets the original design specifications for strength, durability, and corrosion resistance.

The practical implication for engineers is that flue gas turbine blade repair requires a systematic approach that integrates damage assessment, process design, process execution, and quality verification. The repair process must be qualified through welding procedure qualification (WPQ) and welder qualification (WQ) in accordance with ASME IX or equivalent standards.

The long-term value of this study lies in its demonstration that overlay welding repair can be successfully applied to flue gas turbine blades with appropriate process control and quality verification. This approach can extend the service life of turbine blades by 2-3 times compared to replacement, significantly reducing maintenance costs and improving plant availability.

This practical experience underscores the importance of integrating metallurgical understanding, process engineering, and quality control into a comprehensive repair methodology that ensures long-term reliability and performance of critical rotating machinery components.