Weld Overlay Repair of Flue Gas Turbine Blades

Literature Overview

The study by Wu Guolin (China Petrochemical Corporation, Qianguo Refinery, published in Petrochemical Equipment, 2002) addresses the weld overlay repair of flue gas turbine (FGT) blades, which are critical components in catalytic cracking units (FCCUs) at petroleum refineries. Flue gas turbines are driven by the high-temperature flue gas exiting the regenerator in FCC units, and their blades are subject to extreme thermal cycling, corrosion from sulfur-containing compounds, and erosion from particulate matter. The overlay repair of these blades is essential for extending their service life and maintaining the efficiency of the FCC unit.

Operating Conditions and Failure Modes

Flue gas turbine blades operate under the following extreme conditions:

The primary failure modes of FGT blades include:

1. Hot corrosion: Attack by molten salt deposits (Na₂SO₄, NaCl, Na₂S) that form at temperatures above 600 °C, leading to accelerated oxidation and material loss.
2. Erosion-corrosion: Synergistic effect of particulate impact and chemical attack, resulting in rapid material removal at the leading edge and pressure surface.
3. Thermal fatigue: Repeated thermal cycling causes crack initiation and propagation at the blade root and trailing edge.
4. Creep rupture: At sustained temperatures above 700 °C, the blade material undergoes time-dependent deformation and eventual fracture.

Overlay Repair Procedure

The overlay repair of FGT blades involves the deposition of a corrosion- and erosion-resistant overlay layer on the blade surfaces, typically using gas tungsten arc welding (GTAW) or plasma arc welding (PAW). The overlay material is selected based on the specific operating conditions and the base material of the blade.

Overlay Material Selection

The following overlay materials are commonly used for FGT blade repair:

Welding Process Parameters

The welding process parameters for FGT blade overlay are critical to achieving a sound, crack-free overlay layer:

• Preheat temperature: 150–250 °C for nickel-based overlay materials; 300–400 °C for cobalt-based overlay materials.
• Heat input: 0.5–1.5 kJ/mm for GTAW; 0.3–0.8 kJ/mm for PAW.
• Layer thickness: 0.5–2.0 mm per pass, with a total overlay thickness of 2–5 mm depending on the damage extent.
• Interpass temperature: Maintained below 300 °C for nickel-based materials and below 400 °C for cobalt-based materials to avoid sensitization and cracking.
• Shielding gas: Pure argon (99.99% purity) for GTAW; argon-helium mixture (80/20) for PAW to improve arc stability and penetration.

Microstructure and Performance

The overlay layer produced by GTAW or PAW on FGT blades typically exhibits a columnar grain structure with fine interdendritic precipitates. In the case of Inconel 625 overlay, the microstructure consists of an austenitic matrix with γ′ (Ni₃(Al,Ti)) precipitates and δ (Ni₃Nb) phase particles, which provide solid solution strengthening and precipitation hardening, respectively. The hardness of the overlay layer is typically in the range of 200–300 HV, which provides adequate wear resistance without compromising the ductility and toughness required for thermal fatigue resistance.

The bond strength between the overlay layer and the blade substrate is a critical quality metric. For nickel-based overlay materials on nickel-based superalloy blades, the bond strength is typically in the range of 300–500 MPa, achieved through a combination of metallurgical bonding and mechanical interlocking. The interface between the overlay and the base material is characterized by a thin diffusion zone (5–20 μm) where interdiffusion of alloying elements occurs, creating a gradual transition in composition and properties.

Quality Control and Inspection

The quality of the overlay repair is verified through the following non-destructive testing (NDT) methods:

Additionally, the overlay layer is subjected to hardness testing (Vickers hardness, HV 0.5) to verify that the hardness profile is within the specified range (typically 200–350 HV for nickel-based overlay materials).

Study Insights

The study by Wu Guolin highlights the importance of selecting the appropriate overlay material and welding process for FGT blade repair. The extreme operating conditions of FCC flue gas turbines demand overlay materials with exceptional hot corrosion resistance, erosion resistance, and thermal fatigue resistance. The use of nickel-based superalloys (Inconel 625, Inconel 718) and cobalt-based alloys (Stellite 6) provides the necessary combination of properties, but the welding process must be carefully controlled to avoid cracking, sensitization, and excessive dilution. The study also emphasizes the importance of quality control and inspection, which are essential to ensure the reliability of the overlay repair and prevent premature failure in service. For engineers involved in FGT blade maintenance, a systematic approach to overlay material selection, process parameter optimization, and quality verification is essential to extend blade life and maintain the efficiency of the FCC unit.