Advanced Thermochemical Cleaning Procedures for Structural Braze Repair Techniques

Hot gas path components of modern Industrial Gas Turbines (IGT) are exposed to extreme thermal, mechanical and chemical loading that ultimately leads to their deterioration. Modern GT designs provide for safe operation for a certain operation period. Higher firing temperatures and changing machine loads as a result of the deregulated markets call for highly sophisticated part designs and the use of cost-intensive superalloys. As the lifetime of critical parts is not infinite, they are reconditioned periodically or replaced to regain efficiency losses and to mitigate the risk of unscheduled outages due to hot gas path (HGP) failures. This paper presents advanced thermochemical preparation treatments that form the basis for the subsequent structural repairs, such as high temperature brazing. Before executing any repair step, coated components must be stripped of the consumed and degenerated coatings. Not all of the many techniques that are commonly used can guarantee reproducible and complete removal without damaging the substrate. Recently improved thermochemical techniques, such as a combination of advanced Chemical Stripping and Salt Bath Cleaning, enables the OEM to obtain clean components at low unit costs and for short processing times. In previous approaches, CrF2- and PTFE-based processes were used to clean surfaces and, principally, cracks from oxide scales before welding or brazing was carried out. These preparation techniques were indispensable for reworking superalloys, which cannot be cleaned sufficiently using conventional methods such as exposure under reducing atmospheres at high temperatures. Today, the high versatility of the “Dynamic Subatmospheric Fluoride Ion Cleaning” process (FIC) enables the OEM to run precisely tailored processes, allowing complete freedom to adjust the chemical activity of the gas phase and in so doing fulfil the specific conditions for any superalloy being reworked, even taking into account the varying grade of degradation sustained during service exposure. Weld repairs on superalloys are very sensitive to hot cracking, and high temperature brazing has established itself as a successful method for overcoming this problem. Furthermore, the intensively FIC cleaned surfaces can be regarded as the most important condition to enable a high quality bonding. Other key advantages of braze repairs are the uniform heat input that is possible, the high shape tolerance and the fact that multiple cracks can be simultaneously repaired. In addition, the brazing heat treatment allows controlled adjustment of the microstructural properties. Besides the economic benefits of the treatment, the brazed parts show excellent results in respect of their mechanical integrity. A schematic presentation of the repair sequence described in this paper is shown in the appendix (Fig. 17).