Introduction
Most hot section gas turbine blades and nozzle guide vanes are made from nickel based alloys and may be equi-axed, directionally solidified (DS) or single crystal (SC) investment castings, which have creep properties at elevated temperature that are superior to conventionally cast (CC) alloys.
The main concern regarding repair processes, such as welding or brazing, is the potential loss of elevated temperature mechanical properties (whether CC, DS or SC) in the heat-affected zone (HAZ) or weld. For example, HAZ liquation cracking may occur during welding, and more extensive cracks may form during post weld heat treatment.
Consumable selection
A matching consumable (in terms of strength, not necessarily composition) is not generally used for weld repair of most alloys because of the high risk of weld zone solidification cracking (in addition to HAZ liquation cracking). It is often very difficult to detect these defects by non-destructive testing techniques. Repair with a matching (or similar) consumable is, however, desirable since the most widely used consumable (alloy 625), whilst having good ductility, displays wear, oxidation and creep resistance inferior to typical turbine blade alloys.
It is envisaged that repair with a matching (or similar) consumable may be possible through careful pre-weld and/or post-weld heat treatment of the substrate combined with improved repair processes. It is also widely recognised that criteria for repairability of turbine blades, such as maximum permissible crack depth for repair, would be useful in determining which blades can be repaired and those that should be replaced.
Approved repairs
Repairs are permitted at present on static components (e.g. stator blades, nozzle guide vanes), and in low stress regions of rotating blades (e.g. platforms, shroud seal fins and blade tips). However, OEMs do not permit, and end-users do not accept, the repair of cracks or worn areas on airfoils or roots of rotating blades. It is also recognised that validation of the repair processes is a key issue to be addressed before installation of components that have been subject to novel repairs (novel in terms of location or process). This is of particular concern when the repair is to be located in an area of the blade considered to be of structural importance, such as the airfoil.
The most common materials-related failure mechanisms are low cycle fatigue and creep, and there are industry-standard laboratory tests for testing turbine blade alloys (and repairs thereto) at elevated temperature.
Directionally-solidified repairs
A number of studies have investigated the possibility of propagating directionally-solidified microstructures from the parent blade material into the weld repair, and there is at least one patent in this area for laser weld repair, which relates factors such as pre-heat, solidification-front velocity and welding parameters. However, it is debateable whether this is a necessary requirement for typical repairs, e.g. blade tips, where wear and erosion, rather than creep resistance, are the principal concerns.
Summary
It is widely viewed that the reduction of heat input to the nickel alloy component is the key to eliminating HAZ and weld metal cracking, and that mechanised processes such as laser welding and mechanised TIG welding are key technologies for reaching these goals. A number of the more innovative independent repair companies and OEMs have already installed laser welding systems. The majority, if not all, of these are CO 2 laser welding systems. A limited number of investigators are assessing the advantages of other systems, e.g. Nd:YAG lasers.
Further information
FAQ: What techniques are currently available to weld repair hot section gas turbine blades?
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