Frequently Asked Questions
The high cost of hot section gas turbine blades and nozzle guide vanes as replacement parts and the large numbers of these parts that often need replacing during a major overhaul has established a strong end-user interest in the use of (weld) repaired components. Until the mid-1990s, independent repair companies sustained this new repair market, but increasingly the original equipment manufacturers (OEMs) have recognised the huge size of the market and have responded by acquisition of independent repair companies or by establishing their own repair centres.
Until ten years ago, the majority of permitted repairs were performed by manual processes such as TIG welding (dimensional restoration) or brazing (crack repair). Few mechanised repairs were carried out because although the types of blade damage may be generic, the precise location and scale is generally unique to each blade. However, the mechanisation of turbine blade repair is highly desirable, and more recently, advances in welding technologies have resulted in increasing numbers of applications where weld repairs may be used.
Until recently, the preferred consumable for repair of nickel-based alloys has generally been nickel alloy 625. This alloy has relatively good ductility compared with turbine blade materials, and this property is useful in reducing the risk of weld zone solidification cracking. However, alloy 625 displays wear, oxidation and creep resistance inferior to the blade alloys, and there are number of industrial programmes underway to evaluate higher strength welding consumables and to develop improved welding procedures.
The majority of permitted weld repairs are carried out on gas turbine blade tips. Processes widely used include automated TIG welding, CO2 and Nd:YAG laser welding and plasma welding. There is some debate over preferred consumable type. Whilst wire feed is generally considered easier to control, for many of the newer materials being considered suitable for repair of gas turbine blades, consumables are difficult to manufacture in anything other than powder or perhaps short lengths of wire.
More recently, laser direct metal deposition (DMD) and cold spray have been studied as a means of repairing hot section alloys.
FAQ: What materials issues must be considered when repairing hot section gas turbine blades?
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