Repairing a single crystal20 April 2000
Gas turbine blades are becoming increasingly specialised. The materials involved are increasingly difficult to repair. Andrew Williams, Wood Group Heavy Industrial Turbine Division, Dundee, Scotland
To cope with growing demand for greater gas turbine efficiencies, higher firing temperatures and higher blade stresses, turbine blades are becoming increasingly complex in terms of design criteria. As a result of this extensive technology now being employed, the cost of producing these components has increased significantly. In order to extend the life of hot section components and reduce the overall maintenance cost, repiar has become a major industry driver.
There are many ways of repairing such blades to retain the shape, but few of these methods retain the complex structures of the blades. Wood Group Heavy Industrial Turbine Division has, however, successfully developed techniques capable of repairing these types of component.
Blade manufacturing advances
Over the last few years, the most significant improvements in blade manufacturing materials were obtained by the creation of directionally solidified superalloys and the single crystal superalloy. The weakest points of a material structure are grain boundaries. In the traditional equiaxed form, there are many, randomly orientated crystals, creating numerous grain boundaries, which are open to various fracture mechanisms.
Using special casting techniques, manufacturers found a way to orientate all the grain boundaries in the direction of stress through the blade, from root to tip. This alignment of grain boundaries in directionally solidified materials significantly improved the material’s strength.
The next step was to produce a single crystal material with no grain boundaries at all, to again improve the mechanical properties of the alloy, such as creep, mechanical and thermal fatigue, and environmental degradation. This change has been possible by modifying the chemical composition of the alloy to remove elements included to strengthen grain boundaries, and increasing the elements included to raise precipitation strengthening mechanisms.
These improvements were an enormous challenge to materials manufacturers and casting suppliers to improve their methodology. This advanced technology is more expensive. The cost of new blades has risen dramatically with the advance in technology. One estimate is that a set of blades that formerly cost $0.5 million now cost $2 million. Thus, the next challenge is to find the best way to repair these blades rather than buy new.
Wood Group Gas Turbines is involved in a number of initiatives to develop techniques to meet the future repair requirements of these exotic materials. Many of these projects are run in conjunction with key research organisations to harness the necessary resources and technology, and learn from the experiences of fellow professionals.
These programmes are an important part of the future for the industry. Research into developing further techniques – and better understanding of the current techniques – of repairing directionally solidified materials and single crystal blades is continuing. One added complexity to single crystal repair is the possibility of recrystalisation during any thermal cycling, which would lead to a recreation of the equiaxed structure.
The single most important requirement for the repair of these advanced turbine blades is the need to avoid as far as possible the input of heat which would damage the blades and significantly affect the structure.
A number of techniques have been developed to do this. These include:
Laser powder fusion welding (for details, see MPS, November 1998).
Low pressure plasma spray.
Transient phase restoration.
Thermal barrier coating.
Transient phase restoration
A combination of hydrogen fluoride cleaning and superalloy powder metallurgy techniques have been developed.
Repairs are undertaken in a three-stage process involving cleaning of the component to remove all surface oxides, application of pre-mixed superalloy powder, and finally, a vacuum heat treatment to diffuse and stabilise the repair.
Higher integrity repairs are achieved compared with traditional high temperature braze techniques by cleaning using controlled flowing of hydrogen and hydrogen fluoride gases over the component. This removes tenacious oxides under controlled conditions, allowing total repeatability of the process.
The superalloy powder consists of similar alloying elements to that of the component being repaired. The alloying elements allow the material to melt, solidify and diffuse into the base material during the fusing and diffusion vacuum heat treatment processes. Heat input to the blade is very much reduced in this low pressure environment compared with traditional weld repairs. This technique removes risks of distortion and cracking in the substrate, and provides superior integrity repairs.
Combined with the cleaning process, even deep, tight and craze cracks can be repaired. Great care needs to be taken with the hydrogen fluoride gas, which is very corrosive.
Low pressure plasma spray
Increased turbine inlet temperature has meant that the traditional diffused aluminide-based coatings are inappropriate, since the melting point of the diffusion zone is exceeded.
For the manufacturer, MCrAlY coatings have proven to be the answer, providing oxidation and hot corrosion resistance at the higher temperatures.
Wood Group now has a low pressure plasma spray to apply the MCrAlY coatings with considerable accuracy during the repair process to restore the as-built condition.
The spray equipment has the following characteristics:
152 cm diameter main process chamber.
Dual opposing loading strings with argon-actuated string grippers.
Preheat transfer chambers with 51 cm internal diameter and 79 cm door openings.
Preheat control in each chamber.
Robotic gun control.
Axis movements controlled by programmable logic.
Seven-axis total motion control.
Transferred arc cleaning.
The basic operation of the low pressure plasma spray is for the process chamber to be evacuated to a low vacuum pressure. The turbine blade is extended into the process chamber, where it can be turned through seven axes of motion beneath the jet of spray.
Thermal barrier coating
The increasing complexity of cooling of gas turbine blades results in blades having complex cooling holes. To ensure that coating deposition is even across the whole blade, Wood Group has developed its Chemical Vapour Coating Deposition system. This system is based on allowing the coating to flow around the holes evenly. The best way to do this is for the coating to be in the form of a gas. The allows for static and equal deposition.
Repairing advanced blades
Greater demand to repair blades combined with greater difficulty involved in repairing them results in opportunities for firms which can carry out these repairs. Such firms have to continually develop techniques to allow them to keep up with blade manufacturers.