Engine & Turbine Technology
oxidation and corrosion by the formation of a stable, dense, and tight oxide scale. Depending on the alloy (or the coating), the is scale can be either chromium oxide or aluminum oxide. Industrial gas turbines with very long operating times benefit from high-chromium alloys;turbines in which the alloys are exposed to the highest temperatures benefit fromhigh aluminum contents. Te latter turbines cannot offer the fuel flexibility for which old, ‘low’ -temperature gas turbines were once known. Only chromium oxides can offer reasonable ,protection against attack by sodium sulphate or sodium vanadate salts (‘hot corrosion’).
is possible, often a great deal of additional information can be retrieved from the used and damaged components, especially when there is an opportunity to compare these components with similar components from other turbines and other types and/or brands of turbines. From such experience data, the following comments on generic types of damage can be made. Creep is slow plastic deformation that occurs in a component under stress at high metal temperature. Te creep process gradually exhausts the plastic deformation capability of the component. Up to about two per cent plastic deformation does not usually create serious problems with the alloy. In many cases, however, the accompanying mechanical deformation already exceeds acceptance limits for safe operation. Tis creep damage can be repaired – not
by addressing the material or the root cause of the problem – but by mechanical correction of dimensions, eg, by cutting back and by rebuilding as required. Since the deformation of straightening actions adds up to the creep damage, the straightening process should not be used for critical components. It should be emphasized that in many cases of creep damage, the base material is still in very good shape, eg, it would be pronounced ‘to be in good shape’ by an investigating metallurgist. Tis is not contradictive to a rejection of that same component because of mechanical non-conformance. Practical experience shows that true metallurgical creep damage is a rare phenomenon. Most gas turbine components exhibit a metallurgical creep life that is a multiple of the actual lifetime.
Fig. 2. Deep thermal cycling cracking in the leading edge of a gas turbine blade.
All modern gas turbines, as used in public utilities, operate with high metal temperatures. Tey are protected by high aluminum coatings and need very clean fuel gas and combustion air. Up to about 750°C, the combustion gases only attack the material at its surface or, to some extent, along grain boundaries. At higher temperatures, diffusion in the alloy becomes increasingly important and, consequently, oxygen and other reactive components from the combustion gases can penetrate deep into the surface of the component. In this surface layer, alloying elements react in order of decreasing reactiveness, leading to depletion and formation of internal oxides, nitrides. etc. Because of the high stability of these compounds, this cannot be restored . Te damage created by the mechanisms above
renders the information that is needed to assess the severity of the process creating it. If this damage had been foreseeable in its full extent in the design phase, appropriate design modifications would already have been implemented in that phase. Terefore, pure reverse engineering, in which,
essentially, the same calculations and estimations are made as in the original design phase, cannot be guaranteedto offer a good analysis and solution. It is a valuable instrument to support modification initiatives. If a solution to or a mitigation of a problem
When components are produced, the materials
structure have the right grain size distribution and size and distribution of different phases like carbides and Gamma-prime phase. Tese phases have limited stability and can grow, redistribute, disintegrate, or convert into other phases. Unwanted new phases like s-phase can be formed over time as well. Some degradation is completely reversible; some is not. Fretting is the wear of components that are in low- amplitude vibrating contact. Te slight relative motion creates an ongoing process of diffusion welding and tearing apart of these welds on a microscopic scale. In many alloys, fine particles are thus torn out of
the surface, leading to the typical signs of fretting. Hardness of the alloys is only a factor but not an all- determining parameter. Cobalt-base alloys have better resistance to fretting than nickel-base alloys. In the operation of a gas turbine, good housekeeping is the art of finding the best mcompromise between limited component lifetime and efficient power output of the gas turbine. In other words, gas turbine (hot-section) components are designed for and will be operated to survive an optimised and limited lifetime. Te weakest points of designs will determine the lifetime. By repair or modification of these points, lifetime can be extended. At the same time, these weakest points are usually
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