Assessment of service induced microstructural damage and its rejuvenation in turbine blades
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\
( t p + t ~ ) k m = K(Initial transient behavior) \ep + e d . 2 33"~. It/et )Sin
:
K~(Intragranular creep governed by Y' interparticle spacing)
[-- trice-50,,_ l)]E:m = K2(Intragranular creep governed by mobile dislocation density) and
(t,/s,)km = K3(Grain boundary cavity nucleation) These relationships are able to reveal service induced degeneration effects and can therefore be used to qualify rejuvenated blades. A systematic strategy for designing a HIPping rejuvenation cycle for Ni-base superalloys is presented. Once a rejuvenation cycle is designed, the above-mentioned relationships can then be used to analyze the extent of the rejuvenation of microstructure and creep properties in reheat-treated or hot isostatically pressed service exposed turbine blades. The influence of trace amounts of Zr on creep properties of service exposed IN738LC turbine blades is also highlighted.
I.
INTRODUCTION
IT is well known that creep is the dominant mode of deformation in industrial gas turbine blades at operating stresses and temperatures. In recent years, the possibility of recovering service induced creep damage (Rejuvenation) by hot isostatic pressing (HIPping) has gradually turned into a realityJ a-Sl Several companies now offer commercial rejuvenation services for used turbine blades. I4'51 Many users are also interested in assessing the rejuvenation potential of their used turbine engine components. One of the major difficulties faced by metallurgical engineers in assessing this potential concerns the quantification of microstructural damage incurred by the turbine parts during service and the assessment of the amount of damage repaired upon rejuvenation. An accurate and a reliable analysis of the structure-property correlations in new, used, and rejuvenated components is thus needed to ensure the safe return of rejuvenated blades to service. Traditionally, Larson-Miller (L-M) parameter and life trend diagrams have been used to monitor creep degeneration of turbine engine materials. [6.7] However, analysis based
A.K. KOUL is Research Officer, Structures and Materials Laboratory, NAE-NRC, Ottawa, ON, Canada, K1A 0R6. R. CASTILLO is Senior Metallurgist, Turbine and Generator Division, Westinghouse Canada Inc., Hamilton, ON, Canada. Manuscript submitted July 11, 1986.
METALLURGICAL TRANSACTIONS A
on rupture life (tR) alone ignores the major creep strength parameter of secondary or minimum creep rate. Many relationships between the minimum creep rate (k,,), tR, and eR (creep ductility) are also available for creep data analysis, such as those proposed by Monkman and Grant ISj and Dobes and Milicka. [91 Creep damage in turbine blades is usually assessed by microscopy and creep testing of specimens machined from service exposed blades at a selected stress (o-) and temperature (T) and monitoring parameters such as the tR, eR, and k m . These data are collected over a number of years by removing components from service at regular intervals, and the above-mentioned creep parameters are compared with the specification
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