Assessment of service induced microstructural damage and its rejuvenation in turbine blades

Abstract

Correlations between service induced microstructural degradation and creep properties in investment cast IN738LC turbine blades are discussed. Microstructural degradation in the form of γ’ coarsen-ing, MC carbide degeneration, formation of continuous networks of grain boundary M₂₃C₆ carbides, and the disappearance of serrated grain boundaries are considered in some detail. Their influence on primary (t _p ,ε _p ), secondary (t _s , ε _s ,ε _m ) and tertiary (t_t, ε_t) creep behavior is analyzed through rela-tionships of the form:

$$\left( {\frac{{t_p + t_s }}{{\varepsilon _p + \varepsilon _s }}} \right)\dot \varepsilon _m = K(Initial transient behavior)$$

$$\begin{gathered} \left( {t_{t/\varepsilon _t }^{2 33} } \right)\dot \varepsilon _m = K(Intragranular creep governed \hfill \\ by \gamma ' interparticle spacing) \hfill \\ \end{gathered} $$

$$\begin{gathered} \left[ { - t_{t/(e^{ - 50_\varepsilon } t - 1)} } \right]\dot \varepsilon _m = K_2 (Intragranular creep governed \hfill \\ by mobile dislocation density) \hfill \\ \end{gathered} $$

and

$$\left( {t_t /\varepsilon _t } \right)\dot \varepsilon _m = K(Grain boundary cavity nucleation)$$

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 influ-ence of trace amounts of Zr on creep properties of service exposed IN738LC turbine blades is also highlighted.