EFFECT OF MICROSTRUCTURE ON CREEP RESISTANCE OF HASTELLOY-X

Transition pieces of Hastelloy X in gas turbine combustion are suffered from significant creep deflection with material degradation during the operations. The main purpose of this study is to clarify the effect of microstructural changes on minimum creep rate and rupture life, and is also to develop the creep life prediction method from microstructural observation and the component's operating history. Specimens of Hastelloy X were aged for up to 10(4)h in the temperature range of 750-degrees-900-degrees-C. By using these prior-aged specimens, metallurgical observations and short term mechanical tests were carried out to evaluate the precipitation behaviors and the degree of degradation. The accomplishments of this study were as follows; (1) The amount of intergranular and intragranular precipitates increases during the aging under the temperature of 750-degrees-900-degrees-C. The former one contributes to strengthening the creep resistance and the latter one contributes to weakening it. (2) Minimum creep rate(epsilonm) was explained as functions of volume fraction of precipitates at the as-solutioned (V(o)) and aged (V) conditions, area fraction of intergranular precipitates (rho), applied stress(sigma), aging temperature(T(a)), test temperature (T), and activation energy of aging(Q(a)) and creep (Q(c)). The equation was as followed. epsilon(m) = [B(o)* + A1*(1 - rho)(V - V(o))2exp(-Q(a)/kT(a))] sigma(n)exp(-Q(c)/kT) (3) Creep rupture life was also explained from the minimum creep rate by using the Monkman-Grant equation for the aged material of Hastelloy X.