Both-the initiation and the propagation of creep cracks have been studied in a lCr-lM4.25V steel at 550°C using CT type specimens. The material taken from a large turbine casing was investigated in two conditions: (i) unaged and (ii) after a long exposure in-service time of about 150,OOO h at 540°C. In both cases the material was found to be creep ductile, which is justified in terms of fracture mechanics applied to creeping solids. It is shown that fracture mechanics is unable to provide unique correlations with global load-geometry parameters, either K or C* for all the stages of both crack initiation and crack growth. However there exists a unique correlation between C* and the time to initiation, ti. This correlation does not depend on the initial conditions of the material. During crack growth two stages are defined. Stage I is a transient regime in which C* is almost constant, but the correspondence between the crack growth rate and C* is not unique since largely dependent on the initial loading applied to the specimens.It is shown that the apparent correlation between the crack propagation rate in stage I1 which corresponds to large crack growth rate is doubtful. A simplified method based on reference length and reference stress is used to calculate C* parameter and to simulate the load-line displacement rate. The results obtained from this method are compared to those derived from finite element calculations published in the literature. NOMENCLATURE a = crack length Q, da/dt = crack propagation rate B, E,, B, = gross, effective and net specimen thickness B,, B, , E ; , E, = constants of the constitutive laws C,(t), C*, C&, =creep load parameters E = Young's modulus Fc,(8) = angular function f;,(O) = geometrical tabulated function h, , h, = tabulated functions I,,, = numerical constant J = Rice's integral K = elastic stress intensity factor L, = reference length m(a/w) = limit load ratio n, n, , n ; , nz = constitutive law stress exponents P, Po = load and limit load p , , p i = constitutive law time exponents r = radial distance R, = creep zone size at the crack tip t = time t, = time for creep crack incubation tt, = transition time between small and large scale t , = creep crack growth stage I-stage I1 transition time tps = primary/secondary transition time W = specimen width X, = critical distance an, = dimensionless factor in R, expression yielding conditions 547 FEMS 14jS-C 548 E. M O L I N~ et al. y = constant in reference length expression A, Aexp, Amp = load point displacements and load point 6, E~ = creep strain and VON MISES equivalent displacement rate creep strain i , , = minimum creep strain rate q ( a / w ) = limit load ratio v = Poisson's factor u, C~ = stress, stress tensor u; (0) = geometrical tabulated function uref = reference stress ay = yield stress 8 = angle from the crack plane.

This paper deals with the effect of casting conditions and long-term in-service exposure on the Charpy-V notch and creep properties of a 1Cr-lMo-O.25V steel. The material is investigated under two conditions: (i) as-cast in a thick part which produces a mixed ferrite-bainite microstructure and (ii) as-cast as a small ingot which produces a fully bainitic microstructure. The material from the thick part was taken from either the cold part (-200°C) or the hot part (u54oOC) of a steam power casing which operated for about 150,000 h. A small shift in the ductile-brittle transition temperature is shown after the long exposure time. This shift is related to segregation of phosphorus impurities along the grain boundaries as shown by Auger electron spectrometry. The bainitic microstructure exhibits a much better creep resistance compared to the thick part component material. Moreover, in the latter case, it is shown that long-term in-service exposure induces an increase in creep strain rate and a decrease in creep ductility at least for relatively short times to failure (8 1000 h). These variations in mechanical properties are briefly discussed in terms of microstructural changes. Scanning electron microscopy confirmed that grain boundary cavitation was responsible for decreasing the creep ductility at large times to failure (2 10,000 h).