LCF on turbogenerator rotors and coil retaining rings: material characterization and sensitivity analyses

Abstract

Abstract. Turbogenerator rotors and coil retaining rings (CRRs) are highly loaded components typically subjected to LCF at any machine switch-on and switch-off. The present study aims at LCF characterization of two widely applied steels, 26 NiCrMoV 14 5 (for rotor manufacturing) and 18Mn18Cr (for CRR). Material anisotropy is also considered by performing an extended experimental campaign on specimens machined along different (tangential and radial) directions from trial components. The experimental tests, carried out with the use of a novel testing-constraining device for misalignment auto-compensation and with an original methodology for strain controlling, led to the determination of static, cyclic and fatigue curves for all the investigated cases. The research was completed by sensitivity analyses on the adopted models, thus determining fatigue curve tolerance bands, and by a statistical Analysis of Variance to compare the LCF performance of the different materials along the two considered machining directions. Results showed a significantly better performance of 18Mn18Cr and a weak anisotropy effect, remarkable just at the highest strain values, on a reduced portion of the LCF life range.

A New Loading-Constraining Device for Mechanical Testing With Misalignment Auto-Compensation

“… The linearity and repeatability results were studied first: good linearity was proved by the high values of the linear correlation coefficients (0.96 at the average) and a good repeatability was testified by the reduced value of the standard deviations of the Wheatstone bridge electrical outputs, on average about 10 µV. The relative displacement between the specimen heads was about 0.01–0.02 mm with a 44% reduction in the case of tensile loads and a 30% reduction for compressive forces with respect to the no‐bearing configuration. The relative inclination between specimen ends was lower than 0.05°, with a 28% reduction for tensile loads and a 6% reduction for compressive ones. The percentage bending ε b / ε a tends to decrease, as the applied force increases, to values around 6% (with a reduction by 58 and 50% for tensile and compressive loads, respectively), this value appears to be acceptable also on the basis of the remarks contained in Refs. 6 and 7 . Finally, the described device was used in an extensive low‐cycle fatigue experimental campaign 27 and it always showed a regular behavior that led to well‐repeatable results and consistent with current literature. …”

“…Finally, the described device was used in an extensive low‐cycle fatigue experimental campaign 27 and it always showed a regular behavior that led to well‐repeatable results and consistent with current literature.…”

“…The aforementioned campaign required the execution of several low‐cycle fatigue tests in strain‐controlled conditions with a strain ratio R = −1. Such a campaign represented a good testing ground for the behavior of the studied device, which proved to be regular; moreover, the achieved results 27 showed good repeatability, linearity (referring to the fatigue curve), and consistency with scientific literature in this field. Only one remark must be made: for the highest strain ranges, some effects due to buckling were noticed during the experimentation.…”

“…The described loading‐constraining device was used for an extensive campaign aimed at the characterization of high‐resistance steels for turbogenerator rotor and coil retaining ring manufacturing 26,27 . It was in particular the well‐known 4,5 negative impact of misalignments on the accuracy of the experimentally determined fatigue strengths to motivate to the design, construction, validation, and final application of the here described device.…”

“…This specimen is part of a larger set of specimens, used in an extensive experimental campaign for turbogenerator steel characterization. All the specimens were derived from a portion of a forged rotor, previously subjected to the technological processes of ingot melting with vacuum oxygen decarburization, two‐step forging process, 840°C austenitization, salt bath quench, and 600°C temper 26,27 …”