Issue 38

L. Cui et alii, Frattura ed Integrità Strutturale, 38 (2016) 26-35; DOI: 10.3221/IGF-ESIS.38.04 27 In previous studies, notch behavior on rotor steel of X12CrMoWVNbN10-1-1 was investigated by conventional low cycle fatigue tests without dwell time (LCF-cycles) and creep fatigue tests with hold times at maximum and minimum load on notched specimens [2]. However the knowledge gained from those experiments are still insufficient to exploit the full potential of this steel. This requires experiments on notched specimen to understand the practical implications of applying such an approach to a service situation. Figure 1: Three stage service-type strains cycling at heated surface of power plant components A phenomenological lifetime estimation method which was developed for a multistage service-type creep fatigue loading, specially demonstrates the applicability of rules for synthesis of stress strain path and relaxation including an internal stress concept, as well as mean stress effects. Further, a modified linear damage rule for creep-fatigue life estimation was employed, and a creep fatigue interaction concept covering a wide range of creep dominant loading as well as fatigue dominant loading was also developed. To describe multiaxial stress state at notch, the Neuber hypothesis was applied. In addition, the service-type loading was also analyzed with finite-element-calculation and an equivalent loading is determined. In the end, the lifetime estimation model was validated with the experiments performed in this study. E XPERIMENTAL Material he tested material is a modern ferritic-martensitic stainless steel of type 10Cr-1Mo-1W-V-Nb (German grade X12CrMoWVNbN10-1-1), which is proposed to be suitable for high temperature applications up to 600°C/300bar. Chemical composition and heat treatment of the material are introduced in [3] and listed in Tab. 1. Test specimens were manufactured from the same heat. Because the maximal thermal damage occurs on the heated surface of a rotor, the specimens were taken from locations close to the periphery of production forging with a longitudinal orientation with respect to the axis of the rotor. C Cr Mo W Ni V Nb N Mn Si P X12CrMoWVNbN10-1-1 0.12 10.7 1.04 1.04 0.76 0.16 0.05 0.06 0.42 0.1 0.007 manufacturing segment of a rotor diameter 400 mm x 6500 mm, weight 6000 kg, forged heat treatment austenitization 1050°C 7h / oil + 570°C 10.25 h / air + 690°C 10h / air Table 1: Chemical composition (weight %) and heat treatment of X12CrMoWVNbN10-1-1