16 - Creep fatigue behaviour and crack growth of steels

Larson-Miller (LM) and Orr-Sherby-Dorn (OSD) parameters are widely used when formulating and predicting creep rupture life t_r. The LM constant C and the activation energy Q in the OSD parameter characterize temperature dependence of t_r. Q = Q_LSD (Q_LSD: activation energy for lattice self-diffusion) and C = 20 are often assumed in the formulation. Creep rupture datasets of 11 heats of nickel alloy A617, 304H and 304J1 stainless steels, 6 heats of A3004N aluminum alloy and four Mg-Al alloys are formulated with the parameters for determining C and Q of each dataset. Based on the correlation between C and Q among similar materials, it is discussed what is the C value equivalent to Q = Q_LSD. It is also examined how the C value changes with melting temperature of the material. The value of C equivalent to Q = Q_LSD varies from 14 to 9.5 depending on average values of log(rupture life) and of reciprocal temperature of the dataset. These values of C point out that C = 20 is not equivalent to Q = Q_LSD. The equivalent C values are similar in all the materials independently of their melting temperatures.

The demand for an increase in flexible operation of power plants provides a key challenge. Solutions for new components as well as for units in operation have to be developed to safely allow for more load cycles and faster start-up times. One way to realize this is to improve the lifetime assessment methods by reducing implied conservatisms. The presented work focuses on the description of notch support effects at elevated temperatures for the LCF-regime. At first, a tailored experimental set-up using notched round-bars is introduced. Those tests are performed under global strain control and are equipped with an Alternating Current Potential Drop (ACPD) measurement system. The experimental results show that the crack growth phase up to a so called "technical crack size" is of major relevance to describe notch support effects. That is why within the second part of this paper, a concept is introduced to consider this early crack growth phase within a lifetime assessment approach. Within this concept, fracture mechanical parameters are determined by applying a novel energy-based Finite Element approach. Suggestions are presented to define proper values of the cyclic J-Integral as well as of the amount of Plasticity Induced Crack Closure (PICC). Finally, the concept is applied and validated by comparing the simulation approach with the experimental results.

Steels with varying chromium contents are widely used in steam turbine components and have been introduced steadily in the last decades. The initial aim in the development of such steels is to achieve high performance in creep resistance. Due to the fluctuations of electrical power demand nowadays, power plants are increasingly forced to run at varying utilization levels, which can shift the critical load to the fatigue domain by superimposed creep on the heated surface of components. In the current paper, the creep fatigue behavior of 1%-, 2%- and 10%Cr steels under multiaxial loading is described. The experimental investigation was conducted on steels of the types 1Cr–1Mo–Ni–V, 2Cr–1Mo–W–V and 10Cr–1Mo–1 W–V–Nb–N as representative samples for each of the three steel grades. The experimental database consists of uniaxial as well as biaxial creep fatigue experiments which were conducted on a biaxial cruciform testing machine. Of special interest was a lifetime comparison of experiments under thermomechanical and isothermal loading at the maximum application temperature. A unified viscoplastic constitutive material model with an incorporated damage variable was applied for lifetime assessment. Finally, metallographic investigations contribute to a better knowledge of the evolution of damage and its modeling. The investigation shows slightly different effects on lifetime, dependent on the three steel grades.

The flexibility of steam turbine components is currently a key issue in terms of the fluctuations in the power supply due to regenerative energy. Conventional steam power plants must run at varying utilization levels. Life estimation methods according to standards, e.g. ASME Code N47 and TR, assess the influences of creep and fatigue separately under the assumption of isothermal conditions at the maximum operating temperature. The influence of thermomechanical fatigue (TMF) loading still requires a significant number of experimental studies. Further, the interaction of creep and fatigue is not adequately taken into account. Thus, new lifetime estimation methods are required for the monitoring, re-engineering and new design of power plant components. In this paper, both a phenomenological and a constitutive crack initiation lifetime estimation model for steam turbine components are introduced. The effectiveness of each method is shown by recalculation of uniaxial as well as multiaxial service-type creep–fatigue experiments on high-chromium 10%Cr stainless rotor steel. Finally, the two models are compared with respect to different aspects, such as the type and number of necessary experiments to determine model parameters, the prerequisite for the application and the limitations of each model.

Nowadays, fossil power plants are increasingly required to start up and shut down frequently due to the flexibility of electrical power demand. Thermomechanical fatigue (TMF) induced by temperature transients with superimposed creep on the heated surfaces of components leads to a significant reduction of lifetime. In this paper, the influence of temperature transients on the crack initiation behavior of high-chromium rotor steel of the type X12CrMoWVNbN10-1-1 was studied by performing uniaxial and biaxial service-type TMF experiments. The experiments represent a range of steam turbine cycles with a maximum temperature of 600° C. A significant lifetime reduction was observed on TMF loading compared to isothermal loading under the same mechanical strain cycle. Metallographic examinations have been employed to characterize the associated thermal fatigue damage mechanisms for comparison of the damage evolution under isothermal loading. In particular, the evolution of damage was investigated by systematic metallographic examinations to study the temperature influence on the crack initiation behavior.

Variations in steam temperature due to start-up and shut-down of thick-walled components of power plants induce thermomechanical stresses, which can lead to fatigue and superimposed creep. Subsequently, damage can occur at the heated surface. With respect to component integrity, reliable lifetime assessment methods for critical components are mandatory. Lifetime assessment models have been developed and complex experiments for their verification were conducted. This paper presents a comparison and evaluation of different approaches to reach this target. At first, a phenomenological approach, based on the generalized damage accumulation rule, is introduced. The model takes into account creep rupture damage at stress relaxation as well as fatigue damage. Softening or hardening effects are considered as is an enhanced interaction method for the influence of dwell times and internal stresses. Secondly, a more sophisticated viscoplastic constitutive material model of the type Chaboche is introduced. The material parameters were adapted to steels of the types 1CrMoNiV (1Cr), 2CrMoNiWV (2Cr), 12CrMoV (12Cr) and to a forged 10CrMoWVNbN (10Cr) material. The model allows the recalculation of the material behavior under static and cyclic loads, and partly multi-stage loads. The applicability and accuracy of the models is finally demonstrated with the recalculation of a large variety of different uniaxial and multiaxial experiments on notched specimens and cruciform specimens representing local component loading.