F. Mueller

Creep crack behaviour of a coarse grain nickel-base super alloy

Abstract The paper presents creep crack growth data, obtained for cast nickel-base super alloy IN-738 LC, which represents a typical large coarse grain structure. Long term creep crack growth experiments were performed at 700 and 850°C on side-grooved compact tension and corner crack specimens. In parallel, uniaxial creep tests on smooth specimens were conducted. The coarse grain structure has been examined using electron backscatter diffraction (EBSD). Creep fracture in polycrystalline materials is observed to be typically intergranular and dominates along grain boundaries. These observations were confirmed for the investigated alloy. From metallographic examinations, an effect of grain orientation on creep crack behaviour, however, was not observed. The Nikbin – Smith –Webster (NSW) model, which is based on the creep fracture mechanics parameter C*, and a damage accumulation model, which considers only nominal stress in the ligament, lead to reasonable results when recalculating creep crack growth.

Crack behaviour of 10Cr-steels under creep and creep-fatigue conditions

Abstract High-temperature components with notches, defects and flaws may be subjected to crack initiation and crack propagation under service conditions. To study these problems and to support an advanced remnant life evaluation, fracture mechanics procedures are required. Due to a more flexible service mode of power plants an increase of start up and shut down processes can be observed. Therefore knowledge of life assessment and integrity of such components is decisive. In order to enlighten these problems for steam power plant components, crack initiation time and crack growth rate of modern martensitic steels of type 10CrMoWVNbN were determined at 550 and 600°C in forged and cast conditions. For the experiments, side grooved compact tension, C(T), specimens were used as well as side grooved double edge notched tensile, DEN(T), specimens. Creep/creep-fatigue crack initiation and crack propagation can be described with the usual fracture mechanics parameters C* and KI and a modified two-criteria method was established in order to describe creep fatigue crack initiation.

Influence of Crack-Surface Oxidation on Creep-Fatigue Crack behavior of 1Cr- and 10Cr-Steels

High temperature components with notches, defects, and flaws can introduce crack initiation and crack propagation under service conditions. Fracture mechanics procedures are needed to study crack problems and to support an advanced remnant life evaluation. Since a more flexible service mode of steam power plants causes a higher number of start-up and shut-down events, creep-fatigue crack behavior is decisive for life assessment and integrity of components. Usually, fracture mechanics experiments are carried out under air conditions, although in cases of internal cracks they are not in contact with air. Therefore, it is of interest to realize the degree to which environmental conditions, e.g., crack-tip oxidation, can influence crack initiation and crack growth behavior. In order to reveal problems related to high temperature components, the crack initiation time and crack growth rate were determined in air environment and in a gas with controlled atmosphere on 1CrMo(Ni)V- and 10CrMoWVNbN-steels. Crack initiation and propagation under creep-fatigue conditions have been described with the fracture mechanics parameters C*, KI, and ΔKI and C*. A modified “Two-Criteria-Diagram” was used to describe creep-fatigue crack initiation.

Estimation of fatigue crack growth under complex loading using an accumulative approach

Abstract A model procedure to obtain estimates for thermo-mechanical fatigue crack growth rates based on isothermal test results is presented. This ‘O.C.F.-model’ consists of a linear accumulation of temperature independent fatigue crack growth and two temperature dependent crack growth parts. Temperature dependent crack growth is constituted from creep crack and oxidation damage. For the oxidative damage part, the formation of a γ′-free zone at the crack tip and sub-sequent softening is considered to be determining. The possibility to assess creep crack growth in alloys with very limited creep ductility using linear-elastic fracture mechanics assumptions is investigated and compared to visco-plastic approaches. The isothermal data presented in this paper allows for an estimation of crack growth under both isothermal and anisothermal conditions with and without dwell times. Furthermore it allows assessing the fractions of crack growth caused by the different sources.

Fracture mechanics and testing for crack initiation and growth assessment in coal power plants.

Abstract: This chapter demonstrates state-of-the-art methods for evaluating crack initiation and crack growth in heat-resistant steels and nickel base alloys and their practical application. From investigations over a period of three decades, it has been learned that crack initiation and crack growth are relevant for components subjected to long-term loading which operate under creep, fatigue or creep–fatigue conditions.