Papers by Keyword: Creep Damage

Abstract: Damage and fracture processes in high temperature creep of an investment cast B1914 Ni-based superalloy with the increased amount of boron to 0.08wt.% for high temperature applications were analysed. Constant load creep tests in tension were conducted at temperatures from 800 to under applied stress ranging from 150 to 700 MPa. The microstructure of fractured specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. Microstructure investigation showed that the microstructure of the B1941 superalloy consists of a gamma (γ) phase with a dendritic structure and gamma prime (γ ́) phase with a cuboidal shape. Precipitates of γ ́and a lamellar eutectic, composed of γ/(Mo,Cr,Ni)₃B_2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M₃B₂ boride and matrix.

Abstract: The creep response and stress relaxation of X20 CrMoV12-1 steam piping under diverse operating conditions were simulated using finite element analysis (FEA) code, Abaqus alongside fe-safe/Turbolife software. In the study, steady-state creep and creep analysis characterized by 24 hours daily cycle consisting of a total of 6 hours peak, 4 hours transient and 14 hours off-peak period was considered. Modified hyperbolic sine creep model used in the analysis was implemented in Abaqus via a special creep user-subroutine to compute the stress relaxation and creep behaviour, while the useful service life and creep damage was estimated using fe-safe/Turbolife. The optimum creep strain, stress, damage, and worst life were found at the intrados of the piping, with the steady-state analysis having a higher useful creep life and slower creep damage accumulation. Furthermore, slower stress relaxation with faster damage accumulation was observed in the analysis involving cycles. Finally, a good agreement was obtained between the analytical calculated and simulated rates of the piping.

Abstract: This paper discusses creep void formation and rupture lifetimes in multiaxial stress states for a SUS 304 stainless steel at elevated temperatures. Biaxial and triaxial tension creep tests were performed using a cruciform and a cubic specimen, respectively. These two types of the specimens were designed to achieve uniform equi-biaxial and equi-triaxial stress distributions by a finite element analysis in the gage parts. Void formation at grain boundaries was observed by intermitting biaxial creep tests and by interrupting triaxial creep tests. Creep rupture lifetimes were also obtained in biaxial and triaxial creep tests. Biaxial stresses increase the void formation but give a little influence on a creep rupture lifetime in the correlation with von Mises equivalent stress. Triaxial stresses also increase the void formation and drastically reduce a creep rupture lifetime in the correlation with von Mises equivalent stress. Evident void formation in an equi-triaxial stress condition demonstrates that von Mises equivalent stress is not a suitable measure to evaluate creep damage development in multiaxial stress states. A new equivalent stress is proposed to evaluate creep rupture lifetimes in biaxial and triaxial stress states.

Abstract: The deformation and damage features of a high Mo single crystal Ni-based superalloy during creep at high temperature are investigated by means of measuring creep properties and observing microstructure. Results show that, compared to 4%Mo single crystal nickel-based superalloy, the 6%Mo superalloy displays a better creep resistance, and the creep life of 6%Mo single crystal superalloy at 1040°C/137MPa is measured to be 556 h. In the ranges of applied temperatures and stresses, the creep activation energy of the alloy is measured to be 484.7kJ/mol. Wherein, the deformation mechanisms of the 6%Mo superalloy during steady state creep are dislocations slipping in ϒ matrix and climbing over the rafted ϒ' phase. In the later stage of creep, the deformation mechanism of alloy is dislocations shearing into the rafted ϒ' phase, the alternate activation of dislocations slipping results in the twisted of the rafted ϒ'/ϒ phases, as the creep goes on, to promote the initiation and propagation of cracks along the interface of the twisted ϒ/ϒ' phase perpendicular to the stress axis, up to creep fracture, which is thought to be the damage and fracture features of the alloy during creep at high temperature.

Abstract: Creep damage processes for smooth and notched specimen of austenitic stainless steel through interrupted creep tests using multiple specimens. The material used was 18-8 stainless steel for boiler tube use. The mid-sections of interrupted creep test specimens were observed through SEM(Scanning Electron Microscope) instrumented with EBSD(Electron BackScatter Diffraction patter) equipment. IPF(Inverse Pole Figure) maps, Phase maps and GOS(Grain Orientation Spread) maps were used for investigating creep damage process. For smooth specimen, the relationship between macroscopic creep time fraction and GOS averaged for all pixels showed linearity, while the relationship between creep strain and the averaged GOS showed non-linearity regressed by Green function successfully. For notched specimen, the EBSD maps became noisy possibly due to extensive phase transformation under highly concentrated notch stress. Obtained GOS data for gamma phase only showed non-monotonic change with time and nominal strain. The evaluated local strains in the vicinity of the notch showed relatively small amount, which might cause the very long creep life compared with smooth specimen under the same nominal stress condition.

Abstract: The components used in power plants generally operate at elevated and/or high temperature and are subjected to internal pressure. Under such conditions creep is of a great concern and there is an urgent demand for methods which can be used to predict the creep life. In this work, using our earlier published creep data for advanced creep-resistant T23 and P92 steels, the interrelationship between creep deformation and damage have been analysed by linking them to the identified acting mechanisms, in terms of empirical formulas for the fracture time assessment. The validity and the applicability of various formulas are examined with the objective to gain insight into the creep deformation and fracture behaviour of the steels under investigation.

Abstract: The microstructure and creep behaviour of the welded joints of P92 steel pipe were investigated in order to determine the influence of orbital heat welding technology on the creep resistance. Creep specimens were machined from the welded joints. Tensile creep tests of welded joints were performed at 873 K using different stresses. The microstructure of tested specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. The creep results showed that the creep fracture strain of the welded joints decreases with decreasing value of applied stress. Microstructure investigation showed that fracture behaviour of welded joints is influenced by an enhanced cavity formation at grain boundaries in the heat-affected zone causing lower fracture ductility.

Abstract: Complex metallographic analysis was performed on the steam pipe elbow made of 0.5Cr-0.5Mo-0.3V steel after long-term (more than 240 000 hours) of operation at elevated temperature that revealed the extensive creep damage on the outer surface of the pipe elbow. Metallographic analysis confirmed pronounced creep damage at the outer surface but, at the same time, the non-uniform nature of the cavitation. The density of cavities continuously decreased from outer to inner pipe surface in the most damaged area and rapidly waned along the circumference as well as the length of the elbow. Parallel evaluation of actual extent of the cavitation damage made by metallography and replica methods in various parts of the pipe elbow confirmed that replica method is capable to describe and quantify the cavitation damage of this steel in the same way as metallography, including evaluation of creep damage according to Nordtest NT TR 302.

Abstract: In order to evaluate the safety and integrity of piping with local wall-thinning at elevated temperature, a numerical method for plastic limit load of modified 9Cr-1Mo steel piping is proposed in the present paper. The limit load of piping at high temperature is defined as the load-carrying capacity after the structure has served for a certain time period. The power law creep behavior with Liu-Murakami damage model is implemented into the commercial software ABAQUS via CREEP for simulation, and the Ramberg-Osgood model is modified to consider the material deterioration effect of modified 9Cr-1Mo steel by introducing the creep damage factor into the elasto-plastic constitutive equation. For covering the wide ranges of defect ratios and service time periods, various 3-D numerical examples for the piping with local wall-thinning defects, and creep time are calculated and analyzed. The limit loads of the defected structures under high temperature are obtained through classic zero curvature criterion with the modified Ramberg-Osgood model, and the typical failure modes of these piping are also discussed. The results show that the plastic limit load of piping containing defect at elevated temperature depends not only on the size of defect, but also on the creep time, which is different from the traditional plastic limit analysis at room temperature without material deterioration.

Abstract: In this paper, four types of creep damage models (Kachanov-Robotnov, Liu-Murakami, Cocks-Ashby and Wen-Tu model) were used to study the creep crack growth (CCG) behavior in compact tension (CT) specimen of Hastelloy C-276/BNi-2 brazed joint. The results show that the creep damage model has a great influence on the CCG behavior of brazed joint. The crack-tip stress states, da/dt-C* curves, crack initiation time and rupture life are different for the different damage models. The Kachanov-Rabotnov model can lead to higher CCG rate and shorter rupture life, while the Cocks and Ashby model can reduce CCG rate and prolong the rupture life. The model order in terms of the CCG rate from high to low is K-R, L-M, W-T, C-A model, which is opposite order of crack initiation time. In the simulation of CCG of brazed joint, a precious damage model should be employed for life prediction.