Creep Damage Evaluation of Fine-Grained HAZ in Mod. 9Cr Ferritic Heat-Resistant Steel Weldments

9Cr steel weldments are concerned with evaluation of creep life time and creep rupture mechanism. In fine grain HAZ (FG-HAZ) of weldments, TYPE IV cracking and creep voids occurred at lower stress than rupture stress level of base metal. In the crept specimen, FG-HAZ sometime has large coarsening grains near creep voids. These recovery phenomena are localized in FG-HAZ, and recovered microstructures are dependent on heat input of welding. In this study, creep tests are examined in two types of weldments, and relations between creep life time and coarsened sub-grains or grains have been studied by microstructural changing with EBSP analysis. In crept specimens, boundaries are moved and boundary density is decreasing in the fine-grained HAZ. Maximum grain size and creep life time have linear function, and EBSP can evaluate creep life time of 9Cr weldments. These microstructural changing are considered by morphology of precipitates in the several crept specimens.

Numerical Analysis of the Effect of Diffusion and Creep Flow on Cavity Growth

In this paper, intergranular cavity growth in regimes, where both surface diffusion and deformation enhanced grain boundary diffusion are important, is studied. In order to continuously simulate the cavity shape evolution and cavity growth rate, a fully-coupled numerical method is proposed. Based on the fully-coupled numerical method, a gradual cavity shape change is predicted and this leads to an adverse effect on the cavity growth rates. As the portion of the cavity volume growth due to jacking and viscoplastic deformation in the total cavity volume growth increases, the initially spherical cavity evolves to V-shaped cavity. The numerical results are physically more realistic compared to results in the previous studies. The present numerical results suggest that the cavity shape evolution and cavity growth rate based on an assumed cavity shape, whether spherical or crack-like, cannot be used in this regime due to transitional coupled growth mechanisms.

Partitioned Cyclic Fatigue Damage Evolution Model for PB-Free Solder Materials

This study presents an approach to predict the degree of material degradation and the resulting changes in constitutive properties during cyclic loading in viscoplastic materials in micro-scale applications. The objective in the modeling approach is to address the initiation and growth of distributed micro-damage, in the form of micro-cracks and micro-voids, as a result of cyclic, plastic and creep deformations of material. This study extends an existing micromechanics-based approach, developed for unified viscoplastic models [Wen, et al, 2001], which uses dislocation mechanics to predict damage due to distributed micro-scale fatigue crack initiation [Mura and Nakasone, 1990]. In the present study, the approach is extended to a partitioned viscoplastic framework, because the micro-scale mechanisms of deformation and damage are different for plastic and creep deformation. In this approach, the model constants for estimating cyclic damage evolution are allowed to be different for creep and plastic deformations. A partitioned viscoplastic constitutive model is coupled with an energy partitioning (E-P) damage model [Oyan and Dasgupta, 1992] to assess fatigue damage evolution due to cyclic elastic, plastic and creep deformations. Wen’s damage evolution model is extended to include damage evolution due to both plastic and creep deformations. The resulting progressive degradation of elastic, plastic and creep constitutive properties are continuously assessed and updated. The approach is implemented on a viscoplastic Pb-free solder. Dominant deformation modes in this material are dislocation slip for plasticity and diffusion-assisted dislocation climb/glide for creep. The material’s behavior shows a good correlation with the proposed damage evolution model. Damage evolution constants for plastic and creep deformation were obtained for this Pb-free solder from load drop data collected from the mechanical cycling experiments at different temperatures. The amount of cyclic damage is evaluated and compared with experiment.

Creep Deformation Behavior and Microstructural Degradation During Creep of Pre-Strained 25Cr-20Ni-Nb-N Steel

The purpose of this study is to clarify the creep deformation behavior and microstructural degradation during creep of pre-strained 25Cr-20Ni-Nb-N steel (TP310HCbN), which has the highest creep strength among austenite stainless steels used for boiler tubes. The creep rupture strengths of the 20% pre-strained materials tested at 650°C under 210 MPa and 180 MPa were higher than those of solution-treated materials. However, the long time creep rupture strengths of the 20% pre-strained materials tested at 700°C and 750°C were lower than those of solution-treated materials. Thus, the creep strengths of the prestrained materials depend on test temperature and stress. Furthermore, the minimum creep rate of the 20% pre-strained materials and re-solution-treated materials tested at 650°C under 300MPa were 1.2 × 10−9 and 1.6 × 10−8 s−1, respectively. Thus, the minimum creep rate of the 20% pre-strained materials was lower than for re-solution-treated materials. The creep strengthening mechanism of the pre-strained materials at 650°C was considered to be that high-density dislocations were maintained until the late stage of creep. On the other hand, the creep rupture strengths of the 20% pre-strained materials were lower than those of solution-treated materials tested at over 700°C because of agglomeration and coarsening of precipitates and the recovery of dislocations.

Creep-Fatigue Damage and Crack Initiation for a Mod 9Cr-1Mo Structure With Weldments

A structural test and evaluation on creep-fatigue damage, and creep-fatigue crack initiation have been carried out for a Mod. 9Cr-1Mo steel structural specimen with weldments. The conservatisms of the design codes of ASME Section III subsection and NH and RCC-MR codes were quantified at the welded joints of Mod.9Cr-1Mo steel and 316L stainless steel with the observed images from the structural test. In creep damage evaluation using the RCC-MR code, isochronous curve has been used rather than directly using the creep law as the RCC-MR specifies. A y-shaped steel specimen of a diameter 500mm, height 440mm and thickness 6.35mm is subjected to creep-fatigue loads with two hours of a hold time at 600°C and a primary nominal stress of 30MPa. The defect assessment procedures of RCC-MR A16 guide do not provide a procedure for Mod.9Cr-1Mo steel yet. In this study application of σd method for the assessment of creep-fatigue crack initiation has been examined for a Mod. 9Cr-1Mo steel structure.

Determination and Modeling of the Creep Propagation on High Chromium Steel

Modified 9Cr-1Mo steel (T91) is a candidate material for pressure vessels and for some internal structures of GCR (Gas Cooled Reactors). In order to validate this choice, it is necessary, to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr-1Mo steel has been undertaken at CEA in order to characterize the behavior of this material and of its welded junctions. In this program, the role of the Laboratory for structural Integrity and Standards (LISN) is to develop high temperature defect assessment procedures under fatigue, creep and creep-fatigue loadings, to validate the existing methods (developed on austenitic stainless steels as 316L(N) for the fast reactors) and to get new experimental data on Mod 9Cr-1Mo steel. This paper presents the experimental program undertaken to develop defect assessment under creep loading and describes the main results obtained. Then a creep propagation law is proposed for the Mod 9Cr-1Mo steel at 550°C. To validate the experimental interpretation, a numerical analysis with a 3D finite element model is proposed and allows to model the propagation of the crack. Finally, a comparison of the experimental and the numerical data and in particular of the C* value is investigated.

Characteristic of Crack Growth Life for W Added 9Cr Ferritic Heat Resistant Steel Under the Conditions of High Temperature Creep-Fatigue Multiplication Based on Non-Equilibrium Science

The W added 9Cr ferritic heat resistant steel ASME grade P92, developed as a boiler tube material, is used under the conditions of creep-fatigue multiplication. In this paper, using P92 steel, crack growth tests under the conditions of creep-fatigue multiplication were conducted and the effects of cycle-dependent and time-dependent mechanisms on the crack growth life tf were investigated. Furthermore, on the basis of the concept of non-equilibrium science, the multiple effects of creep and fatigue on the crack growth life tf were clarified.

Void Growth Simulation of a Steam Turbine Casing Material Under Creep and Creep-Fatigue Loading

High temperature components in thermal power plants are subjected to creep and creep-fatigue loading where creep voids initiate and grow on grain boundaries. Development of a quantitative evaluation method of the void growth is important for reliable maintenance of these components. In this study, creep and creep-fatigue tests were carried out at 600 °C on a 1Cr-Mo-V casting steel. Creep damaged materials were produced by interrupting the creep tests and microstructure of the damaged materials were observed carefully by a scanning microscope. The creep-fatigue tests were also conducted in a scanning electron microscope, and continuous observation of void growth behavior during the tests was made. From the observations, spherical shape voids initiate and grow up to their length of 2μm on grain boundaries at initial stage of damage, and then these voids change their shape to crack-like to grow until their length reaches around 10μm under both the creep and the creep-fatigue conditions. Under the same stress level, the void growth rate in the creep-fatigue condition was faster than that in the creep condition indicating acceleration of void growth rate by cyclic loading. Previously proposed void growth simulation model, in which the void growth was controlled by diffusion and power law creep, was modified to express acceleration of the void growth by the cyclic loading. Void growth behavior within a certain area under both the creep and the creep-fatigue condition were simulated by the modified program. Predicted void growth behaviors agreed with observed ones. The void growth behavior of an actual turbine casing was also simulated and void growth behavior was discussed based on the result.

Regulatory Review Results on Allowable Tensile Stress Values of Creep Strength Enhanced Ferritic Steels

In order to evaluate long-term creep strength and to review current allowable tensile stresses of creep strength enhanced ferritic steels, a committee was organized in Japan Power Engineering and Inspection Corporation. In 2004FY and 2005FY, creep test data of Gr. 122, Gr. 91, Gr. 92, Gr. 23 and KA-SUS410J2TB steels were collected and analyzed by means of region splitting procedure in the committee. Based on the analysis, the allowable tensile stresses were reviewed in accordance with METI regulatory base. And the master curves for creep rupture life evaluation of welds were set forth furthermore based on the data analysis.

An Anisotropic Creep Damage Model for Anisotropic Weld Metal

Past studies from creep tests on uniaxial specimens and Bridgman notch specimens, for a P91 weld metal, showed that anisotropic behaviour (more specifically transverse isotropy) occurs in the weld metal, both in terms of creep (steady-state) strain rate behaviour and rupture times (viz. damage evolution). This paper describes the development of a finite element (FE) continuum damage mechanics methodology to deal with anisotropic creep and anisotropic damage for weld metal. The method employs a second order damage tensor following the work of Murakami and Ohno [1] along with a novel rupture stress approach to define the evolution of this tensor, taking advantage of the transverse isotropic nature of the weld metal, to achieve a reduction in the number of material constants required from test data (and hence tests) to define the damage evolution. Hill’s anisotropy potential theory is employed to model the secondary creep. The theoretical model is implemented in a material behaviour subroutine within the general-purpose, non-linear FE code ABAQUS [2]. The validation of the implementation against established isotropic continuum damage mechanics solutions for the isotropic case is described. A procedure for calibrating the multiaxial damage constants from notched bar test data is described for multiaxial implementations. Also described is a study on the effect of uniaxial specimen orientation on anisotropic damage evolution.