Takumi Tokiyoshi

Prediction of Type IV creep failure of a seam-welded mod. 9Cr-1Mo elbow based on microscopic damage simulation

Abstract Utilising the random-fracture-resistance model of grain boundaries, micro-macro combined creep damage simulation was applied to the prediction of the distribution of small defects in the FGHAZ (fine-grained heat-affected zone) of longitudinal welds in an actual-size elbow of modified 9Cr-1Mo (9Cr-1MoVNb) steel subject to internal pressure at 923 K. Based on the simulation results, a prediction scheme for the final rupture life of welds was considered using the damage mechanics concept together with effective stress. The applicability of nonlinear fracture mechanics was also discussed, assuming the initial crack length determined from the microscopic simulation results. The results thus obtained are summarized as follows: As the simulation results showed, the peaks of small defect density in the subsurface could be predicted, corresponding well with the observed results. Final failure life prediction based on the damage mechanics concept was found to be applicable, by considering both the final failure surface connecting the weakest grain boundaries and the effective stress against this surface. The fracture mechanics approach was also found applicable when assuming the initial crack length from the high peaks of the simulated small defects in the last stage of creep life.

Experimental observation of creep damage evolution in seam-welded elbows of mod. 9Cr-1Mo steel

Seam-welded elbows and straight pipe of the same size as actual high temperature reheat piping for boiler applications were manufactured, and internal pressure creep testing was conducted. Uniaxial creep testing was also performed in order to compare creep damage behaviours. Comparing the observed creep damage evolution and stress analysis results, a creep damage estimation method was discussed. Creep damage distribution varied depending on ovality in the cross sections of the seam-welded elbows. It is important that FEM models for creep analysis incorporate measurement results for the cross-sectional shape of seam-welded elbows. Using the nominal stress of the specimens and the average creep rupture data from small size uniaxial welded joint tests, a simplified prediction method was discussed, which is applicable to creep rupture time prediction for both large creep specimens and seam-welded elbows.

Development of LBB Assessment Method for Japan Sodium Cooled Fast Reactor (JSFR) Pipes (5): Crack Growth Assessment Method for Pipes Made of Mod.9Cr-1Mo Steel

For sodium pipes of Japan Sodium cooled Fast Reactor (JSFR), the continuous leak monitoring will be adopted as an alternative to a volumetric test of the weld joints under conditions that satisfy Leak-Before-Break (LBB). The vessels of JSFR are connected by thin wall pipes with a large diameter made of modified 9Cr-1Mo steel and the internal pressure of the pipes is very low. Modified 9Cr-1Mo steel has relatively large yield stress and small work hardening coefficient compared to the austenitic stainless steels which are currently used in the conventional plants. Therefore, these material characteristics of modified 9Cr-1Mo steel must be taken into account in LBB assessment, as well as geometrical and structural features of JSFR pipes. In order to demonstrate LBB aspects of the JSFR pipes, the authors have proposed a LBB assessment flowchart and developed assessment methods of unstable fracture and crack opening displacement (COD) for the thin wall pipes with large diameter made of modified 9Cr-1Mo steel. This paper studies the master curve to estimate the crack length when a postulated initial crack unexpectedly grows and penetrates the pipe thickness. In order to obtain the fatigue crack and creep crack growth characteristics of modified 9Cr-1Mo steel pipes, fatigue crack and creep crack growth tests were conducted using compact tension (CT) specimens and crack growth rates for both fatigue and creep at elevated temperature were obtained. Based on the obtained material characteristics and the results of a series of crack growth calculations, a relationship between the penetrated crack length and the ratio of membrane to total stress, so called as master curve, was proposed. In this study, master curves were proposed for pipes made of modified 9Cr-1Mo steel as a function of pipe geometry, i.e. the ratio of radius to thickness.Copyright

Effect of Repair Welding on the Creep Rupture Strength of an Aged Weldment

Weldments in high-energy piping of fossil power plants are known to suffer extensive creep damage over the course of long-term operations. Repair welding after the removal of the deteriorated areas is considered as a potential method for improving the life of such weldments. Very few papers, however, are reported concerning the influence on creep rupture behaviour of repair welding performed at the end of the creep life of the weldments. In this paper, creep test of large weld specimens was carried out to create substantial numbers of creep voids, and these specimens were then repair-welded using various methods. On the basis of the results of subsequent creep rupture tests and stress analyses of these test specimens, the influence of repair welding on the creep rupture strength of deteriorated weldments was discussed.

Description of Inelastic Behavior of Perforated Plates Based on Effective Stress Concept

High-temperature heat exchangers are well known as important components of high-temperature nuclear reactors such as High-Temperature Gas-Cooled Reactors and Fast Breeder Reactors. Perforated plates in heat exchanges are one of the critical parts from a viewpoint of creep-fatigue damage.

Creep rupture behaviour of circumferentially welded mod. 9Cr–1Mo steel pipe subject to internal pressure and axial load

The multi-axial creep strength of circumferential welds in power piping, including failure mode and failure life against a wide range of stress ratios and stress levels, was newly examined in this paper. The creep rupture behaviour of modified 9Cr–1Mo steel (9Cr–1Mo–VNb steel; ASME P91) pipe with a circumferential weld subject to combined internal pressure and axial load was experimentally investigated at 650 °C. The test results, with several kinds of stress ratios of macroscopic axial and hoop stress can be summarised as follows. Along with the increased stress ratio (axial/hoop), both failure location and failure mode changed from a base metal failure caused by hoop stress to an FGHAZ (fine-grained heat affected zone) failure due to axial stress. The stress ratio where the failure mode changed was ‘0.8’. Strength reduction in FGHAZ failure by axial stress should be considered in the structural design of circumferential welds. A series of FEM creep analysis was carried out to discuss the relation of the failure mode to both the local stress distribution and the damage. The failure mode variation along with the increase in the stress ratio is also discussed based on simplified ductile creep failure analysis.

Estimation of Fracture Mechanics Parameters Under Displacement-Controlled Elastic Plastic Fatigue Conditions

Accurate prediction of the residual life of components under long-term service is urgently needed due to requirements for the reduction of the maintenance costs of thermal power plants. Quantitative prediction of crack growth under displacement-controlled conditions such as thermal fatigue is a key to the fulfillment of this need. In a previous paper, the present authors proposed a simplified prediction method of fracture mechanics parameters such as J and C* of a perforated plate under thermal fatigue, on the basis of the reference stress approach under displacement-controlled conditions. In this paper, the fracture mechanics parameters of a CCT specimen and structural models of CrMoV cast steel under displacement-controlled conditions were numerically and experimenttally examined on the basis of the proposed method. The ratio of the elastic and the inelastic crack opening displacement, as well as the ratio of the elastic and the inelastic load point displacement, was used to correlate the fracture mechanics parameters in the elastic and inelastic region. As a result of the study, the proposed method based on displacement was found to be insensitive to small variation in the predicted results of macroscopic load in comparison with the original reference stress method, and is considered to be applicable to displacement-controlled conditions such as thermal fatigue.Copyright