International Journal of Pressure Vessels and Piping
Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel
Introduction
Mod.9Cr–1Mo steel has been widely used for high temperature structures such as boiler components in ultra-supercritical (USC) thermal power plants operating at about 600 °C. It is also a candidate material for a steam generator, intermediate heat exchanger and secondary piping of a liquid metal reactor, and a pressure vessel and some internal structures of gas cooled reactors due to its high creep strength. However, it is found that the creep properties of the welded joints are inferior to those of the base metal and weld metal due to the Type IV damages forming in the fine-grained HAZ paralleling the fusion line at high temperature [1], [2]. Scientists and researchers have been investigating the mechanisms of Type IV fracture both from the microstructural approaches and mechanical approaches [3], [4], [5]. However, mechanisms of Type IV fracture have not been fully understood. The complicated stress state distributed in the welded joint affects the formation and growth of creep voids prior to the crack growth in fine-grained HAZ, which occupies a large fraction of creep life.
During the past years, several creep damage models were proposed in order to describe the nonlinear creep behavior of initially isotropic solids at high temperatures. After Kachanov first devised the continuum damage mechanics to evaluate the creep life of a structure in 1958 [6], the damage mechanics have been applied for predicting creep damage and lives of components under complicated stress conditions [7]. Hyde et al. predicted the creep damage distributions and crack growth life of circumferential pipe welds by damage mechanics using creep data of parent, weld and HAZ of new and serviced materials [5]. Eggeler et al. investigated the effect of hoop, axial and radial stress distributions of a P91 pressure vessel with welds using the creep data of base metal, weld metal and simulated inter-critical HAZ [4]. Bauer et al. analyzed the stress–strain situations and multiaxial stress state in various pipe models including welds for predicting creep life, and clarified the effect of creep strength of weld metal [8].
The experimental creep damage distributions and evolutions of high Cr steel welds, however, were scarcely investigated, and the comparison between experimental creep damage evolutions and numerical analysis was scarcely conducted. In the present paper, we have investigated the formation and growth processes of Type IV creep damage (voids and cracks) quantitatively using the large scale thick welded joints with double U grooves of Mod.9Cr–1Mo steel. The failure mechanism is discussed, comparing the experimental results with the stress–strain distributions and stress triaxial condition in the welded joint calculated by finite element method (FEM).
Section snippets
Creep tests
The material investigated in the present study is a Mod.9Cr–1Mo steel plate of 25 mm in thickness. The chemical compositions of base metal and the welding filler are given in Table 1. The plates were welded by using gas tungsten arc welding (GTAW) method with double U grooves. The post weld heat treatment (PWHT) adopted was to keep temperature at 745 °C for 1 h. The simulated fine-grained HAZ specimens were produced by rapid heating (60 °C/s) to the peak temperature 900 °C and followed by the gas
Creep properties of welded joint, base metal and simulated HAZ
The creep test results for the welded joints, simulated fine-grained HAZ (FG-HAZ) and base metal (BM) of the Mod.9Cr–1Mo steel at 550, 600 and 650 °C are shown in Fig. 6. The failure location of welded joint is shown with subscript in this figure. The creep rupture times of the simulated fine-grained HAZ were smaller more than one order than those of base metal at all temperatures. At 550 °C, the fracture location of welded joint was base metal for the applied stress of 240 and 220 MPa, and it
Discussion
From Fig. 13, comparing the results of numerical analysis with the creep voids measurement, the stress triaxial factor is reasonable to explain why the fine-grained HAZ is the most damaged zone in the whole welded joint, because it accelerates the creep void formation and growth. However, the distributions of maximum principal stress and stress triaxial factor in the fine-grained HAZ shown in Fig. 14, Fig. 15 are not thoroughly consistent with the experimental void distributions of Fig. 8,
Conclusions
In the present paper, damage processes in the fine-grained HAZ of thick welded joint of Mod.9Cr–1Mo steel during creep were investigated. Stress–strain distributions in the welded joint were computed by FEM analysis with a three-dimensional, three-material model (base metal, simulated fine-grained HAZ and weld metal). The results can be summarized as follows.
- (1)
It is found that the creep voids of Mod. 9Cr–1Mo steel weld form at the early stage of creep rupture life (0.2 of life), the number of
Acknowledgements
The present study includes the result of “Development of damage prevention technology for welded structures in the next-generation high temperature nuclear plant” entrusted to Central Research Institute of Electric Power Industry by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).
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