Micro–macro creep damage simulation for welded joints

Abstract

Assessment of Type IV creep damage in pipe welds is important for residual life prediction of fossil power plants. Actual creep damage is firstly microscopic, such as the initiation and coalescence of small defects, and lastly macroscopic, such as the propagation of crack-like defects. In this paper, an outline of the micro -macro combined creep damage simulation on the basis of the grain-boundaryfracture-resistance model is shown, and is applied to the creep damage simulation of both low-alloy steel welds and high-chromium steel welds. Firstly modelling of the simulation for low-alloy steel welds such as 2.25Cr -1Mo steel (ASME P-22) is discussed, and application examples are shown such as creep rupture specimens in the laboratory and welds in actual power piping. Secondly a trial is carried out to reproduce the number density of small defects in the heat-affected zone of welded joints of modified 9Cr -1Mo steel (ASME P-91). The possibility of predicting the microscopic damage is demonstrated. Figure 13 Comparison between observed and simulated results for number density of small defects (P-91).

Creep damage analysis of simulated-HAZ notched bar specimens of modified 9Cr-1Mo steel

“…It remains difficult, however, to determine an optimal method for expressing both the microscopic and macroscopic phenomena of actual Type IV damage. While the grain-boundary-fracture-resistance model, which describes the initiation, growth and coalescence of creep cavities leading to macro-crack formation (Kawashima et al 2003, Igari et al, 2011, provides mechanism-based simulation results for cavity density, this cannot be easily extended to general structures which experience multiaxial stress states. A damage mechanics approach is appropriate for general structures, but the relation between the damage parameter and cavity density is not clear.…”

“…Type IV creep damage in the FGHAZ (fine-grained heat affected zone) consists of creep cavities found in the subsurface of the piping. The mechanism of damage development is considered as follows from the perspective of continuum mechanics (Kawashima et al, 2004, Igari et al, 2011. The creep strain rate of the FGHAZ is faster than that of either the neighboring weld metal or base metal, and this can cause a tri-axial constraint in the FGHAZ depending on the difference in the creep strain rate.…”

Type IV creep crack initiation and propagation in mod.9Cr-1Mo steel welds

“…However, the correspondence between the damage variable and the number density of creep voids (creep void density in brevity), i.e., the physical meaning of the damage variable, has not been sufficiently examined. Fukahori et al [12,13] carried out damage simulations of large-uniaxial cross welds using a random-fracture-resistance model of grain boundaries (random-fracture-resistance model in brevity), which can express initiation and coalescence of defects (voids) of the order of the grain size. Creep void density distributions in the large-uniaxial cross welds was predicted with the help of stress distributions determined from FE creep analyses, and final rupture life was also predicted using a damage mechanics concept.…”

A Creep Damage Model for High-Temperature Deformation and Failure of 9Cr-1Mo Steel Weldments

“…Cavities usually nucleate at triple junctions or at the particles located on grain boundaries. Grain deformation and cavity coalescence are the main crack formation mechanism in 9Cr-1Mo [3,[11][12][13].…”