Micro-Macro Combined Simulation of the Damage Progress in Low-Alloy Steel Welds Subject to Type IV Creep Failure

Abstract

A simulation method for microscopic creep damage at grain boundaries in the fine-grain heat-affected zone of low-alloy steel welds involving high energy piping was proposed on the basis of the combination of elastic-creep FEM analysis and random fracture resistance modeling of the materials. First, the initiation and growth-driving forces of small defects were concretely determined based on microscopic observation of the damage progress at the grain boundaries of the material, taking into account dependence on stress and temperature. Then, a simulation procedure combining the stress distribution from elastic-creep FEM and the random fracture resistance model was proposed, and this procedure was applied to the simulation of the microscopic damage progress in a welded joint model test and in actual power piping. The results in terms of the simulated number density of small defects throughout the wall thickness were in good agreement with the observed results.

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

“…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.…”

Review of Creep Cavitation and Rupture of Low Cr Alloy and its Weldment

“…Furthermore, Kawashima et al [20] reported that for 2.25%Cr1%Mo steel the creep ruptures lifetime depends on the cavity nucleation rate and cavity growth size. Creep damage accumulates due to the initiation and growth of extensive cavitation at the prior austenite grain boundaries [20]. Cavity formation predominates during the initial transient and individual cavities appear to nucleate on grain boundary carbides [20].…”

Micro–macro creep damage simulation for welded joints

“…The results of parameter identification for FGHAZ are summarized as follows: the distribution of the grain boundary length [L] was determined as the normal distribution with a mean value of 10.5 mm and a standard deviation of 5 mm on the basis of the microscopic observation of the FGHAZ; the distribution of the initial value of fracture resistance [R 0 ] with a mean value of 0.5 and a standard deviation of 0.1607 was determined as shown in Figure 3(a) on the basis of the time history of the number density of small defects at a specific location of FGHAZ in the creep test of a welded joint; the stress dependence of F shown in Figure 3(b) was determined through a combination of the distribution [R 0 ] and the microscopic observation of a point with a prescribed stress; the parameter K is given in the literature [2,17].…”

“…(1) Creep damage simulation [2,12] of 40 mm-thick welded joint subject to tensile stress of 39.2 MPa at 898 K.…”

“…Damage in the coarse-grain HAZ (Type III) and that in the fine-grain HAZ (Type IV) compose the main focus for P-22, with the latter Type IV damage being more important since the maximum damage occurs not on the surface, but at the subsurface of the piping. Figure 1 [2] illustrates the relation between the number density of small defects and the creeprupture time fraction together with microscopic damage progress in the fine-grain HAZ (FGHAZ) at prescribed creep-rupture time fractions of 50%, 60% and 75%. In the Type IV damage mode, small defects with an average size of 10.5 mm (the same size as the grain) initiate and coalesce together.…”