Microscopic Simulation and Life Prediction of High Cr Steel Welds Subject to Type IV Creep Damage

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

“…According to the grain-boundary-resistance modeling simulation results by Fukahori et al (2013Fukahori et al ( , 2017, micro-cracks initiate by the coalescence of creep cavities when the creep cavity density reaches 4,000/mm 2 . Based on this result, the critical value of creep damage variable D cr is defined as below:…”

“…Before simulating the multi-pass welding process, the test material was normalized (1,050℃×30min/25mm thickness) and tempered (780℃×30min/25mm thickness). Although the heat affected zones of welded joints are generally made up of CGHAZ (coarse-grained heat affected zone), FGHAZ and inter-critical HAZ depending on the thermal history during the welding process, a simplified three-material model incorporating weld metal, FGHAZ and base metal is often adopted in the creep analysis of welded joints (Fukahori et al, 2013). This is based on the following assumptions: the creep deformation properties of the CGHAZ with small width (in the case of P91) are equivalent to those of weld metal; and the properties of the inter-critical HAZ are equivalent to those of base metal.…”

“…According to the microscopic observation of cavity initiation in P91 FGHAZ by Masuyama et al (2013), small sub-micron cavities initiate at triple points or precipitates at the boundaries of FGHAZ grains; these cavities grow and link to completely surround a grain and thus create cavities with the size of the FGHAZ grains, which subsequently coalesce with neighboring grain-size cavities, leading to the formation of microcracks. The number density of grain-size cavities (cavity density, for brevity) is usually used as a measure of creep damage in P91 welded joints (Fukahori et al, 2013, Komai, et al, 2016. Nondestructive inspection of the Takashi HONDA*, Takuya FUKAHORI*, Toshihide IGARI*, Yasuharu CHUMAN*, Takumi TOKIYOSHI* and Alan CF COCKS** surface and subsurface of pipe welds is carried out to examine the current damage, such as the cavity density, and the results are employed to guide simulation-based predictions.…”

“…For the cavity nucleation model, the proposal by Gonzalez and Cocks (2014) is adopted, which considers the randomness of grain-boundary-facet orientations in a polycrystalline material and provides a damage growth criterion in terms of the creep strain rate and tri-axiality factor. A critical value of the damage parameter, corresponding to the initiation of micro cracks by the coalescence of creep cavities, is determined from the grain-boundary-resistance modeling simulation results (Fukahori et al, 2013. Before applying this method to a welded joint with tri-axial constraint and a hardness distribution in the FGHAZ, it is better to confirm its applicability to the simple condition of a simulated FGHAZ notched bar specimen with a uniform hardness and a tri-axial constraint due to the notch.…”

“…Nondestructive inspection of the Takashi HONDA*, Takuya FUKAHORI*, Toshihide IGARI*, Yasuharu CHUMAN*, Takumi TOKIYOSHI* and Alan CF COCKS** surface and subsurface of pipe welds is carried out to examine the current damage, such as the cavity density, and the results are employed to guide simulation-based predictions. Several approaches can be taken in terms of damage simulation for Type IV failure, such as: continuum mechanics together with the stress-based time fraction or the ductility fraction of the material; creep fracture mechanics using the C* parameter (Yatomi et al, 2003, Segle et al, 2000; creep cavitation mechanics assuming diffusion (Rice, 1981); use of a grain boundary-resistance model (Fukahori, et al, 2013; and/or damage mechanics (Hyde et al, 2003, Yao et al, 2007. It remains difficult, however, to determine an optimal method for expressing both the microscopic and macroscopic phenomena of actual Type IV damage.…”