[en] The small punch (SP) testing method has been used to assess the reliability of industrial facilities such as fusion reactor structures and power-generation systems. Conventional SP tests have evaluated flow properties, transition temperature, fracture strain, and other mechanical properties by analyzing load-deflection curves. However, previous research has not used a fracture-mechanics approach that considers flaws, stress analysis and fracture toughness. In this study, in order to obtain the fracture toughness based on fracture mechanics by SP tests, a sharp notch was machined into the central part of the SP specimens. A stress-intensity factor for sharp-notched SP specimens is proposed from analysis of stress fields near the crack tip. The crack initiation point was determined by analysis of the load-deflection curves combined with acoustic emission signals. The fracture toughness of SA 508 Class 3 steel was successfully evaluated by using the load at the crack initiation point

[en] This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite-austenite (M-A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed

[en] Small punch test is used to assess the reliability of industrial facilities such as fusion reactor structures and power generation systems. Conventional small punch tests evaluate transition temperature and fracture strain by analyzing the load-deflection curves and deformation behaviors, respectively. However, previous research did not consider fracture mechanical concepts such as flaw, stress analysis and fracture toughness. In this study, to obtain more reliable fracture characteristics based on linear elastic fracture mechanics in small punch tests, a pre-crack was introduced to small punch specimen. Stress and deformation behaviors near the crack tip were analyzed and the stress intensity factor was derived. Using the load at crack initiation point, the fracture toughness of SA 508 C1.3 steel was successfully evaluated

[en] Failure Assessment Diagram (FAD) which includes all failure mode from linear elastic fracture to plastic collapse was used for integrity and Fitness-For-Service (FFS) assessment of natural gas pipeline. Since tensile properties and fracture toughness of base metal, weld metal and Heat Affected Zone (HAZ) are required for using FAD, tensile test and fracture toughness test of each region were performed. Especially, the evaluation of representative properties of HAZ enables to assess FFS accurately by using HAZ-based FAD when crack-like flaws exist in HAZ. This HAZ-based FAD has originality in comparison with current codes, which assess integrity by using material properties of weld metal or base metal when flaws exist in HAZ

[en] Highlights: ► Fracture resistance of API X65 steel weld HAZ was systematically investigated. ► Variations in DBTT and CTOD toughness within HAZ were explored. ► Results were discussed with the additional data from simulated HAZ specimen tests. - Abstract: The fracture resistance of heat-affected zones (HAZs) in girth welded joint of API X65 steel pipeline was systematically investigated. While the change in Charpy impact energy has been typically evaluated in previous studies, here the variations in ductile-to-brittle temperature (DBTT) and crack-tip opening displacement (CTOD) within HAZ were explored. A series of experiments revealed that both values vary dramatically (i.e., DBTT increases and CTOD decreases) as the location approaches the fusion line (FL) and thus the region adjacent to FL exhibited the lowest CTOD and highest DBTT, possibly due to the increasing portion of coarse-grained HAZ. Interestingly, however, even the FL regions still showed moderate toughness at −40 °C ∼ room temperature. Microstructural analysis and additional impact tests using simulated HAZ specimens suggested a possibility that fine-grained HAZs with higher toughness may suppress the brittle fracture from neighboring coarse-grained region.