Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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specimen size effect represents a potentially dangerous situation when using fracture toughness data from small specimens to design a structure requiring larger material thickness than the specimen size tested. In the transition regime, a crack in a small specimen may exhibit stable ductile tearing, while a large specimen could fail by brittle fracture and have much lower toughness properties than predicted by the small-scale test as shown in Fig. 4. In addition, tests with unstable crack growth, such as a pop-in event, tend to be most sensitive to specimen thickness. Most steels used in heavy structural applications have section thicknesses insufficient to maintain plane strain conditions at test temperatures corresponding to service conditions. Test specifications recommend the test specimen to be the full thickness of the material to be used in service, less the minimum amount of machining to produce the specimen. By testing specimen geometry near full thickness, the measured toughness is representative of inservice conditions. Weld and HAZ effects In addition to the factors discussed above that may influence fracture toughness as measured in laboratory test specimens, microstructural factors must be considered when performing toughness tests on the weld HAZs of ferritic materials. Fabrication of welds in steel can lead to the development of small, localized, and scattered regions of low toughness in the HAZ called local brittle zones (LBZs). These are discrete microstructural regions in the HAZ that exhibit lower resistance to fracture initiation than the surrounding HAZ or weld material. LBZs can often produce fracture initiation under near linear elastic conditions during fracture toughness testing. For example, in terms of CTOD, fracture toughness can be of the order of 0.01-0.03 mm in the LBZ when the adjacent material can have a Pass 1 Pass 2 Eliminated HAZ Altered HAZ Pass 1 Unaltered HAZ Pass 2 A3 isotherm A1 isotherm Intercritically reheated CGHAZ (IRCG) Subcritically reheated CGHAZ (SRCG) Unaltered CGHAZ, Pass 1 Note: Shaded region indicates the etched HAZ. Unaltered CGHAZ, Pass 2 Unaltered FGHAZ, Pass 2 Unaltered ICHAZ, Pass 2 Unaltered SCHAZ, Pass 2 Fig. 6 — HAZ microstructural regions in a multi-pass weld[12]. 16 ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 toughness of greater than 0.4 mm at the same test temperature. These effects are typically more pronounced at test temperatures in the ductile-to-brittle transition regime and in situations where specimen size can maximize constraint at the crack tip in test specimens. Figure 6, taken from API RP-2Z[12], illustrates the HAZ microstructure generated during multipass welding with conventional arc welding processes such as submerged arc welding, flux-cored arc welding, or gas metal arc welding. LBZs are often found in the intercritically and subcritically reheated HAZ zones and are the result of reheating a zone of unaltered, coarse-grained HAZ with subsequent weld passes. Note that the LBZs are located very close to the weld fusion line, typically within 0.5 mm, although this depends on the welding heat input level and other welding process parameters. LBZs will not be encountered when testing other HAZ regions or when performing base material fracture toughness tests. Theoretical and experimental work demonstrates that the likelihood of an LBZ to promote low toughness is related to LBZ length present along the test specimen crack tip. Although a test specimen may contain LBZs, the fatigue crack tip may not intersect an LBZ of sufficient length or in a position of high constraint (i.e., the LBZ might be located near one of the specimen's free edges where low constraint will occur) and consequently have little influence on the resulting measured toughness value. In addition, if the material adjacent to and ahead of the LBZ has sufficient fracture toughness to arrest a brittle fracture initiating from an LBZ, the risk of brittle fracture in the larger structure will be measurably reduced. Specimen aspect ratio effect Another factor that may influence measured toughness values is the specimen geometry in terms of cross-section dimensions. The difference between (B2B) and (BB) SENB specimen geometries has less effect on the measured fracture toughness than specimen thickness or a/W effects. Here, B refers to material thickness. Numerous studies have assessed the effect of specimen aspect ratio and most of these investigations consider the SENB geometry. A summary of some of those studies follows. Dawes[9] reported that distinguishing between test results from (B2B) and (BB) geometries in the transition regime when using an a/W ratio of 0.5 is difficult. Sorem[13] reported increased scatter in CTOD results using the (BB) specimen geometry compared to (B2B) specimen geometry. Sorem[13] also indicated that the lower bound CTOD results from both (B2B) and (BB) geometries were similar if a sufficient number of samples were tested. However, the average CTOD result was higher for the (BB) geometry compared to (B2B) geometry. The effect of the specimen aspect ratio on the toughness properties of weld regions in the transition regime was reported by Machida[14]. The (B2B) geometry produced lower toughness results compared to the (BB) specimen geometry in the transition regime. Machida determined that the lower test results from the (B2B) geom-

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