It is well known that liquid metal embrittlement (LME) cracking can occur during the resistance spot welding (RSW) of zinc-coated high-strength steels. These cracks can be classified into two types, based on location, namely outer cracks inside or outside the electrode indentation region, and inner cracks around the weld area. Recent research has proposed that the latter cracks can be inhibited by controlling weld schedules, though there have been a few reports concerning especially the effects of these cracks on joint properties. In this study, an X-ray CT scan and a tensile-shear test were carried out consecutively with 10 mm-wide RSW joints in order to visualize the cracks in 3 dimensions, and to clarify their effects on tensile-shear strength (TSS). The cracks in the RSW joints of the zinc-coated ultra-high strength steel could be clearly observed by the scan. The results showed that the TSS decreased to 40% by the small interface cracks even with the maximum depth of 170 μm.

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This study aims to guide a method for assessment of brittle fracture performance of a multi-planar welded pipe structure that will be adopted as substructure of an extremely large-scale infrared ray telescope (Thirty Meter Telescope, TMT). In this structure, the brittle fracture from narrow part of welds between pipes that include a crack like welding defect can most likely occur under large-scale seismic load at a minimum service temperature (around －15°C). The tensile fracture testing for a “narrow-weld joint specimen” that has a crack in coarse-grained heat affected zone (CGHAZ) of final welding pass is conducted, and the brittle fracture occurred from the CGHAZ at a temperature 40°C lower than the service temperature. This test temperature is employed assuming a fracture toughness deterioration due to the large scale seismic loading. The fracture load is predicted based on the Weibull stress concept, where the Critical Weibull stress distribution of the CGHAZ is identified from testing and FE-analysis conducted for standard fracture toughness specimens at －30°C. Predicted results taking ductile crack growth into account show a good agreement with the experimental results. This result implies that the Weibull stress concept can be applicable for brittle fracture assessment of the “narrow-weld joint specimen” where a notch is located in the CGHAZ irrespective of temperature as long as brittle fracture occurs. Namely, it is expected that the Weibull stress concept is applied for fracture assessment of a multi-layer welded pipe structure subjected to large-scale seismic loading.

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High-strength steels (HSSs) have been increasingly used in car bodies in order to simultaneously achieve the weight reduction and high collision safety of vehicles. In resistance spot welding, which is widely used for joining car bodies, low cross tension strength (CTS) of joints using HSSs is a problem. In our previous study, we focused on FEW (friction element welding) of HSSs to improve CTS and reported that the poor local ductility of the area quenched from two-phase temperature region (inter-critically annealed and quenched area) due to the presence of soft ferrite (α) and hard martensite (M) causes a decrease in CTS of FEW. The local ductility of steels with dual phase structure is known to be affected by the microstructural morphology. Therefore, in this study, we prepared joints of FEW with both the equiaxial α (C30) and the acicular α (C30L) in dual phase structure by controlling parent structures prior to the inter-critical annealing and quenching during FEW. C30L exhibited the higher CTS than C30. C30L fractured along the thickness direction of the lower sheet from the inter-critically annealed and quenched area near the edge of the joint to the softened area of heat affected zone, while C30 fractured along the inter-critically annealed and quenched area; however, the dimples were observed on the fracture surfaces irrespective of materials. Taking into account the fact that C30 and C30L showed no difference in the hardness distribution of the joints and the hardness difference between α and M hardness, it was suggested that morphology of α plays an important role in determining CTS. It is inferred that the local strain concentration in the inter-critically annealed and quenched area is relaxed by the presence of fine acicular α, which leads to the improvement of the local ductility, i.e. CTS.

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The effects of carbon and chromium on the block size of acicular ferrite (AF) were investigated, focusing on cementite formation. Block size of the AF refined with increasing carbon content and coarsened with increasing chromium content. The effects of carbon and chromium on cementite size were inverse. The block size did not necessarily become finer with increasing yield stress of the γ-phase and driving force of the γ→α phase transformation. On the other hand, the block size became finer regardless of the carbon and the chromium content when the sympathetic/ autocatalytic nucleation frequency increased and the cementite size increased. This suggests that carbon depleted zone increased with cementite formation and promoted the sympathetic/ autocatalytic nucleation. The effect of the cementite formation on the number of the sympathetic/ autocatalytic nucleation sites were quantified.

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