Why is the sample taken horizontally? What is the indicator for low-temperature impact energy?

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Why are 20R steel with a nominal thickness δn greater than 25 mm, and 16MnR, 15MnVR with δn greater than 38 mm, as well as 18MnMoNbR, 13MnNiMoNbR and Cr-Mo steel plates of any thickness, subjected to Charpy (V-notch) low-temperature impact testing when the design temperature is below 0°C? Why is the sample taken horizontally? What is the indicator for low-temperature impact energy? Brief answer: For domestic steel grades such as 16MnR, 15MnVR, and 15MnVNR, when their thickness reaches a certain level, or for steel plates of grades 18MnMoNbR and 13MnNiMoNbR with higher strength levels at any thickness, the temperature at which ductility transitions may occur can be in the range of -19.9 to 0°C – a situation that is very dangerous. Yet these materials are not treated as low-temperature materials. To avoid this problem, V-notch impact tests on the base material and test plates at the aforementioned temperature range must be conducted to verify whether they meet the design requirements. Due to uneven chemical composition or the presence of impurities during the casting of steel ingots, these uneven areas and impurities extend along the direction in which the metal is stretched after hot rolling, resulting in what are known as \"flow lines\" or fibrous structures. As a result, the mechanical properties of the metal exhibit anisotropy: the mechanical properties parallel to the flow lines are higher than those perpendicular to them, with greater differences observed in plasticity and toughness. Therefore, the standards for manufacturing pressure vessel steel plates specify the mechanical properties measured in the transverse direction, which have lower values, as the criteria for impact resistance, in order to ensure the safe use of the material. The index for low-temperature impact energy is: Akv at 20R ≥ 18J ； For Q345R and 15MnVR, Akv≥20J; for 18MnMoNbR and 13MnNiMoNbR, Akv≥27J.