GB/T20801.2-2020 Material selection for low-temperature and low-stress or low-temperature stress-reduction conditions

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I. There are two understandings regarding the material selection for low-temperature stress-reduction conditions: The first understanding is that carbon steels (including carbon-manganese steels) suitable for such conditions, and which are exempt from impact testing, can be used at minimum temperatures 11°C lower than those specified in Tables 6 and A.1; however, the minimum temperature must not be lower than -30°C. For example, for a certain GC2-grade industrial pipeline with steel pipes made in accordance with GB/T8163, of diameter φ108×4 and made of 20# steel, Table A.1 shows that the minimum operating temperature at which impact testing is not required is >-20°C. Assuming that the pipeline is under a \"low temperature stress condition,\" the minimum operating temperature at which the steel pipe is exempt from impact testing, in accordance with 8.1.3.3 a), is -30°C. Second interpretation: Steel grade 20# belongs to carbon structural steel. According to the provisions of 8.1.3.3/a)/3), carbon steel materials applicable to the \"low-temperature stress reduction condition\" do not include carbon structural steel. Therefore, even if the aforementioned pipes meet the \"low-temperature stress reduction condition\", the minimum operating temperature at which steel pipes of grade 20# are exempt from impact testing is still >-20°C. II. Understanding of material selection for low-temperature and low-stress conditions: (1) Carbon structural steel used as bolt material is not suitable for low-temperature and low-stress conditions; for example, in a certain GC2-grade industrial pipeline, the steel pipe is of type GB/T8163, with dimensions of φ25×2, and is made of 20# steel. First interpretation: For the aforementioned pipeline, according to Table 6, since it is not possible to prepare small-diameter samples, the minimum operating temperature at which the impact test can be waived is -46℃ ； Assuming that the aforementioned pipeline is under \"low-temperature and low-stress conditions\", the minimum operating temperature at which the impact test can be waived is -104°C. Second interpretation: 20# steel belongs to carbon structural steel. According to the provisions of 8.1.3.3/b), carbon steel materials applicable to \"low-temperature and low-stress conditions\" do not include carbon structural steel. Therefore, even if the pipeline is subject to \"low-temperature and low-stress conditions\", the minimum operating temperature at which the aforementioned φ25×2 20# steel pipe is exempt from impact testing remains -46°C. (2) GC1-class pipelines are not suitable for low-temperature and low-stress conditions. Which of the above understandings are correct and which ones are incorrect?