Why is △T not subtracted in the verification for low-temperature brittle fracture per Appendix F of GB/T150.3?
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Dear teachers, regarding the allowable operating temperature of materials, taking Q245R as an example, the lower limit specified in the new version of GB/T150.2-2024 is -20°C. Meanwhile, Appendix F of the new version of GB/T150.3-2024, which deals with the assessment of materials against brittle fracture at low temperatures, specifies -10°C according to table F.2. Thus, there is a discrepancy between the lower temperature limits set out in GB/T150.2-2024 and those in Appendix F of GB/T150.3-2024; two people have asked this question in the past couple of days. However, according to Appendix F of GB/T150.3-2024, to determine the minimum operating temperature, one first refers to tables F1-F3 to find the correction value △T, then uses table F.2 to determine Tmat; the minimum allowable temperature is then given as (Tmat-△T). Both parties saw that in table F.2, the Tmat value for Q245R is -10°C, which means the lower limit specified for Q245R is -10°C, and this conflicts with the lower limit of -20°C specified in GB/T150.2-2024. I was puzzled: if neither of the two people saw it, why is there still a △T to be subtracted? Appendix F specifies that the minimum allowable temperature is (-10-△T), not simply -10°C; what needs to be verified is also (-10-△T)-20. Was it really the case that two people missed this?
The calculation of the minimum service temperature for materials in Appendix F of GB/T150.3-2024 does indeed include the step of subtracting the correction factor △T. This is extremely important, as the role of △T is to adjust the critical temperature for brittleness of the material according to specific conditions, in order to ensure the safety of the material under the intended conditions of use. If the Tmat value from Table F.2 is used directly (which is -10°C for Q245R) without considering △T, the safety of the material may be overestimated. In short, the minimum operating temperature should be Tmat minus ΔT, that is, (-10°C - ΔT). Therefore, when calculating the actual minimum operating temperature, it is indeed necessary to subtract △T from Tmat; only in this way will the resulting value be correct and in line with the minimum allowable operating temperature for materials specified in GB/T150.2-2024 (which is -20°C for Q245R). This isn’t a missed step; it’s a step that must be carried out. .
What if △T is close to 0? In the absence of specific examples, it must be considered in terms of the limit state