The issue of the upper design temperature limit for carbon steel and its tendency to graphitize

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Clause 3.6.2 of GB/T 150.2 states that when carbon steel and carbon-manganese steel are used for extended periods at temperatures above 425°C, the tendency of the carbide phases in the steel to graphitize must be taken into account. Graphitization refers to a change in the microstructure of a material, resulting in a decrease in its strength, ductility, and/or creep resistance. This provision regarding \"attention to the graphitization tendency of steel\" likely originates from the American standard API 571 \"Mechanisms of Failure in Refinery Equipment\"; a similar definition can be found in section 3.34.1 of the latest version, API 571-2020 (i.e., the third edition). There is a small detail: the temperature range specified in API 571-2020 is 425°C–595°C, whereas in the previous version, API 571-2011 (i.e., the second edition), it was 427°C–593°C. Section 6.4 of China’s GB/T 30579-2014 \"Identification of Damage Modes in Pressure Equipment\" (a revised draft of this standard is now available) describes graphitization, with the temperature range being 427°C–596°C. Is 425°C then the upper design temperature limit for carbon steel? In fact, this is not the case; attention should be paid to two key terms in the provisions of the aforementioned standards – “above 425°C” and “long-term use”. It would be too arbitrary to consider only the design temperature, without conducting any further analysis regardless of the operating temperature. Furthermore, clause 3.6.1 of GB/T 150.2 also specifies the upper limit for the operating temperature of steel, that is, the maximum design temperature, as the highest temperature corresponding to the allowable stress given in the standard. Additionally, on the standard allowable stress table, the area to the right of the thick line indicates that the allowable stress is determined by the high-temperature endurance strength limit, and it does not mean that use is prohibited in that area. It should be noted, however, that according to Note 3 of 4.6.2.2 in GB/T 150.1, the allowable stress on the right side of the thick line cannot be used when determining the pressure for the withstand test.