Regarding whether the MDMT of 1.25Cr-0.5Mo material is ≤ -18℃ [
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API RP 934‑E, version 2018, sets the impact temperature of the material at or below MDMT, whereas the older version (2010) specified –18°C. I don’t really understand why it was changed in this way. I specialize in welding and have no knowledge of design; I would like to ask the designers here: for 1.25Cr-0.5Mo materials (such as SA-387 GR.11 Cl.1), is it possible for the MDMT value to be ≤ -18°C under normal circumstances?
It is possible, and the value might even be low; factors such as the lowest average monthly temperature in the area over the years, whether the equipment has insulation, and whether low-temperature conditions occur during startup and shutdown processes need to be taken into account. If all the above factors are taken into account and a temperature of -18°C will not be reached, then there is no need to set such a low requirement for the shock temperature; it can be increased. The answer is not necessarily comprehensive