Engineering guide
The Minimum Design Metal Temperature (MDMT) is the lowest metal temperature for which a pressure vessel can be certified per ASME BPVC Section VIII Division 1 without (or with) impact testing. It governs material selection, testing scope and the value stated on the nameplate. This module determines the MDMT and whether an impact test is required — per UCS-66, UCS-66.1 and UCS-68 for carbon and low-alloy steels and per UHA-51 for austenitic and high-alloy steels.
The core of the procedure is Figure UCS-66: for each material group (curve A to D), it delivers, as a function of the governing thickness, a basic exemption temperature below which construction without impact testing is not permitted. If the component is not fully stressed, UCS-66.1 grants a temperature reduction based on the stress ratio; postweld heat treatment (PWHT), where not otherwise required, provides a further credit per UCS-68. For bolts and nuts, the separate exemption temperatures of General Note (c) to UCS-66 apply.
You need to calculate the MDMT for every ASME design — especially critical for low-temperature applications, unpressurized cool-down scenarios (gas depressurization, Joule-Thomson effect) and when re-rating existing vessels for colder operating cases.
Scope
- MDMT for carbon and low-alloy steels (UCS-66)
- MDMT for austenitic / stainless steels (UHA-51)
- MDMT for bolting (UCS-66 General Note c)
Typical workflow
Determine the governing thickness per component
For each pressure-retaining component, the governing thickness is determined per UCS-66(a) — for welded joints usually the thinner of the connected thicknesses, for non-welded parts one quarter of the thickness, with special rules for flanges, tubesheets and flat heads.
Assign the material curve
The material is assigned to one of the curves A to D of Figure UCS-66: curve A for rimmed or otherwise unfavorable materials, curve D for the toughest (e.g. normalized fine-grain steels). From the curve and the governing thickness follows the basic exemption temperature.
Temperature reduction based on the stress ratio
If the component is only partially stressed at the MDMT, a reduction of the exemption temperature is determined per Figure UCS-66.1 from the ratio of the actual to the allowable stress (coincident ratio); at a ratio of 0.35 or less, the impact test obligation is waived down to −155 °C (−48 °C without further verifications per older editions; the module uses the limits of the current edition).
Check the PWHT credit per UCS-68
If postweld heat treatment was performed even though the code does not require it, the exemption temperature for P-No. 1 materials may be lowered by a further 17 °C (30 °F).
Evaluate the special cases of austenitics and bolting
For austenitic steels, the exemption rules of UHA-51 apply (typically down to −196 °C without impact testing, depending on material, filler metal and temperature), and for bolts and nuts the exemption temperatures of General Note (c) to UCS-66. The module reports the governing MDMT per component; the vessel MDMT is the warmest of these values.
Frequently asked questions
What is the difference between the MDMT and the minimum operating temperature?
The MDMT is a design and certification quantity: the lowest metal temperature at which the full coincident pressure may be applied, and it appears on the nameplate. The minimum operating temperature is a process quantity. The MDMT must cover all operating, upset and ambient cases — including cool-down by depressurization, cold feed streams or low ambient temperatures during unpressurized start-up.
Why may the impact test be waived at low utilization?
Brittle fracture requires, besides low temperature and a notch, a sufficient tensile stress. If the actual stress is significantly smaller than the allowable stress, the brittle fracture risk drops accordingly. Figure UCS-66.1 translates this physics into a temperature reduction; at a stress ratio of 0.35 or less, the most far-reaching exemption applies. Important: what governs is the stress in the coincident temperature-pressure condition, not the design stress.
For a nozzle in a thick shell, which thickness is the governing one?
The governing thickness depends on the weld detail: for butt-welded parts it is the thinner of the two connected wall thicknesses at the joint under consideration. A thin-walled nozzle in a thick shell is therefore assessed via the nozzle thickness, the shell itself via its joints. Each component thus receives its own exemption temperature; the worst one determines the vessel MDMT.
Do austenitic steels need impact testing at all?
Often not: UHA-51 exempts common austenitic materials down to very low temperatures (typically −196 °C and, depending on the case, below), because they have no pronounced ductile-brittle transition temperature. The exemption depends, however, on carbon content, filler metal, solution annealing condition and the actual temperature — in particular, the weld metal and the heat-affected zone can be subject to testing even though the base material is exempt.
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