Engineering task and calculation objective
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.
Standard and calculation basis: ASME BPVC VIII-1 UCS-66 / UCS-66.1 / UCS-68 / UHA-51: 2025
Calculation 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)
Calculation 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.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Specified MDMT | – | – |
| Design temperature | – | – |
| Design pressure | – | – |
| Material | – | – |
| Nominal thickness tn | – | – |
| Corrosion allowance c | – | – |
| Required thickness tr | – | – |
| Joint efficiency E | – | – |
| Flat part thickness tfl | – | – |
| Material curve | – | – |
| Joint / part type | – | – |
| PWHT performed (not required) | – | – |
| Material is P-No. 1 | – | – |
| Thickness of joined part 2 | – | – |
| Ratio method | – | – |
| Applied membrane stress S* | – | – |
| Allowable stress at MDMT S | – | – |
| Max. allowable pressure MAP | – | – |
| Governing thickness tg | – | – |
| Basic exemption temp. (Fig.UCS-66) | – | – |
| PWHT credit (UCS-68) | – | – |
| E* = max(E; 0.80) | – | – |
| Coincident ratio | – | – |
| Temp. reduction (Fig.UCS-66.1) | – | – |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Impact test required | – | – |
Calculation options
Material curve
1 · 2 · 3 · 4
Joint / part type
1 · 2 · 3 · 4 · 5 · 6
PWHT performed (not required)
0 · 1
Material is P-No. 1
0 · 1
Ratio method
1 · 2 · 3
Impact test required
0 · 1
Carbon content > 0.10 %
0 · 1
Material type
1 · 2 · 3 · Duplex · 5 · 6 · 7
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.