ASME B31.3 section 323.2 requires the designer to verify that the materials which meet other requirements of the code are suitable for service throughout the operating temperature range. The minimum temperature limits for various materials are provided in Table A-1 of the code and indicate the design minimum temperature for which the material is normally suitable without impact testing other than that required by the material specification.

At low temperatures carbon steel material is susceptible to brittle fracture. The code requires the designer to take into consideration brittle fracture rules for material selection of piping and various other components. By taking these rules into consideration the risk of brittle fracture of materials can be mitigated in design.

The capability of a material to withstand brittle fracture is termed as toughness. Tough materials require relatively larger amounts of energy to intitiate and propogate a crack. Toughness depends on the strength and ductility of material. Finer grain size in material imparts it more toughness. Lower carbon content gives tougher material but also reduces yield strength. Manganese and Nickel improve toughness whereas Oxygen, Sulphur and Molybdeum decrease toughness. Commonly used methods to measure toughness of a material are the Charpy Impact tests and Drop-weight tests.

Minimum Temperature without Impact Testing

Previous versions of ASME code permitted use of carbon steel piping down to -29°C without impact testing. However, this unrestricted application down to -29°C was limited from 1994 versions of ASME B31.3. Table A-1 of ASME B31.3 lists the design minimum temperatures for which the material is normally suitable without impact testing. In some cases, for e.g. in case of API 5L Grade B material, the code refers to alphabet "B" instead of minimum temperature. Table A-1 also refers to Note-6 which states "For carbon steels with a letter designation in the minimum temperature column, see para. 323.2.2(e) and the applicable curve and notes in Fig. 323.2.2A."

Minimum Temperature

Figure below shows the minimum temperature - wall thickness curves A,B,C & D and is based on Figure 323.2.2A and Table 323.2.3A of ASME B31.3. If a design minimum temperature and thickness combination is on or above the curve, impact testing is not required for that combination. Hovering on the curves below will display the minimum temperature and wall thickness combination.

The lowest temperature for Curve-A is -9.4°C, for Curve-B is -28.9°C, and for Curve-C and D is -48.3°C. However, since the 2020 edition, the use of Curve-B limit of -29°C for carbon steels is further limited by qualification of impact tested welding procedures as clarified in the next paragraph.

For Category D fluid service any carbon steel material may be used up to a minimum temperature of -29°C (-20°F) without impact testing. For API 5L Grade B material, Curve B is applicable. This means that API 5L Grade B with wall thickness of 12.7mm may be used up to a minimum temperature of -29°C. As the wall thickness increases above 12.7mm the curve shows lower minimum temperatures. For example for materials (API 5L Grade B) that reference Curve-B, pipe wall thickness of 15.9mm can be used down to a minimum temperature of -16.7°C without impact testing and not -29°C as permitted by the old versions of the code. 

For ASTM A333 Grade 6, Table A-1 does not refer to any alphabet designation and specifies the minimum temperature as -45.6°C (-50°F). Hence in piping specifications that have wall thickness over 12.7mm and are intended to be used for low temperature applications down to -45.6°C, it may be preferable to use ASTM A333 Grade 6 material which requires impact testing at -50°F as per ASTM material specification. 

Impact Testing of Welds

For steel materials, impact testing of welds including manufacturing welds is required under the following conditions as per the ASME 31.3 2020 edition:

Either the base material is required to be impact tested


If the design minimum temperature is lower than -18°C (0°F). 

Note that until the ASME B31.3 2018 edition, weld metal deposits required impact testing in accordance with para 323.3 if design minimum temperature was -29°C (-20°F) or lower. This limit has been changed in the 2020 edition. As per the 2020 edition, the weld metal of the welding procedure qualification test coupon shall be impact tested at or below the design minimum temperature of less than -18°C except when the following condition is satisfied.

Impact testing of the weld metal is not required when the classification of the welding consumables according to the filler metal specification requires impact testing at or below the design minimum temperature. 

How does this impact the MDMT of piping classes?

If the ASME B31.3 2020 Edition has to be applied, piping classes with design minimum temperature of -29°C will have to use impact tested welding consumables for piping fabrication. Accordingly, if the welding consumables are not impact tested at minimum design temperature of -29°C, the design minimum temperature for such piping classes will need to be specified as -18°C.

Can API 5L Grade B pipe be used below -29°C without Impact Testing

As per note-3 of Table 323.2.2, impact testing is not required if the design minimum temperature is below −29°C (−20°F) but at or above −104°C (−155°F) and the stress ratio defined in Fig. 323.2.2B does not exceed 0.3.

If the stress ratio falls between 0.3 and 1, section 323.2 (d) provides further exemption on use of carbon steel material without impact testing below the temperature limits specified in Table A-1 or Figure 323.2.2A subject to meeting the following conditions:

  • Piping is not in elevated temperature service.
  • Piping is not expected to experience shock loading or thermal bowing.
  • Piping does not contain welds between dissimilar materials.
  • Stress ratio as defined in Figure 323.2.2B is less than one.

The stress ratio is defined as the maximum of

  • Hoop stress over basic allowable stress
  • Combined stress over basic allowable stress
  • Operating pressure over pressure rating for pressure rated components.

If the calculated stress ratio is between 0.3 and 1, the impact testing temperature can be reduced according to figure 323.2.2B using the stress ratio.