Low alloy steel ASTM A333 Gr.6 seamless pipes are widely used in the propane dehydrogenation (PDH) process for the transmission of liquid propane and liquid propylene. The boiling points are -42.1°C and -47.7°C for propane and propylene, respectively. The ASTM A333 Gr.6 steel is working at such a low temperature that Charpy V-notch impact toughness must be taken into account.
According to ASME B31.3 – 2018: Process Piping, the minimum design metal temperature (MDMT) for which ASTM A333 Gr.6 is normally suitable without impact testing other than that required by the material specification is -45°C [-50°F]. However, the boiling point of propylene is -47.7°C, which is beyond the lower limit. According to para. 323.2.2 (h) of the Code, the MDMT may be reduced to a temperature no colder than -48°C [-55°F] by the temperature reduction provided that “the maximum obtainable Charpy specimen has a width along the notch of less than 2.5 mm” and “the stress ratio is 0.3 or less”. Under these conditions, the ASTM A333 Gr.6 low alloy steel can be used without impact testing requirements. If any of the above conditions cannot be met, specimens shall be taken from base metal, weld metal and heat affected zone and qualified by impact testing at the design minimum temperature.