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As per ASME B31.3, the basic allowable stress of a material is minimum of the following:

• 1/3rd of tensile strength at design temperature.
  2/3rd of yield strength at design temperature.
  100% of average stress for a creep rate of 0.01% per 1000 hours.
  67% of average stress for rupture at the end of 100000 hours.
  80% of minimum stress for ruptures at the end of 100000 hours.

For austenitic stainless steel or nickel alloys the lower of yield strength and 90% of yield strength at temperature.