BS 1501-151-28A stress allowables and ASTM equivalent 1

Out of BS 1501 1964 the following:

Grade 151-28A does not have allowable stresses at high temperature. Room temperature properties are :
Rm20 = 44.1 kg/mm2
Re20 = 25.2 kg/mm2 up to 32 mm thick , 23.9 kg/mm2 for thicker plates.

Grade 151-28B however does have hot values. The material is similar to 28A however normalised.
Room temperature values are same as 28A.
Yield at 650 F (343 C) = 15.2 kg/mm2 up to 32 mm thick.
Yield at 700 F (371 C) = 14.89 kg/mm2 up to 32 mm thick.
For allowable stress determination you have of course take into account a safety factor as per applicable Code.

The equivalent ASTM would be A 515 Gr. 60