Be careful of your units, you are switching between deg F and deg C.

Your Design Temp is 1100 deg C (2012 deg F)

You report B31.3 SA at 649 deg C (1200 deg F) as 13.8 MPa (2 ksi). It appears B31.3 does not give SA values for this material above 1200 deg F.

Material Supplier SA at 1100 deg C (2012 deg F) is 5.1 MPa (0.74 ksi)

The material supplier's SA number does not seem out of line. You need to satisfy yourself that it is appropriate to use this material at this high temperature ( 1100 deg C / 2012 deg F).