The allowable stress in sustained case is Sh, so it is confirmed that the sustained stresses are within the allowable at that temperature. But due to local yielding because of expansion there may be reduction in pipe diameter(may be minute) because of which the section modulus get disturbed (decreases- as section modulus decreases bending stress increases). Whether the sustained stress calculated considers this effect (section modulus) as it considers other mechanical,erosion and corrosion allowance. If this is not the case then won't the sustained stresses be more in hot condition than calculated.

Thanks in advance,
Rajagopal