Its been a while, but the list of allowable ASTM materials permitted to be used in ASME sect I , their allowable stresses, and permitted temprature range are based on certified lab data and required lifetimes based on the tested values of yield, UTS, and creep life, without explicit consideration of oxidation, corrosion, and fatigue damage. Those latter 3 damage mechanisms are imputed to be the responsiblilty of the designer and operator. The confidence of the creep life and assigned high temperature stress is related to the certified lab's test data ; one cannot allow operation at temperatures above which there is not confidence in the data or theory as for newer alloys since there is only short term test data.
Each material has a variation in properties due to practical methods of fabricating the materials and structures that use the material , including weld properties. One cannot allow operation above the critical temperatures for which metallurgical changes occur, just as post fabrication heat treatment temperatures are not allowed to exceed certain critical temperatures to avoid change the metallurgy to one that was not within the lab's test range.
While the code rules allow the allowable stress determination to be based on the calculated expected mid wall metal temperature, the heated boiler parts, during operation, are heated by the combusted fuel gases and the outer diameter temperatures are above the mid wall metal temperatures, so there needs to be a margin to account for that operating temperature difference between mid wall and OD, such that the OD material does not undergo metallurgical changes that degrade properties.
One example may be carbon steel boiler tubes within the boiler setting absorbing heat from combusted fuel gases. Graphitization is a metallurgical change that can embrittle the outside of the heated tube if that OD temperature exceed perhaps 825 F , so some grades of carbon steel tubes have a max listed mid wall temperature limit of 750 F with the implied margin between OD and mid wall of (825-750) = 75 F.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick
