yield of steel

That would depend upon the specific steel (carbon, alloy, high strength, ultra-high strength, etc.). Determine the application then select the type of steel based on its strain / temperature characteristics.

For most steels, some yield strength reduction occurs at any elevated temperature above ambient or room. The reduction again depends upon the type. Ultra high strength steels used for things like rocket motor casings, firearms, mortars, aircraft landing gear, airframes and other high temperature applications would be least affected.

The condition of the steel is also a factor, as if it were cold worked, or quenched, then a tempering treatment or low temperature stress relief would modify the yield strength.

Tell us the specific grade and condition and we can better assist.

milo

For piping materials, ASME B31.3 Tables A-1, A-1B have tabular data. High carbon steel starts loosing strenght after 300 F. Low carbon and stainless loose after 100 F, but they do not do loose as quickly has high carbon steels.