Selection of steels for petroleum refinery vessels and piping is determined by the most economical combination of code allowable stresses and ability to resist service deterioration under operating conditions. Temperature is a major factor in selection because it effects both strength and corrosion rate; refinery operating temperatures may range from -50 degrees F or lower to 1200 degrees F or higher depending on the process. Hydrogen deteriorates some steels over much of this range but the mechanism of attack varies with temperature. Design stresses for many steels are constant in the range -20 degrees F to 650 degrees F, but at and below atmospheric temperatures ferritic steels may undergo a notch-brittle transition; the probability of this hazard must be recognized. From 650 degrees F to 1200 degrees F, ferritic steels containing up to 9 percent chromium and 1 percent molybdenum resist most forms of deterioration successfully. The higher chromium ferritic steels have little merit. Above 1200 degrees F, and for severely corrosive conditions at lower temperatures, austenitic stainless steels must be used.

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