When designing pressure vessels for low temperature, The BBN Steel engineer finds that notch toughness ranks high in importance, because a part or structure will generally fail due to a notch or other stress concentration. Test results measure the steels capacity to absorb energy, and thus signify its ability to resist failure at points of local stress concentration.
Fatigue limit of steel also must be considered. At low temperatures, systems are usually subjected to dynamic loads, and structural members to cycle stresses. Examples include vessels that frequently undergo pressure changes and large structures and mobile equipment that experience extreme stress imposed by packed snow or high winds. Other considerations include heat conductivity and thermal expansion.
A516 Gr.60 carbon steels have a better weldability, greater toughness, and higher strenght with low coefficients of termal conductivity than alloy steels. The ASTM A516, one of the most frequently used group of carbon steels, have tensile strengths ranging from 379 MPa to 586 MPa minimum. The big advantage of A 516 steels is their low initial cost.
Compared with A516, A442 class steel have higher carbon and manganese in plates less than 25.4 mm thickness, and lower manganese beyond 25.4 mm. However, applications for A 516 Grades 55 and 60 duplicate those of A442. They are easier to fabricate than A442 grades because carbon content is lower.
