Steels for Low-Temperature
When designing low-temperature systems or equipment, the 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.
Carbon steels have a better weldability, greater toughness, and higher strenght with low coefficients of termal conductivity than alloy steels. The A 516, 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 A 516, A 442 class 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 A 442. They are easier to fabricate than A 442 grades because carbon content is lower.
Higher strength with good notch toughness is available in carbon steels A 537 Grade A and A 537 grade B. Their can be earlier normalized or quenched and tempered to raise yield and tensile strength and impact toughness beyond those of the A 516`s. Table 1 shows mechanical properties at low temperatures for some typical ASTM carbon steels.
Table 1. Specifications for Low-temperature Steels
|Designation||Lowest usual service temperature, (°C)||Min Yield Strength (MPa)||Tensile Strength (MPa)||Min Elongation, L0= 50 mm (%)||Uses|
|A442 Gr. 55||-45||221||379 – 448||26||Welded pressure vessels and storage tanks; refrigeration; transport equipment|
|A442 Gr. 60||-45||221||414 – 496||23|
|A516 Gr. 55||-45||207||379 – 448||27|
|A516 Gr. 60||-45||221||414 – 496||25|
|A516 Gr. 65||-45||241||448 – 531||23|
|A516 Gr. 70||-45||262||483 – 586||21|
|A517 Gr. F||-45||690||792 – 931||16||Highly stressed vessels|
|A537 Gr. A||-60||345||483 – 620||22||Offshore drilling platforms, storage tanks, earthmoving equipment|
|A537 Gr. B||-60||414||551 – 690||22|
|A203 Gr. A||-60||255||448 – 531||23||Piping for liquid propane, vessels, tanks|
|A203 Gr. B||-60||276||482 – 586||21|
|A203 Gr. D||-101||255||448 – 531||23||Land-based storage for liquid propane, carbon dioxide, acetylene, ethane and ethylene|
|A203 Gr. E||-101||276||482 – 586||21|
|A533 Gr. 1||-73||345||552 – 690||18||Nuclear reactor vessels where low ambient toughness required for hydrostatic testing; some chemical and petroleum equipment|
|A533 Gr. 2||-73||482||620 – 793||16|
|A533 Gr. 3||-73||569||690 – 862||16|
|A543 Gr. 1||-107||586||724 – 862||14||Candidate material with high notch toughness for heavy-wall pressure vessels|
|A543 Gr. 2||-107||690||793 – 931||14|
Since a variety of low-temperature steels are available, the engineer must consider the advantages each has to offer according to the application. The cost-strength ratio is but one factor; others, such as welding and fabrication costs, have equal or greater bearing on final costs. However, heat-treated carbon grades are often used for low-temperature services. Besides offering excellent low-temperature toughness plus fabricability, these grades are lower in initial cost.