Steels for Cryogenic and Low-Temperature Service

Carbon and alloy grades for low-temperature service are required to provide the high strength, ductility, and toughness in vehicles, vessels, and structures that must serve at -45°C and lower. Because a number of steels are engineered specifically for service at low temperature (about -100°C), selecting the optimum material calls for thorough understanding of the application and knowledge of the mechanical properties that each grade provides.

At temperature below ambient, a metals behavior is characterized somewhat by crystalline structure. The yield and tensile strengths of metals that crystallize in the body-centered cubic from iron, molybdenum, vanadium and chromium depend greatly on temperature. These metals display a loss of ductility in a narrow temperature region below room temperature.
The tensile strength of metals with face-centered cubic structures - aluminum, copper, nickel and austenitic stainless steel - is more temperature dependent than their yield strength, and the metals often increase in ductility as temperature decrease.

Transformation occurring in compositions that are normally stable at room temperature, but metastable at cryogenic temperatures, can greatly alter their behavior. For example, the combination of gross plastic deformation and cryogenic temperatures can cause a normally ductile and tough stainless steel, such as 301, 302, 304, 321, to partially transform to bcc structure, resulting in an impairment of ductility and toughness. A fully stable stainless steel 310 cannot be transformed at cryogenic temperatures.

The 300 series steels offer a fine combination of toughness and weldability for service to the lowest temperatures. In the annealed condition, their strength properties are adequate for ground-based equipment but inadequate for lightweight structures. For aerospace applications, fabricators can take advantage of the alloys strain-hardening characteristics and use them in highly cold-worked condition. The principal shortcomings of cold-worked materials are: low weld-joint efficiencies caused by annealing during welding and the transformation to martensite that occurs during cryogenic exposure. Selection of fully stable grade type 310, overcomes the transformation problem. Precipitation-hardening A286 stainless has even higher strength when cold worked before aging.

The only alloy steel recommended for cryogenic service is 9% nickel steel. It is satisfactory for service down to -195°C and is used for transport and storage of cryogenics because of its low cost and ease of fabrication. Other alloy steels are suitable for service in the low-temperature range. The steels A201 and T-1 can suffice to -45°C, nickel steels with 2.25% Ni can suffice to -59°C, and nickel steels with 3.5% Ni to -101°C.

Steel for Cryogenic Service: An Example

Designers of cryogenic assemblies base their stress calculations on the room-temperature properties of the material. The reason is that it is the highest temperature the material will encounter. And it stands that if a higher-strength material that stands up to super cold conditions were available, designers might specify it.

At 26°C austenitic stainless steel has tensile and yield strengths that are 172 MPa greater than the corresponding strengths for type 304 stainless. At -100°C its tensile and yield strength exceed those of type 304 by 550 MPa and 276 MPa respectively.

A grade with following chemical composition shows good mechanical properties at cryogenic temperatures:

C - 0.072%
Mn - 16%
P - 0.02%
S - 0.008%
Si - 0.41%
Ni - 5.85%
Cr - 17.8%
N - 0.36%
Fe - Remainder
(The composition is given for plates with 12.7mm thickness)

The material combination of high strength, good toughness, and weldability should prompt designers to specify it for welded pressure vessels for the storage of cryogens.

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

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.

Pipeline Steels for Low Temperature Uses

Steels for natural gas pipelines must meet more demanding requirements than that used for oil. For example they carry compressed gas at -25°C to -4°C, making crack growth and brittleness a problem in the severe artic environment. Achieving low-temperature notch toughness, grain size control, and low sulfur content were among major problems in developing the steel, particularly since economic feasibility had to be considered.

Hot-rolled steels present a good opportunity to cut both cost and weight if the cost per unit strength could be reduced. As strength of high-strength, low-alloy steels rise, toughness usually drops.

In steel alloyed with molybdenum, manganese and columbium, which is use for these pipe-lines, molybdenum raises both strength and toughness. Carbon is reduced to make columbium more soluble, and to improve weldability and impact strength. Steels with small and large amount of columbium have similar precipitation kinetics; higher strengths are produced by larger quantities of columbium. Columbium also promotes hardenability, which is needed to develop an acicular-ferrite microstructure. Manganese, along with molybdenum, helps to inhibit transformation to polygonal ferrite on the steel.

Where sulphur cannot be kept low, however, rare earth additions will control the shape of the sulfide inclusions. During hot working, grain refinement is enhanced because columbium has a grain-boundary pinning effect. This effect makes it possible to produce a highly substructured austenite prior transformation, which helps in assuring transformation to fine grained acicular ferrite.

Contributing to high strength and good impact resistance is the transformation mechanism - austenite changes to fine-grained acicular ferrite, which is further strengthened by the precipitation of columbium carbonitride. Other advantages include good formability and most important, excellent weldability.

Aside from pipeline, this steel can be used in the automotive, railroad, heavy equipment, construction and shipbuilding industries, application areas which the keynote is low cost per unit strength. Because of their inherently good strength-toughness relationship, the manganese-molybdenum-columbium steels may well satisfy this requirement.