Stainless Steels for Cryogenic Applications

Sažetak:

During the Cryogenic Tempering Process a material such as steel goes through a phase change that transforms the crystal lattice structure from body-centered cubic to face-centered cubic. The face-centered cubic structure has less space available for interstitial defects and results in a stronger, more durable material.

All structural metals undergo changes in properties when cooled from room temperature to temperatures below 0°C (32°F) temperatures in the “subzero” range. The greatest changes in properties occur when the metal is cooled to very low temperatures near the boiling points of liquid hydrogen and helium. However, even at less severe subzero temperatures encountered in arctic regions, where the temperatures may fall to as low as -70°C (-95°F), carbon steels become embrittled.

The austenitic stainless steels such as 304 (1.4301) and 316 (1.4401) are however “tough” at cryogenic temperatures and can be classed as “cryogenic steels”. They can be considered suitable for sub-zero ambient temperatures sometimes mentioned in service specifications sub-arctic and arctic applications and locations, typically down to -40°C. This is the result of the 'fcc' (face centered cube) atomic structure of the austenite, which is the result of the nickel addition to these steels.

The austenitic steels do not exhibit an impact ductile to brittle transition, but a progressive reduction in Charpy impact values as the temperature is lowered.

Figure 1: Crystal lattice structures and cryogenic tempering.

During the Cryogenic Tempering Process a material such as steel goes through a phase change that transforms the crystal lattice structure from body-centered cubic to face-centered cubic. The face-centered cubic structure has less space available for interstitial defects and results in a stronger, more durable material.

Subzero Characteristics of Austenitic Stainless Steels

Austenitic stainless steels have been used extensively for subzero applications to -269°C (-452°F). These steels contain sufficient amounts of nickel and manganese to depress the Ms-temperature into the subzero range. Thus they retain face centered cubic crystal structures on cooling from hot working or annealing temperatures. The tensile strengths of chromium-nickel austenitic stainless steels increase markedly with decreasing temperature; yield strengths also increase but to a lesser degree.

The austenitic steels are very well suited to cryogenic service. Consider the effect of cryogenic temperatures on the tensile properties of the austenitic stainless steels shown in Table 1. This table lists the mechanical properties of four austenitic stainless steels (Types 304, 304L, 310 and 347) used in cryogenic service at room temperature (75°F), -320°F (-195.5°C) and -425°F (-254°C).

Note that the high ductility (elongation and reduction of area) of the austenitic stainless steels is retained at cryogenic temperatures. Note also that the yield and tensile strengths of these steels increase as the temperature decreases, with larger increase occurring in tensile strength than in yield strength. This behavior is probably due to the slightly higher amount of carbon in Type 310 compared to the other steels listed. Interstitial elements, such as carbon, are known to have marked effects on the yield strength exhibited at low temperatures.

Table 1: Mechanical properties of austenitic steels at different temperatures [3]

AISI type Testing Temperature [°C] Yield Strength [MPa] Tensile Strength [MPa] Elongation Reduction in area [%]
304 24 227 586 60 70
304 -195.5 393 1416 43 45
304 -254 439 1685 48 43
304L 24 193 586 60 60
304L -195.5 241 1340 42 50
304L -254 233 1516 41 57
310 24 310 658 60 65
310 -195.5 585 1085 54 54
310 -254 796 1223 56 61
347 24 241 620 50 60
347 -195.5 284 1282 40 32
347 -254 313 1450 41 50

As indicated by the ductility exhibited by the austenitic steels, the toughness of these steels is excellent at cryogenic temperatures. The results of Charpy V-notch impact tests on Types 304, 304L, 310 and 347 conducted at room temperature (75°F/24°C), -320°F (-195.5°C) and -425°F(-254°C) are shown in Table 2. The toughness of these four steels decreases somewhat as the temperature is decreased from room temperature to -320°F, but on the further decrease -425°F the toughness remains about the same. It should be noted that the toughness of these steels is excellent at -425°F and that the toughness of Types 304 and 310 is significantly higher than those of Types 304L and 347.

Table 2: Transverse Charpy V-notch impact strength of some austenitic stainless steels

  Energy absorbed (J)
AISI type 27°C -195.5°C -254°C
304 209 118 122
304L 160 91 91
310 192.5 121 117
347 163 89 77

Cryogenic Applications

There are a number of petrochemical processes that require operation at low temperature. In ethylene plants, for example, the product is separated by fractional distillation at sub-zero temperature, and towers, drums, piping and heat exchangers may be exposed to temperatures as low as -120°C (-184°F).

The materials most frequently used for operation in sub-zero temperatures are aluminum, carbon steel, 3% or 9% nickel steels and austenitic stainless steels. Austenitic stainless steels are generally employed where the temperatures are below -196°C (-321°F) for the construction of pipes, pumps and valves. Because of their excellent combination of mechanical and physical properties, austenitic stainless steels are being considered for load-bearing structures of large superconducting magnets for plasma containment in magnetic fusion experiments at cryogenic temperatures.

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