Stainless Steels for Cryogenic Applications __ KEY to METALS Article

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Stainless Steels for Cryogenic Applications

Abstract:

During the Cryogenic Tempering Process a material such as steel goes through a

phase change that transforms the crystal lattice s tructure from body-centered cubic toface-centered cubic. The face-centered cubic s tructure has less s pace available for

interstitial defects and results in a stronger, more durable material.

All s tructural 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 s evere subzero temperatures

encountered in arctic regions, where the temperatures may fall to as low as -70°C (-95°F),

carbon s teels become embrittled.

The austenitic stainless s teels such as 304 (1.4301) and 316 (1.4401) are however “tough”

at cryogenic temperatures and can be classed as “cryogenic s teels”. 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-centeredcubic. The face-centered cubic structure has less space available for interstitial defects and

results in a s tronger, more durable material.

Subzero Characteristics of Austenitic Stainless Steels

Austenitic stainless s teels have been used extens ively 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 s trengths of

chromium-nickel aus tenitic s tainless steels increase markedly with decreasing

temperature; yield s trengths also increase but to a lesser degree.

The austenitic s teels 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 lis ts the mechanical properties of four austenitic stainles s s teels (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 aus tenitic stainless

steels is retained at cryogenic temperatures. Note also that the yield and tens ile s trengths 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 sl ightly 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 s trength exhibited at low

temperatures.

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

AISI type TestingTemperature

[°C]

Yield Strength[MPa] TensileStrength

[MPa]

Elongation Reduction inarea [%]

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 and310 is significantly higher than those of Types 304L and 347.

Table 2: Transverse Charpy V-notch impact strength of som e austenitic stainles s s teels

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 inmagnetic fusion experiments at cryogenic temperatures.

Date Published: Apr-2010





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Stainless Steels for Cryogenic Applications __ KEY to METALS Article