Stainless Steels for Cryogenic Applications

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 M_s-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.