What is Cryogenic Stainless steel?
Steels, which are used at low temperatures, are called cryogenic steels, low-temperature resistant steels, or steels that are tough at sub-zero temperatures. Their most important property is sufficient toughness at operating temperature. Associated with this, is a considerable sensitivity to brittle fracturing. Furthermore, they are characterized in some cases by high strengths and their suitability for welding. These materials are used particularly in machinery for liquefying gas, for pipes, pumps, fittings of the cooling industry as well as for bearing, transport, and pressure vessels of the liquidated gases.
Materials for Cryogenic Applications
For low-temperature applications, various materials can be chosen depending on the operating temperature.
These also include non-ferrous materials such as:
– Copper and copper alloys
– Nickel and nickel alloys
– Aluminium and aluminium materials
The table given below shows the various materials for Cryogenic applications in the industries:
To improve its toughness at subzero temperatures, the element nickel, in particular, is added to the steel, in addition to other materials-technical measures. If the nickel content is below 10% and if no further property-determining elements (like e.g. chromium) are added, the microstructure remains ferritic-pearlitic at room temperature under conditions of equilibrium.
Influences of Nickel on Low temperatue properties & toughness
Nickel is the only element, through which the toughness of an iron-based material can be improved even at falling temperatures. By adding nickel, the curve of the impact energy moves to the left over the temperature line, i.e. the transition temperature and with it therefore the risk of the formation of brittle fractures is moved to lower temperatures. Assume, the transition temperature of steel with 2% nickel and 0.15% carbon is about -120 °C. Steel with 13% nickel and 0.01% carbon on the other hand does, up to a temperature of -196°C, not anymore display up a drop in the course of the curve due to its austenitic microstructure (and therefore also no embrittlement at low temperatures.)
Nickel also brings about a lowering of the A1 and A3 temperature, with which a considerable supercooling of the austenite transformation is associated. As a result, steels alloyed with sufficient nickel can already tend to form martensite (cubic martensite) during air cooling.
Welding guideliness for Cryogenic steel
The successful welding of nickel-alloyed pressure vessel steels that are tough at subzero temperatures depends on a range of variables. These include, in particular:
– the steel grade,
– the manufacturing process and the delivered state,
– the remanent (remaining) magnetism,
– thermal conductivity and thermal expansion,
– the welding process and the filler metal,
– the heat controlling and the heat input and the
– cooling rate.
The choice of filler metals depends on the following criteria:
– toughness properties of the base materials,
– application temperature,
– weldability with the alternating current –> thorough knowledge of the magnetic specifics of the 9 % steel (remanent magnetism),
– welding position –> suitability for out-of-position welding
– hot crack resistance and
– type of powder in submerged arc welding.
Selection of Welding consumables
The selection of welding consumables for welding cryogenic steels depends on the material chemistry. Mostly matching types of welding consumables are used. The welding consumable shall be having the:
– Very similar linear expansion coefficient as base material.
– For metal arc welding, hydrogen-controlled filler metals are to be used.
– For mixed compounds with austenites, nickel-based filler metals are to be preferred.
Preheat & Interpass temperature
– Pre-heating temperature: is to be adapted to the plate/sheet thickness, the temperature should not go below 100°C.
– Interpass temperature: 180 °C
Note: If the interpass temperature is higher, the toughness properties of the heat-affected zone are influenced negatively at operating temperature due to retarded cooling (risk of coarse grain formation).
In addition, cooling rates that are too high have a disadvantageous impact on the toughness of the welded joint (risk of formation of brittle martensite).
Heat Treatment for cryogenic Stainless Steel
Cryogenic stainless steel can lose its advantageous properties due to the detrimental effects of the welding process. The adverse effects caused by the welding can result in higher ferrite content in the weld metal as well as a heat-affected zone.
If the impact energy needs to meet high requirements around the fusion line, a quenched and tempered (Q & T) base material is recommended. To counter the adverse effect of the welding, the welded component can be treated by solution annealing.
Ferrite Control & Ferrite content Limits
For austenitic stainless steels, many standards prescribe ferrite limitations. For example, ASME III specifies a minimum of 5FN for service above 427°C (800°F), or 3-10FN for service over 427°C (800°F); while API 582  specifies a minimum of 3FN (although it is noted that for cryogenic service lower FN may be required). It has been discovered that by adjusting the weld metal ferrite of SMAW electrodes in the range 25FN, the 0.38mm (0.015inch) lateral expansion requirement may be met.