Stainless steel filler metals for cryogenic applications

Q: What considerations need to be made for filler metal selection for fabricating stainless steel tanks containing liquid gasses?

A:The cryogenic temperature range is defined as from −150°C to absolute zero, or −273°C (the temperature at which molecular motion theoretically ceases). Table 1 details the liquefaction temperatures of various gases, which range from 150°C for oxygen to −269°C for helium. Transportation of these liquids requires the materials used in the fabrication process to have sufficient mechanical properties in terms of yield strength and toughness at extreme low temperatures. For example, LNG in storage is usually kept at a temperature of -170°C, (7°C below its liquefaction temperature). The mechanical testing requirements are usually specified at a testing temperature of -196°C, which is the liquefaction temperature of nitrogen. This method adds an extra margin of safety for the mechanical requirements on the fabrication, while using the testing temperature associated with liquid nitrogen is a practical and available method for the preparation of the test pieces. Ferritic, martensitic, and duplex stainless steels become brittle at low temperatures in the same way as low alloy steels. The ductile to brittle transition temperature (DBTT) is a critical property in materials science, indicating the temperature at which the toughness of a material shifts from ductile to brittle behavior very rapidly. Below this temperature, materials tend to fracture with minimal plastic deformation, while above it, they exhibit ductile properties with significant deformation before fracture. As illustrated in Figure 1, body-centered cubic (BCC) steels have a narrow transition window; they quickly become brittle and fracture easily.

Austenitic stainless steels, however, have a face-centered cubic (FCC) structure and are said to be ‘tough’ at low temperatures. They do not exhibit a rapid ductile to brittle transition, but rather a progressive reduction in Charpy impact toughness values as the temperature is lowered. They are classed as ‘cryogenic steels’ and take more energy to fracture at low temperatures.

As mentioned, the atomic structure of austenite is face-centred cubic (Figure 2). Essentially, the structure formed is because of the addition of nickel to the steel. It is this closely packed structure that gives austenitic steels their toughness at low temperatures.
Ongoing development of austenitic filler metals has resulted in modification of standard grades with enhanced results in cryogenic applications. Table 2 shows the comparison of mechanical test results between standard filler metals and cryogenic products of the same stainless steel grade.
The chemistry and microstructure of the Exaton 19.12.3.L CRYO grade is such that it ensures a very low ferrite level of FN 2 – 3 (WRC 92). This property significantly improves the resistance of the weld metal to micro-fissuring. Whilst the tensile and yield strength is around the same as the standard 316L, the biggest improvement is in the impact toughness at cryogenic temperatures, resulting in an increase to 90J @ -196°C, up from >32J for a standard grade (it should be noted that the current specification requirement for TUV approval of Charpy impact toughness @ -196° is 32J; however, the typical requirement specified by an end user is 40J).
Further testing is currently in progress on the new cryogenic grade filler metal to obtain Charpy impact test results at -269°C. The objective is to prove the integrity in the weldments installed in liquid hydrogen equipment.
This filler metal research is important, as hydrogen is another liquid gas that is becoming more prevalent as a fuel source. For example, the UK Government has a strategy to develop 5GW of hydrogen production capacity by 2030. The liquefaction temperature of hydrogen is -253°C, so the requirements for the mechanical properties and testing temperatures will necessitate that liquid helium will need to be used, which is a significant step up in the design parameters used for LNG.

About this Tech Article

This tech article appeared in Stainless Steel World, March 2024 magazine. To read many more articles like these on an (almost) monthly basis, subscribe to our magazine (available in print and digital format) – SUBSCRIPTIONS TO OUR DIGITAL VERSION ARE NOW FREE.

Every week we share a new Featured Story with our Stainless Steel community. Join us and let’s share your Featured Story on Stainless Steel World online and in print.