Stainless Steel World Magazine April 2024 | Page 40

Meet the columnist

Peter Stones

IEng MWeldI IWE / EWE As part of the ESAB Specialty Alloys Group , Peter is technical support for stainless and nickel alloy filler metals . Peter is actively involved with TWI and is a non-executive director of The Welding Institute . Peter worked for Sandvik for 10 years and was Global Product Manager for Sandvik Welding up to 2018 , when ESAB purchased the filler metals business . Contact : peter . stones @ esab . com

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

Fracture energy

FCC ( ex . Austenitic Stainless Steels )

BCC ( ex . Low Alloy Steels )

Figure 1 . FCC stainless steels continue to exhibit toughness at low temperature , where BCC structures have a rapid brittle transformation .

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 facecentered 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

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

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