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• The 0.2% proof stress and ultimate tensile strength at 4.2 K for 35% pre-strained specimens increased respectively around 86% and 18% compared to those of the as-received material. • The elongation at fracture at both RT and 4.2 K decreased for the pre-strained 316LN SS compared to those of the as-received material. • The pre-strained 316LN SS fractured at 4.2 K exhibits a ductile fracture mode. In the present work, influences of room temperature tensile pre-strain on cryogenic tensile properties of 316LN austenitic stainless steel (SS) were investigated. The tensile properties of the 316LN SS with various tensile pre-straining amounts of 15%, 25% and 35% were measured at both room temperature (RT) and liquid helium temperature (4.2 K). Results indicated that both 0.2% proof strength (R p0.2) and ultimate tensile strength (R m) at both RT and 4.2 K of the pre-strained 316LN SS significantly increased compared to those of the as-received material. The R p0.2 and R m at 4.2 K for 35% pre-strained specimens increased around 86% and 18%, respectively, compared to those of the as-received material. However, the elongation at fracture (A) at both RT and 4.2 K decreased for the pre-strained 316LN SS compared to those of the as-received material. The fractured morphologies of the pre-strained 316LN SS were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Dimples in the surface fractured at both RT and 4.2 K of the as-received and pre-strained 316LN exhibited a ductile fracture mode. Moreover, fractographic results indicated that the pre-strain treatment considerably decreased the ductility at both RT and 4.2 K. TEM analyses revealed that pre-strain treatment at RT led to dislocation accumulation and stress-induced martensite phase occurred at 4.2 K, which interpreted the cryogenic tensile properties of the pre-strained 316LN SS. [ABSTRACT FROM AUTHOR]

• Fatigue crack propagation behavior of 316LN tungsten inert gas (TIG) weldment and weldment with post weld heat treatment (PWHT) was investigated at liquid helium temperature. • Weldment with PWHT results in the highest fatigue crack propagation resistance, whereas the base material demonstrates the lowest fatigue crack propagation resistance. • The crack featured with a transgranular propagation characteristics for the base material, the weldments without and with PWHT. • More amount of strain-induced martensite near the crack tip in the base material compared to in the as-welded weldment and in the weldment with PWHT. • The strain-induced martensite transformation plays a crucial role in determining the fatigue crack propagation behavior at cryogenic temperature. In the present work, fatigue crack propagation behavior of 316LN tungsten inert gas (TIG) weldment and weldment with post weld heat treatment (PWHT) was investigated at liquid helium temperature. Results indicates that the weldment with PWHT results in the highest fatigue crack propagation resistance, whereas the base material demonstrates the lowest fatigue crack propagation resistance. Microstructure evolution reveals that the fatigue crack propagation is characterized as transgranular fracture for the base material as well as the as-welded weldments and the weldment with PWHT. Moreover, the crack propagation path of the weldments shows a zigzag crack extension, especially for the weldment with PWHT, whereas the crack extension of the base material is relative straight. Electron backscattered diffraction reveals more amount of strain-induced martensite near the crack tip in the base material compared to in the as-welded weldment and in the weldment with PWHT, which is considered to be a crucial factor in the fatigue crack propagation behavior at cryogenic temperature. [ABSTRACT FROM AUTHOR]

In the present work, a cryogenic mechanical property testing system conduction-cooled by two G-M cryocoolers was developed. The testing sample can be cooled from room temperature to 2.7 K within 7.5 h. The sample was first cooled down to 11.1 K directly by the two G-M cryocoolers and then cooled down to 2.7 K by decompressing the chamber. Instead of liquid helium, the cooling process is characterized by cooling with recycled helium gas as heat transfer medium. The heat load of the system was analyzed and optimizations were adopted in terms of material selections and design. The static load capacity of the system reaches 200 kN and the fatigue load capacity can reach 50 kN. This system can be installed onto an electronic universal testing machine or a fatigue testing machine to characterize static tension, fracture mechanics or fatigue properties at tunable low temperatures. Tensile properties of 316L austenitic stainless steels at 4.2 K were tested with the system and the results were compared with those obtained by cooled using liquid helium, which demonstrates high reliability. [ABSTRACT FROM AUTHOR]