Determination of cryogenic fracture characteristics of austenitic stainless steel JN1 welded joint and heat-treated materials by means of small punch testing method

Fracture characteristics of a TIG welded joint and heat-treated materials of JN1, which is an austenitic stainless steel recently developed for use as a cryogenic structural material, were evaluated by the small punch (SP) testing method using miniaturized specimens at 77 K and 4 K. The area under the load-displacement curve up to the maximum load was defined as the SP energy to evaluate the fracture characteristics of the materials in SP tests. Fracture surfaces were also observed using a scanning electron microscope. Solution-treated JN1 steel showed ductile fracture behavior with a high SP energy value even at 4 K, and serrations were observed in the load-displacement curve at 4 K. In the weld metal and fusion line specimens of the TIG welded joint, serrations also occurred at 4 K, but the fusion line specimens showed lower SP energy values than weld metal and solution-treated JN1 steel at 77 K and 4 K. In addition, the fusion line specimens showed pop-in behavior at 4 K, that is, cracks approximately 0.1-1 mm long were induced by pop-in in the vicinity of the fusion line at the early stage of loading. JN1 materials heat-treated under the conditions of 650 °C-5 h and 700 °C-1 h showed similar fracture characteristics to those of the fusion line specimens at 77 K and 4 K except for the cracks induced by pop-in. With increasing temperature and time of heat treatment, SP energy decreased significantly, and the fracture surface showed a transition from ductile dimple to brittle intergranular cracking. It is shown that the SP testing method is useful for evaluating the fracture behaviors of microscopic regions such as the fusion lines of welds at cryogenic temperatures.