Fracture properties of a 25 Mn austenitic steel and its welds at 4 K

Tensile properties, fracture toughness, and fatigue crack growth rates of 40-mm and 80-mm-thick plates of a 25Mn-5Cr-1Ni steel were measured in liquid helium at 4 K. Similar tests were conducted on 40-mm-thick shielded-metal-arc welds with a composition of 25Mn-2.8Ni-6.5Cr-1.4Mo. The tensile yield strengths at 4 K were 895 MPa for the base metal and 961 MPa for the weld metal; these values are much higher than the yield strength at room temperature (282 MPa for the base metal). Fracture mechanics tests were conducted using 25.4-mm-thick compact tension specimens. The fracture toughness values measured by the J-integral method were 249 MPa √m for the base metal and 158 MPa √m for the weld. The fatigue crack growth rates of both the base metal and the weld were similar to the fatigue crack growth rates of 316 stainless steel plate over the stress intensity range evaluated, 29 to 80 MPa √m. The results indicate that the 25Mn-5Cr-1Ni alloy has a favorable combination of strength, toughness, and fatigue crack growth resistance at 4 K. The fracture surface of representative fracture toughness specimens of the plate and the weld were examined by scanning electron microscopy, transmission electron microscopy, and magnetic measurements; they were ductile and contained no evidence of α' martensite