Fractographic studies on dynamic failure in electron-beam-welded joints in austenitic steel

Abstract

1. In single-phase ASS of 06Kh23N28MDT type, reducing the temperature from 20°C to-60°C causes the failure to go over from viscous to quasiviscous, which affects the cracking resistance. 2. When there is deformation-activated austenite-martensite transformation, which sets in in 06Kh23N28MDT steel at −196°C, the metal fails in a viscous fashion, and the fracture has prominent pit microrelief without structural orientation, which raises the dynamic viscosity. 3. Two-phase 12Khl8N9T steel contains a small amount of platey ferrite and is characterized by viscous fracture at +20°C and brittle fracture at −196°C. 4. Increase in the amount of δ ferrite and a transition of it to vermicular from in Kh18N2M3T steel leads to quasibrittle failure in the interdendrite region and to structural orientation of the microrelief at 20°C and −60°C, which is seen as a reduction in the dynamic viscosity. 5. The chemical inhomogeneity in the austenite near the austenite-ferrite boundaries in this steel leads at −196°C to destabilization of the austenite, with strong plastic strain ahead of the crack tip causing the austenite-martensite transformation. The fractures then have prominent viscous regions, in which the failure has occurred by the pore coalescence mechanism. That type of crack propagation raises the dynamic viscosity. 6. Fractographic studies confirm the previous demonstration in [1] of effects from the primary and secondary structures on the dynamic failure viscosity of metal in electron-beam welded joints in austenitic and austenitic-ferritic stainless steeels.