Fracture Toughness Testing and Toughening Mechanisms of Some Commercial Cobalt-Free HardfacingAlloys

Hardfacing alloys are weld deposited to provide a wear resistant surface for stmctural base materials. Commercial low cobalt hardfacing alloys are being evaluated to reduce plant activation levels. Since hardfacing alloys typically must be resistant to cracking to assure adequate in service performance, fracture toughness is a ~ critical material property. Fracture toughness (KJ measurements of Fe-base, NLbase, and Co-base hardfacings were performed in accordance with ASTM E399-90 procedure in an effort to identi~ a tough cobalt-free alternative. Reduced scatter in KICdata was observed for the Fe-base hardfacings, and the 95% lower bound KICvalues were generally higher than the Ni-base hardfacing alloys. Preliminary crack growth data obtained during pre-cracking indicate that the Ni-base hardfacings possess better fatigue crack growth resistance. However, none of the Fe-base or NLbase hardfacings have KICvalues that are comparable to the reference Co-base IiZrdfacing. The test specimens were machined from thick (0.5”) weld deposits, and the rnicrostructures of the test specimens are compared with the . more prototypic, thinner deposits. Microstructural and fiactograpbic examinations are used to characterize the fracture mechanisms and delineate the operative toughening mechanisms. Crack deflection and crack bridging toughening mechanisms are shown to be relevant for most of the commercial hardfacings.