Improving the ductility-dip cracking resistance of Ni-base alloys

Abstract

Many austenitic alloys undergo a severe ductility drop at intermediate temperatures, which may result on the solid state phenomenon known as ductility-dip cracking (DDC). This intermediate temperature intergranular cracking has been recognized as a grain boundary sliding creep-like phenomenon. Several factors have been identified to control the DDC susceptibility of these austenitic alloys, including grain size, grain boundary orientation to the applied force, impurity segregation to the grain boundaries, grain boundary tortuosity, and intergranular precipitation. This work studies the effect of Nb and Ti additions to Ni-base filler metals 52(ERNiCr-3) and 82(ERNiCrFe-7) on the DDC susceptibility. Strain-to-fracture (STF) test were performed on Ni-base filler metals 52 (0.25%Mn–8.9%Fe–29.1%Cr–0.5%Ti–0.7%Al–Ni) and 82 (2.75%Mn–0.7%Fe–20.1%Cr–2.6%Nb–0.5%Ti–Ni) with different amounts of Nb and Ti additions. Optical and electron microscopy were used to characterize the microstructures and fracture surfaces. Segregation of the added elements to the solidification grain and subgrain boundaries was verified. The Nb addition to FM-52 caused the precipitation of eutectic NbC carbides. The additions of Ti caused the formation of large Ti(C,N) and the additions of Ti and Nb caused the formation of (TiNb)(CN) eutectic-like precipitates in both weld metals. When large amounts of Nb and Ti were added to FM-52, colonies of acicular and blocky precipitates were observed. Solidification cracks formed when large amounts of Nb and/or Ti were added. On the other hand, small Nb and Ti additions caused an important reduction of DDC susceptibility. The Nb- and Ti-rich precipitates pinned the migrated GBs causing an increase in the GB tortuosity. For the temperatures and strains tested, Nb and Ti additions resulted in a general increase in the threshold strain required to initiate solid state cracking during the strain-to-fracture test.