Ductility-Dip Cracking Susceptibility of Austenitic Alloys

摘要

A new test technique, the strain-to-fracture (STF) test, has been developed to evaluate and quantify elevated temperature, solid-state cracking that occurs in austenitic alloys. This test has been used to determine the minimum strain required to induce grain boundary cracking in a range of alloys and allows temperature-strain "envelopes" to be developed that describe ductility-dip cracking susceptibility. In this study, Type 310 stainless steel, Ni-base Alloy 690, and the super-austenitic alloy AL-6XN were evaluated. A significant difference in the strain-to-fracture behavior was observed among these alloys. Metallurgical analysis revealed that intergranular cracking occurred preferentially along fusion zone migrated grain boundaries (MGBs). Cracking was most severe in materials that were free of second phases or precipitates and accordingly had the straightest MGBs. Recrystallization was observed at test temperatures near the upper end of the ductility-dip temperature range (DTR) and was accompanied by a recovery of ductility. At similar temperatures, the fracture behavior of all three materials was similar. At low temperature within the DTR, samples were macroscopically flat with evidence of ductile intergranular fracture. This transitioned to a wavy fracture surface with slip lines and thermal striations at high temperatures.