Development of the Strain-to-Fracture Test for Evaluating Ductility-Dip Cracking in Austenitic Stainless Steels and Ni-Base Alloys

Abstract

The strain-to-fracture test has been developed as a reproducible and robust test technique for evaluating susceptibility to ductility-dip cracking (DDC) and other elevated temperature cracking phenomena. Samples are tested over a range of temperature and strain, producing a temperature-strain “envelope” in which cracking occurs. Threshold strain for fracture (Emin) and the ductility-dip temperature range (DTR) can then be determined from these envelopes and used to compare susceptibility among materials. The test is very flexible and allows for changes in the testing parameters and material conditions that permit the factors that affect cracking to be identified. In this investigation, the weld metals of three austenitic stainless steels, two Ni-base alloys, and two Ni-base filler metals have been tested using the strain-to-fracture test. The stainless steels include type 304, type 310, and the superaustenitic grade AL-6XN. The Ni-base alloys tested were alloy 690 and C-22. Filler metal samples produced using the gas-tungsten arc welding (GTAW) process included alloys 52 and 82. Alloy 690 and filler metal 52 were found to be the most susceptible to DDC with a low threshold strain and wide DTR. Type 310 stainless steel exhibited a similar DTR to alloy 690 but had a higher threshold strain. Type 304 stainless steel was found to be the most resistant. Metallographic evaluation of strain-to-fracture samples revealed that cracking occurred preferentially along migrated grain boundaries (MGBs) in the weld metal. Cracking was most severe in the weld metals that were free of second phases or precipitates and exhibited very straight MGBs. The high resistance of type 304 was related to the presence of ferrite in the weld deposit. Recrystallization was observed at temperatures near the upper end of the DTR and was accompanied by a recovery of ductility.