Development of the Strain-To-Fracture Test to Study Ductility-Dip Cracking in Austenitic Alloys
The strain to fracture test has been developed as a robust testing technique for ductility-dip cracking (DDC) and other elevated temperature cracking phenomenon. Samples are tested over a range of temperature and strain, producing temperature-strain cracking envelopes. Threshold strain for fracture (Emin) and the ductility-dip temperature range (DTR) can then be extracted from these envelopes and used to comparesusceptibility among materials. The test is robust and allows for changes in the testingparameters and material condition to develop a fundamental understanding of factors thataffect cracking.Three austenitic alloys were tested using the strain-to-fracture test: Type 310 stainless steel, Ni-base Alloy 690, and AL-6XN. Alloy 690 was found to be the most susceptible to DDC with a low threshold strain and wide DTR. Type 310 stainless steel produced a similar DTR to Alloy 690 but exhibited a higher threshold strain. AL-6XNwas found to have the greatest resistance to DDC with a narrow DTR although the threshold strain was still fairly low.Metallographic samples revealed intergranular cracking occurred preferentially along 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 temperatures near the upper end of the DTR and was accompanied by a recovery of ductility.Fracture surfaces of samples for all three materials were similar in appearance when run at the same temperature. At low and medium temperature of the DTR, samples were macroscopically flat. At high temperatures there was evidence of slip lines and thermal striations.
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