Effect of 475oC embrittlement on the fatigue behaviour of a duplex stainless steel

Abstract

The low cycle fatigue behaviour of a duplex stainless steel was studied in standard heat treated and embrittled (by aging at 475°C for 100 hours) condition by mechanical testing, scanning and transmission electron microscopy. The fatigue crack initiation behaviour was studied in the embrittled condition by interrupting stress-controlled fatigue tests, identifying the crack initiation sites and relating them to the crystallographic parameters obtained from EBSD-OIM scans. The aging treatment at 475°C resulted in the precipitation of a' in the ferritic phase. Impact energy drops from 260 Joules in the annealed condition to 8 Joules in the aged condition. The drop in impact energy was caused by the inability of the ferritic phase to form deformation twins in the embrittled condition, which was confirmed by TEM examination. From the low cycle fatigue experiments conducted at A C/2 = 4. Ox 10-', 6. Ox 10-', 8.Oxl0-3 and 1.Ox10"2, it was established that the deformation curves in the annealed condition has three discernible stages: (i) cyclic hardening, (ii) cyclic softening and (iii) cyclic saturation and in the aged condition has two discernible stages: (i) cyclic hardening, and (ii) cyclic softening till final failure for all values of strain amplitude. A change in slope is observed in the cyclic stress-strain curve in the aged condition as compared to the standard heat treated condition. In the range of strain amplitude employed, in the aged condition, fatigue life is longer at lower plastic strain amplitude, decreases and becomes similar at intermediate plastic strain amplitude and becomes shorter at higher plastic strain amplitude in comparison to the standard heat treated condition. The gradual decrease in fatigue life with increase in plastic strain amplitude in the aged condition was attributed to the rapid cyclic softening caused by disappearance of the a' precipitates. From the fatigue crack initiation studies conducted at Ao12 = 400 MPa and 500 MPa, it was established that the crack initiation sites are the slip markings corresponding to { 111 } plane traces in the austenitic grains at Dof2 = 400 MPa and more cracks were observed to initiate at E3 CSL boundaries in the austenitic grain at Do12 = 500 MPa in the aged condition. The major resistance to crack growth came from the ay phase boundary.