Damage Evolution and Crack Nucleation during High-Cycle Fatigue in Duplex Stainless Steel

M.C. MARINELLI ; U. KRUPP; M. KUEBBELER ; I. ALVAREZ- ARMAS,; S. HEREÑÚ

Oxford

Encuentro; The 8th Meeting of TC2 on Micromechanisms of the European Structural Integrity Society; 2012

ESIS, Mansfield College, University of Oxford

Duplex stainless steels (DSS) have found widespread use in the chemical and petrochemical industries. This is mainly due to their relatively high strength and toughness as well as high resistance to corrosion and stress corrosion. These remarkable properties enhance the use of DSS in technological applications related to cyclic loading. However, these steels embrittle when they are exposed in the temperature range of 280-500ºC limiting their applications in this range of temperatures. The 475°C-heat treatment causes the formation of small bcc Cr-rich a´ particles of a lattice parameter between that of pure Fe and pure Cr by spinodal decomposition. The increase of the Vickers hardness from 280HV to 465HV in the embrittled ferrite grains seems to strengthen the ferrite-austenite phase boundaries. The fatigue damage mechanism in duplex steels depends strongly on the variation of the strength of the two phase?s austenite and ferrite and on the applied plastic strain amplitude. For studying the effect of this embrittlement on HCF, stress-controlled cyclic tests were conducted on 1.4462 (German standard) DSS in two different heat treatment conditions (annealed, 475°C-embrittled) and scanning electron microscopy (SEM) observations in combination with automated electron backscattered diffraction technique (EBSD) measurements were analyzed. These studies have revealed that the fatigue limit is substantially increased by the embrittling heat treatment. For both heat treatment conditions cyclic plastic activity usually begins firstly in the austenite phase. As cycling proceeds, plastic deformation is present in both phases in the annealed material while in the embrittled DSS only some ferritic grains show plastic deformation near the phase boundaries. Therefore, in the embrittled DSS microcracks nucleate at a/a and a/g boundaries and then propagate along slip markings formed successively in the austenitic and ferritic grains, while in the as received material they nucleate mostly along slip bands in the ferrite and in austenite. The experimental data have been used in combination with a numerical short-crack model, which take the real two-phase microstructure and its elastic/plastic anisotropy into account and permit to describe quantitatively the propagation behaviour of microstructurally short fatigue cracks.