Cyclic deformation and fatigue behaviour of 7Mo-0.5N superaustenitic stainless steel-stress-strain relations and fatigue life

The cyclic deformation characteristics and fatigue behaviour of a superaust enitic stainless steel with composition Fe-25Cr-22Ni-7.6Mo-3Mn-0.46N (wt%) have been investigated. Detailed studies were performed on cyclic hardening /softening behaviour, hysteresis loops, waveform, fatigue lifetime, and int ernal as well as effective stresses during cyclic straining in the total st rain amplitude range 2.7.10(-3) - 1.0.10(-2) Special attention is paid to t he role of nitrogen and the interaction between nitrogen and molybdenum. Im mediate cyclic softening takes place at small strain amplitudes, whereas ha rdening occurs during the first few cycles at large strain amplitudes follo wed by softening. For all strain amplitudes a virtually stationary state de velops after about 10% of the lifetime with only a weak decrease of the pea k stresses. In the cyclic stress-strain curve the material hardens linearly during multi step testing, whereas single step testing leads to excessive hardening at the largest strain amplitudes. During strain cycling the inter nal stresses develop like the total stresses, while the effective stresses decrease with increasing number of cycles for all strain amplitudes and als o diminish with increased strain amplitude. This behaviour is discussed in terms of developing dislocation structures, studied in an accompanying pape r. A double slope behaviour in Coffin-Manson diagrams is observed. The fati gue lifetime resembles that of AISI 316 with 0.29 wt% nitrogen at high stra in amplitudes but is shorter at lower strain amplitudes. However, in stress controlled situations the superaustenitic material is superior. (C) 2001 A cta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.