THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MECHANICAL BEHAVIOUR OF A DUPLEX STAINLESS STEEL: Part II. FATIGUE LIFE AND FATIGUE CRACK GROWTH

Abstract

The mechanical behaviour of AISI 329 steel has been investigated for ageing times up to 20,000 h at temperatures of 475,425, 375, 325 and 275°C. The study has concentrated on the changes in the response to cyclic strains, in the low-and the high-cycle fatigue regimes, and in the resistance to fatigue crack propagation as a function of temperature and time of ageing.It is shown that ageing increases the fatigue resistance in the high-cycle fatigue regime, but the opposite occurs in the low-cycle fatigue regime. Ageing increases the LEFM threshold stress intensity factor range for fatigue crack propagation which reaches high values in these alloys, and is influenced by the fatigue load ratio. Crack closure contributes to the LEFM threshold stress intensity factor range for crack propagation only in the annealed condition of the AISI 329 steel. NOMENCLATURE a = crack length b, c = fatigue strength and ductility exponent, respectively K,, = closure stress intensity factor N , = number of cycles to failure A& = crack tip opening displacement E, E' = Young's modulus and Young's modulus in plane strane, respectively /? = constant AK,= stress and effective stress intensity factor ranges, respectively AK,,, = threshold stress intensity factor range Acr = stress range E; = fatigue ductility coefficient of = fatigue limit u; = fatigue strength coefficient by, cr; = yield stress and cyclic yield strength, respectively

Fatigue crack propagation in cast duplex stainless steels: thermal ageing and microstructural effects

“…Fatigue crack propagation micro‐mechanisms have only scarcely been investigated. More numerous studies deal with FCGB in hot‐ or cold‐rolled duplex stainless steels containing larger amounts of ferrite (45–62%) 14–23 . It has been shown that, in these materials, the fatigue crack growth rate (FCGR) is increasing with thermal ageing and with the amount of ferrite and with the associated embrittlement.…”

“…Microstructural effects have also been evidenced but all the authors do not agree on their origin. For instance, the frequency of crack deviations along the crack path can be either higher 13 , 20 or lower 9 in the unaged condition than in the aged condition. In wrought duplex stainless steels, Marrow and King 17 observed crack deviation at all interfaces for crack propagating perpendicularly to the elongated phases.…”

“…Up to now, the effect of ageing on crack closure in duplex stainless steels is not well established. Moreover, only RICC and TICC mechanisms have been studied 11 , 18–20 , 23 …”

“…More numerous studies deal with FCGB in hot-or coldrolled duplex stainless steels containing larger amounts of ferrite (45-62%). [14][15][16][17][18][19][20][21][22][23] It has been shown that, in these materials, the fatigue crack growth rate (FCGR) is increasing with thermal ageing and with the amount of ferrite and with the associated embrittlement. Microstructural effects have also been evidenced but all the authors do not agree on their origin.…”

“…Moreover, only RICC and TICC mechanisms have been studied. 11,[18][19][20]23 . Nevertheless, in this ductile material, plasticity should also induce crack closure.…”