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

Abstract

A B S T R A C TThe ferrite phase of cast duplex stainless steels becomes embrittled after thermal ageing, leading to a significant decrease in fracture properties. In the present paper, the influence of ageing and solidification structure on the fatigue crack growth rates (FCGRs) and on the fatigue crack growth mechanisms in a cast duplex stainless steel is studied. FCGRs measured at room temperature increase slightly after ageing at 400 • C, due to ferrite cleavage and to the resulting irregular shape of the crack front. The crack propagates without any preferential path by successive ruptures of ferrite and austenite phases. The macroscopic crack propagation plane depends on the crystallographic orientation of the ferrite grain. Secondary cracks can appear due to the complex solidification structure. This in turn influences the FCGR. The fatigue crack closure level decreases with increasing ageing. This can be explained by a decrease in the kinematic cyclic hardening of these materials.Keywords cast duplex stainless steels; crack closure effect; crack propagation; EBSD; fatigue. N O M E N C L A T U R Ea = crack length B = specimen thickness K = K max -K min K eff = K max -K op K max = stress intensity factor at maximum load K min = stress intensity factor at minimum load K op = stress intensity factor at opening load P = applied load P max = maximum applied load P min = minimum applied load P op = crack opening load R = isotropic part of hardening W = specimen width X = kinematic part of hardening α = ferrite γ = austenite ε = total strain ε p = plastic strain σ = stress I N T R O D U C T I O NDuplex stainless steels containing up to 30% ferrite (α) in an austenite (γ ) matrix are used for the fabrication of pipes in nuclear power plants. These materials combine the qualities of each phase: the ferrite resistance to corrosion and to hot cracking and the austenite toughness. 1 However, it has been established that microstructural transformations occur in ferrite after long time exposures at the service temperature, i.e., about 300 • C.

Thermal Aging Phenomena in Cast Duplex Stainless Steels

“…[2][3][4][5][6][7]9 However, a complete embrittlement (or zero ductility) above room temperature has not been observed in the thermally aged CASS alloys for LWR conditions. Many past studies have been carried out at temperatures higher than the common operating temperature range, i.e., in accelerated aging conditions, to obtain detectable property change within short time periods, [2][3][4][5][6][7][10][11][12][13][14][15][16][17][18][19][20][21][22] and it has become customary to simulate the microstructural processes in aging during service by aging at 400°C. [10][11][12][13][14][15][16][17][18][19][20][21][22] However, as a variety of aging mechanisms are found to strongly depend on aging temperature, we need to be conscious on the relevance of those accelerated aging experiments to the actual service conditions in LWR power plants.…”

“…Further, numerous aging experiments in the past have been performed at or above 400°C to investigate microstructural evolution and mechanical behaviors and to predict long-term CASS behaviors. [2][3][4][5][6][7][10][11][12][13][14][15][16][17][18][19][20][21][22] However, such highly-accelerated aging treatment above the operation temperature range is yet to be thoroughly validated for simulation of the lower temperature (300°C) phenomena in the current fleet of nuclear power plants. 9 The current consensus might be that, although the vast majority of the current nuclear power plants are expected to be operational for extended lifetimes of 60 years or potentially longer, 1 the behaviors of CASS alloys during such prolonged periods have remained largely unknown.…”

Damage and dynamic strain aging in a thermal aged cast duplex stainless steel

“…The microstructure studied by optical microscopy (Fig. 2) confirms the complex morphology of the ferrite ðaÞ and austenite ðcÞ [19]: the two phases are bipercolated and linked by crystallographic orientation relationships which are close to those given by Kurdjumov and Sachs [20], typically [1 1 0](1 1 1)ck[1 1 1](1 1 0)a.…”

“…Columnar grains are oriented along AE1 0 0aea crystallographic directions. Equiaxed ones reveal no preferential orientation [19]. Their size is very large: generally around 2 or 3 mm in diameter.…”

Cyclic deformation and fatigue cracking behaviors of Cu–28wt%Ag binary alloy

“…In many alloys, such as hypoeutectic Al-Si binary alloy, alpha/beta titanium alloys and duplex stainless steel, the short crack nucleation was along the interphase interfaces [35][36][37][38][39][40][41][42], and the internal stresses associated with strain incompatibility may be the origin of crack initiation. Pineau et al [43,44] carried out numerous tests on the fatigue and creep properties of various steels. Recently, some Cu-Ag binary alloys with high strength and good conductivity have been developed by cold-rolling or drawing [45][46][47].…”