Evaluation of environmental effects on fatigue crack growth behaviour of a high strength steel in a saline solution with cathodic protection
Introduction
In most of the studies devoted to fatigue crack growth behaviour of metals in a corrosive environment, ambient air is taken as the reference environment. This choice is justified by the fact that air is the most usual working environment for structural materials. It ensues that the fatigue behaviour of these materials is well documented in this environment, which appears consequently as a natural reference. This is the case in many works [1], [2], [3], [4] that have been published on the effects of a saline solution on the Fatigue Crack Growth Rates (FCGRs) of long cracks in structural steels under cathodic protection.
The typical fatigue crack growth behaviour obtained in these conditions, for example by Vosikovsky [1], is schematically represented on Fig. 1. This figure is valid for a constant load ratio and does not account for the strength level of the steel. In the ΔK range where the curve obeys a Paris law: da/dN=C.ΔKm, the effect of a saline solution compared to air is generally observed above a critical ΔK value. This effect is characterised by the higher value of m in the corresponding Paris law. It means that the crack growth rate enhancement as ΔK increases is stronger in the aqueous solution than in air. This regime is followed by the occurrence of a ‘plateau’ for which the FCGR in NaCl with cathodic protection remains nearly constant (m≈0). According to Vosikovsky [1], the lower is the frequency, the higher is the FCGR value associated with the plateau. Others authors [2], [5] established that this FCGR value also increases when the applied cathodic potential becomes more and more negative. In the region below the plateau, where the difference between the da/dN curves in the aqueous solution and air tends to increase with ΔK, environmental effects are attributed to an embrittlement by hydrogen resulting from water reduction near the crack tip [1], [4]. This mechanism is associated with brittle fracture surfaces. According to FCGRs curves, two conclusions are generally proposed. Firstly, a critical ΔK value is required to activate the environment-assisted mechanism due to hydrogen. Secondly, the effect of hydrogen increases with ΔK or with the plastic zone size.
Nevertheless, these interpretations are based on the implicit assumption that environmental effects in air can be neglected. However, it is known that air is a very active environment with regard to fatigue crack growth if the results are compared with those obtained in a high vacuum, in which the action of gaseous species are strongly limited [6]. Consequently, if the comparison with results obtained in air remains useful, from an engineering point of view, it does not allow a good comprehension of environmental effects in saline solutions. This requires a comparison with a truly inert environment.
The present study is devoted to the fatigue crack propagation of natural cracks in a high strength steel cycled in 3.5% NaCl solution with cathodic protection. Cathodic protection potential and fatigue test frequency have been selected in relation to the use of this steel in offshore applications (jack-up platforms). Previous work by Huneau and Mendez [7] on the same steel has already shown the respective effects of air and NaCl solution with cathodic protection on fatigue lifetimes, crack initiation kinetics and damage mechanisms, in comparison to the results obtained in vacuum.
The present work intends to go further in fatigue-environment interactions by investigating the propagation of self-initiated small cracks. The size of these cracks evolves from 100 to 1000 μm. They correspond however to the incubation period of long cracks generally considered in fatigue crack propagation studies. Environmental effects are analysed through a comparison with the fatigue behaviour obtained in a high vacuum. Fatigue tests were also performed in air, which is considered here as another active environment.
Section snippets
Material
The tested steel denoted SE702 is a member of the Superelso family. This structural high strength steel, manufactured by Industeel (ex Creusot–Loire Industrie), is equivalent of the ASTM A517 Grade Q. The chemical composition is indicated in Table 1. The mechanical properties of the material were recorded as follows: 0.2% yield stress 780 MPa, ultimate tensile strength 860 MPa, elongation 19%. This quenched and tempered steel exhibits a tempered refined martensite microstructure illustrated in
Fatigue crack growth rates curves
FCGRs curves established in air and in vacuum at 20 Hz and in air and in NaCl/CP at 0.17 Hz are plotted in Fig. 4. For each test conditions, the crack growth evolution obeys a Paris law (da/dN=C.ΔKm). The corresponding C and m values are reported in Table 2. This table also indicates the initial crack depth at which fatigue crack growth tests were started.
The corresponding da/dN−ΔK results obtained at 20 Hz in air and in vacuum are in good agreement with the effective Paris laws, da/dN−ΔKeff,
Environmental effects in air
Results in air confirm that this environment is highly active since FCGRs can be amplified by more than one order of magnitude when compared to vacuum. The values of the parameters m and C of the Paris law determined in air at 20 Hz are comparable to those obtained by Hénaff et al. for long fatigue cracks on another quenched and tempered steel [12]. When compared to results obtained in vacuum, the influence of air (20 and 0.17 Hz) on FCGRs is maximal at low ΔK, and then significantly decreases as
Summary and conclusions
Fatigue crack propagation of natural small cracks in the high strength steel SE702 was investigated in different environments. From this study, the following results have been obtained:
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In 3.5% NaCl solution at 0.17 Hz, hydrogen produced by cathodic protection strongly enhances FCGRs compared to vacuum. But, contrary to accepted belief based on a comparison with the results obtained in air, the influence of hydrogen does not increase with ΔK. Indeed, the present study shows that hydrogen effects
Acknowledgements
The Région Poitou-Charentes is acknowledged for its financial contribution to the grant of B. Huneau. The authors are also grateful to IFREMER-Brest for financial and technical support, and particularly to D.Choqueuse for his advises concerning the electrochemical experiments. Finally, the authors would like to thank Pr. G. Hénaff for fruitful discussions on environmental effects on fatigue behaviour.
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