Corrosion Fatigue Behavior of Austenitic Stainless Steel in a Pure D2O Environment

Strain-hardened austenitic stainless steels are commonly used as structural materials in drilling equipment because they meet the demanding requirements in terms of mechanical, magnetic, and chemical properties necessary for drilling technologies in subterranean energy resources exploration. Drilling operational conditions might become a challenge for the integrity of these materials due to the cyclic loading the drillstring is subjected to, in combination with the downhole temperature, and the corrosivity of the drilling fluid. In this research work, the relationship among the pitting corrosion resistance of one Mn-stabilized austenitic stainless steel and its corrosion fatigue behavior has been determined by means of electrochemical methods, advanced surface characterization, and corrosion fatigue testing in brines of near-neutral pH with different chloride contents at room temperature (RT) and 150°C. It has been determined that the corrosion fatigue behavior of the investigated CrMn stainless steel is strongly affected by its susceptibility to pitting corrosion. The synergistic effect between the corrosive environment and the mechanical load depends upon the applied stress amplitude and the pitting resistance of the material. The corrosion fatigue behavior of the austenitic stainless steel at RT was synergistically affected by the environmental and loading conditions at low stress amplitudes. In contrast, the large susceptibility to pitting of the material at 150°C has a significant detrimental effect on its corrosion fatigue behavior when subjected to high stress amplitudes. The observed damage mechanism at 150°C can be described as pitting-induced corrosion fatigue because pit propagation controlled the corrosion fatigue behavior of the CrMn stainless steel. The obtained experimental results have shown that the pitting resistance, assessed for instance by multiple electrochemical methods, could in cases where pitting susceptibility has a large influence on the environmentally sustained cracking mechanism, be used as an indicator of the expected corrosion fatigue behavior of the material. As demonstrated in this study, however, results from accelerated electrochemical testing solely might have a limited prediction capability of long-term corrosion behavior.

Download Full-text

Effects of Surface Finish and Loading Conditions on the Low Cycle Fatigue Behavior of Austenitic Stainless Steel in PWR Environment for Various Strain Amplitude Levels

Volume 3: Design and Analysis ◽

10.1115/pvp2009-78129 ◽

2009 ◽

Cited By ~ 7

Author(s):

Jean Alain Le Duff ◽

Andre´ Lefranc¸ois ◽

Jean Philippe Vernot

Keyword(s):

Stainless Steel ◽

Austenitic Stainless Steel ◽

Strain Amplitude ◽

Surface Finish ◽

Low Cycle Fatigue ◽

Fatigue Behavior ◽

Complex Loading ◽

Loading Conditions ◽

Test Results ◽

Cycle Fatigue

In February/March 2007, The NRC issued Regulatory Guide “RG1.207” and Argonne National Laboratory issued NUREG/CR-6909 that is now applicable in the US for evaluations of PWR environmental effects in fatigue analyses of new reactor components. In order to assess the conservativeness of the application of this NUREG report, Low Cycle Fatigue (LCF) tests were performed by AREVA NP on austenitic stainless steel specimens in a PWR environment. The selected material exhibits in air environment a fatigue behavior consistent with the ANL reference “air” mean curve, as published in NUREG/CR-6909. LCF tests in a PWR environment were performed at various strain amplitude levels (± 0.6% or ± 0.3%) for two loading conditions corresponding to a simple or to a complex strain rate history. The simple loading condition is a fully reverse triangle signal (for comparison purposes with tests performed by other laboratories with the same loading conditions) and the complex signal simulates the strain variation for an actual typical PWR thermal transient. In addition, two various surface finish conditions were tested: polished and ground. This paper presents the comparisons of penalty factors, as observed experimentally, with penalty factors evaluated using ANL formulations (considering the strain integral method for complex loading), and on the other, the comparison of the actual fatigue life of the specimen with the fatigue life predicted through the NUREG report application. For the two strain amplitudes of ± 0.6% and ± 0.3%, LCF tests results obtained on austenitic stainless steel specimens in PWR environment with triangle waveforms at constant low strain rates give “Fen” penalty factors close to those estimated using the ANL formulation (NUREG/6909). However, for the lower strain amplitude level and a triangle loading signal, the ANL formulation is pessimistic compared to the AREVA NP test results obtained for polished specimens. Finally, it was observed that constant amplitude LCF test results obtained on ground specimens under complex loading simulating an actual sequence of a cold and hot thermal shock exhibits lower combined environmental and surface finish effects when compared to the penalty factors estimated on the basis of the ANL formulations. It appears that the application of the NUREG/CR-6909 in conjunction with the Fen model proposed by ANL for austenitic stainless steel provides excessive margins, whereas the current ASME approach seems sufficient to cover significant environmental effects for representative loadings and surface finish conditions of reactor components.

Download Full-text