Oxide-Induced Crack Closure: An Explanation for Near-Threshold Corrosion Fatigue Crack Growth Behavior
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dominant in dry, oxygen-free atmospheres or at high load ratios (where there is no plasticity-induced closure) provides a mechanism for increased crack closure. By resulting in an earlier contact between the fracture surfaces during the closing portion of the load cycle, closure loads are raised and the effective stress intensity range at the crack tip reduced, i.e. the presence of the oxide debris effectively raises Kmm. This mechanism for environmentally-affected crack growth, however, is specific only to stress intensity ranges where oxide thicknesses are of the order of crack tip opening displacements (CTOD), e.g. at near-threshold levels. The object of the present paper is to report new evidence supporting the concept of oxide-induced crack closure, and to comment on its significance to nearthreshold corrosion fatigue in pressure vessel steels tested in air, water, hydrogen and helium atmospheres. In particular, a characterization of crack flank oxide deposits is made using ESCA and Auger spectroscopy. Further, additional contributions to enhanced crack closure at low stress intensities resulting from varying fracture surface morphology are discussed in the light of the mechanical, microstructural and environmental behavior patterns commonly observed at near-threshold levels.
EXPERIMENTAL PROCEDURES The two pressure vessel steels investigated were both 2 l/4Cr-1 Mo steels, namely ASTM A542 Classes 2 and 3 (hereafter referred to as SA542-2 and SA542-3 respectively), of composition shown in Table I. The SA542-2 steel, which was supplied as 25 m m thick plate, was fully martensitic following water quenching from 927 ~ prior to tempering (1 V2 h) at 638 ~ The lower strength SA542-3 was supplied as 175 mm thick plate, and after water quenching from 954 ~ and tempering (7 h) at 663 ~ the microstructure was found to be fully bainitic ( ~ 3 pct ferrite), uniformly through the thickness. A few additional tests were performed on SA5422, reaustenitized at 950 ~ water quenched and tem-
ISSN 0360-2133/81/0811-1435500.75/0 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICALSOCIETY OF AIME
VOLUME12A, AUGUST 1981--1435
pered (l h) at 690 ~ to give a lower strength martensitic structure (referred to as SA542-2/T690). Ambient temperature mechanical properties are listed in Table II. Fatigue crack propagation experiments were performed with 12.7 m m thick compact specimens,* ma* Plane strain conditions were maintained, based on the criteria that cyclic plastic zone sizes did not exceed 1/15 of test-piece thickness. chined in the T-L orientation between quarter and mid-thickness sections of the plates. Testing was performed on 50 kN Instron electro servo-hydraulic machines, operating under load control at 50 H z (sine wave) at load ratios R of 0.05 and 0.75. Tests were conducted at ambient temperature in moist air (30 pct relative humidity), distilled water, and high purity dehumidified hydrogen and helium gases. Hydrogen and helium atmospheres were maintained at 138 kPa pressure in a small O-ring sealed chamber, clampe
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