Corrosion Fatigue of an Hsla Steel

Abstract

Single-pitted specimens of an HSLA steel, were tested in laboratory air and in 1 M NaCl solution to study the influence of a corrosive environment on its fatigue life.The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM showed the locations of fatigue crack initiation. The mechanisms of corrosion fatigue were studied by performing tests in 1 M NaCl at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be unimportant in the corrosion fatigue of HSLA steel in this study. The 1 M NaCl corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced fatigue behaviour. A non-damaging pit depth was found. NOMENCLATURE a = material constant C = pit shape-related constant D = pit diameter E = elastic modulus E' =potential er = fracture strain h = pit depth i = current density K = strength coefficient Kt = elastic stress concentration factor Kr = experimental fatigue notch factor Kr = calculated fatigue notch factor L, = left-side crack length L2 = right-side crack length NT = total fatigue life Ni = surface crack initiation life Ni,T = fraction of fatigue life devoted to surface crack initiation life N p = surface crack propagation life n = strain hardening exponent R = stress range r = pit radius R.A. = reduction of area S, = ultimate tensile stress S, = 0.2% proof stress S,, = upper yield stress AK = stress intensity factor AS = applied stress range a, = fracture stress "Deceased.625

Influence of Pitting Corrosion on Fatigue and Corrosion Fatigue of Ship and Offshore Structures, Part II: Load - Pit - Crack Interaction

“…In the tests [19] the pit aspect ratio values changed in the range from 1/3.6 to 1/2.1 with their average value of about 1/3. Hence the stress concentration factor values changed in the range from 1.85 to 2.2 with their average value equal to 1.98 .…”

“…The non-damaging pit depth values are very different even for similar materials: e.g. for weldable HSLA steels the values amounted to 110 μm [19] or below 40 μm [21]; for Al-alloys the smallest depth of a pit in which fatigue cracks were initiated , amounted to 60 μm [2], 20÷30 μm [8] or 2 μm [5], hence the non-damaging pits for Al-alloys were smaller than for steels. The above mentioned scatter of the data suggests that not only the pit depth but also another factors may affect pit-tocrack transition.…”

“…the ratio of the pit depth (a p ) and pit breadth (the diameter D). The following formula has been proposed [19] for the stress concentration factor (K t ) at a pit:…”

“…The stress concentration at a pit is determined in global scale by general pit curvature, but on the local scale by the curvature strictly in the site where crack is initiated [3]. In order to examine a role of roughness of pit walls for crack initiation in a weldable HSLA steel, many pits were chemically polished to make their bottom more smooth [19]. The life of specimens with more smooth pits was longer by about 50% than those with rough (non-polished) pits.…”

“…medium-carbon steels after different heat treatment [11][12][13][14][15], or even high -carbon steels [16,17] or a martensitic stainless steel [18]. Only a few papers concern weldable low-alloy, low-carbon steels applicable to ship and offshore structures [4,[19][20][21][22]. Although this paper concerns mainly ship and offshore structures, a limited amount of data on relevant materials makes that a model to be used for predicting fatigue life of such structures must be partly based on conclusions drawn from the tests on materials not applicable to ship and offshore structures.…”