Corrosion Fatigue of Simulated C-Mn Steel HAZs in Sour Produced Fluids

Abstract

Steel Catenary Risers are subjected to both fatigue loading (from waves and tides) and corrosive environments (internal and external). The financial benefit of using C-Mn steels for SCRs is significant, therefore there is a need to establish the limitations of the material. These limitations are likely to be controlled by the HAZ, as HAZ microstructures are typically more susceptible than parent steel to cracking during exposure to a sour environment, especially if they have high hardness. Samples of API 5L X65 line pipe steel were heat treated to provide materials exhibiting two microstructures comparable to those seen in girth weld HAZs with two levels of hardness. Fatigue tests were then performed in a sour environment to determine the influence of microstructure, frequency and crack depth on the observed crack growth rate. Tests were performed using a ‘frequency scanning’ technique which involved maintaining a constant ΔK by continually monitoring and shedding the applied load range as crack length increased during the test. Frequency was varied in the range 10–0.01Hz. As there was a concern that the effect of frequency might be masked by an effect of crack depth, a second series of tests was also carried out on each microstructure to explicitly investigate the effect of this latter variable. A stress ratio of 0.5 was used to ensure a relatively high mean stress, to simulate the presence of a tensile residual stress. Test data suggested that harder material exhibited a higher fatigue crack growth rate in the sour environment, and that frequency had only a small effect over the range examined. A second series of tests suggested that crack growth rate was independent of crack depth for crack depths greater than 6mm, although there was some evidence that for shallower flaws the crack growth rate may be higher. It is suggested that when performing fracture mechanics calculations using fatigue crack growth rate data, it is important to ensure that the latter are associated with tests performed on material exhibiting a comparable microstructure to the component being modelled. For relatively shallow flaws it may be the case that crack growth rates in a sour environment are higher (for a given ΔK) than for longer flaws. It should therefore be recognised that in certain applications, defect assessments which use crack growth rate data derived from tests using deeply-cracked specimens may result in non-conservative life predictions.