Work carried out in the recent past by members of the International Cyclic Crack Growth Rate Group (ICCGRG) has demonstrated the close connection between the occurrence of environmentally assisted cracking (EAC) in low-alloy steels in a pressurized water environment and the presence of sulfide inclusions in the steel. Due to the length of time that experimental work can take, coupled with the sometimes poor repeatability of tests, work was undertaken on a computer model of “sulfide particle” distributions and their interaction with a hypothetical crack front.
The model is basically simple in concept, proceeding in the following manner. An array of randomly located spherical particles, each assumed to consist of a large number of subparticles is set up within a matrix. A crack, exposed to a water environment, is then grown through this matrix, particles intercepted by the crack dissolving; this is modeled by the spreading out of subparticles. Additional factors such as the rate of particle dissolution and rate of subparticle loss from the system are also incorporated into the model. The model is set up to calculate the number of subparticles that congregate at the crack tip. The single premise adopted is that greater than a specified number of subparticles at the crack tip is sufficient to bring about EAC.
In the work discussed here, consideration is given to the relative importance of material sulfur content and second-phase particle size. It was found that the latter had a much greater influence on subparticle numbers at the crack tip than the former. From this, it is concluded that the setting of a bounding curve in Section XI of the ASME Code for modern low sulfur steels would depend more on the occurrence of smaller sulfide particles than the lower sulfur content in such steels. Also considered was the variation of the occurrence of EAC with cycling frequency and stress intensity factor range (ΔK). From the boundaries to the EAC regime, data are obtained that can be applied to the setting of upper bounding curves for the ASME Code.