PSI - Issue 26

Myroslava Hredil et al. / Procedia Structural Integrity 26 (2020) 409–416 Hredil et al. / Structural Integrity Procedia 00 (2020) 000 – 000

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1. Introduction

Main gas pipelines are considered by as long term operated objects, which are subjected during their service to combined action of operational stresses and aggressive environment. Operational stresses are suggested to be sustained static stresses from the pressure of transported hydrocarbons. Zvirko et al. (2016) and Nykyforchyn et al. (2019) pointed out that such operational condition of pipe steels lead to their susceptibility to stress corrosion cracking (SCC) which is the main cause of pipeline failures. However, it should be noted that pipelines can also be under action of cyclic stresses (for instance, due to temperature changes), which is especially noticeable for over-ground sections of pipelines. Corrosive environments in turn can affect pipeline not only from the outside, due to soil corrosion in places with damaged protective coatings), but also from the inside, as shown by Hredil et al. (2010), inducing pipe steel hydrogenation. It is evident from a review of Ohaeri et al. (2018) that hydrogen causes degradation of pipe steels which is also proved by Gabetta et al. (2008), Gredil (2008). Since it is impossible to avoid hydrogenation of the main gas pipeline steels due to the corrosive influence of aggressive components of transported hydrocarbons, the actual task is to evaluate the consequences of these harmful effects on the workability of pipelines (including in the presence of cyclic components of pipeline loading). In this regard, this work is aimed at studying the features of the fatigue crack growth in pipeline steels degraded in operational and laboratory conditions, taking into account the impact of soil solution on the external pipe surface.

2. Regularities of fatigue crack growth for different pipeline steels

2.1. Effect of long term operation

The changes in fatigue crack growth characteristics of the steels after their long term operation were analyzed. The effect of operational degradation was not revealed in the Paris region of the fatigue crack growth curves for all tested steels while some changes were observed in their near-threshold regions. Namely, threshold values of stress intensity factor (SIF) range were lower for all operated steels comparing to the corresponding values for as-received ones (see Fig. 1). The maximum effect of operational degradation was determined in the case of the steel 17H1S, and the minimum in the steel X70 (values above the bars indicate a decrease of  K th due to degradation). Thus, the main attention during fractographic investigations has been paid to peculiarities of the near-threshold fatigue crack growth in different steels in the as-received state and after their operation.

Fig. 1. Threshold values of SIF Δ K th for pipe steels of different strength depending on their state tested in air.

Peculiarities of near-threshold fatigue crack growth in air have been analyzed on the steel 17H1S in as-received state and after 30 years of operation on the gas main, since the effect of operational degradation for this steel was the most noticeable. It was noted that fracture mechanism of the steel at the near-threshold section of the fatigue crack growth curve is generally the same for both tested steels. First of all, the change in orientation of local elements on the fracture surfaces from grain to grain is noticeable (Fig. 2). However, the fracture surface of the steel in the initial