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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Considering the fracture surfaces of 17H1S steel in both initial and operated states after the tests in NS4 solution at a crack growth rate of ~ 10 -9 m/cycle, obvious traces of the corrosive medium action can be noted in the form of brightly shining corrosion products (especially along the contact zones of the crack edges, Fig. 4a, b). Significant oxidation of the fracture surfaces of the specimens indicates the creation of favorable conditions for wedging of the crack edges by corrosion products due to their larger volume, as shown by Suresh et al. (1981). It was suggested that the wedging effect of the corrosion products for the operated steel is more pronounced due to a higher fracture relief of this steel; therefore, retardation of crack growth in the near-threshold section of the fatigue crack growth curve is more noticeable.

a b Fig. 4. Fracture surfaces of 17H1S steel in the as received state (a) and after long term operation (b) after the tests in NS4 solution. Results of fatigue crack growth tests of 17H1S steel in NS4 solution revealed also some acceleration of the crack growth in the Paris section of the fatigue crack growth diagram. The effect of corrosive environment was more appreciable for the long-term operated steel. (Fig. 3). This could be explained fractographically by appearance of intergranular facets on the fracture surfaces of both types of steel, which usually is concerned with hydrogenating effect of environment. The maximum embrittling effect of hydrogenation is manifested by cracking along the grain boundaries (Fig. 4). Additionally, more essential secondary cracking along grain boundaries is observed in the case of the operated steel. (Fig. 4b). Therefore, combined action of  K (within the 2nd section of the fatigue crack growth diagram) and corrosive environment enabled visualization of in-service damages along grain boundaries or, at least, indication of cohesion weakening between adjacent grains caused by long term operation of steel. Moreover, decohesion between ferrite and pearlite layers in 17H1S steel at the microscale revealed earlier by Krechkovs’ka et al. (2019) is fractographically confirmed in the form of areas with a lamellar structure of perlite surrounded by ferritic grains. Steel X70. Similar results have been obtained for the operated steel X70. In this case, fatigue crack growth in NS4 solution was also a little faster. Increase in fracture relief at the beginning of the 2nd section of the fatigue crack growth curve was also confirmed for the operated X70 steel in a corrosive medium comparing with the tests in air (Fig. 5a,b). Typical fatigue relief consisting of festoons elongated in the main direction of the crack front, with classic fatigue striations perpendicularly to festoons (Fig. 5а) was observed on the fracture surface of the operated steel after the test in air. However, intergranular cracking (as in the case of the steel 17H1S) due to cohesion weakening between adjacent grains was revealed after the test of the same long-term operated steel in the environment (Fig. 5b). Corrosive environment additionally reduces the resistance to intergranular fracture. It is noteworthy that the combined effect of a corrosive environment and long-term operation is less noticeable on the fracture surfaces of X70 steel than for 17H1S steel, which is consistent with the results of the mechanical testing.

3. Methodic aspects of fatigue testing of pipe steels

3.1. Method simulating in-service conditions of gas pipelines

A methodic approach to fatigue testing of pipe steels was proposed which allows approximating to operating con ditions of the main pipelines due to simulation of the steel hydrogenation without its contact with corrosion environment.