PSI - Issue 2_B

Balázs Fekete et al. / Procedia Structural Integrity 2 (2016) 2164–2172 Author name / Structural Integrity Procedia 00 (2016) 000–000

2169

6

Results of dislocation density measurement in 08Ch18N10T steel interrupted fatigued samples are shown in Fig. 4(c). Total dislocation density increased at the beginning of fatigue life and then gradually decreased. Dislocations were concentrated in PSBs and the walls of cells, which act as strong obstacles for dislocation motion, and channels and cells are cleared of dislocations as a result of the annihilation of screw dislocation during stress cycling.

Fig. 4. Investigation of the kinetics of the fatigue process for 08Ch18N10T (a) TEM micrograph in the initial state (b) TEM micrograph in the 50% fatigued state (c) evolution of the dislocation density with fatigue

3.2. Fractography Samples of 15Ch2MFA steel tested under ISO conditions usually show one or two major cracks and several smaller ones. The major cracks often originated at the joint between the thermocouple and the specimen. The number of large cracks decreases with increasing total strain amplitude, while the number of smaller cracks grows. These are focused near the ends of the major cracks and form a network (Fig. 5(a)). The origins of the cracks are situated on the test bar surfaces. They have a semi-oval shape (Fig. 5(b)). The fracture surfaces are fine grained in appearance and covered with a very fine blue oxide layer. The total lengths of the major cracks are up to 4 mm.

Fig. 5. (a) Surface of test specimen, 15Ch2MFA, ISO, TSA = 1.5% (b) Fracture surface of cracks, 15Ch2MFA, ISO, TSA = 0,6% (c) Fracture surface near the origin of a crack, 15Ch2MFA, ISO, TSA=0.6% (d) Dependence of the distance between striation lines on the length of crack for 15Ch2MFA