4. Initiation and Growth Behavior of Small Inner Crack

In this chapter, using austenitic stainless steel SUS304 and steam turbine rotor material 1Cr-1Mo-1/4 V forged steel, creep-fatigue interruption tests were conducted at high temperatures and low tensile strain rates. The small cracks that initiated and grew inside the specimens were observed on the longitudinal split sections along the stress axis, and their changes with time were clarified. In addition, the size of intergranular cavities, which are considered to be closely related to the inner cracking type creep-fatigue fracture, was measured. The main results obtained can be summarized as follows. Small inner cracks are initiated and grow with a temporal and spatial distribution on grain boundaries nearly perpendicular to the stress axis. However, there are few cases in which neighboring cracks coalesce more than one grain size (about 50 µm) distance apart in the direction of the stress axis. Small inner cracks initiate in the early stage of the lifetime. The density of the cracks increases rapidly after the mid-life. Three-dimensional observation of a 0.75N_f specimen of SUS304 shows that the areal crack density of the longitudinally split section is about 35 cracks/mm² at this point, but the volumetric crack density is approximately 1400 cracks/mm³, which is very high. Multiple cavities are initiated and grow in a series on the same grain boundary, and they merge to form a crack with the length of a grain boundary as a unit. The difficulty of crack initiation is almost independent of the grain boundary length, but cracks tend to be initiated at grain boundaries that are perpendicular to the stress axis. Crack initiation is dominant until late in the fatigue life. Small inner crack growth also progresses discretely with one grain boundary length as a unit due to the growth and coalescence of cavities on almost one grain boundary adjacent to the crack tip and is similar to crack initiation in mechanism. However, there is no significant difference in the crack length distribution up to the late fatigue life, and no significant crack growth is observed. At the late stage of fatigue life, the overall crack length distribution rapidly shifts toward the longer side, suggesting that a large number of small cracks distributed on almost the same cross section grow slightly to induce mutual coalescence and rapidly form a main crack.