Gigacycle Fatigue Properties of High-Strength Steels According to Inclusion and ODA Sizes

Abstract

Gigacycle fatigue tests were conducted for several versions of JIS-SCM440 low-alloy and JIS-SUJ2 bearing steels using 20-kHz ultrasonic fatigue testing to elucidate the relationship of the inclusion size and type to fish-eye fracture properties. The total number of tested specimens was over 200. Most of the specimens revealed Al 2 O 3 or (Cr, Fe) 3 C inclusion-originating types of fisheye fractures, while TiN inclusions and the matrix also caused fish-eye fractures in some specimens. Based on these fatigue test results, 10 9 -cycle fatigue limits were estimated according to inclusion size by resorting the obtained data points according to their inclusion sizes at the fisheye fracture origin. The estimated fatigue limits revealed saturation when the inclusion sizes were smaller than 15 lm, while those fatigue limits depended on the inclusion sizes to the -1/6th power in the case of inclusions above 15 lm in size. The saturation of the 10 9 -cycle fatigue limits was considered to be caused by the effects of the optically dark areas (ODAs). Moreover, the fatigue limits also depended on inclusion type. In comparing the Al 2 O 3 and (Cr, Fe) 3 C inclusions, the key features causing the difference in the fatigue limits were likely to be bonding between the inclusion and the matrix, i.e., the (Cr, Fe) 3 C inclusions were tightly bonded to the matrix, unlike the Al 2 O 3 inclusions, although both inclusions were of the hard type.

Influence of inclusion type on the very high cycle fatigue properties of 18Ni maraging steel

“…This influence of crack initiation mechanism on VHCF properties is well in accordance with the literature. For high carbon-chromium steel SUJ2, Furuya et al [32] found longer lifetimes and higher endurance limits for specimens featuring crack initiation at Al 2 O 3 -inclusions and interface fracture than for specimens with (Cr,Fe)C-inclusions where the fractured particles acted as crack starters. For the same high carbon-chromium steel, Spriestersbach et al [33] found higher necessary stress intensity amplitudes to propagate a crack to fracture in the VHCF regime, when the crack started at a fractured interface between AlCaO-or CaO-inclusions and matrix than when it started at a fractured TiNinclusion.…”

“…Equations used to predict the VHCF strength typically consider the projected area of the inclusion and the Vickers hardness of the matrix [31]. More recent investigations show, however, that also the type of inclusion is important for the harmful influence of inclusions on VHCF strength [32,33].…”

Gigacycle Fatigue Properties of Double-Melted SCM440 Steel and Size Effects

“…When using conventional specimens, however, the difference between the two materials is small, so the results from conventional specimens tend to be misleading, as seen in our past research. 28) Hence, using larger specimens is strongly advisable to be able to correctly evaluate the gigacycle fatigue properties of highstrength steel. This also applies to the evaluation of inclusion sizes when an inclusion inspection employs fatigue testing.…”

“…One study indicated that the gigacycle fatigue strength of double-melted and single-melted steels were equivalent in spite of the fact that double-melting reduces inclusions. 28) This result perhaps needs review using larger specimens, since conventional specimens are less reliable. The author already has the test results for single-melted steel, 25) so the review would be achieved by simply adding the new results for double-melted steel.…”

A model to predict S–N curves for surface and subsurface crack initiations in different environmental media

“…In addition, it is commonly believed that the maximum inclusion size exists, below which no crack would initiate at inclusions. The maximum inclusion sizes are 5 lm [38], 8 lm [39] and 15 lm [40] for different steels tested in air. Yang et al [41] proposed an empirical expression to calculate the maximum size for high strength steel, i.e.…”