Subsurface Crack Initiation and Propagation Mechanism in the Super-Long Fatigue Regime for High Speed Tool Steel, JIS SKH51, by Fracture Surface Topographic Analysis

Influences of Manufacturing‐Related Microstructural Variations on Fatigue in Carbide‐Rich Tool Steels

“…Extensive investigations of the VHCF properties of various highstrength steels under cantilever-type rotating bending and axial loading were performed by Shiozawa et al, [27][28][29][30][31] Lu et al, [32,33] and Shimatani et al [34][35][36][37] The information provided in literature can contribute in the following to a deeper classification and understanding of fatigue in high-strength and high-hardness steels.…”

“…Several crack initiation and early crack growth mechanisms are reported in the literature, such as crack initiation by local accumulation of fatigue damage at inclusion-matrix inferfaces, [3][4][5][6]21] hydrogen-assisted crack growth originated at inclusions, [3,7,8] and crack initiation through microcarbide decohesion mechanism from the matrix. [27][28][29] Shiozawa et al [27][28][29][30][31][32] comprehensibly demonstrated and investigated the mechanism of this microcarbide decohesion in the stress field of carbides and NMI.…”

“…They concluded that in VHCF and UHCF regime, this decohesion mechanism forms areas of conspicuously high surface roughness around crack-initiating discontinuities in highstrength steels. [27][28][29] This GBF mechanism through "discrete exfoliation of fine microcarbides" around NMI or larger primary carbides located in deeper volume regions is the dominant failure mode in the VHCF and UHCF regime for tool steels at low stress amplitudes. [27][28][29][30][31][32] The GBF is described as a region around the inclusion rich in microcarbides and shows a very rough and granular morphology.…”

“…[27][28][29] This GBF mechanism through "discrete exfoliation of fine microcarbides" around NMI or larger primary carbides located in deeper volume regions is the dominant failure mode in the VHCF and UHCF regime for tool steels at low stress amplitudes. [27][28][29][30][31][32] The GBF is described as a region around the inclusion rich in microcarbides and shows a very rough and granular morphology. [27][28][29] Multiple microcracks initiate by decohesion of spherical microcarbides from the matrix and different microcracks coalesce with each other forming a microcrack network during further fatigue process.…”

“…[27][28][29][30][31][32] The GBF is described as a region around the inclusion rich in microcarbides and shows a very rough and granular morphology. [27][28][29] Multiple microcracks initiate by decohesion of spherical microcarbides from the matrix and different microcracks coalesce with each other forming a microcrack network during further fatigue process. [27][28][29] After the GBF formation process, an interior crack, considered as long crack, can grow radially regarding long crack growth law in Paris regime and form the fish-eye area on the fracture surface.…”