Influence of surface residual stresses on gigacycle fatigue response of high chromium cold work tool steel

Abstract

The effect of surface compressive residual stresses (RS) induced by surface grinding and polishing on the gigacycle fatigue behavior of medium-carbon high-chromium alloy cold work tool steel was evaluated. Two test series were performed: Specimens of series I revealed high compressive RS of about -800 MPa at the surface, resulting from grinding with fine emery paper, which treatment had definitely a beneficial influence on the fatigue endurance strength. The existence of surface RS was also revealed to be responsible for the location of the fatigue crack initiation. High compressive RS favored internal crack origins. In this case crack nucleation sites were primary carbide clusters in the interior of the specimen, forming so-called fish-eyes at the fracture surface. In contrast, specimens of test series II had only very low RS, which enabled crack initiation near/at the surface at primary carbides/clusters. Furthermore, it has been shown that the high initial RS are prone to partial relaxation through cyclic loading for which the mechanisms are currently unknown. In this case near-surface induced failure was obtained. It was possible to confirm the experimentally obtained data by the use of the concept of local fatigue strength as function of effective RS. The relaxation of high initial RS was experimentally confirmed through RS measurements at runout specimens (10 10 cycles without failure

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

“…This is confirmed by other authors as well. [13][14][15][16][17][18][19][20][21][22][23][24][25][26]38] As a result, the fatigue strength can scatter strongly since the heterogeneous carbide microstructures in ingot cast and hot worked tool steels show strong deviation of carbide sizes comparing rolling and transverse directions. [12] Beiss, [11] Sohar et al, [16] and Randelius et al [25,26] confirmed this anisotropy effect and specify that smaller carbides are found in rolling directions of ingot cast and hot worked materials, thus up to 60% higher fatigue strength can be achieved.…”

“…[12,25,26] In addition, surface conditions and treatments can affect the fatigue performance of tool steels, e.g., different hard machining and polishing processes or surface treatments and finishing techniques. [17,20,26,[33][34][35][36][37] Sohar et al [13][14][15][16][17][18][19][20] focused on the effects of manufacturing, carbide microstructure and compressive residual stresses (CRS) on the UHCF strength and failure mechanisms. They separate three different failure modes for ingot cast tool steels depending on the present residual stresses at the specimen surface.…”

“…They separate three different failure modes for ingot cast tool steels depending on the present residual stresses at the specimen surface. [13,14,17] Internal failure mode occurs at large primary carbides and clusters till 10 5-6 cycles when high CRS are present. [13,14,17,18] If only low CRS were introduced to the specimens, a near-surface crack initiation at primary carbides and an internal volume fish-eye failure mode under granular bright facet (GBF) formation occur in the region of 10 7-10 cycles.…”

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Here, D is a factor depending on the defect location (surface or bulk), while α depends on the hardness. Studies by Meurling et al, [12] Sohar et al, [13][14][15][16][17][18][19][20] Kazymyrovych et al, [21,22] Ekengren et al, [23] Bergström et al, [24] and Randelius et al [25,26] investigated the influences of microstructure properties on HCF (N G ¼ 10 7 ), very-HCF (VHCF) (N G ¼ 10 8-9 ), and ultra-HCF

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The influence of tension mean stress to the crack initiation process at the very high cycle fatigue regime for a high strength steel

“…Some investigators like [1,2,3] have shown, that compressive residual stresses at the surface of specimens due to local plastic deformation or local plastic deformation and reduction of surface roughness, increases the lifetime also in the very high cycle regime. From these results it is known, that the compressive mean stress inhibits the crack initiation at or close to the surface, until the influence is compensated by crack initiation in the volume of the specimen.…”