Enhancement of the fatigue limit at HCF- and VHCF-loadings with the help of thermomechanical treatment at the temperature of maximum dynamic strain ageing
Final Report Abstract
The project showed that a thermomechanical treatment (TMT) in the temperature range of carbon-induced dynamic strain ageing is suitable to improve the fatigue lifetime as well as the fatigue strength of the quenched and tempered steels 42CrMo4 and 100Cr6 under both high cycle fatigue (HCF) and very high cycle fatigue (VHCF) conditions. The results indicate that the positive effects of the TMT can be attributed to a stabilization of the dislocation structure around inclusions. Fractography revealed that TMT specimens show increased maximum stress intensity factors around crack-initiating internal inclusions when compared to nontreated specimens with similar fatigue lifetimes, thus confirming the positive effect of the TMT in both HCF and VHCF regime. In spite of the improved fatigue properties after TMT, the characteristic formation of a fine granular area (FGA) could be observed around critical inclusions in the VHCF regime, which is seen as the reason for the subsequent fracture. However, for ultrasonic fatigue testing, the TMT specimens showed a tendency to off-centered fracture with respect to the longitudinal axis of the specimen. In the maximally loaded volume, which has seen the optimal TMT, inclusions resulting in a higher stress intensity factor than the inclusions leading to off-centered fracture could be found. Selected area diffraction analysis of the proximate surroundings of such non-fracture-initiating inclusions showed neither formation of finer grains nor incipient cracks. The absence of both is related to the stabilizing effect of the TMT. For surface-induced fracture, the lifetimes of TMT specimens were shorter than for specimens in the initial state. The reason for this could be identified as redistribution of near-surface compressive residual stresses into low tensile residual stresses due to the mechanical part of the TMT. A subsequent shot peening process resulted in compressive residual stresses comparable to the initial state and led to higher lifetimes of TMT specimens when compared to the initial state also for surface-induced fracture. In order to elucidate the mechanisms during a TMT further, some specimens were treated with the pure thermal treatment or pure mechanical treatment of the TMT. For both variants, the fatigue properties did not improve when compared to the initial state. Hence, the mechanism of the TMT resulting in a stabilized dislocation structure and improved fatigue properties is the combination of thermal and mechanical loading in the temperature range of maximum dynamic strain ageing.
Publications
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Properties of the Fine Granular Area and Postulated Models for Its Formation during Very High Cycle Fatigue—A Review. Applied Sciences, 10(23), 8475.
Sippel, Jan Patrick & Kerscher, Eberhard
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Comparison of the Internal Fatigue Crack Initiation and Propagation Behavior of a Quenched and Tempered Steel with and without a Thermomechanical Treatment. Metals, 12(6), 995.
Khayatzadeh, Amin; Guth, Stefan & Heilmaier, Martin
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Influence of a Thermo-Mechanical Treatment on the Fatigue Lifetime and Crack Initiation Behavior of a Quenched and Tempered Steel. Metals, 12(2), 204.
Khayatzadeh, Amin; Sippel, Jan; Guth, Stefan; Lang, Karl-Heinz & Kerscher, Eberhard
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Influence of a thermomechanical treatment on the lifetime of a high strength steel in very high cycle fatigue. ADVANCES IN FRACTURE AND DAMAGE MECHANICS XX, 2848, 020014.
Sippel, Jan & Kerscher, Eberhard
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Influence of fracture toughness and defect size on vibration characteristics and resulting stress amplitude of AISI 52100 and AISI 4140 during ultrasonic fatigue testing. Procedia Structural Integrity, 47, 608-616.
Sippel, Jan Patrick & Kerscher, Eberhard
