Boundaries for increasing the fatigue limit of the bearing steel SAE 52100 by thermomechanical treatments

AbstractHigh end applications often require high strength materials especially regarding the fatigue limit. It is still an open question what the highest fatigue limits are and how to increase them further in high strength steels. A thermomechanical treatment (TMT) in the temperature range of maximal dynamic strain ageing is an auspicious way to increase the fatigue limit of bearing steel SAE52100. Successful attempts have been published for various materials states. The present investigation deals with the influence of non-metallic inclusions on the fatigue limit and the possibility to increase the fatigue limit by TMT. The fatigue limit of high strength steels increases by reducing the size of the non-metallic inclusions during steelmaking. We found that TMT increases the fatigue limit of a high strength materials state only in cases in which the non-metallic inclusions are the starting points of the fatigue cracks. In the case of high cleanliness of the steel the origin of the fatigue cracks may be at any position in the matrix. If there are no critical non-metallic inclusions a TMT did not increase the fatigue limit further. The reasons for this behaviour will be explained and the border of the fatigue limit of high strength steels will be discussed

Surface Erosion of Carbon Steel 1045 During Waterjet Peening

The present study investigates the effect of waterjet treatment on the surface characteristics of the carbon steel 1045. The effect of waterjet treatment parameters namely number of jet passes and pressure was investigated. An increase in the number of jet passes as well as pressure leads to a higher roughness and more erosion of the surface. The damage features consist of various fracture mechanism modes occurred at the initial and evolved damage stage. The ferrite phase experienced more damage than the pearlite phase. However, the damage was more concentrated along the grain boundaries. The shearing force from the jet lateral flow raised the circumferential rim and created lateral cracks and sub-tunnels which might eventually be removed in the subsequent jet passes. The hardness of the treated specimens increased with an increase in the number of jet passes and pressure

Corrosion Fatigue Studies on a Bulk Glassy Zr-Based Alloy under Three-Point Bending

Corrosion fatigue (CF) tests were carried out on bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 (Vitreloy 105) samples under load-controlled three-point bending conditions with a load ratio of R = 0.1 in 0.01 M Na2SO4 + 0.01 M NaCl electrolyte. During cyclic testing, the bar-shaped specimens were polarized in situ at constant potentials and the current was monitored. Three different anodic potentials within the interval between the pitting potential EP and the repassivation potential ER and three different load amplitudes were applied. In some cases, in situ microscopic observations revealed the formation of black corrosion products in the vicinity of the crack tip during anodic polarization. Fractographic analysis revealed a clear distinction between two modes of crack growth characterized by smooth dissolution induced regions on the one hand and slim fast fracture areas on the other hand. Both alternating features contributed to a broad-striated CF fracture surface. Moreover, further fatigue tests were carried out under free corrosion conditions yielding additional information on crack initiation and crack propagation period by means of the open circuit potential (OCP) changes. Thereby, a slight increase in OCP was detected after rupture of the passive layer due to bare metal exposed to the electrolyte. The electrochemical response increased continuously according to stable crack propagation until fracture occurred. Finally, the fracture surfaces of the CF samples were investigated by energy dispersive X-ray with the objective of analyzing the elemental distribution after anodic dissolution. Interestingly, anodic polarization at a near repassivation potential of −50 mV vs. saturated calomel electrode (SCE), which commands a constant electric potential of E = 0.241 V vs. standard hydrogen electrode (SHE), led to favorable effects on the fatigue lifetime. In conclusion, all results are conflated to a CF model for bulk glassy Vitreloy 105 under anodic polarization in chloride-containing electrolyte and compared to the previously proposed stress corrosion mechanisms under similar conditions. © 2017 Grell, Wilkin, Gostin, Gebert and Kerscher

Corrosion Fatigue Studies on a Bulk Glassy Zr-Based Alloy under Three-Point Bending

Corrosion fatigue (CF) tests were carried out on bulk glassy Zr52.5Cu17.9Al10Ni14.6Ti5 (Vitreloy 105) samples under load-controlled three-point bending conditions with a load ratio of R = 0.1 in 0.01 M Na2SO4 + 0.01 M NaCl electrolyte. During cyclic testing, the bar-shaped specimens were polarized in situ at constant potentials and the current was monitored. Three different anodic potentials within the interval between the pitting potential EP and the repassivation potential ER and three different load amplitudes were applied. In some cases, in situ microscopic observations revealed the formation of black corrosion products in the vicinity of the crack tip during anodic polarization. Fractographic analysis revealed a clear distinction between two modes of crack growth characterized by smooth dissolution induced regions on the one hand and slim fast fracture areas on the other hand. Both alternating features contributed to a broad-striated CF fracture surface. Moreover, further fatigue tests were carried out under free corrosion conditions yielding additional information on crack initiation and crack propagation period by means of the open circuit potential (OCP) changes. Thereby, a slight increase in OCP was detected after rupture of the passive layer due to bare metal exposed to the electrolyte. The electrochemical response increased continuously according to stable crack propagation until fracture occurred. Finally, the fracture surfaces of the CF samples were investigated by energy dispersive X-ray with the objective of analyzing the elemental distribution after anodic dissolution. Interestingly, anodic polarization at a near repassivation potential of −50 mV vs. saturated calomel electrode (SCE), which commands a constant electric potential of E = 0.241 V vs. standard hydrogen electrode (SHE), led to favorable effects on the fatigue lifetime. In conclusion, all results are conflated to a CF model for bulk glassy Vitreloy 105 under anodic polarization in chloride-containing electrolyte and compared to the previously proposed stress corrosion mechanisms under similar conditions. © 2017 Grell, Wilkin, Gostin, Gebert and Kerscher

Influence of a Thermo-Mechanical Treatment on the Fatigue Lifetime and Crack Initiation Behavior of a Quenched and Tempered Steel

A thermo-mechanical treatment (TMT) at the temperature of maximum dynamic strain aging has been optimized and performed on quenched and tempered steel SAE4140H (German designation: 42CrMo4) in order to improve the fatigue limit in the high cycle fatigue (HCF) and and very high cycle fatigue (VHCF) regimes. Fatigue tests, with ultimate cycle numbers of 107 and 109, have shown that the TMT can increase both the fatigue lifetime and the fatigue limit in the HCF and VHCF regimes. The increased stress intensity factors of the critical inclusions after the TMT indicate that the effect can be attributed to a stabilized microstructure around critical crack-initiating inclusions through the locking of edge dislocations by carbon atoms during the TM

Molecular Simulation Study on the Influence of the Scratching Velocity on Nanoscopic Contact Processes

The influence of the scratching velocity on mechanical and thermal properties of a nanoscopic contact process was studied by molecular dynamics simulations. Simulations with different scratching velocities were conducted in dry and lubricated systems. The contact process consisted of a lateral scratching of a spherical indenter on a planar substrate. All molecular interactions were described by the Lennard-Jones truncated and shifted potential. The forces on the indenter, the coefficient of friction and the work done by the indenter as well as the power applied on the indenter were sampled. Furthermore, an analysis of thermal properties was conducted: The change of the energy of the substrate, the indenter and the fluid was evaluated and the local temperature field was determined. The forces, the coefficient of friction and the work done by the indenter show practically no influence of the scratching velocity. The work done by the indenter was found to be the same for all velocities. As a consequence, the power supplied to the system depends linearly on the scratching velocity, which affects the temperature of the contact zone. As expected, the presence of a lubricant reduces the temperature of the substrate in the vicinity of the contact

Effect of Multiple Passes Treatment in Waterjet Peening on Fatigue Performance

The influence of waterjet peening on the residual stresses and fatigue performance of AISI 304 is investigated. The specimen surfaces were treated with multiple jet passes. The fatigue strength was evaluated using an alternating bending fatigue tester. The results of XRD measurements showed that a higher amount of compressive residual stresses is induced in the treated specimens. This strengthening layer is limited within the first 100 �m below the surface, which had been confirmed by micro hardness measurements. Even though the treated specimens showed compressive residual stresses the fatigue limit is lower than that of the untreated specimens. The roughness of the surface and the resulting notch effect seems to be stronger than the positive effect of the hardened layer

Influence of Waterjet Peening and Smoothing on the Material Surface and Properties of Stainless Steel 304

The present study investigates a combined effect of waterjet peening and smoothing on the surface of austenitic stainless steel 304. An analysis of surface finish and the change in hardness were evaluated. The waterjet treatment on the surface was conducted in steps with multiple passes. Initially, the surface was treated with a higher energy and later with a lower energy of the water droplets. The surfaces treated with multiple steps of decreasing energy produced a smoother surface with lower peak heights and a slightly higher increase in the hardness than the surface treated with only a single step. The hardening layer was also maintained during the later step treatment. The combined action of surface hardening and smoothing using multiple steps in waterjet treatment is useful in increasing the hardness and reducing the roughness of the surface