Wechselverformung und Gefügeänderungen von Ck 15 und Ck 45

Microstructural Changes and Cyclic Deformation The crack initiation starts due to weakening and strengthening process during rotating bending. However a smaller plastic deformation amplitude is noticed at the same nominal stress compared to tension-compression stressed specimens. This results in a higher fatigue life. The different cyclic deformation behaviour was proofed by SEM (rotating bending specimens showed a lower slip line density compared to tension-compression specimens at the same nominal stress) and TEM investigations (the rotating bending specimens showed a smaller dislocation density at the same nominal stress). Furthermore it is showed, a correlation of cyclic stress strain data σ(ε_pls) between tension-compression and rotating bending specimens exists. This is also valid for the Manson-Coffin-relationship. the relation between lg ε_pls and lg N_B depends on the material (Ck 15, Ck 45) but not on the state of stress.     

Eigenspannungen in der Oberfläche gas- und badnitrierter Stahlproben

Residual stresses in the surface of gas and bath nitrided steel specimens . Residual stresses in the steels Ck 45, 34 Cr 4, 34 CrAl 6, and Ck 15 were measured by X-ray diffraction. Cylindrical specimens of the heat-treated steels were nitrided in ammonia-gas and in a Tenifer®-salt- bath. The course of residual stresses of Ith order was determined by removing thin layers. With the carbon steel Ck 15 ist shown the effect of tempering on residual stresses in bath-nitrided and quenched specimens.     

Stress Controlled Cyclic Deformation of Ck 15-steel at intermediate life

Experimental investigations have been carried out to study the cyclic deformation behaviour of Ck 15-steel in the intermediate life under constant stress amplitudes, 24 kp/mm² ? σ_a ≥ 30 kp/mm², in axial pushpull fatigue. In all the cases, an initial softening process followed by a hardening process has been observed during fatigue life. The plastic strain amplitudes near fatigue life of the specimens are linearly dependent on the stress amplitudes. From the experimental results, it has been observed that both the fatigue ductility exponent and the fatigue strength exponent have a value of around 0.32. Experimental observations on Ck 22 steel are in excellent agreement with these findings.     

Lokale Last- und Eigenspannungsverteilungen an rißbehafteten Oberflächen nach Korrosionsermüdung von vergütetem Ck 45

Distributions of Loading and Residual Stresses in Cracked Surfaces of Corrosion Fatigued Quenched and Tempered Steel Ck 45 Corrosion fatigue in the active state leads to a great number of short cracks and corrosion pits in the surface of metallic materials. This paper deals with the resulting distributions of loading and residual stresses in the case of bending fatigue. The material investigated was the quenched and tempered CBN-ground steel Ck 45. Results presented clearly indicate that stress distributions around cracks and corrosion pits are very inhomogeneous. Corrosion pits lead to a relaxation of manufacturing induced residual stresses. In the loaded state as a consequence of notch effects, maximum stresses are observed, which explains the importance of corrosion pits as crack initiation sites.     

Modellierung des Einflusses von Temperatur und Verformungsgeschwindigkeit auf die Fließspannung von Ck 45 bei Temperaturen T ≲ 0,3 Ts

Modelling of the temperature and strain-rate dependance of the flow stress of Ck 45 at temperatures T ≲ 0.3 T_s After presenting the theoretical background of thermally activated dislocation slip an evaluation method is discussed, which allows to describe the influence of temperature and strain-rate on the flow stress. For this purpose it is neccessary to measure the flow stresses in tensile tests as well as the thermal flow stress jumps in strain-rate jump tests at different temperatures and strain-rates. The quality of this method is shown using experimental results obtained at specimens of normalized Ck 45 (SAE 1045). The constitutive law allows a reliable extrapolation of flow stress values to strain-rates up to 10⁺⁴ s⁻¹. The influence of plastic strain on the constitutive constants of the modelling law is discussed.     

Untersuchungen zur Schwingungsrißkorrosion von unlegierten Stählen in wäßrigen Medien

Although a lot of work is done up to now in investigating corrosion fatigue many questions about the corrosion fatigue process of steel are still open. This applies for instance to the quantitative proportion at the complete damaging process which the corrosion strain component will assume and it applies moreover to special details of the crack initiation stage. The damaging process of corrosion fatigue of steel in the active state is generally supposed to proceed in the following three stages: “formation of pits”, “crack initiation” and “crack propagation”. The objective of the present study was at first to get by statistical means a quantitative assumption about the influence of the corrosive medium and secondly to get information about the contribution of pitting to the crack initiation process. Tension-compression experiments were conducted with specimen from steels Ck 15 and Ck 35 in the normalized state both without a corrosive influence (inert spindle oil) and with corrosive influence (desalted water, 3% NaCl-solution). It was found that the damaging process in its principal course is not affected by the kind of the corrosive medium but is hardly influenced by the extent of the load-amplitude at a given load-frequency.     

Größeneinfluß und Dauerfestigkeitseigenschaften unter Besonderer Berücksichtigung optimierter Oberflächenbehandlung

Size Effect and Fatigue Properties with Respect to Optimized Surface-Treatment. A hyperbolic function describes the geometrical size effect of notched specimens made from heat treated steel. An estimation of fatigue properties of components under one level fatigue tests is possible, if there are comparable materials and surface properties. The fatigue properties of specimens are well described by standardized stress-N graphs. The slope of the stress-N graphs in the range of load cycle depends on the concentration factor and not on the size effect. The fatigue properties of components are largely increased by thermal and mechanical surface strengthening. For the determination of the improvement of fatigue properties it is important to known the initiation of cracking. The improved fatigue properties of inductive surface hardened smooth specimens can be explained by the initiation of cracking below the surface. Mechanically strengthened notched specimens start cracking on the surface. The increase of fatigue properties for these specimens is explained by compressive residual stresses. The fatigue properties of notched specimens can be improved by the optimisation of mechanical strengthening, to higher values than for smooth surface strengthened specimens. This is due to compressive residual stresses. They decrease the tensile stresses which are responsible for crack propagation. If the tensile stress is below fatigue limit for initiation of cracking the crack arrests immediately.     

Einfluß positiver und negativer Mittelspannungen auf das zyklische Spannungs-Dehnungsverhalten von normalisiertem Ck 45

Influence of Positive and Negative Mean Stresses on the Cyclic Stress-Strain Behaviour of Normalized Steel SAE 1045 In technical parts cyclic loads are often superimposed by static load components. In such cases, mean stresses influence the cyclic deformation behaviour and produce cyclic creep effects. Single step push-pull fatigue tests were carried out with normalized specimens of the steel SAE 1045. Using cyclic deformation curves and cyclic creep curves, the influence of different mean stresses will be discussed.     

Festwalzen und Schwingfestigkeit

Deep Rolling and Fatigue Strength The fatigue properties of specimens and components are largely increased by deep rolling. Depending on geometrical shape of components and material strength the compressive residual stresses and the increased surface hardness made by deep rolling have a different effect on the improvement of fatigue strength. The fatigue properties of smooth specimens and components with a sufficient toughness can be raised by increase of surface hardness, whereas in case of notched parts the influence of permanent compressive residual stresses is dominant. The application of deep rolling in case of crankshafts shows a clear superiority of mechanical strengthening procedure to thermal surface strengthening. If there are some reasons to improve the wear behaviour beside the fatigue strength it is commendable to combine thermal and mechanical surface strengthening.     

The fatigue strength of centrifugally cast spheroidal graphite iron pipe

The fatigue strength of centrifugally cast spheroidal graphite (SG) iron pipe was investigated. A parallel series of tests were carried out both on plain plate specimens which were extracted from an iron pipe, and on notched iron specimens. These results were compared with results for rolled steel beams, which were made from a steel with a tensile strength similar to the SG iron [1]. It was found that the strengths at a life of $\text{2 × 10}{}^6$ cycles differed only by 20% between the SG iron pipe and the rolled steel beam, whereas those of plain plate specimens of the two materials differed by 38%. The fatigue failure in the SG iron pipe initiated from the inherent gas pores existing in the inner surface of the pipe, while the fracture in the rolled steel beams originated from external notch defects. Thus, the steel beam appeared more sensitive to the external notches than the SG iron pipe, when the notch size was smaller than 1 mm. However, it was revealed from the fatigue tests on notched plates that, as the notch became severer, the fatigue strength of SG iron became more affected by the notches than did that of the steel. A fracture mechanics analysis indicated that this was because the fatigue crack growth rate for SG iron was three times as high as that for steel.     

Beeinflussung des Dauerschwingverhaltens gekerbter Proben durch unterschiedlichen Verschleißzustand der zur Kerbherstellung verwendeten Werkzeuge

Influence on the fatigue behavior of notched specimens by different wear and tear of the tools used for producing the notches . In fatigue-specimens of × 5 CrNi 18 9 and Ck 45 notches had been produced by sharp and blunt tools. The fatigue strength of this specimens from austenitic steeel was smaller when the notches had been produced by sharp and blunt tools. The fatigue strength of this specimens from austenitic steeel was smaller when the notches had been produced by sharp tools. Specimens of ferritic steel on the contrary had a greater fatigue strength, when the notches had been produced by blunt tools. Statistical methods had been used for planning and evaluation of the tests. Therefore appear differences in the strength reduction factor ß_k between in conventional manner determined statements and the here calculated ones.     

Randschichtzustände von normalisiertem und vergütetem 42 CrMo 4 nach konsekutiven Kugelstrahl- und Festwalzbehandlungen

Near surface materials states of normalized as well as quenched and tempered SAE 4140 after consecutive shot peening and rolling treatments The near surface states of normalized as well as quenched and tempered SAE 4140 after consecutive shot peening and rolling treatments are presented and discussed. Especially in the normalized state separately shot peened or rolled specimens reveal characteristic differences in the near surface dislocation structures and in the thickness of the subsurface areas influenced by the mechanical treatments. In case of the consecutive application of shot peening and rolling, a sequence effect occurs in such a way that the finally applied surface treatment determines the microstructure of the very surface and the subsurface materials layers.     

Kraft- und Wegmessungen als Grundlage für das Verständnis des Kerbschlagbiegeversuchs

Load-Displacement Diagrams – a Basis for the Understanding of Notched Bar-Impact Tests . The absorbed energy as well as the effect of temperature on absorbed energy during the recording of load – displacement diagrams of notched bar – impact tests of mild steels, Ck 10, Ck 35, Ck 45, Ck 60 and C 85W 2 in normalized state were analysed. The considerable values of relation between discontinuity in nature of characteristic load and displacement of load displacement diagrams and irregularity of fracture toughness temperature curves were obtained there. From these investigations one can define characteristic temperatures (transition temperatures), which are not arbitrary, but physically founded and which describe the behaviour of materials in different temperature-ranges. These characteristic temperatures, above all, the crack arrest temperature could also be defined after fractographical and microfractographical investigations. The absorbed energy temperature curves were also determined arithmetically with the application of simple methode of area computation of load displacement diagrams. Therefore, it can be made sure that the available curves are correct and determined on principle of physics.     

机械滚压对A473M钢疲劳性能的影响

采用机械滚压对A473M马氏体不锈钢轴套材料进行表面处理,研究滚压工艺对其力学性能的影响。采用SEM、白光干涉仪、X射线衍射仪、显微硬度计、EBSD、拉伸试验机和疲劳试验机分别对试样表面形貌、表面粗糙度、残余应力、显微硬度、拉伸性能和疲劳性能进行系统表征。结果表明:滚压加工试样表面的粗糙度明显降低,仅为车削加工的1/5;滚压加工在材料近表面引入残余压应力,其值最高可达946 MPa,沿深度方向逐渐减小,残余压应力层深度约为200μm,表面硬度提高30%左右,硬度影响层深度可达200μm;抗拉强度、屈服强度和伸长率分别提升了40%,22%和8%,疲劳寿命由基体材料的5.4×10^4周次提高到1×10^7周次。采用滚压加工后材料的力学性能明显提升,疲劳寿命显著增加。     

Abrasiver Verschleiß nitrocarburierter Stähle

Abrasive wear of nitrocarburized steels Diffusion coatings have been used increasingly to improve surface properties of several machine components for tribological applications. In this investigation, the abrasive wear of various diffusion coatings has been studied. These coatings developed by three different nitrocarburizing processes, namely salt-bath, gas and plasma nitrocarburizing, were conducted on five various steel substrates. The substrates were St 52-3, Ck 45, 42 CrMo 4, 30 CrNiMo 8 and X 20 Cr 13. The thickness of the compound layers and their porosity were measured using optical microscopy. The structure of compound layers was characterized using an X-ray diffractometer, and their surface roughness by a stylus profilometer. The surface hardness measurements and the hardness profile were made according to the Vickers principle. The abrasion test was carried out using a pin-on-table machine. The predominant wear mechanisms have been evaluated using scanning electron microscopy. It was found that the employed nitrocarburizing processes have improved the abrasive wear resistance of all tested coating-substrate-combinations. In general, the abrasive wear rate decreases with an increase in the hardness of the compound layer as well as its thickness. The best coating-substrate-combinations to improve abrasion resistance were salt-bath and gas nitrocarburized specimens of the substrate X 20 Cr 13. Plasma nitrocarburized specimens of the substrate St 52-3 and Ck 45 were the worst.     

Anisotropy of the fatigue behavior of extruded and rolled magnesium alloys

Fatigue tests were conducted using both extruded and rolled magnesium alloys AZ31 to study anisotropy of the fatigue behavior of the alloys. For the above purpose, two types of specimens whose longitudinal axes are parallel (E specimen) and perpendicular (T specimen) to the extrusion direction were prepared for the extruded Mg alloy. For the rolled Mg alloy, three types of specimens, whose longitudinal axes are parallel (R specimen) and perpendicular (T and S specimens) to the rolling direction, were prepared. S–N curves and crack propagation characteristics for both the extruded and rolled specimens with different longitudinal directions were studied to investigate the effects of the texture, microstructures and residual stresses on the fatigue behavior of the magnesium alloy AZ31. Anisotropy of the fatigue behavior was observed for both the extruded and the rolled magnesium alloys. In the extruded Mg alloy, differences in both fatigue lives and fatigue limit exist between E and T specimens. The fatigue resistance for the former is superior to that of the latter. In the rolled Mg alloy, lesser differences were found between R and T specimens. However, fatigue lives of the S specimens were clearly shorter than those of the former at the high stress amplitudes above the fatigue limit. In the extruded Mg alloy, the rod-like microstructure observed is an important factor contributing to the anisotropy of the fatigue behavior of the alloy. While in the rolled Mg alloy, the texture induced during processing is considered to play a dominant role in the anisotropy of the fatigue behavior of the alloy.     

The effect of steady torsion on fatigue crack growth in shafts

Most of catastrophic mechanical failures in power rotor shafts occur under cyclic bending combined with steady torsion: Mode I (ΔK_I) combined with Mode III (K_III). An analysis of the influence of steady torsion loading on fatigue crack growth rates in shafts is presented for short as well as long cracks. Long cracks growth tests have been carried out on cylindrical specimens in DIN Ck45k steel for two types of testing: rotary or alternating bending combined with steady torsion in order to simulate real conditions on power rotor shafts. The growth and shape evolution of semi-elliptical surface cracks, starting from the cylindrical specimen surface, has been measured for several loading conditions and both testing types. Short crack growth tests have been carried out on specimens of the same material DIN Ck45k, under alternating bending combined with steady torsion. The short crack growth rates obtained are compared with long crack growth rates. Results have shown a significant reduction of the crack growth rates when a steady torsion Mode III is superimposed to cyclic Mode I. A 3D Finite Element analysis has also shown that Stress Intensity Factor values at the corner crack surface depend on the steady torsion value and the direction of the applied torque.     

Experimental investigation and life prediction of hot corrosion pre‐exposure on low‐cycle fatigue of a directionally solidified nickel‐base superalloy

Low‐cycle fatigue tests were conducted on the directionally solidified nickel‐base superalloy DZ125 at 850 °C in the unexposed and exposed specimens for 2, 15, 25 and 50 h in hot corrosion environment. The pre‐exposed specimen exhibited a lower life than unexposed specimens. Fatigue cracks in the unexposed specimens are initiated from defects near the surface, while the cracks of exposed specimens preferentially occur on the surface. Hot corrosion damage in fatigue life was found to be associated with the reduction of the bearing area. A novel life prediction methodology based on continuum damage mechanics was proposed to predict the experimentally observed decrease in low‐cycle fatigue life with increasing prior exposure time.     

Effect of surface roughness on low-cycle fatigue life of Cr---Mo---V steel at 550 °C

Low-cycle fatigue tests have been carried out on Cr---Mo---V steel specimens with two different modes of surface roughness at 550 °C. The fatigue life of specimens with a rough surface is approximately half of that of specimens with a smooth surface over the plastic strain range investigated. From interrupted tests, it is seen that low-cycle fatigue behaviour is largely influenced by the crack initiation process even for a high strain range and the decrease of fatigue life in specimens with a rough surface is mainly due to a reduction in the number of cycles for crack initiation.