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.     

Schwingfestigkeitssteigerung durch Festwalzen

Improvement in Fatigue Properties by Means of Deep Rolling Deep rolling has an almost 60 years old tradition and today it is an important process especially for automobile structural parts. The development of deep rolling was greatly influenced by the Institute of Material Science of the Technical University in Darmstadt. After a short historical review the general mechanism of mechanical strengthening is discussed. Then new experimental results on ductile cast irons and a case hardened steel are given. The improvement in fatigue strength of notched specimens out of these materials is even higher than that observed on steel in previous investigations. However the experiments also indicate that mechanical strengthened parts obviously have no real fatigue limit. Measurements of crack propagation on nodular cast iron specimens show that the propagation rate of cracks in the notch root is extremely reduced by compressive residual stresses. However no real crack arrest could be observed.     

Residual stress influence on fatigue lifetimes of electroplated AISI 4340 high strength steel

Residual stresses play an important role in the fatigue lives of structural engineering components. In the case of near surface tensile residual stresses, the initiation and propagation phases of fatigue process are accelerated; on the other hand, compressive residual stresses close to the surface may increase fatigue life. In both decorative and functional applications, chromium electroplating results in excellent wear and corrosion resistance. However, it is well known that it reduces the fatigue strength of a component. This is due to high tensile internal stresses and microcrack density. Efforts to improve hard chromium properties have increased in recent years. In this study, the effect of a nickel layer sulphamate process, as simple layer and interlayer, on fatigue strength of hard chromium electroplated AISI 4340 steel hardness – HRc 53, was analysed. The analysis was performed by rotating bending fatigue tests on AISI 4340 steel specimens with the following experimental groups: base material, hard chromium electroplated, sulphamate nickel electroplated, sulphamate nickel interlayer on hard chromium electroplated and electroless nickel interlayer on hard chromium electroplated. Results showed a decrease in fatigue strength in coated specimens and that both nickel plating interlayers were responsible for the increase in fatigue life of AISI 4340 chromium electroplated steel. The shot peening pre-treatment was efficient in reducing fatigue loss in the alternatives studied.     

Cold rolling influence on residual stresses evolution in heat-treated AA7xxx T-section panels

This article describes a novel cold working method for relaxing residual stresses in extra-long quenched T-section panels. Distortion led by quench-induced residual stresses in components is usually a great concern for the aviation industry. In this study, the influence of cold rolling on the residual stresses in a scale-down quenched AA7050 T-section specimen is experimentally and numerically investigated. An integrated numerical model was built to predict the quenching and the subsequent cold rolling processes. High levels of compressive stress around the surfaces of quenched T-profile specimens and the tensile surface residual stresses (RS) in the quenched and cold rolled specimens were predicted via numerical analysis, the surface stresses magnitude and distribution have, therefore, been quantified via X-ray diffraction (XRD) technique. The deflection of quenched specimens experienced 1.5% and 3% cold rolling was measured as well. In addition, to examine the cold rolling effect on the mechanical properties of T-profile specimens, hardness tests were carried out on quenched and cold rolled T-section specimens. It concludes that with 1.5% cold rolling the residual stresses in the core part of quenched material can be effectively relieved to an extent of lower than 83 MPa, with limited change for the material properties.     

Oberflächenbehandlung nach spanender Formgebung durch ein einfaches Zusatzverfahren

A supplementary mechanical treatment to strengthen surfaces after machining In this work, a surface treatment is presented in which a hard metal tool insert with rounded edges is pressed on a machined surface at low speed and optimized pressure. This treatment can be executed on the same lathe used previously for machining. The effects on surface topography and surface layer properties (microhardness and X-ray peak halfwidth profiles, as well as residual stresses) have been quantitatively determined. In copper specimens, the surface treatment leads to compressive residual stresses and to strong surface layer deformation and, thus to an increase of near-surface hardness. This effect decreases the amplitude of plastic deformation in the surface layer during fatigue and results in a 25% increase of the fatigue strength at 10⁷ cycles. In Al-2024 T6, the surface treatment causes high compressive residual stresses, an increase in superficial hardness and smoothing of the surface topography. This results in an improvement of the fatigue strength at 10⁷ cycles of 48%.     

Messung von Festwalzeigenspannungsverteilungen an Bauteilen

Measurement of Deep-Rolling Residual Stress-Distributions on Components In many practical cases surface layers of components are the most heavily stressed material zones. In order to improve the fatigue strength and the wearability of components thermal, thermochemical and mechanical surface treatment methods like induction hardening, case hardening, nitriding, shot peening and deep-rolling are applied for several years. The effect of these methods depends on the increase of strength and on the production of compressive residual stresses in the heavily stressed surface layers. The mechanical processes also affect the surface roughness which may be reduced by choosing suitable process parameters. Due to the mechanical surface treatment by deep-rolling fatigue strength improvements up to 200% are possible [1], and the compensation of the notch effect was observed for notched specimen [2]. In the industry the deep-rolling is applied on e. g. crankshafts, screws, valve shafts and actuation shafts. The direct control of surface layer properties, like residual stresses and hardening, by x-ray measurements on axial sections of the components is presented here as a method for quality insurance of rolled parts. The measuring method may also be used to verify results of FEM-simulations. FEM-simulations are applied to estimate the modification of the residual stresses due to preparation.     

Optimierung von Schwingfestigkeitseigenschaften beim Oberflächendrücken gekerbter Umlaufbiegeproben unter Berücksichtigung der Probengröße

Optimisation of Fatigue Properties by Surface-Rolling of Notched Specimens of Different Size The fatigue properties of notched specimens can be largely improved by surface-rolling. It is important to know the material strengthening in the rolling contact to improve the conditions of the surface-rolling process. A computer program is presented, which allows a determination of rolling pressures to any geometries of specimens. As a result of the fatigue investigations it is shown that surface-rolled notched specimens improve the fatigue limit above the fatigue strength of smooth specimens independed on the stress concentration. The improvement of endurance limit of notched specimens with non propagated cracks is based on the effect of constant compressive residual stresses.     

Verhalten laserschockverfestigter und festgewalzter Randschichten der Ti‐Legierung Ti‐6Al‐4V bei schwingender Beanspruchung unter erhöhten Temperaturen

Residual stress stability and near‐surface microstructures in high temperature fatigued mechanically surface treated Ti‐6Al‐4V It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly‐stressed metallic components. Deep rolling is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of deep rolling on the low‐cycle and high‐cycle fatigue behavior of a Ti‐6Al‐4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near‐surface microstructures. Preliminary results on laser shock peened Ti‐6Al‐4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ～450 °C, i.e., at an homologous temperature of ～0.4 T/T_m (where T_m is the melting temperature). Based on cyclic deformation and stress/life (S/N) fatigue behavior, together with the X‐ray diffraction and in situ transmission electron microscopy observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti‐6Al‐4V at such higher temperatures, despite the almost complete relaxation of the near‐surface residual stresses. In the absence of such stresses, it is shown that the near‐surface microstructures, which in Ti‐6Al‐4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.     

Zum Mechanismus der Wechselfestigkeitssteigerung durch Werkstoffverfestigung und Druckeigenspannungen nach einer Oberflächenbehandlung

The Improvement of Fatigue Limit as a Result of Hardening and Macrostresses Due to a Surface Treatment Surface treatments, that increase the hardness as well as induce surface residual macrostresses, are universaly able to improve the fatigue limit. It is shown, that depending on the shape of specimens both effects together are responsible for the raise of the fatigue strength, which is in contrast to former opinions. The increase of hardness increases the stress required for crack initiation and is thus decident for unnotched specimens, whereas in this case the influence of permanent residual stresses is relatively smaller. Notched specimens of sufficient stress concentration factor k_t are determined by the crack propagation conditions, which can be controlled decisively by mean loads. The increase of hardness improves the resistance against crack initiation proportional to the 1/k_t portion of the unnotched fatigue limit, but cracks remain nonpropagating as long as a certain minimum alternative stress, which can be raised by compressive residual stresses, is not exceeded. Depending upon concentration factor, mean compressive load and hardness the transition from crack initiation to crack propagation as the criterion for fatigue fracture can be estimated by several fatigue-strength-diagrams, which are evaluated for specimens of constant hardness but are valid for surface hardened specimens as well.     

Randschichtzustand der Mg-Basislegierung AZ 31 nach mechanischen Oberflächenbehandlungen

Near surface properties of Mg-alloy AZ 31 after mechanical surface treatments Near surface materials properties are investigated for Mg-alloy AZ 31 after different shot peening or deep rolling treatments resp. Surface topography as well as depth distributions of hardness, residual stresses and X-ray interference line half-width values are analysed for individual process parameters. In addition, stability of residual stress distributions during fatigue loading is investigated.     

Modeling and experimental validation of the surface residual stresses induced by deep rolling and presetting of a torsion bar

During the production of torsion bars, two different mechanical processes of inducing the residual stresses into the torsion bar are used: the presetting of the torsion bar and the deep rolling of the torsion bar. The process of presetting the torsion bar is carried out by twisting the torsion bar to the desired angle and releasing it to the new residual angle position. With controlled overstraining, favorable residual shear stresses are induced into the torsion bar, so the material is strain hardened and the yield point of the material is shifted and increased in the stress and strain space. The objective of the deep rolling process is to introduce compressive residual stresses into near-surface regions in order to increase the fatigue strength of the torsion bar. These two processes influence each other. The final level of residual stresses depends on the production sequence of these two processes and the production parameters of each process. The correct production sequence of these two operations and distribution of beneficial residual stress was simulated using the finite element (FE) method. To validate this model, the predicted surface residual stresses were compared by the X-ray diffraction (XRD) measurements of residual stresses.     

Changes in mechanical properties of vehicle components after strengthening under low-amplitude loads below the fatigue limit

Changes in the mechanical properties of vehicle components were investigated after strengthening under low-amplitude loads below the fatigue limit (SLAL). The strengths of vehicle components including fatigue strength, static strength and yield strength were significantly increased by SLAL. The surface hardness of the front axle, composed of a kind of low strength steel without heat treatment, was enhanced with increased strengthening and damaging. The change of surface hardness was reversed completely for the transmission gear, composed of a kind of high strength steel with surface heat treatment. The natural frequency of the transmission gear was appreciably increased by SLAL.     

Fatigue characteristics of SAE52100 steel via ultrasonic nanocrystal surface modification technology

Ultrasonic nanocrystal surface modification (UNSM) technology is a novel surface modification technology that can improve the mechanical and tribological properties of interacting surfaces in relative motion. UNSM treatment was utilized to improve the wear resistance fatigue strength of slim bearing rings made of SAE52100 bearing steel without damaging the raceway surfaces. In this study, wear and fatigue results that were subjected to different impact loads of the UNSM treatment were investigated and compared with those of the untreated specimen. The microhardness of the UNSM-treated specimens increased by about 20%, higher than that of the untreated specimens. The X-ray diffraction analysis showed that a compressive residual stress of more than 1,000 MPa was induced after the UNSM treatment. Also, electron backscatter diffraction analysis was used to study the surface structure and nanograin refinement. The results showed that the rolling contact fatigue life and the rotary bending fatigue strength of the UNSM-treated specimens increased by about 80% and 31%, respectively, compared to those of the untreated specimen. These results might be attributed to the increased microhardness, the induced compressive residual stress, and the nanocrystal structure modification after the UNSM treatment. In addition, the fracture surface analysis showed that the fish eye crack initiation phenomenon was observed after the UNSM treatment.     

Kugelstrahlen von Spaltbiegeprofilen mit ultrafeinkörnigen Gradientengefügen

Linear bend splitting and linear flow splitting are innovative methods to produce bifurcated profiles with ultrafine grained (UFG) microstructures in an integral style. Linear bend split profiles exhibit high potential for lightweight applications, due to their bifurcations and the high strength of the ultrafine grained microstructures, which develop at the surface of the work piece. The presence of the ultrafine grained microstructure is accompanied by a duplication of hardness and strength and a markedly increase of the fatigue properties, compared to the untreated material. Because of their high strength, ultrafine grained materials exhibit increased potential for the formation of compressive residual stresses. Therefore, shot peening of ultrafine grained microstructures could result in an increased fatigue resistance. The results clearly show that shot peening, despite optimized shot peening parameters, does not lead to an increase of the fatigue resistance. Compared to the untreated ultrafine grained microstructure, the fatigue resistance of shot peened material is even lower. The lower fatigue resistance is probably caused by the roughness of the shot peened surface, which overcompensates the compressive residual stresses.     

Application of the local strain approach on a rolling point contact model

A newly developed cold forming process, the linear flow splitting process, allows the bifurcation of thin metal sheets by severe plastic deformation (SPD). This process produces flanges with an ultra-fine grained (UFG) microstructure, which is characterized by an increased hardness and lower surface roughness in comparison to the material in as-received state. The technology is researched within the Collaborative Research Centre 666 (CRC 666) “Integral Sheet Metal Design with Higher Order Bifurcations”. The increased hardness and reduced surface roughness of the flanges make this technology suitable for manufacturing linear guide rails. In order to achieve the required reliability and fatigue strength, the assessment of the performance of the linear flow-split profiles with gradient microstructure is of great importance. When considering rolling contact fatigue, the examination of the mechanical behavior of the side of the flange with higher hardness due to the UFG microstructure is required. In order to reduce time and costs, a numerical method to determine the rolling contact fatigue behavior would be useful minimizing the normally applied experimental effort. The present work aims to check the applicability of the local strain approach for the evaluation of the rolling contact fatigue behavior of linear flow split components by a Finite Element (FE) analysis. The Hertzian theory is only valid for homogeneous materials and cannot be directly applied to the flanges with gradient microstructure. Therefore an accurate modeling of the material in order to take into account the gradient in the mechanical properties is required. In this paper the characterization of the material behavior under axial cyclic loading will be presented. These results will be used for the Bergmann damage parameter to analyze the rolling contact fatigue behavior. Some parameters required for the application of the Bergmann approach are determined by a numerical simulation of the rolling contact. Results are compared to rolling contact fatigue tests performed on a recently developed test rig under the assumption of rolling without sliding and neglecting the effect of surface roughness and residual stresses.     

FE‐Festwalzsimulation und Dauerfestigkeitsberechnung festgewalzter Bauteile mit Konzepten der Schwingbruchmechanik

FE‐Simulation of Fillet Rolling and Fatigue life Calculation based on Fracture Mechanics Concepts Fillet rolling is a method which significantly improves the fatigue strength of members. Residual stresses induced in the surface layer during the fillet rolling process are able to retard or prevent crack propagation. For fatigue strength prediction of fillet rolled notched members a fracture mechanics based concept is described. It consists of three parts: • Finite element simulation of the fillet rolling process to calculate the residual stresses • Simulation of residual stress redistribution due to cyclic load • Assessment of fatigue cracks starting from notch roots and propagating under compressive residual stresses by means of fracture mechanics.     

Mechanical and corrosion fatigue behaviors of gradient structured 7B50-T7751 aluminum alloy processed via ultrasonic surface rolling

In this work, ultrasonic surface rolling process(USRP) was utilized to produce a gradient structured layer on 7 B50-T7751 aluminum alloy, and the mechanical properties and corrosion fatigue behavior of treated samples were studied. These results reveal that underwent USRP, a 425 ~m thick gradient structure and a 700 ~m deep compressive residual stress field are created, aluminum grain size become fine(~ 67 nm),and the corrosion rate of treated surface reduces by 60.08% owing to the combined effect of compressive residual stress and surface nanocrystallization. The corrosion fatigue strength is enhanced to 117% of that of 7 B50 Al alloys by means of USRP due to the introduced compressive residual stress, which is considered as the major favorable factor in suppressing the initiation and early propagation of corrosion fatigue cracks. Besides, the gradient structure is an important factor in providing a significant synergistic contribution to the improvement of corrosion fatigue performance.     

Fatigue and mechanical characteristics of nano-structured tool steel by ultrasonic cold forging technology

Ultrasonic cold forging technology (UCFT) utilizing ultrasonic vibration energy is a method to induce severe plastic deformation to a material surface, therefore, the structure of the material surface becomes a nano-crystal structure from the surface to a certain depth. It improves the mechanical properties; hardness, compressive residual stress, wear and fatigue characteristics. Applying UCFT to a rolling process in the steel industry is introduced in this study. First, the UCFT specimens of a tool steel (SKD-61/equivalent H13) are prepared and tested to verify the effects of the UCFT in a variety of mechanical properties, the UCFT is applied to the trimming knives in a cold rolling process. It has been determined that UCFT improves the mechanical properties effectively and becomes a practical method to improve productivity and reliability by about two times compared with the conventionally treated tooling in the trimming process in a cold rolling line.     

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。     

激光冲击处理对Ti6Al4V力学性能的影响

通过对钛合金Ti6Al4V的激光冲击处理,研究了激光冲击处理工艺对钛合金Ti6Al4V力学性能的影响.实验表明:激光冲击处理能有效提升Ti6Al4V的力学性能,在激光功率密度由1.15GW/cm2增加到2.31GW/cm2过程中,其冲击波峰值压力线性增加,表面最大残余压应力也相应增大,最高达-264MPa,表面硬化层的显微硬度高达510Hv,硬化层深度约为0.25mm,经过激光冲击处理后硬度相对于原始钛板提高了64%,随着激光能量的增加,冲击区域的抗拉强度极大增强,塑性降低.