Prediction of Fatigue Lifetime under Multilevel Cyclic Loading Based on a Short Crack Growth Model in a Low Carbon Steel SAE 1017

This contribution deals with the initiation and growth of short fatigue cracks in cyclic loaded notched specimens made of the 0.15 wt% carbon steel SAE1017. On the basis of experimentally determined data, a damage model based on cyclic crack growth has been developed, which accounts for the anomalous behaviour of short fatigue cracks. In this model the initiation and propagation of a critical crack is regarded as damage. This approach allows to calculate fatigue life for constant amplitude tests as well as for multilevel tests and irregular loading. Deviations from Miner's rule, which have been often observed for two‐level tests, are attributed to the varying fraction of crack initiation and propagation phase for different loads. The inaccuracy of Miner's rule deduced from two‐level tests is of secondary importance for service life calculation when compared with the negligence of amplitudes below the fatigue limit. The proposed model yields shorter service life than the elemental version of Miner's rule.     

Thermal Fatigue Testing of Tool Materials for Thixoforging of Steels

Semisolid processing, already a well established manufacturing route for the production of intricate, thin‐walled aluminium and magnesium parts with mechanical properties as good as forged grades, faces a major challenge in the case of steels. The tool materials must withstand complex load profiles and relatively higher forming temperatures for thousands of forming cycles for industrial application to be attractive. Since the forming pressures are much lower than those encountered in conventional forging, the principle die failure mechanism in steel thixoforging is expected to be thermal fatigue. Hence, suitable materials able to withstand the steel thixoforming environment for an economically acceptable life, can be best identified with a thermal fatigue test. Such a test is described in the present work. A novel CrNiCo and a nickel‐base superalloy, reported to exhibit superior thermal fatigue resistance in demanding tooling applications, was tested under thermal fatigue conditions encountered in the thixoforming of steels.     

The Effect of Post Fatigue Tensile Loading on Mechanical Properties and Failure Behaviour of Steel

In this research work the early fatigue damage characteristics up to 50% of life has been investigated at near‐endurance stress amplitude in a medium carbon steel with hardened & tempered and normalized conditions by application of post fatigue tensile loading. During fatigue loading microvoids were generated at the interphase‐interface through vacancy clustering. Directional localized coalescence of these microvoids generated ‘void sheets’ in the normalized steel and ‘craters’ in the hardened & tempered steel. These defects acted as the potential sites for failure during post fatigue tensile loading deteriorating the mechanical properties. This was, therefore, clearly indicated that when a high tensile load is superimposed on near‐endurance fatigue loading, material may fail much earlier than its actual fatigue life. The normalized steel suffered from more reduction in ductility than the hardened & tempered steel indicating more susceptibility to fatigue damage.     

Semi‐Solid Rheoforging of Steel

To produce steel components with complex shapes excessive machining is necessary frequently since high pressure die casting of steel is not industrially applied. Forming steel in the semi‐solid state can in principle produce new components and geometries which cannot be realised by conventional closed die forging. Semi‐solid forging of steel combines the possibility of producing geometries not conventionally forgeable in one forming operation and of adding further functions during the same operation. In previous investigations on thixoforming of steels, the semi‐solid steel was generated by reheating precursor material billets. An alternative approach for generating semi‐solid steel from the liquid state with subsequent forging operation is presented in this paper for the first time. The steel grades X210CrW12 cold work tool steel and 100Cr6 bearing steel are molten and driven into a globular semi‐solid state using a cooling slope and a cup. By cooling the steel into the semi‐solid range instead of heating it, the required process temperatures are lower than in the process route via heating. Therefore, the load on the dies in a semi‐solid forging operation is decreased. Suggestions for the respective layout of the process are made for both steel grades. Future potentials and challenges to be solved are discussed, showing advantages especially in the field of high melting point alloys such as steels. This technique enables to produce pre‐shaped semi‐solid billets to optimise the materials flow and the homogeneity of the mechanical properties.     

Investigating and Understanding the Cyclic Fatigue,Deformation, and Fracture Behavior of a Novel High Strength Alloy Steel: Influence of Orientation

In this paper, the results of a recent study aimed at understanding the influence of orientation on high cycle fatigue properties and final fracture behavior of alloy steel Pyrowear 53 is presented and discussed. This alloy steel has noticeably improved strength, ductility, and toughness properties compared to other competing high strength alloy steels having a near similar chemical composition and processing history. Test specimens of this alloy steel were precision machined and conformed to the specifications detailed in the ASTM standards for tension testing and stress‐controlled cyclic fatigue tests. Test specimens were prepared from both the longitudinal and transverse orientations of the as‐provided alloy steel bar stock. The machined test specimens were deformed in cyclic fatigue over a range of maximum stress and under conditions of fully reversed loading, i.e., at a load ratio of ?1, and the number of cycles‐to‐failure recorded. The specific influence of orientation on cyclic fatigue life of this alloy steel is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to characterize the intrinsic features on the fatigue fracture surfaces. The conjoint influence of microstructure, orientation, nature of loading, and maximum stress on cyclic fatigue life is discussed.     

应变幅对H13热作模具钢等温疲劳行为的影响

对H13热作模具钢试样进行600 ℃等温疲劳实验，通过显微维氏硬度计、金相显微镜（OM）、超景深显微镜和扫描电子显微镜（SEM）等设备研究了0.7%，0.9%和1.1%三种不同应变幅对疲劳行为的影响。结果表明:应力应变滞后回线呈现对称性，应变幅越大，滞回环面积越大。H13钢在实验中呈现循环软化的特征，应变幅越大，疲劳寿命越短，1.1%应变幅试样寿命约为0.7%应变幅试样的61.2%。应变幅的增加对裂纹萌生和扩展起促进作用，1.1%应变幅试样裂纹扩展最明显。高温非真空实验条件下，材料表面产生的氧化物也会促进裂纹扩展。疲劳后试样微观组织发生明显的长大和粗化，较大应变幅对碳化物析出有更大的助力，还会加速材料软化。有应变幅试样显微硬度远低于无应变幅试样。      

Residual stress relaxation of welded steel components under cyclic load

It is a well known fact that the fatigue strength and the life of welded steel components are affected, to a considerable extent, by residual stresses distributed around their weldments. When externally applied load is superimposed on residual stresses, unexpected deformations and failure of the components can occur. These residual stresses are not constant, but are relaxed or redistributed during in‐service. Under monotonic load relaxation takes place when the sum of external and residual stress locally exceeds the yield stress of material used. It is noteworthy that under cyclic load the residual stress is considerably relieved by the first or the early cycles of load, and then is gradually relaxed with increasing loading cycles. Although many investigations in this field have been carried out, the phenomenon of and mechanism of stress relaxation are still not clear, and only a few comprehensive models have been proposed to predict amount of relaxed residual stress. In this study, the characteristics of residual stress relaxation under monotonic and cyclic load were investigated, and a model for quantitatively predicting the amount of residual stress relaxation is proposed.     

Induction Heat Treatment of Sheet‐Bulk Metal‐Formed Parts Assisted by Water–Air Spray Cooling

In order to produce components with massive secondary functional elements from sheet metal bulk forming operations, termed sheet‐bulk metal forming, can be applied. Owing to high, three‐dimensional stress and strain states present during sheet‐bulk metal forming, ductile damage occurs in the form of micro‐voids. Depending on the material flow properties, tensile residual stresses can also be present in the components' formed functional elements. During service, the components are subjected to cyclic loading via these functional elements, and tensile residual stresses exert an unfavorable influence on crack initiation and crack growth, and therefore on the fatigue life. Following the forming process, temperature and microstructurally related compressive residual stresses can be induced by local heat treating of the surface. These residual stresses can counteract potential crack initiation on the surface or in the subsurface regions. In the present study, the adjustability of the residual stress state is investigated using a workpiece manufactured by orbital cold‐forming, which possesses an accumulation of material in its edge region. Based on residual stress measurements in the workpiece's edge region using x‐ray diffractometry, it is possible to verify the compressive residual stresses adjusted by varying the cooling conditions.     

Thixocasting Steel Hand Tools using Al2O3‐coated Steel and Molybdenum Dies

Forging is state‐of‐the‐art for producing hand tools on an industrial scale. Due to high demands on the stiffness and the fracture toughness, high‐strength forging steels are used to provide cavity‐free components with high mechanical load capacity. Moreover, forging is a cost‐effective mass production process but, in spite of all its advantages, it has its limitations, e.g. in the freedom of designs. However, because of the extreme thermal loading (particularly with regard to permanent moulds) and the frequently unavoidable casting defects, hand tools are not cast. By means of thixocasting steel, technical difficulties can be reduced and new options are provided which allow the manufacturing of components with much higher complexity than that using forging. Through near‐net shape production, manufacturing steps and costs can be reduced. Furthermore, steels, which are difficult to forge but nonetheless have high potential for specific applications (such as high strength or corrosion resistant steels), can also be processed. In cooperation with industrial partners, X39CrMo17 stainless steel size 17 combination spanners were thixocast. Forming dies were designed and optimized by simulation, the hot forming X38CrMoV5 tool steel as well as the molybdenum alloy TZM were selected as the tool alloys. The dies were treated by a plasma nitriding process and subsequently coated with crystalline Al₂O₃ protective coatings by plasma‐enhanced chemical vapor deposition (PECVD). During the experiments, combination spanners were successfully cast in the semi‐solid state. Cast parts were heat‐treated to enhance the components' toughness, which was subsequently measured by Charpy impact and tensile tests.     

4Cr5MoSiV1热作模具钢700 ℃的低周疲劳行为

采用轴向应变幅控制的低周疲劳试验研究了总应变幅对4Cr5MoSiV1热作模具钢700 ℃低周疲劳行为的影响,包括循环应力响应行为、循环应力应变行为、循环迟滞回线和应变疲劳寿命行为等。结果表明:随着总应变幅从0.2%增大到0.6%,4Cr5MoSiV1钢在700 ℃时循环应力响应均表现为先循环硬化再循环软化的特性,并且应力幅最大值从220 MPa增大到308 MPa。同时,随着总应变幅的增大,4Cr5MoSiV1钢在700 ℃下的低周疲劳寿命由6750循环周次降低到210循环周次,且其过渡寿命约为1313循环周次。疲劳断口形貌分析结果显示,高温低周疲劳过程中裂纹主要萌生于试样表面处,且随着应变幅增大,裂纹源逐渐增多,疲劳条纹间距变宽,其断裂方式由韧性断裂转变为脆性断裂。透射电镜分析结果显示,循环软化可能与板条结构转变为胞状结构、基体发生位错湮灭、碳化物的析出和粗化有关。      

Biaxial Low Cycle Fatigue Behavior and Martensite Formation of a Metastable Austenitic Cast TRIP Steel Under Proportional Loading

In this paper the biaxial low cycle fatigue behavior under proportional loading of a recently developed metastable austenitic stainless cast steel is presented. Total strain controlled tests were carried out on a 250 kN biaxial servohydraulic tension‐compression testing machine equipped with a biaxial orthogonal extensometer to measure the principal strains in the gauge area of the used cruciform specimens. The principal stresses were determined based on the compliance after the load reversals. The low cycle fatigue behavior under biaxial synchronous loading is compared to the uniaxial behavior. Therefore, biaxial single step tests and a biaxial multiple step load increase test were carried out. The dependence of the stress state on the cyclic deformation curves, cyclic stress‐strain curves and the formation of martensite are described. Finally, the fatigue life relationship according to Basquin and Manson‐Coffin was determined and compared to the Smith, Watson and Topper damage parameter, which provides a satisfactory fatigue life prediction.     

High Temperature Tribological Studies on Surface Engineered Tool Steel and High Strength Boron Steel

The popularity of hot sheet metal forming processes in the recent years has necessitated research efforts to improve tool life and control the friction level during hot forming operations. In this work, the tribological properties of tool steel and ultra high strength boron steel (UHSS) pairs at elevated temperatures have been studied by using a special hot sheet metal forming test rig that closely simulates the conditions prevalent in the real process. This test involves linear unidirectional sliding of a preheated UHSS sheet between two tool steel specimens where new workpiece material is continuously in contact with the tool surface. The study is aimed at investigating different surface treatments/coatings applied on either the tool or sheet surface or on both. The results have shown that it is possible to control the coefficients of friction through surface treatments and coatings of the tool and workpiece materials. The application of a coating onto the sheet material has a greater influence on the friction compared to changing the tool steel surface. After running‐in, the investigated tool steel variants show almost similar frictional behaviour when sliding against the same sheet material. Although coating the UHSS sheet reduces friction, it abrades the tool surface and also results in transfer of the sheet coating material to the tool surface.     

盐雾腐蚀对HRB400E抗震钢筋低周疲劳和力学性能的影响

摘要：采用质量分数为5%的NaCl溶液在盐雾腐蚀箱进行30~90d加速腐蚀试验,研究了盐雾腐蚀对HRB400E钢筋低周疲劳行为和拉伸性能的影响。然后采用轴向位移控制模拟地震载荷对腐蚀钢筋开展了低周疲劳和拉伸试验,获得循环响应特征曲线和应变 寿命曲线。结合SEM断口形貌观察,分析钢筋的低周疲劳断裂机制。结果表明：盐雾腐蚀对钢筋的质量和尺寸有明显影响,钢筋表面产生腐蚀坑;钢筋的力学性能随腐蚀时间增加而降低,腐蚀90d的断裂伸长率下降率达461%,屈服强度在腐蚀30d以后可能就不再满足标准要求;腐蚀明显削弱了钢筋的抗循环载荷性能,导致低周疲劳寿命下降;腐蚀会减小钢筋的裂纹扩展区面积并加速裂纹扩展。     

Experimental Analysis of Crack Growth in GFRP Reinforced Concrete

For decades, bridge slabs have been troubled by the corrosion of steel reinforcement. The unique corrosion resistance of glass fiber-reinforced polymer (GFRP) bars makes them a promising alternative to steel bars. Experiments have been conducted to investigate the bond performance of GFRP reinforced concrete under constant amplitude cyclic fatigue loading. Each specimen was an identical length beam with a single GFRP bar at the bottom, intended to simulate a transverse strip of a typical bridge deck slab. The crack growth was monitored for specimens of different widths, simulating different transverse reinforcement spacings. Up to 2?million?cycles of cyclic loads were applied at 100% typical service load levels. No fatigue failure was encountered in the testing. The effects of moderate overloads were also investigated.     

Estimating the working life of steel beams on the basis of their mechanical properties

Forged steel 50 rollers are used in the roughing group of wire and small-rod mills. In intense rolling, the roller pin often breaks, since the pin experiences the maximum stress. It is established that the fracture is of fatigue type, and three disintegration zones are identified. Crack propagation is due to the stress in the crack tip. The rate of crack propagation in cyclic loading depends on the structure of the metal, the grain size, and the mechanical properties of the rollers. The unbroken rollers are characterized by high values of the impact strength, initial and final temperatures of ingot forging, and σ_y/σ_B. An empirical dependence of the number of roller rotations to pin fracture on the mechanical properties of the material and the loads is established. Small-grain structure of the metal matrix and purity of the metal ensure a good combination of strength and plasticity.     

Simplified numerical approach for the thermo-mechanical analysis of steelmaking components under cyclic loading: an anode for electric arc furnace

The need of a strong improvement of productivity and reliability led the adoption of advanced modeling techniques in the design of steelmaking plants components. In this work a procedure based on a finite element simulation is proposed in order to perform a durability analysis of an anode for electric arc furnace. This component undergoes cyclic thermal loads, which also produce a partial melting of one part, meanwhile the other is maintained at almost constant temperature by a cooling system. A simplified, but effective, procedure is developed to take into account steel melting during the heating phase. Considering the cyclic loading conditions, several material cyclic plasticity models, and their effect on the thermal fatigue behavior, are also systematically investigated. The proposed approach permits the component fatigue life to be assessed by a simple and fast uncoupled thermo-mechanical simulation in steady-state conditions.     

热轧逆相变退火中锰钢的疲劳性能

 为了研究循环载荷对含奥氏体钢微观组织和力学性能的影响,对逆相变处理中锰钢进行了疲劳性能研究,采用SEM、EBSD、XRD等表征了微观组织,采用单轴拉伸试验表征了疲劳前后的力学性能。研究结果表明,在应力比R为-1的试验条件下,逆相变中锰钢中值疲劳极限为378 MPa,疲劳极限与抗拉强度之比σ-1/R_m为0.48;中锰钢超细晶粒尺寸特征有利于阻碍二次疲劳裂纹扩展,亚稳奥氏体的转变行为受应力幅影响较大;在中值疲劳极限的应力水平下,亚稳奥氏体和单轴拉伸性能受循环载荷影响不大。     

Low cycle fatigue properties of modified 9Cr-1Mo ferritic martensitic steel weld joints in sodium environment

Mod.9Cr-1Mo ferritic-martensitic steel is the material chosen for the steam generator of the Prototype Fast Breeder Reactor being built at Kalpakkam, India. The use of sodium as a heat transfer medium for Liquid Metal Fast Breeder Reactors (LMFBRs) necessitates a comprehensive understanding of the effects of dynamic sodium on the Low Cycle Fatigue (LCF) behaviour of structural components. Moreover welds being the weak links in any structure, it is necessary to evaluate the LCF behaviour of joints in sodium environment, more so in Mod.9Cr-1Mo steel because of the well established Type — IV cracking in this material. With this aim in view, a programme has been initiated to evaluate the LCF properties of weld joints of this steel in dynamic sodium environment. A facility has been developed in-house for mechanical property evaluation in dynamic sodium. LCF tests conducted in flowing sodium environment at 823 and 873 K showed a similar trend in cyclic stress response in air and sodium environments exhibiting a continuous cyclic softening behaviour. The fatigue lives are significantly improved in sodium environment when compared to the data obtained under identical testing conditions in air environment. The lack of oxidation in sodium environment is considered to be responsible for the delayed crack initiation and consequent increase in fatigue life. Comparison with RCC-MR code shows that the design curve based on air tests is conservative.     

Ni-based superalloy as a potential tool material for thixoforming of steels

Abstract

Semisolid processing, already a well established manufacturing route for the production of intricate, thin walled aluminium and magnesium parts with mechanical properties as good as forged grades, faces a major challenge in the case of steels. The tool materials must withstand complex load profiles and relatively higher forming temperatures for thousands of forming cycles for this near-net shape process to be attractive for steels on an industrial scale. The potential of a Ni-based superalloy, Inconel 617, reported to exhibit superior thermal fatigue resistance in demanding tooling applications, was investigated. The response to thermal cycling of this alloy at high temperatures was compared with that of X38CrMoV5 hot work tool steel widely used in the manufacture of conventional forging dies. The favourable thermophysical properties of the latter were completely negated by its limited temper resistance, while the Inconel 617 alloy responded to thermal cycling by the usual heat cracking mechanism.     

Process Design and Tooling Configuration for Precision Forging of High Performance Components

Precision forging is a production process which uses forming technology for manufacturing near‐net shaped, highly loaded components. In comparison to conventional forming and machining production processes, an improvement in material savings and a significant shortening of the process chain can be obtained. Subsequent to the forming and the integrated heat‐treatment process, often only a final hard‐finishing of specific functional surfaces with minimum cutting volumes is necessary. The increased demands on dimensional and geometric accuracy from precision forgings result in increased requirements from the applied forging tools, the forming machines as well as from the specific design and control of the process. Therefore the Finite Element Analysis (FEA) is an appropriate simulation tool. Aims of the current research are the development of strategies for comprehensive process design and shrinking correction as well as the enhancement of the machines and tooling technologies for precision forging. Precise tool design requires a detailed knowledge about material specific heat balance resulting in a process and tool related heat transfer.