Improvement of life prediction in AISI H11 tool steel by integration of thermo-mechanical fatigue and creep damage models

In hot forging operations, the die surfaces and the nearest surface layers of the die undergo mechanical and thermal cycles which significantly influence their service life. The real thermal and mechanical cycles have been previously investigated in forging plants by measurements and numerical simulation, and a reasonable variation window of process parameters has been determined. A new simulative test applied to AISI H11 hot working die steel has been used to generate failure data in conditions similar to those of the forging dies, but under a more controlled and economical method. Fracture surfaces of specimens for different tests observed by scanning electron microscopy (SEM) indicate that both thermo-mechanical fatigue (TMF) and creep phenomena can be considered to be main damage mechanisms and their contribution to the failure differs as testing conditions vary. As a result of the experiments, the failure is affected by both thermo-mechanical cycle and resting time at high temperature. Therefore, the authors developed a new lifetime prediction model obtained by combining the damage evolution laws, at each cycle, for pure creep and pure TMF. This combination was based on the linear accumulation rule. The damage evolution law for pure creep is obtained by modifying Rabotnov's law in order to suit the case of actual hot forging cycles, where temperature and stress vary widely. The damage evolution law for pure TMF is based on a generalization of the Wöhler–Miner law. This law is modified in order to take into account the presence of thermal cycle and thermal gradient. Comparison between the experimental cycles to failure and the predicted ones was performed using tests excluded in the determination of the coefficients. The conclusion was that the accuracy of prediction appears to be quite good and that the linear accumulation and interaction of TMF and creep is confirmed.     

The characteristics of fatigue under isothermal and thermo-mechanical load in Cr–Ni–Mo cast hot work die steel

ABSTRACT High temperature isothermal fatigue (IF) and in-phase thermo-mechanical fatigue (TMF) tests in load control were carried out in cast hot work die steel. At the same load amplitude, the fatigue lives obtained in the in-phase TMF tests are lower than those obtained in the isothermal tests. Observations of fracture surface and the response of stress–strain reveal that cyclic creep in the tensile direction occurs and the intergranular cracks dominate in TMF tests, whereas cyclic creep in the compressive direction occurs and the path of the crack growth is mainly transgranular in IF tests. A model of life prediction, based on the Chaboche law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method, the lifetime prediction gives results corresponding well to experimental data.     

Thermo-mechanical fatigue testing of a rhenium-free single-crystalline super alloy

Abstract

Due to high temperatures and mechanical loads, cracks are initiated in aero engine turbine blades which limit the cyclic life of these components. The materials used for components which underlie high thermal and mechanical load are single crystalline (SX) nickel based super alloys that in most cases contain a certain amount of rhenium. Dramatically increasing Re prices lead to the development of Re-free alloys.

In this work, low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests were carried out on the Re-free single crystal M-247LC SX. The test results are shown and a model based on crack propagation was used to predict LCF and TMF life. It was shown, that the modeling results fit properly for out-of-phase TMF and LCF life while for in-phase TMF differences between calculated life and experiments occur due to a different mechanism of fracture.     

Structural changes of radial forging die surface during service under thermo-mechanical fatigue

Radial forging is one of the modern open die forging techniques and has a wide application in producing machine parts. During operation at high temperatures, severe temperature change associated with mechanical loads and the resultant wearing of the die surface lead to intense variation in strain on the die surface. Therefore, under this operating condition, thermo-mechanical fatigue (TMF) occurs on the surface of the radial forging die. TMF decreases the life of the die severely. In the present research, different layers were deposited on a 1.2714 steel die by SMAW and GTAW, with a weld wire of UDIMET 520. The microstructure of the radial forging die surface was investigated during welding and service using an optical microscope and scanning electron microscope. The results revealed that, after welding, the structure of the radial forging die surface includes the γ matrix with a homogeneous distribution of fine semi-spherical carbides. The weld structure consisted mostly of columnar dendrites with low grain boundaries. Also, microstructural investigation of the die surface during operation showed that the weld structure of the die surface has remained without any considerable change. Only dendrites were deformed and broken. Moreover, grain boundaries of the dendrites were revealed during service.     

Tutorial: Industrial Split-plot Experiments

Many industrial experiments involve two types of factors: those that are hard-to-change and those that are easy-to-change (ETC). Hard-to-change (HTC) factors have levels that are difficult and/or expensive to change. As a result, the experimenter would prefer to run the experiment in such a manner as to minimize the number of times that he/she must change the levels of these factors. Unfortunately, it is precisely the changing of these levels that provides the information about the effects of the HTC factors. Consequently, when we minimize the number of times we change the levels of these factors, we also minimize the relevant information about their effects.

This paper summarizes the structure and the analysis of industrial split-plot experiments. The purpose of this article is to teach practitioners how to identify split-plot experimental conditions, how to run the experiment efficiently, and then how to analyze the results. The article illustrates both first-order and second-order experiments. The first four sections provide a basic background on experimental design and an introduction to first-order split-plot experiments. The remainder of this article contains more advanced topics dealing with second-order, split-plot experiments.     

Effect of forging conditions and annealing temperature on fatigue strength of two-phase titanium alloys

This paper reports on the results of studies on the influence of deformation degree and temperature in the die forging process and annealing temperature on fatigue strength of forgings made of two-phase, martensitic titanium alloys (Ti–6Al–4V and Ti–6Al–2Mo–2Cr). Dilatometric and metallographic studies have been carried out along with fatigue tests. The influence of phase composition and microstructure obtained after cooling at different rates on fatigue strength has been determined.     

Probabilistic prediction of high cycle fatigue reliability of high strength steel butt‐welded joints

The aim of this paper is to develop a probabilistic approach of high cycle fatigue (HCF) behaviour prediction of welded joints taking into account the surface modifications induced by welding and the post‐welding shot peening treatment. In this work, the HCF Crossland criterion has been used and adopted to the case of welded and shot peened welded parts, by taking into account the surface modifications which are classified as follows: (i) the compressive residual stresses, (ii) the surface work‐hardening, (iii) the geometrical irregularities and (iv) the superficial defects. The random effects due to the dispersions of: (i) the HCF Crossland criterion material characteristics (ii) the applied loading and (iii) the surface modifications parameters are introduced in the proposed model. The HCF reliability has been computed by using the ‘strength load’ method with Monte Carlo simulation. The reliability computation results lead to obtain interesting and useful iso‐probabilistic Crossland diagrams (PCD) for different welding and shot peening surface conditions. To validate the proposed method, the approach has been applied to a butt‐welded joint made of S550MC high strength steel (HSS). Four types of specimens are investigated: (i) base metal (BM), (ii) machined and grooved (MG) condition, (iii) As welded (AW) condition and (iv) as welded and shot peened (AWSP) condition. The comparison between the computed reliabilities and the experimental investigations reveals good agreement leading to validate the proposed approach. The effects of the different welded and post‐weld shot peened specimen's surface properties are analysed and discussed using the design of experiments (DoE) techniques.     

Modeling fatigue crack and spalling for rolling die under hot milling

In hot milling process, rolling die is subjected to nonsteady conditions which can rise the combinations of fatigue and spalling damage mechanism. An understanding about the failure mechanism of the rolling die is essential under hot rolling process. Fatigue crack growth and spalling process are governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic‐plastic fracture mechanics, an analytical model are presented in this paper to determine the elliptical crack growth rate and spalling damage mechanism. The model includes new proposed constitutive equations for fatigue and spalling crack growth. To verify the models, finite element simulation and experimental data are considered. The results show good agreement with finite element simulation and experimental data.     

Validation of procedures for fatigue life assessment of a steel pressure vessel

A full‐size pressure vessel, made of steel plate P355NL1 (EN 10028‐3), was tested under repeated internal pressure until its failure was observed. Also, four representative structural details of the tested pressure vessel were fatigue tested under load control with a stress ratio of R= 0. These structural details are basically two seam‐welded joints, namely a butt‐welded joint and a joggle‐welded joint, one plate attachment using fillet‐welded joints and a nozzle‐to‐plate connection. S–N curves were generated for these details based on both nominal and structural stresses. These curves are critically compared with those proposed in pressure vessel design codes like the ASME VIII – Division 2, the PD 5500 and the recently approved EN procedures, the EN 13445 standard. Finally, predictions of the fatigue life of the pressure vessel, obtained using the previously referred procedures and the experimentally derived design curves are critically compared with the observed life of the vessel.     

一种新型非调质钢弯曲疲劳性能的试样尺寸效应

使用液压伺服疲劳试验机考察一种新型(汽车前轴用)Nb+V复合微合金非调质钢的疲劳行为,绘制出S-N曲线并分析了疲劳断日特征,研究了其三点弯曲疲劳性能的试样尺寸效应及其原因.结果表明,试样的尺寸对非调质钢的三点弯曲疲劳性能有显著的影响,其三点弯曲疲劳极限随着试样尺寸的减小而增加,但是试样尺寸对疲劳试样的断口形貌几乎没有影响;在三点弯曲疲劳试验中,试样尺寸效应源于试样内部的应力梯度,小尺寸试样的应力梯度比大尺寸试样的高.     

基于有限元模拟的四通阀体多向模锻工艺研究

针对阀体传统制造工艺效率低、锻件性能差、成本高等问题,本文提出采用先进的多向模锻工艺进行阀体制造,并以一种不等径四通阀体为典型对象,结合Deform-3D数值分析进行工艺研究.首先,根据不等径四通阀体的结构特征,确定了分模面,并依此设计了两种锻件图;其次,根据锻件图设计了 3种不同的多向模锻结构,并分别确定了模具的加载...     

双频激振下带V形缺口轴的疲劳寿命研究

针对金属轴类零件在实际复杂工况下易产生应力集中而发生疲劳破坏的问题,利用双频激振系统,研究带V形缺口轴的疲劳寿命随缺口几何参数的变化规律。首先,提出了促进轴疲劳裂纹萌生的激振频率控制曲线,同时采用响应曲面法中的Box-Behnken设计法对V形缺口的夹角、圆角半径和深度进行三因素三水平的实验设计;其次,建立了疲劳寿命多元回归预测模型,并采用方差分析法对模型进行可靠性评价;最后,利用响应曲面和等高线图分析了缺口的夹角、圆角半径和深度对轴疲劳寿命的影响规律,并进行了预测模型的应用。研究结果表明：疲劳寿命预测值与实测值之间的误差在4.2%以内,预测精度较高,预测模型可靠;缺口几何参数对疲劳寿命从大到小的影响次序是缺口深度、缺口圆角半径、缺口夹角,以圆角半径和深度的交互作用对轴疲劳寿命的影响最为显著。研究结果可为金属轴类零件的抗疲劳设计提供重要参考。     

渗碳Cr-Ni高强硬度合金钢超高周疲劳寿命预测

为了给渗碳合金钢提供一种有效可行的超高周疲劳寿命预测方法,在应力比为0和0.3两种情况下,对渗碳Cr-Ni高强硬度合金钢展开疲劳试验研究.通过对试样断口的微观组织观测,发现渗碳层与基体材料中均有非金属夹杂的存在;通过对裂纹萌生位置和疲劳断口形貌的观察,将疲劳失效分为带有细晶粒区(Fine Granular Area,F...     

热作模具钢在高温热机械应力循环下的疲劳断裂行为

研究了热作模具钢在应力控制下的等温疲劳和同相热机械疲劳寿命，发现在相同的应力幅下，同相热机械疲劳寿命低于上限温度的等温疲劳寿命。通过研究疲劳过程中的循环应变响应和疲劳断口特征时发现，等温疲劳条件下，滞后环朝压缩方向发展，疲劳裂纹主要为穿晶萌生与扩展；在热机械疲劳条件下，滞后环朝拉伸方向发展，疲劳裂纹主要沿晶萌生与扩展。这是导致同相热机械疲劳寿命低于等温疲劳的主要原因。     

Low cycle fatigue life enhancement of 316 L stainless steel by nitrogen alloying

Tests carried out at room temperature on 316 L stainless steels with different nitrogen contents show that nitrogen improves the low cycle fatigue resistance of the materials. However, saturation occurs when nitrogen content is above 0.12 weight per cent. The microstructural aspect is also studied; the deformation is more difficult and more planar when nitrogen is present. Moreover, nitrogen delays the formation of cells. A single relation, derived from the Manson-Coffin formula, describes the low cycle fatigue behaviour of these steels by taking into account plastic strain range and nitrogen content.     

Corrosion fatigue life prediction of a steel shaft material in seawater

Corrosion fatigue behaviour of a medium strength structural material was studied in air and in 3.5% NaCl solution. Emphasis was placed on the study of corrosion pit formation and the development of cracks from pits. Pitting and crack propagation were quantified throughout the fatigue loading thereby allowing a model to be developed that included the stages of pitting and the pit-to-crack transition in order to predict the fatigue life. The results showed that a large number of corrosion pits with small size form at a very early stage in the fatigue lifetime. The number of pits and subsequent cracks was found to be higher at higher stress levels leading to multiple crack development and coalescence. When compared to air, fatigue life in a corrosive environment was significantly reduced at low stress levels due to pitting damage, indicating a dominant role of corrosion over that of mechanical effects. The corrosion fatigue model proposed shows good agreement with the experimental test data at lower stress levels but predicts more conservative lifetimes as the stress increases. Kitagawa–Takahashi diagram was produced for both test environments where it is indicated that the fatigue limit can be eliminated in a corrosive environment.     

In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in‐phase (IP) and out‐of‐phase (OP) thermomechanical fatigue (TMF) tests cycling from 400 °C to 700 °C under full reverse strain‐controlled condition were investigated. Stress‐mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high‐temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi‐cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter.     

深海钢悬链立管时域疲劳寿命预估研究

研究了钢悬链立管(SCR)时域非线性动力分析用于疲劳寿命预估。随着海洋工业不断往深海进发,过去十年内钢悬链立管成为众多油田开发的优选方案。然而,对于钢悬链立管设计而言疲劳是个关键难题。钢悬链立管的非线性动力特性显著,与频域方法相比,时域分析能够模拟非线性水动力载荷以及结构的非线性特性。本文数值模拟了海洋环境载荷作用下SCR的非线性结构动力响应,利用S-N曲线对立管进行整体疲劳寿命预估。对影响SCR疲劳寿命的各参数进行敏感度分析,输入的敏感度分析参数包括拖曳力系数、浮力因子、海床土体硬度等,研究表明疲劳寿命的预估结果与这些参数相关,所给出的结论便于设计人员更好理解钢悬链立管动力特性,选择合适的参数用于立管疲劳设计。     

Fatigue assessment and fatigue life calculation of quenched and tempered SAE 4140 steel based on stress–strain hysteresis, temperature and electrical resistance measurements

In this paper, mechanical stress–strain-hysteresis, temperature and electrical resistance measurements are performed for the detailed characterization of the fatigue behaviour of quenched and tempered SAE 4140 steel used for many applications in the automotive industry. Stress-controlled load increase and constant amplitude tests (CATs) were carried out at ambient temperature on servo-hydraulic testing systems. The applied measurement methods depend on deformation-induced changes of the microstructure in the bulk material and represent the actual fatigue state. The plastic strain amplitude, the change in temperature and the change in electrical resistance can be equally used for an assessment of baseline fatigue properties in generalized cyclic deformation curves as well as in generalized Morrow and Coffin–Manson curves. On the basis of comprehensive experimental fatigue data, the physically based fatigue life calculation method ‘PHYBAL’ based on the generalized Basquin equation was developed. S–N (Woehler) curves calculated with ‘PHYBAL’ using data from only three fatigue tests agree very well with the conventionally determined ones.     

Approaches to fatigue life assessment applied in the very high cycle regime

Designers and calculation engineers are becoming increasingly interested in the latest results on very high cycle fatigue (VHCF). Often, the influence of loading with a very high number of cycles on component behaviour is estimated conservatively, but the exact safety margin is unknown. This paper gives an overview of failure mechanisms in the HCF‐ and VHCF‐regions and of material and component related influences, which have to be considered in the fatigue life assessment. The state of the art of design codes, recommendations from the literature and initial investigations on variable amplitude loading in the VHCF‐regime are presented. This review indicates that further research activities are necessary to improve fatigue life assessment in order to allow a reduction of safety margins.