镍基粉末高温合金中夹杂物导致裂纹萌生和扩展行为的研究

采用扫描电镜原位拉伸和原位疲劳的方法,跟踪观察了人工植入Al2O3夹杂物的镍基粉末高温合金P/M Rene95中夹杂物导致裂纹萌生、扩展乃至断裂的过程,结果表明,无论是在单轴拉伸还是低周疲劳实验中,裂纹均首先萌生于脆性非金属夹杂物Al2O3处,大于一定尺寸的夹杂物,还会使该裂纹扩展成为导致合金断裂的主裂纹,从而大大降低合金的屈服强度、断裂强度及低周疲劳寿命.     

热处理工艺对50CrVA高强弹簧钢超高周疲劳损伤机制的影响

研究了50CrVA高强弹簧钢在不同热处理状态下(淬火+中温回火和退火)的超高周疲劳破坏行为及其裂纹萌生机理。结果表明,50CrVA高强弹簧钢在107~109循环周次内发生疲劳破坏,两种热处理状态的S-N曲线下降形态不同,均未出现疲劳极限。热处理工艺改变50CrVA的微观组织,从而影响超高周疲劳阶段(寿命107周次)的疲劳破坏损伤机制:经淬火+中温回火处理材料破坏多起源于内部夹杂物,夹杂物周围存在的应力场与溶质原子发生弹性交互作用,吸引间隙原子向其周边扩散、富集,使间隙原子富集区材料性能下降,导致裂纹在内部夹杂物处萌生;退火热处理后材料微观组织对间隙原子向材料内部夹杂物扩散起到阻碍作用,所以超高周疲劳裂纹易于在材料表面萌生。     

非金属夹杂物对纯镍N6焊接接头组织性能的影响

以纯镍N6为主要研究对象,借助光学显微镜(OM)、扫描电镜(SEM)、能谱分析仪(EDS)、X射线衍射仪(XRD)及电解萃取装置等手段分析了非金属夹杂物对纯镍N6焊接接头结构及性能的影响.结果表明:纯镍N6试样断裂与非金属夹杂物尺寸、形貌、数量等因素有直接的关系;非正常断裂试样的断口表现为沿晶断裂,断口上夹杂物主要由Al、Si、Ca、Mg、O、Fe等元素组成;对试样的金相组织与夹杂物进行分析发现非正常断裂试样中夹杂物数量及尺寸均大于正常试样,杂物数量大于50个/mm2且夹杂物尺寸大于10 μm的试样均发生断裂;夹杂物主要为氧化铝类(Al2O3、SiO2)、硅酸盐类、氮化物类(AlN、TiN、Ti(C,N))及复相夹杂物;原位拉伸试验表明,断裂的主要原因是在金属基体中存在的夹杂物诱发裂纹源,在应力的作用下裂纹扩展导致断裂.综合分析表明,纯镍N6焊接接头在生产中的断裂是由脆性夹杂物的存在而引起的.     

汽轮机转子钢常温与600℃超高周疲劳行为研究

利用自主研发的高温超声疲劳实验系统,开展Cr Mo W转子钢常温及600℃下的超高周疲劳实验。为新型超超临界汽轮机转子提供高温超高周疲劳数据,实验结果表明高温会极大降低转子钢的疲劳强度。S-N曲线在常温及600℃下均呈现连续下降型,且600℃下S-N曲线在整个疲劳寿命周次内保持一定下降趋势。断口分析发现,常温下疲劳寿命107周次试件的疲劳裂纹以内部萌生为主,600℃下疲劳破坏的内部夹杂萌生方式与表面萌生方式均分布于整个疲劳寿命。夹杂物尺寸分析表明,高温降低疲劳裂纹内部萌生夹杂物的临界尺寸。     

夹杂物对粉末高温合金疲劳性能的影响

粉末高温合金FGH95是发动机用的优选材料。通过对该合金的疲劳性能试验和扫描电镜微观观察,发现合金中含有由铝、氧、镁、钛、硅和钇等元素组成的夹杂物。无论室温或者高温条件下,合金的疲劳裂纹源均从夹杂物处萌生,并且夹杂物尺寸越大,距离表面越近,所对应的疲劳寿命越低。     

Q345B宽带钢边缘裂纹分析

采用化学成分分析、力学性能测试、显微组织观察和夹杂物检验等方法对有边缘裂纹的Q345B宽带钢进行分析。发现是由于铸坯存在FeS,MnS等夹杂物,轧制中这些夹杂物及过高锰含量导致裂纹的进一步扩展,最终形成边缘裂纹。     

余热处理钢筋剪切开裂分析

通过金相检验、扫描电镜观察、夹杂物分析和工艺分析等手段对牌号为HRB335,规格为击Ф22mm的余热处理钢筋剪切开裂的原因进行了分析。结果表明：是由于连铸坯内部裂纹中存在的夹杂物导致铸造裂纹在轧制过程中不能被焊合,从而引起了钢筋剪切开裂。     

20CrMoH钢齿轮开裂原因

在某20CrMoH钢齿轮渗碳处理过程中,其齿轮根部出现裂纹。采用宏观观察、化学成分分析、金相检验、扫描电镜及能谱分析等方法分析了齿轮开裂的原因。结果表明：材料晶界处分布枝晶状的硫化锰,该类夹杂物属于Ⅱ类硫化物,形成于第二脆性区的温度区间,由稳定的共晶反应生成,硫化锰夹杂物的形成与S、O元素含量及冷却速率有关;夹杂物破坏了基体的连续性,应力集中处萌生微裂纹并不断扩展,最终导致20CrMoH钢齿轮发生开裂。     

U75钢轨闪光焊接头轨腰断后伸长率不合格原因分析

采用化学成分分析、力学性能测试、金相检验和断口分析等方法,分析了U75V钢轨闪光焊接头轨腰拉伸断后伸长率不合格的原因。结果表明:钢轨接头和钢轨母材轨腰大量密集分布MnS夹杂物(评级达到3.0级),长条状的MnS夹杂物经闪光焊顶锻后,随金属流线偏转45°方向,当与拉伸过程中的45°最大剪切应力平行时,相当于MnS夹杂物受到横向拉伸剪切,裂纹容易在夹杂物中开裂和扩展,故断后伸长率很低。     

某风电机组行星齿轮断齿原因

某风电机组行星齿轮在运行约2a后发生断齿.采用化学成分分析、宏观观察、断口分析、金相检验、扫描电镜及能谱分析等方法,分析了该齿轮断齿的原因.结果表明:该行星齿轮原材料中存在大量聚集的夹杂物,在齿轮服役过程中,夹杂物附近易产生应力集中,从而在该处萌生裂纹,齿轮表面烧伤加速裂纹扩展,最终导致齿轮发生疲劳断裂.     

Effect of non-metallic inclusions on butterfly wing initiation,crack formation,and spall geometry in bearing steels

Non-metallic inclusions such as sulfides and oxides are byproducts of the bearing steel manufacturing process. Stress concentrations due to such inclusions can originate cracks that lead to final failure. This paper proposes a model to simulate subsurface crack formation in bearing steel from butterfly-wing origination around non-metallic inclusions until final failure. A 2D finite element model was developed to obtain the stress distribution in a domain subjected to Hertzian loading with an embedded non-metallic inclusion. Continuum Damage Mechanics (CDM) was used to introduce a new variable called Butterfly Formation Index (BFI) that manifests the dependence of wing formation on depth. The value of critical damage inside the butterfly wings was obtained experimentally and was used to simulate damage evolution. Voronoi tessellation was used to develop the FEM domains to capture the effect of microstructural randomness on butterfly wing formation, crack initiation and crack propagation. Then, the effects of different inclusion characteristics such as size, depth, and stiffness on RCF life are studied. The results show that stiffness of an inclusion and its location have a significant effect on the RCF life: stiffer inclusions and inclusions located at the depth of maximum shear stress reversal are more detrimental to the RCF life. Stress concentrations are not significantly affected by inclusion size for the cases investigated; however, a stereology study showed that larger inclusions have a higher chance to be located at the critical depth and cause failure. Crack maps were recorded and compared to spall geometries observed experimentally. The results show that crack initiation locations and final spall shapes are similar to what has been observed in failed bearings.     

Experimental study of the cracking behavior of specimens containing inclusions (under uniaxial compression)

This paper presents the results of an experimental investigation on the cracking behavior of brittle heterogeneous materials. Unconfined, uniaxial compression tests were conducted on prismatic gypsum specimens containing either one, or two, inclusions. These inclusions were of different strengths, stiffnesses shapes, and sizes. Emphasis was placed on crack coalescence processes associated with specimens containing an inclusion pair, as this was the primary objective of the research. Some observations reported in this study compare well with those of other researchers as the overall cracking sequences are similar. On the other hand, the amount of debonding observed in this study at the inclusion interface is significantly less than what was previously observed. Moreover, the extent of shear crack growth at an inclusion boundary increased substantially in specimens containing two inclusions, compared to those with single inclusions.     

基体裂纹-异型夹杂相互作用焦散线实验研究

该文通过透射式静态焦散线方法利用三点弯曲梁断裂实验对异型夹杂与基体裂纹的相互作用进行研究。首先得到不同夹杂情况下I型裂纹尖端的焦散斑图,引入焦散斑纵横轴长之比β反映焦散斑在夹杂作用下的畸变特性;其次,提取相应的焦散斑特征尺寸,并得到I型裂纹的应力强度因子K_I;最后,基于不同夹杂情况下裂尖焦散斑、裂尖应力强度因子与裂尖和夹杂之间距离的关系,揭示不同夹杂对裂纹尖端应力场奇异性影响规律。实验研究结果为含异型夹杂结构的强度设计和断裂性能评估提供实验依据。     

Effects of inclusions on formation of acicular ferrite and propagation of crack in high strength low alloy steel weld metal

Abstract

High strength low alloy steel was welded by gas shielded arc welding process without preheating. Microstructural characteristics of the weld metal, morphology of inclusions and crack propagation paths were investigated by means of optical microscopy and scanning electron microscopy. The chemical composition of the inclusion and element distribution across the inclusion were analysed via energy dispersive spectroscopy system. Results indicated relatively large inclusions with diameters of about 0·6–0·8 μm are much more effective in providing nucleation sites for acicular ferrite transformation and refining the microstructure within austenite grain than small ones with diameters of about 0·3–0·5 μm. When the main crack tip encountered inclusion, more crack paths would be initiated from the interface between inclusion and acicular ferrite plates.     

Detrimental effect of nitride and aluminium oxide inclusions on fatigue life in rotating bending of bearing steels

Abstract

Rotating bending fatigue tests were performed on hardened AISI type 52100 bearing steel. Fracture surfaces after testing at a stress amplitude of 950 MPa showed that the Ti(C,N) inclusions which caused fatigue failure were significantly smaller than the corresponding alumina inclusions. The smallest crack initiating Ti(C,N) inclusion had a size of 3 μm and the smallest alumina inclusion was 17 μm. It was also shown that fatigue life was significantly shorter for a steel which showed cracked alumina inclusions on the fracture surfaces than for a steel which had non-cracked inclusions. Finite element calculations were performed to determine the driving forces of short cracks at Ti(C,N) and alumina inclusions. Two configurations were studied in each case, based on both non-cracked and cracked inclusions. The calculations incorporated heat treatment simulation and cyclic loading with successive growth of cracks. It was found that the Ti(C,N) configurations gave the highest driving forces for crack growth. The alumina configuration with a non-cracked inclusion gave the lowest driving force. It was concluded based both on experimental evidence and theoretical considerations that Ti(C,N) inclusions are more detrimental to fatigue life than alumina inclusions of the same size. It is their shape and thermal properties which make Ti(C,N) inclusions more detrimental than alumina inclusions. Internal cracking of alumina inclusions leads to reduced fatigue life.     

铍中Be2C及硅基夹杂对铍材伸长率的影响

铍材伸长率是目前限制铍材使用的关键因素.对高伸长率和低伸长率铍材的拉伸断口进行对比分析,发现在铍材的断裂源处往往含有非金属夹杂,夹杂造成的微裂纹是铍材解理断裂的裂纹核心.通常情况下,这些夹杂是以硅为基的杂质,一般不会造成铍材伸长率的明显降低.而当杂质中存在碳元素时,形成脆、硬稳定的Be2C,因Be2C难以变形,其断裂强度较铍材更低,因此在Be2C处容易位错集束而加重应力集中,并沿Be2C内部生成主裂纹,该主裂纹与原有的微裂纹合并后,裂纹长度加长,导致铍材解理速度加快,伸长率显著降低.     

Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 10⁷ cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.     

Fatigue crack growth through particulate clusters in polycarbonate material

The interaction of a crack with a perfectly bonded inclusion or a cluster of inclusions in polycarbonate matrix was investigated through both numerical simulations and fatigue tests. Stress intensity factors (K_I) were evaluated by boundary element method for several particle sizes, position and finally for inclusion cluster as a precursor study for the experiments. The numerical simulation has shown the crack tendency to circumvent the inclusions with consequential reduction of the growth rate. Fatigue crack growth tests were carried out on several particle-filled specimens at constant value of the applied stress intensity factor range (ΔK_Iapp) highlighting the crack delay due to the presence of the stiff second phase. The experiments demonstrated that the inclusion effect on the crack growth rate can be explained with a model based on the crack shielding effect in which the particle would act to reduce the effective stress intensity factor at crack tip (K_Ieff). Finally, the crack growth rate was predicted with an analytical model, and then compared to that obtained by the fatigue testing. Possible explanations for differences are discussed.     

Modeling fatigue crack nucleation at primary inclusions in carburized and shot-peened martensitic steel

The mechanics of high cycle fatigue crack nucleation (formation of a stable crack that can grow away from the influence of the notch root of the inclusion) at subsurface primary inclusions in carburized and shot-peened martensitic steel subjected to cyclic bending is investigated using three-dimensional (3D) finite element (FE) analysis. FE models are constructed using a voxellation technique to address the shape, size, and distribution of primary inclusions within clusters. The critical depth for fatigue crack nucleation is predicted considering the gradient in material properties arising from carburization, prestrain and compressive residual stress distribution due to shot peening, and the gradient of applied bending stress. The influence of inclusion shape and interface condition (intact or debonded) with the matrix on the driving force for fatigue crack nucleation is examined. It is observed that the inclusion shape has minimal influence on the predicted results while the effect of the interface condition is quite significant. For partially debonded interfaces, the predicted critical depth from surface for fatigue crack nucleation agrees qualitatively with experimental observations.     

3D modeling of subsurface fatigue crack nucleation potency of primary inclusions in heat treated and shot peened martensitic gear steels

A computational strategy is developed to characterize the driving force for fatigue crack nucleation at subsurface primary inclusions in carburized and shot peened C61^® martensitic gear steels. Experimental investigation revealed minimum fatigue strength to be controlled by subsurface fatigue crack nucleation at inclusion clusters under cyclic bending. An algorithm is presented to simulate residual stress distribution induced through the shot peening process following carburization and tempering. A methodology is developed to analyze potency of fatigue crack nucleation at subsurface inclusions. Rate-independent 3D finite element analyses are performed to evaluate plastic deformation during processing and service. The specimen is subjected to reversed bending stress cycles with R = 0.05, representative of loading on a gear tooth. The matrix is modeled as an elastic–plastic material with pure nonlinear kinematic hardening. The inclusions are modeled as isotropic, linear elastic. Idealized inclusion geometries (ellipsoidal) are considered to study the fatigue crack nucleation potency at various subsurface depths. Three distinct types of second-phase particles (perfectly bonded, partially debonded, and cracked) are analyzed. Parametric studies quantify the effects of inclusion size, orientation and clustering on subsurface crack nucleation in the high cycle fatigue (HCF) or very high cycle fatigue (VHCF) regimes. The nonlocal average values of maximum plastic shear strain amplitude and Fatemi–Socie (FS) parameter calculated in the proximity of the inclusions are considered as the primary driving force parameters for fatigue crack nucleation and microstructurally small crack growth. The simulations indicate a strong propensity for crack nucleation at subsurface depths in agreement with experiments in which fatigue cracks nucleated at inclusion clusters, still in the compressive residual stress field. It is observed that the gradient from the surface of residual stress distribution, bending stress, and carburized material properties play a pivotal role in fatigue crack nucleation and small crack growth at subsurface primary inclusions. The fatigue potency of inclusion clusters is greatly increased by prior interfacial damage during processing.