夹杂物特征参数对拉伸载荷下X80管线钢中裂纹萌生及扩展影响的原位观测

 采用扫描电镜原位观测的方法,跟踪观察了拉伸载荷作用下X80管线钢中不同尺寸、形态的夹杂物导致裂纹萌生与扩展的微观力学行为。结果表明：在拉伸载荷作用下,夹杂物面积越大,夹杂内萌生的裂纹条数越多,第1条裂纹萌生时的应力越小;夹杂物的形态和面积决定第1条裂纹的萌生位置;当夹杂物呈不规则形态且棱角较多时,裂纹在萌生初始阶段就向基体扩展,但之后继续加载时裂纹不会继续在基体中扩展;夹杂物对X80管线钢的拉伸性能无显著影响。     

A strain-based fracture model for stress corrosion cracking of low-alloy steels

The stress corrosion cracking (SCC) behavior of 4135 steel under different heat treatments is analyzed in an attempt to relate microstructural characteristics with macroscopic measurements of SCC resis-tance, especially the very impressive improvements associated with changes from intergranular (IG) to transgranular (TG) fracture paths. Considering that local hydrogen embrittlement at the crack tip causes SCC processes, a local cracking criterion, based on a critical strain depending on hydrogen concentration, is assumed to control the process. Stress corrosion cracking is viewed as a discontin-uous series of unstable crack extensions through the locally embrittled regions. The model developed on this basis explains the macroscopic behavior observed at the threshold situation and partially at stage II propagation and clarifies the role of the metallurgical variables in each of the types of fracture detected.     

Effect of oxygen on near-neutral pH stress corrosion crack initiation under a simulated tape coating disbondment

A test setup that enables investigating the synergistic effects of tape coating disbondment, cathodic protection (CP) and low-frequency cyclic loading is used to determine the effect of oxygen on near-neutral pH stress corrosion crack initiation. After low-frequency corrosion fatigue tests, results show that surface micro-cracks initiate under the simulated tape coating disbondments at regions with insufficient CP potentials, not more remote locations completely free from CP as commonly believed. The presence of 1% oxygen in the composition of the purging gas has binary effects on the crack initiation under the simulated tape coating disbondment. On one hand it increases the size of the surface micro-cracks initiated on the steel surface at locations with some but insufficient CP potentials; and on the other, it reduces the number of surface micro-cracks at regions with no CP because of an extension of the general corrosion region deep inside the simulated coating disbondment.     

无镍高氮不锈钢弯曲疲劳裂纹萌生与扩展行为

 采用四点弯曲疲劳试验研究了不同应力水平下无镍高氮不锈钢的疲劳行为,并对材料疲劳裂纹的微观形貌、萌生位置及扩展路径进行了分析。结果表明,试验钢疲劳为多裂纹起裂,随着应力水平的升高,裂纹总长度逐渐增加,当应力水平接近材料屈服极限时,裂纹长度趋于稳定;裂纹大多数在滑移带处萌生,裂纹在扩展过程中产生了扭曲、偏移和分叉现象;裂纹在晶内主要沿单滑移带或多滑移带交替扩展,穿过晶界或孪晶界时大多发生了偏转。     

Effects of sodium chloride and shot peening on corrosion fatigue of AISI 6150 steel

Reversed-bending fatigue tests of quenched and tempered AISI 6150 steel were conducted in dry air and in aqueous 3 pct NaCl. The 3 pct NaCl environment drastically reduced fatigue life but two different shot peening treatments were found to improve the corrosion fatigue life over that of unpeened samples. Multiple fatigue crack initiation occurred at very distinct locations in both the unpeened and the peened specimens fatigued in 3 pct NaCl. Fatigue crack propagation from each initiation site occurred first on flat facets normal to the stress axis and then by a more ductile mechanism after the initiation facets had linked. The average size of the corrosion fatigue initiation facets in the peened specimens was much smaller than that of the unpeened specimens; however, the number of initiation sites was greater in the peened specimens. It is believed that the beneficial effect of the shot peening results from significantly reduced early fatigue crack propagation rates in the compressive residual stress layer at the surface. Formerly Research Assistant, Materials Department, University of Wisconsin-Milwaukee     

A unified mechanism of stress corrosion and corrosion fatigue cracking

A mechanism of stress corrosion cracking (SCC) is outlined in which anodic dissolution at film rupture sites relieves strain hardening and reduces the fracture stress at the crack tip. Experimental evidence is cited to suggest that relief of strain hardening occurs by interaction of subsurface dislocations with divacancies generated by the anodic dissolution. A transgranular crack propagates by accumulation of divacancies on prismatic planes which then separate by cleavage under plane strain conditions at the crack tip. At appropriate metallurgical and chemical conditions, anodic dissolution and/or divacancy migration may be enhanced at grain boundaries, leading to an intergranular failure mode. Evidence is also available to indicate that cyclic loading relieves strain hardening. Relief of strain hardening by combined cyclic loading and corrosion accounts for the higher incidence of corrosion fatigue cracking (CFC) without the requirement of any critical dissolved species. Data on fatigue of stainless steel at elevated temperature in both vacuum and air provide additional support for the proposed mechanism.     

氢致钢内部疲劳裂纹萌生和扩展的有限元分析

对氢致钢内部疲劳裂纹的萌生和扩展进行了数值模拟.首先用有限元法分析了氢在疲劳载荷作用下向钢中缺陷处扩散富集的过程,然后计算得到氢含量分布结果.根据夹杂理论将氢富集区视为在缺陷附近分布的弹性夹杂,用有限元法计算得到的氢含量场求出夹杂处的应力强度因子,进而建立疲劳裂纹萌生和扩展的判据.比较了在不同加载条件下氢致疲劳裂纹萌生和扩展的规律.用梯形法修正了Sofronis和McMeeking的瞬态扩散有限元公式,发现用梯形法可以缓解加载初期较高的浓度梯度和应力梯度引起的计算结果震荡的情况,这对于计算开裂判据是十分重要的.最后讨论了提高模拟精度和改进模型的方法.      

B级船板钢形变断裂过程的原位研究

 在扫描电子显微镜下观察分析了B级船板钢试样在原位拉伸过程中的断裂行为以及低温脆性断口中二次裂纹的扩展。结果表明：在拉伸变形过程中,微裂纹首先在试样缺口处形成,然后沿铁素体+珠光体界面扩展。加载过程中,多边形铁素体发生塑性变形,裂纹在基体内以“Z”字型扩展。在低温脆性断裂区,二次裂纹以穿晶方式通过铁素体基体,在扩展到珠光体区域时有时沿铁素体和珠光体的界面扩展,有时穿过珠光体区域扩展。     

Crack propagation in a pipe steel subjected to cyclic loading after long-term operation

The mechanical properties (under static and cyclic loading) and damage of a product pipeline made of 3G steel have been studied after its long-term operation. The ultrasound velocity in a damaged steel and the fatigue crack length on the sample surface are determined at various stages of macrocrack initiation. At the stage of main-crack initiation, the cumulative curves of fatigue microcrack distribution are well described by both exponential and power functions, with their exponents decreasing with increasing stress amplitude. The majority of fatigue microcracks have a length close to the average grain size.     

Initiation of stress corrosion cracks in aluminum alloys

Studies on initiation of stress corrosion cracking (scc) were conducted on notched rods of aged Al-8.6Mg, Al-21.5Zn and Al-2.6Mg-6.3Zn alloys tested in aqueous solutions and water-containing ethanol solutions under free corrosion conditions. It was found that there is a definite scc initiation period (t) preceding crack propagation. The crack initiation rate (1/t) increased with increasing initial stress intensity (K_I,) at the notch. The crack initiation rates were thermally activated with apparent activation energies of Q ≈ 40 to 60 kJ/mol in the ethanol based solutions and Q ≈ 109 kJ/mol in aqueous solutions. Cathodic hydrogen precharging prior to scc decreased the subsequent crack initiation period. The results were discussed in relation to a hydrogen mechanism of scc and shown to be entirely consistent with the lattice decohesion model developed by Oriani. Formerly Graduate Student with the University of British Columbia     

Corrosion Fatigue Crack Initiation in a Mode II Notch Specimen

The stress intensityK _II of a Mode II specimen was calculated using a finite element methodvia theJ integral. The site, direction, and the threshold value for crack initiation from the notch under cyclic Mode II loading in air, in water, and under dynamic charging with hydrogen were investigated. The results showed that the Mode II fatigue crack in a high strength steel initiated at or close to the site of the maximum principal stress, rather than at the site of the maximum shear stress, and the subsequent crack growth was oriented approximately normal to the direction of the maximum principal stress. The site and direction of crack initiation in water and under dynamic charging with hydrogen were similar but different from that in air. The threshold values for crack initiation in air, in water, and under dynamic charging were 28.8, 12.3, and 10.2 MPa m^1/2, respectively. The fracture surface of a corrosion fatigue crack in water and under dynamic charging consisted of intergranular facets at low ΔK _II values but of quasi-cleavage at higher ΔK _II values and were different from those in air.     

真空轧制825合金/X65钢复合板的组织性能

 针对油气输送领域对耐H2S腐蚀油气复合管线的需求,采用真空轧制复合技术成功制备出825镍基合金/X65高强管线钢复合板。真空轧制复合技术是基于真空电子束焊接和热轧复合所开发出的一种新型复合技术,在高真空、高温和强塑性变形条件下,复合界面实现优异的冶金结合。采用X65/825合金/825合金/X65的4层对称复合轧制模式,并对复合界面的微观组织和力学性能特征进行分析。研究表明,复合界面连续平直,无孔洞和裂纹等缺陷,镍、铬和铁元素在界面两侧发生明显的扩散,另外复合界面生成一条连续的厚度约为1 μm的TiC薄带,在结合界面离散分布少量的颗粒状Al2O3化合物。界面平均剪切强度为404 MPa,拉剪断裂在复合界面处。     

Fatigue behavior in the potentiostatic passive corrosion regime of the iron-base superalloy A-286

Surface crack initiation and propagation behavior of the iron-base superalloy A-286 were tested using smooth hourglass specimens under passive corrosion conditions. In the very underaged (VUA) condition, cracks were initiated at slip steps (stage I) and later propagated in a stage II mode, resulting in a minimum or dip in crack growth rate. For the highly overaged condition (HOA), cracks initiated at inclusions, and only stage II was observed; therefore, anomalies in crack growth behavior were not observed. A planar slip distribution, observed in VUA, was associated with a reduced crack growth rate. In general, the VUA microstructure was superior to HOA, as it exhibited an impressive combination of excellent mechanical properties, decreased susceptibility to corrosion, good resistance against crack initiation, and low crack growth rate, under corrosion fatigue conditions. Possible reasons for such behavior are discussed. Formerly Postdoctoral Associate, Carnegie Mellon University Formerly Professor, Carnegie Mellon University     

Microstructural effects on high-cycle fatigue-crack initiation in A356.2 casting alloy

The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution) were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than ∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated at a pore or oxide of comparable size.     

Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel

The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.     

Effects of pre-existing grain boundary microvoid distributions on fracture toughness and fatigue crack growth in low alloy steel

The role of dispersions of pre-existing grain boundary microvoids is investigated in fracture toughness and fatigue crack propagation behavior in a low alloy steel. Microvoid damage is achieved by prior exposure of the steel to gaseous hydrogen atmospheres at high temperatures and pressures, where carbon within the steel reacts with ingressed hydrogen to nucleate methane bubbles along prior austenite grain boundaries (hydrogen attack). It is shown that, whereas the crack initiation and crack growth toughness (i.e. K_Ic and the tearing modulus) are severely degraded, even for comparatively mild degrees of microvoid damage, rates of sub-critical crack growth by fatigue remain relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary microvoids, which promotes crack growth by lowering the intrinsic resistance of the microstructure, and the resulting tortuous crack paths, which extrinsically retard crack growth at low stress intensities by lowering the local crack tip “driving force” (crack tip shielding). As shielding effects are minimized at high stress intensities, the degradation in intrinsic toughness is related to changes in ductility by means of a stress-modified critical strain model for ductile fracture, where the presence of small microvoid clusters is shown to promote coalescence through the easier onset of plastic strain localization. Fatigue behavior, conversely, is dominated by extrinsic shielding mechanisms and is modeled in terms of two-dimensional models of crack deflection and roughness-induced crack closure.     

Crack Growth Modeling and Life Prediction of Pipeline Steels Exposed to Near-Neutral pH Environments: Dissolution Crack Growth and Occurrence of Crack Dormancy in Stage I

This investigation was initiated to provide governing equations for crack initiation, crack growth, and service life prediction of pipeline steels in near-neutral pH (NNpH) environments. This investigation has focused on the crack initiation and early-stage crack growth. The investigation considered a wide range of conditions that could lead to crack initiation, crack dormancy, and crack transition from a dormant state to active growth. It is concluded that premature rupture caused by stress cracking at a service life of about 20 to 30 years previously observed during field operation could take place only when the worst conditions responsible for crack initiation and growth have been realized concurrently at the site of rupture. This also explains the reason that over 95 pct of NNpH cracks remain harmless, while about 1 pct of them become a threat to the integrity of pipeline steels.     

汽车排气系统用铁素体不锈钢的耐热性研究

利用实验室条件下冶炼的Fe-Cr-Nb-Mo铁素体不锈钢,对其高温强度和热疲劳性能进行了研究。研究结果表明:实验钢的高温抗拉强度和屈服强度优于或基本相当于目前所使用的铁素体不锈钢的性能;实验钢的热疲劳裂纹从V形缺口处萌生,夹杂物为裂纹萌生优先区域;热疲劳裂纹长度和扩展速度随热疲劳上限温度的升高而增大;热疲劳裂纹优先沿晶界扩展,高温氧化是疲劳裂纹扩展的主要因素。