Underlying mechanisms of the stress generation in surface coatings

This paper was aimed at providing the underlying thermo-mechanical mechanisms in relation to the residual stresses in surface coatings. Four different topics were involved, namely the analytical model for predicting the residual stress, the accuracy of Stoney's equation, the bifurcation behavior for a coating system, and the optimal design for improving the adhesion of a coating. All these topics were in relation to the curvature and the misfit strain created during the coating deposition process, the material properties and dimensions of the coating and substrate.     

Intergranular corrosion resistance of nickel-based alloy 690 weldments

This study focused on the temperature distributions and thermal histories of alloy 690 weldments, as well as the residual stress, grain boundary character distribution, degree of sensitization, and carbide precipitation, to evaluate the combined effects of these factors on the intergranular corrosion resistance of the weldments. Both laser beam welding and gas tungsten arc welding were performed for comparison. The results show that laser beam welding, with its narrower temperature distribution and rapid heating/cooling, resulted in more low energy Σ (1 ≦ Σ ≦ 9) boundaries, lower residual stress, and considerable suppression of Cr₂₃C₆ carbide formation. Therefore, laser weldment had a low degree of sensitization, and thus the intergranular corrosion resistance was significantly improved.     

基于正交试验设计的高温管道焊接工艺优化

P91耐热钢焊接接头广泛用于电厂管道,厚壁管道焊接残余应力分布比较复杂,而焊接接头残余应力的大小对其高温环境下运行的蠕变又有着较大的影响.文中运用正交试验设计方法,以焊接残余应力为评价指标,对P91耐热钢管道焊接的工艺参数进行优化没计,然后采用大型有限元分析软件ABAQUS对最优焊接工艺的焊接残余应力进行数值模拟,获得了P91厚壁管道焊接接头的残余应力分布状况.结果表明,焊接速度对焊接残余应力的影响最为显著,其次电弧电压、焊接电流和坡口间隙等.研究结果为优化高温管道焊接工艺,有效控制焊接残余应力提供了可能.     

冷喷涂改善2219铝合金熔焊接头残余应力新方法

目的采用冷喷涂技术改善2219铝合金变极性钨极氩弧焊(VPTIG)焊接接头残余应力。方法采用冷喷涂技术在8 mm厚2219铝合金VPTIG焊接接头表面进行Cu涂层制备,分析冷喷涂前后接头微观组织、力学性能及残余应力的变化,探究冷喷涂过程中高速碰撞颗粒的"喷丸效应"与加热气体的"热效应"对接头残余应力的改善作用及内在机制。结果冷喷涂前后,接头整体的微观组织无明显变化,但在焊缝表面(Cu涂层与焊缝结合面处)观察到明显的塑性变形。冷喷涂后,焊缝区上表面显微硬度提高,作用深度约2 mm(余高处);接头抗拉强度表观上稍有降低,这是由于涂层对焊缝的拉伸应变局部化改变所致,"喷丸效应"与"热效应"单独作用下,接头抗拉强度无明显变化,所有拉伸试样均从焊趾处沿着熔合区发生断裂。焊态接头残余应力分布总体呈拉应力状态,在焊缝处最大,经过冷喷涂后,焊缝残余拉应力峰值从约200MPa降到约24MPa,大部分区域从拉应力变为压应力,且分布均匀性变好。结论冷喷涂技术可以显著改善VPTIG焊接接头残余应力分布。冷喷涂粒子的"喷丸效应"对基体残余应力的改善作用显著。高温气流的"热效应"虽未能显著降低残余应力,但使应力分布变得均匀。     

激光冲击强化ZCuAl10Fe3Mn2合金的热疲劳裂纹生长行为

为了提高高温构件的热疲劳性能、减少表面裂纹,研究激光冲击对ZCuAl10Fe3Mn2合金硬度、表面形貌、残余应力和热疲劳性能的影响.采用扫描电子显微镜(SEM)和能谱仪(EDS)分析合金的显微组织和裂纹形貌.结果表明:在4 J脉冲能量下,激光冲击能显著改善合金的热疲劳性能.在热应力和交变应力的作用下,试样缺口附近组织氧...     

Dual-Layer Oxidation-Protective Plasma-Sprayed SiC-ZrB<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Carbon Nanotube Coating on Graphite

Graphite is used in high-temperature gas-cooled reactors because of its outstanding irradiation performance and corrosion resistance. To restrict its high-temperature (>873 K) oxidation, atmospheric-plasma-sprayed SiC-ZrB₂-Al₂O₃-carbon nanotube (CNT) dual-layer coating was deposited on graphite substrate in this work. The effect of each layer was isolated by processing each component of the coating via spark plasma sintering followed by isothermal kinetic studies. Based on isothermal analysis and the presence of high residual thermal stress in the oxide scale, degradation appeared to be more severe in composites reinforced with CNTs. To avoid the complexity of analysis of composites, the high-temperature activation energy for oxidation was calculated for the single-phase materials only, yielding values of 11.8, 20.5, 43.5, and 4.5 kJ/mol for graphite, SiC, ZrB₂, and CNT, respectively, with increased thermal stability for ZrB₂ and SiC. These results were then used to evaluate the oxidation rate for the composites analytically. This study has broad implications for wider use of dual-layer (SiC-ZrB₂/Al₂O₃) coatings for protecting graphite crucibles even at temperatures above 1073 K.     

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

An investigation has been undertaken to establish the effect of surface preparation method on the susceptibility of a 304 stainless steel to stress corrosion cracking under simulated atmospheric corrosion conditions. MgCl₂ was deposited onto four-point bend specimens, which were then placed in a chamber with a relative humidity of 45% and temperature of 60 °C. These test conditions were designed to reflect external exposure of stainless steel components in industrial plant, including nuclear reactor components, situated in a coastal region, but with the severity of the exposure conditions enhanced to allow discrimination of the effect of surface preparation in a short timescale (up to 1500 h). Four surface preparation methods were evaluated: transverse grinding, longitudinal grinding, transverse dressing using an abrasive flap wheel, and transverse milling. For each case, surface topography, surface defect mapping, near-surface microhardness mapping, residual stress and electron back-scattered diffraction measurements were undertaken. Stress corrosion cracks were observed for the ground and milled specimens but not for the dressed specimens, with cracks apparently originating at corrosion pits. The density of cracks increased in the order: transverse ground, milled and longitudinal ground, with the cracks notably much smaller in length for the transverse ground condition. The propensity for cracking could be linked to the high residual stress and apparent nanocrystalline microstructure at the surface. There was a greater propensity for pitting to initiate at local defect sites on the surface (laps, deeper grooves). However, the tendency was not overwhelming, suggesting that other factors such as more general roughness or the distribution of MnS inclusions had an influence, perhaps reflecting the severity of the environment.     

Microstructure,hardness and residual stress distribution in maraging steel gas tungsten arc weldments

Abstract

The distribution of residual stresses due to welding has been studied in maraging steel welds. Gas tungsten arc welding process was used and the effect of filler metal composition on the nature of residual stress distribution has been investigated using X-ray diffraction technique with Cr K_α radiation. Three types of filler materials were used, they include: maraging filler, austenitic stainless steel and medium alloy medium carbon steel filler metal. In the case of maraging steel weld, medium alloy medium carbon filler, the residual stress at the centre of the weld zone was more compressive while, less compressive stresses have been identified in the heat affected zone of the parent metal adjacent to the weld metal. But, in the case of austenitic stainless steel filler the residual stresses at the centre of the weld and heat affected zone were tensile. Post-weld aging treatment reduced the magnitude of stresses. The observed residual stress distribution across the weldments has been correlated with microstructure and hardness distribution across the weld.     

基于晶体微观有限元方法的多相复合涂层高温摩擦应力场模拟

对高温摩擦磨损工况下多相复合涂层的热-力耦合应力场的模拟研究尚不充分。基于Voronoi多边形建立NiCr-Cr₃C₂-CaF₂/BaF₂多相复合涂层的晶体微观有限元模型,模拟复合涂层中各相的占比、分布形态和热-力学参数,求解得到热-力耦合工况下的von Mises应力和第一主应力分布。结果表明：在高温摩擦工况下,多相复合涂层的应力显著高于均匀涂层,尤其是在硬质相尖端附近易产生局部高应力区域,改善相的形态将锐角钝化能够有效缓解局部高应力现象;热-力耦合应力场与黏结相和硬质相的弹性模量密切相关,通过调节各相模量能够有效调控复合涂层的Mises应力和拉应力值。基于微观有限元方法的热-力耦合应力场模拟可为高温摩擦磨损工况下多相复合涂层的优化设计提供理论依据。     

液压支架主要承载构件焊接残余应力对比分析

针对综合机械化采煤工程中的安全性问题,以高强钢液压支架的掩护梁和底座为研究对象,应用X射线法对焊后及热处理后的残余应力水平进行评价.对液压支架掩护梁和底座在焊接后热处理后进行两次残余应力现场实测,分别选择各构件特殊部位6～ 10个区域(40多个测点)进行研究,定量给出了残余应力的总体水平及应力分布规律.结果表明,焊接后底座应力水平普遍高于掩护梁,最高应力为429.4 MPa(0.62ReL),掩护梁最高应力为348 MPa(0.50ReL).经过整体热处理,残余应力明显下降,底座及掩护梁残余应力最大值降幅为37％及71％;平均值降幅为46％及76％,新型热处理工艺对降低焊接残余应力有显著效果.     

车削残余应力方向性实验

为了了解车削残余应力分布方向性规律,对调质处理和淬火处理的45号钢进行精、粗车削加工,然后测量和观察不同试样的残余应力圆周分布特点,了解材料力学性能和切削条件对残余应力方向性的影响。研究结果显示:残余应力分布具有方向性,车削残余应力轴向最大,切向最小;残余应力方向性主要由机械效应决定,热效应影响不大;残余应力不同方向的大小和变动率受工件材料力学性能和加工条件影响;轴向残余应力对车削工件影响最大。     

The strengthening effect of (Ni,Fe)Al precipitates on the mechanical properties at high temperatures of ferritic Fe–Al–Ni–Cr alloys

Strengthening through a homogeneous distribution of a second phase is a concept that is widely employed in high-temperature materials. The most prominent among this group are nickel-based superalloys which owe their high-temperature strength to finely dispersed Ni₃Al particles. Similar microstructures can be obtained in the Fe–Al–Ni–Cr system with B2-ordered (Ni,Fe)Al precipitates in a ferritic matrix. These precipitates lead to an increase of high-temperature strength compared to conventional iron-base high-temperature alloys. However, secondary precipitates form during air cooling from high temperatures and affect the ductility. The results show that the ductility can be improved by a two-step aging treatment. Within the stress and temperature range investigated, the dependence of the secondary creep rate on the applied stress of aged alloys can be described by a power law if a threshold stress is introduced.     

A method of modeling residual stress distribution in turning for different materials

This paper introduces a more comprehensive experimental model which has the capability of predicting residual stress profile. The main advantage of this model over the existing models that it provides the effect of machining parameters on maximum residual stress and determines both the location and depth of this maximum residual stress. Five different materials namely; stainless steel-304, steel-37, 7001 and 2024-aluminum alloys and brass were machined by turning utilizing one of experimental design techniques based on response surface methodology. Tensile strength of these materials and both cutting speed and feed rates are considered as three input parameters affecting residual stress distribution. The residual stress distribution in the machined surface region was determined using a deflection-etching technique. It is proposed here that the residual stress profile is a deterministic function of the three input parameters used. Also, it is postulated that the residual stress profile along the depth beneath surface is a polynomial function of the depth beneath surface and the coefficients of this polynomial are, in turn, functions of the input parameters. The model has been developed and has been checked for accuracy.     

The Effect of Residual Stress on the Distortion of Gray Iron Brake Disks

Thermal distortion of gray iron brake disks due to residual stress and its effect on brake vibrations were studied. The residual stress of heat- and non-heat-treated gray iron disks was measured using neutron scattering. Dynamometer tests were performed to measure the friction force oscillation caused by the disk runout during brake applications. High-temperature tensile tests were carried out to find out possible plastic deformation due to residual stress during brake applications. The results showed that the average residual stress of the heat-treated disk (47.6 MPa) was lower than that of the non-heat-treated disk (99.6 MPa). Dynamometer tests at high temperatures (up to 600 °C) indicated that the residual stress pronounced the runout: the increase in disk runout after the tests for the non-heat-treated sample was more than twice that for the heat-treated sample. This difference correlated well with the neutron scattering results and the dimensional changes after a separate vacuum heat treatment. The high-temperature tensile tests showed severe reductions in yield strength at 600 °C, suggesting that disks produced with no stress relaxation could be deformed during severe braking.     

基于弹性模量变化的7A52铝合金VPPA-MIG复合焊接残余应力测试

采用虚拟仪器和NI数据采集卡搭建了一种以小孔法为核心的残余应力测试系统,分析了7A52铝合金VPPA-MIG复合焊后残余应力的分布情况. 为降低弹性模量误差对最终测量结果的影响,通过实测复合焊接接头不同区域的弹性模量,拟合弹性模量随测量点位置变化的曲线来修正弹性模量误差. 针对10 mm厚7A52铝合金板材,完成了VPPA-MIG复合焊接残余应力测试试验. 结果表明,焊缝两侧各区域上的残余应力分布基本关于焊缝对称,熔合区出现最大拉应力,最大横向残余应力σ_y与纵向残余应力σ_x分别为118和223 MPa. 从熔合区至热影响区,残余应力均为拉应力,逐渐减小且高于焊缝中心的残余应力. 与单MIG焊相比,复合焊的最大横向残余应力与纵向残余应力大于MIG焊,但高应力区比MIG焊窄.     

Residual Stress Measurement and Calibration for A7N01 Aluminum Alloy Welded Joints by Using Longitudinal Critically Refracted (<Emphasis Type="Italic">LCR</Emphasis>) Wave Transmission Method

Residual stress measurement and control are highly important for the safety of structures of high-speed trains, which is critical for the structure design. The longitudinal critically refracted wave technology is the most widely used method in measuring residual stress with ultrasonic method, but its accuracy is strongly related to the test parameters, namely the flight time at the free-stress condition (t ₀), stress coefficient (K), and initial stress (σ₀) of the measured materials. The difference of microstructure in the weld zone, heat affected zone, and base metal (BM) results in the divergence of experimental parameters. However, the majority of researchers use the BM parameters to determine the residual stress in other zones and ignore the initial stress (σ₀) in calibration samples. Therefore, the measured residual stress in different zones is often high in errors and may result in the miscalculation of the safe design of important structures. A serious problem in the ultrasonic estimation of residual stresses requires separation between the microstructure and the acoustoelastic effects. In this paper, the effects of initial stress and microstructure on stress coefficient K and flight time t ₀ at free-stress conditions have been studied. The residual stress with or without different corrections was investigated. The results indicated that the residual stresses obtained with correction are more accurate for structure design.     

重型燃机喷嘴壳体及遮热板热障涂层剥落机制

目的 探究重型燃机喷嘴壳体及遮热板热障涂层剥落机制,为该部件的全寿命管理提供参考。方法 采用等离子喷涂方法,分别制备以06Cr25Ni20不锈钢和Hastelloy X合金为基材的热障涂层试验件,并结合水淬热冲击表征方法与瞬态热力耦合仿真方法,表征热障涂层水淬后的剥落状态,获得热障涂层残余剪应力的分布状态随基材和服役工况的变化行为,揭示热障涂层在多层热失配工况下的剥落机制。结果 在水淬热冲击条件下,2种不同基材的热障涂层试验件表现出类似的剥落行为,但由于基材热膨胀系数的差异,以06Cr25Ni20不锈钢为基材的热障涂层的残余剪应力(70.1 MPa)比Hastelloy X合金基材的热障涂层(52.7 MPa)更大,热冲击寿命更短。在梯度温度载荷下,2种不同基材热障涂层试验件的失效模式不同,前者的最大残余剪应力为39.2 MPa,后者为25.7 MPa。结论 在2种温度载荷下,以Hastelloy X合金为基材的热障涂层具有较低的残余应力和较长的服役寿命。此外,水淬热冲击可以快速表征热障涂层的寿命行为,但其失效模式与实际梯度温度载荷下的失效模式仍有一定区别。     

Finite Element Simulation for Welding Residual Stress and Creep Damage of Welded Joint

基于大型有限元软件Abaqus及其用户子程序(Umat)功能, 开发了焊接残余应力与蠕变损伤耦合计算程序, 对高温用焊接接头残余应力作用下的蠕变损伤行为进行有限元模拟, 并与无焊接残余应力状态下的蠕变损伤情况 进行比较. 研究结果表明, 在炉管焊接状态下, 焊接残余应力最大值集中在焊缝和热影响区处, 并且轴向与环向 残余应力较高. 在高温环境下, 焊接接头初始态焊接残余应力较高, 虽然在短时间内应力松弛到较低的水平, 但 其蠕变损伤仍较大程度地受焊接残余应力的影响, 蠕变损伤分布与焊接残余应力的分布基本一致.     

超声振动辅助滚压参数对钛合金表面残余应力的影响

结合有限元分析和实验研究,基于与普通滚压工艺的对比,对超声振动辅助滚压强化钛合金表面的残余应力场进行分析,获得了沿改性层深度方向的残余应力变化和瞬态应力分布云图。结果表明:与普通滚压相比,超声振动辅助滚压可改变滚压头和材料表面接触力的作用方式,使应力波沿着材料深度方向动态传播,从而产生更深的残余压应力影响层,并导致压应力层下移。在实验参数条件下,最大残余压应力值和表面残余应力值都随着静压力的增大而显著增大;随着主轴转速的增大,最大残余压应力值明显单调增加,而最大残余压应力层深度逐渐减小;振幅对残余应力场的影响不十分显著。     

Microstructure,high-temperature deformability and oxidation resistance of a Ti5Si3-containing multiphase MoSiBTiC alloy

A Ti₅Si₃-containing multiphase MoSiBTiC alloy with a composition of 38Mo–30Ti–17Si–10C–5B (at.%) was designed and produced by arc-melting. The alloy was composed of five phases—Mo solid solution (Mo_ss), Mo₃Si, Mo₅SiB₂ (T₂), Ti₅Si₃ and TiC—and consistently has good thermal stability at least up to 1700 °C. The density of the alloy was ∼7.0 g/cm³, which is considerably smaller than that of Ni-base superalloys. Microstructure was carefully examined and microstructural anisotropy was confirmed. The anisotropy was considered to be generated by thermal gradient during the solidification process. Microcracking was remarkable across the primary Ti₅Si₃ phase, which was caused by thermal expansion anisotropy of the Ti₅Si₃ phase. High-temperature deformability was examined by high-temperature compression tests at 1500 °C. Two kinds of loading axes were chosen for the compression tests with respect to the microstructural anisotropy. The alloy exhibited a peak stress of 450–550 MPa, followed by good deformability at the testing temperature. Microstructure refinement and reduction in microcrack density were observed after hot working. Oxidation tests were conducted on the alloy at 1100 °C and 1300 °C for 24 h. The oxidation curves demonstrated that rapid mass loss finished within several minutes. After that, the mass loss began to slow down and then the specimens' mass decreased almost linearly with increasing testing time. Cross-section observation indicated that oxygen propagated through Mo_ss, whereas T₂ and Ti₅Si₃ phases acted as barriers against oxygen attack during the tests. In addition, it was found that the alloy gained better oxidation resistance after high-temperature deformation, suggesting a positive effect of phase refinement on its high-temperature oxidation resistance.