Mechanical and Structural Properties of Similar and Dissimilar Steel Joints

The mechanical properties of specimens from similar and dissimilar weld joints were examined. A ferritic steel (St37-2) and an austenitic stainless steel (AISI 304) were joined by the gas tungsten arc weld (GTAW) process using an austenitic filler metal. Mechanical and metallographic properties of the specimens were obtained by means of microhardness testing, tensile testing, bending fatigue testing, and light optical and scanning electron microscopy. The highest microhardness values were recorded on the ferritic–austenitic dissimilar weld joint, whereas the highest tensile strength and bending fatigue life were obtained with the austenitic–austenitic joints. Ferritic and pearlitic structures were observed in the microstructure of the ferritic–ferritic joint. The microstructures of austenitic–austenitic and austenitic–ferritic joints showed small recrystallization grains in addition to the typical austenitic and ferritic structures. Scanning electron microscopy was used to observe the fracture surfaces of the specimens and the origins of the fatigue cracks.     

Failure analysis of connecting bolts and location pins assembled on the plate of main-shaft used in a locomotive turbochanger

Three connecting bolts, three location pins and navel of turbo-disk fractured, which were assembled on the plate of the main-shaft used in a locomotive turbochanger. Detailed fractographic study and metallurgical analysis were focused on the trouble bolts. The fatigue fracture is the main failure mechanism of the bolts. Appearance of the surface decarburization layer in the thread tip and root regions of the three failed bolts make the hardness at the thread regions decrease intensely so that the fatigue cracks initiated form the root at the first engaged thread. Surface damage morphology with cutting, wear and plastic deformation features was found on the working flanks of the engaged threads. Other components fractured in succession after the trouble bolt fractured.     

机车车钩断裂分析

某机车在运行中其车钩发生断裂。对断裂车钩进行了断口观察、化学成分复验、力学性能测试和夹杂物及显微组织检验。结果表明，该车钩退火处理前已存在的铸造裂纹成为疲劳源，退火处理时的过热又造成钩体表面氧化和严重脱碳，尤其钩腕处的晶粒度达到2～3级，大大降低了车钩的力学性能，在综合因素作用下导致车钩疲劳断裂。     

高强度低屈强比铆螺钢轧制工艺的研究

为消除螺栓在热处理中产生的各种缺陷,缩短生产周期,使铆螺钢免热处理.采用热模拟试验机、实验室轧机和多工位冷镦机对铆螺钢轧制实验,铆螺钢原料和成品分别在拉力试验机和万能试验机上进行拉力试验,并对其组织进行了分析.结果表明,铆螺钢经过控轧控冷,获得具有多边形铁素体、细片状珠光体、粒状贝氏体、残余奥氏体和少量MA岛的多相组织.由于控轧控冷后的多相组织及TRIP效应,改善了螺栓的强韧性.铆螺钢因低的屈强比可以直接由热轧棒材冷镦成螺栓,螺栓无需最终热处理,产品的力学性能满足8.8级螺栓国家标准的相应要求.     

Mechanical behavior of cast SiCp-reinforced Al-4.5%Cu-1.5%Mg alloy

SiC_p-reinforced Al-4.5%Cu-1.5%Mg composite specimens were processed by vigorous stirring of the carbide in a semi-solid alloy slurry, followed by remelting and casting (stir-casting). The tensile and fatigue properties were evaluated in the as-cast and in the heat-treated conditions. In monotonic tensile testing, reinforcement with SiC_p produced a substantial increase in the work hardening of the material. This increase became more significant with increasing volume fraction of carbide. The yield and ultimate tensile strength, and the elastic modulus of the material, increased with heat-treatment and volume fraction of carbide at the expense of ductility. These properties are inferior to those of other reinforced, more complex aluminum alloys processed by other methods. In stress-controlled fatigue tests under fully reversed (R = −1) bending conditions, the fatigue life of the composite was longer than that of the unreinforced specimen at intermediate and lower stress levels. At higher stress levels the improvement was negligible. In heat-treated reinforced alloy specimens the fatigue strength at 1 × 10⁷ cycles decreased with increasing carbide particle size. With solid solution and precipitation strengthening, as well as carbide dispersion strengthening of the alloy, the crack growth threshold stress intensity factor K_th, increased, as did the crack initiation time and the crack growth rate.     

机械滚压对A473M钢疲劳性能的影响

采用机械滚压对A473M马氏体不锈钢轴套材料进行表面处理,研究滚压工艺对其力学性能的影响。采用SEM、白光干涉仪、X射线衍射仪、显微硬度计、EBSD、拉伸试验机和疲劳试验机分别对试样表面形貌、表面粗糙度、残余应力、显微硬度、拉伸性能和疲劳性能进行系统表征。结果表明:滚压加工试样表面的粗糙度明显降低,仅为车削加工的1/5;滚压加工在材料近表面引入残余压应力,其值最高可达946 MPa,沿深度方向逐渐减小,残余压应力层深度约为200μm,表面硬度提高30%左右,硬度影响层深度可达200μm;抗拉强度、屈服强度和伸长率分别提升了40%,22%和8%,疲劳寿命由基体材料的5.4×10^4周次提高到1×10^7周次。采用滚压加工后材料的力学性能明显提升,疲劳寿命显著增加。     

Properties of Wood Fibre-Polypropylene Composites: Effect of Wood Fibre Source

This study examined the effect of type of wood fibre source on the physical and mechanical properties of wood fibre-polypropylene composites. Wood flour, fibres of heat-treated wood and pellets were used as sources of wood fibres in the manufacturing process. All studied wood fibre-polypropylene composites were made from 75% wood, 22% recycled polypropylene (PP) and 3% maleated polypropylene (MAPP). Wood fibre-polypropylene composites were compounded in a conical twin-screw extruder. Water absorption and thickness swelling were studied. Mechanical properties of the composites were characterised by tensile, flexural, and impact testing. Micromechanical deformation processes were investigated using scanning electron microscopy done on the fractured surfaces of broken samples. The durability of composites exposed to three accelerated cycles of water immersion, freezing and thawing was examined. The results showed that the density of the composites was a key factor governing water absorption and thickness swelling. A significant improvement in tensile strength, flexural strength, and Charpy impact strength was observed for composites reinforced with heat-treated fibre compared to composites reinforced with pellets and especially to wood flour reinforced composites. The flexural strength and dimensional stability performance reduced after exposure to freeze-thaw cycling for all composites, but the degree of these changes was dependent on the wood fibre source.     

Analysis of catastrophic failure of axial fan blades exposed to high relative humidity and saline environment

One blade of an axial fan that uses air from a marine atmosphere of high relative humidity for cooling a gas turbine failed, with catastrophic consequences for the system, the fracture type is brittle. Root cause analysis of the failure involved the application of non-destructive testing, chemical and mechanical characterization of the material of the blade, the fractographic analysis of fracture surfaces, the finite element modeling of the stress condition of the blade, the use of metallographic techniques for the identification of the manufacturing process of the component, and the nature of the interaction of the structural components of the blade with the surrounding environment. It was found that: (1) the alloy of the blade corresponds to Al-2024 without heat treatment, manufactured by directional solidification; (2) the fracture mechanism is low-cycle fatigue or high tensile stress, the initiation is in a inaccessible area and covered with corrosion products originated by a process of intergranular cracking of the alloy, as a consequence of saline products deposition from the marine atmosphere in the zone.     

高加抽汽管疏水阀气动执行机构螺栓断裂原因分析

某核电厂大修期间,发现高压加热系统中疏水阀气动头与阀门本体连接的6颗内六角沉头螺栓中有5颗发生了断裂。通过分析断裂螺栓材料、力学性能、显微组织和断口等,并结合阀门实际运行信息,认为疏水阀气缸中的工作压力超过公称压力致使螺栓长期处于高的拉一拉型交变应力状态,从而引起螺栓的疲劳断裂。同时提出了防止该类螺栓在其使用过程中发生类似断裂事故的建议。     

Crystallization behaviour and mechanical properties of rapidly solidified Al<Subscript>87.5</Subscript>Ni<Subscript>7</Subscript>Mm<Subscript>5</Subscript>Fe<Subscript>0.5</Subscript> amorphous alloy

The crystallization behaviour and the mechanical properties of rapidly solidified Al_87.5Ni₇Mm₅Fe_0.5 alloy ribbons have been examined in both as-melt-spun and heat-treated condition using differential scanning calorimetry, X-ray diffractometry (XRD), transmission electron microscopy (TEM), tensile testing and Vicker’s microhardness machine. XRD and TEM studies revealed that the as-melt-spun ribbons are fully amorphous. The amorphous ribbon undergoes three-stage crystallization process upon heating. Primary crystallization resulted in the formation of fine nanocrystalline fcc-Al particles embedded in the amorphous matrix. The second and third crystallization stages correspond to the precipitation of Al₁₁(La,Ce)₃ and Al₃Ni phases, respectively. Microhardness and tensile strength of the ribbons were examined with the variation of temperature and subsequently correlated with the evolved structure. Initially, the microhardness of the ribbon increases with temperature followed by a sharp drop in hardness owing to the decomposition of amorphous matrix that leads to formation of intermetallic compounds     

汽轮机高压外缸结合面双头螺栓断裂分析

某电厂汽轮机高压外缸结合面双头20Cr1Mo1VTiB钢螺栓在安装时发生断裂,通过断口分析、化学成分分析、金相检验以及力学性能测试等方法对螺栓的断裂原因进行了分析。结果表明:该螺栓钢存在严重的显微组织缺陷,力学性能指标不符合技术条件要求,减弱了螺栓的承载能力,螺纹处的应力集中效应会加剧螺栓的缺口敏感性,导致螺栓在安装时于螺纹处发生脆性断裂。     

Schwingfestigkeit von Schraube‐Mutter‐Verbindungen

High cycle fatigue of bolted connections Extensive tests regarding the influences on the fatigue of bolt‐nut‐connections of preloading with torsion, of preloading with yielding, of loading with superimposed bending and of the tested lot are processed. These influences are not yet known according to VDI 2230. New testing devices were designed for these tests, which allow a far less expensive operation and may easily be used for bolts of diameters up to M100 and testing frequencies up to 1000 Hz. The validity of fatigue resistance according to VDI 2230 is specified with respect to the test results. The determined influence of the tested lots is unexpectedly high. The manufacturing process of bolts should be improved to minimize this influence.     

TC4钛合金选区激光熔化与层间激光冲击强化复合工艺

目的 改善激光选区熔化(Selective Laser Melting,SLM)工艺成形的TC4合金的内部缺陷,提高疲劳寿命。方法 选用TC4钛合金为研究对象,提出了SLM结合层间激光冲击(3D-Laser Shock Peening,3D-LSP)与热处理的强化工艺,对复合制造工艺下的微观组织、内部缺陷和力学性能演变进行了研究,并建立了复合强化工艺制造样品的疲劳寿命模型。结果 在激光冲击影响区域内形成了0.2 mm深度的高幅值残余压应力,并在1 mm深度范围内改善了应力场,且显微硬度得到了提升,内部缺陷数量减少了36%,疲劳寿命提升了40%以上。结论 实现了SLM增材制造TC4钛合金的缺陷在线闭合、微观组织改性和疲劳寿命的提升,揭示了层间激光冲击对内部缺陷的闭合机理,为金属SLM复合增材制造的研究与应用奠定了理论基础。     

The effects of fabrication atmosphere condition on the microstructural and mechanical properties of laser direct manufactured stainless steel 17-4 PH

The effects of atmosphere conditions on microstructural and mechanical properties of stainless steel 17-4PH components fabricated by laser direct manufacturing (LDM) were investigated through measurements on phase constitution, porosity, tensile strength, fracture morphology, hardness and evolution of substrate temperature. Results showed that the samples produced in air atmosphere condition possessed higher tensile strength and hardness for both as-deposited and heat-treated states than that in Ar chamber condition, due to dispersion strengthening effect of amorphous oxide particles and nitrogen solution strengthening as a result of higher content of oxygen and nitrogen. The temperature of substrate heat accumulation was higher in Ar chamber condition, leading to dramatically lower porosity and more reverse austenite, which also contributed to the lower strength and hardness.     

Failure and Fracture Analysis of Bolt Assembled on the Fan Used in the Internal Combustion Engine

Three connecting bolts fractured, which were assembled on the fan used in the internal combustion engine. Detailed fractographic study and metallurgical analysis were focused on the fractured bolts. Fractographic and metallurgical studies indicate that the fracture surface and the microstructure of three bolts exhibit similar features. The fatigue fracture is the main failure mechanism of the bolts. Appearance of the micro-cracks in the thread tip of the fractured bolts makes the stress at the thread regions increase intensively so that the fatigue cracks initiated from the thread regions. Due to the presence of dynamic load, the bolts were never properly tightened during installation which should be responsible for the formation of the cracks.     

Mechanical Characterisation of the Effect of Various Forming Processes Applied to Commercially Pure Titanium

This paper illustrates the effect of three forming processes viz. laser forming, mechanical forming and a combination process consisting of a laser forming step followed by a mechanical forming step, on the mechanical properties of commercially pure titanium. All three processes resulted in samples having a similar final radius of curvature. Evaluation of the formed samples includes residual stress measurements, fatigue testing, Charpy impact testing, microhardness measurements and microstructure evaluation. All results were compared to the various forming methods in order to determine which forming method is more suitable for which type of loading condition i.e. impact loading or cyclic loading. Fatigue testing of components produced by the various forming methods revealed that the laser forming process provided the best results when a high load was applied whereas at lower applied loads, the mechanical forming process showed the highest number of cycles to failure. Charpy impact testing done at room temperature and at a sub-zero temperature of − 40 °C revealed that the laser forming process negatively affected the toughness of the material. Residual stress measurements showed that the laser forming process resulte'd in the highest value of surface relieved residual stress values compared to the other two processes.     

电弧增材制造AZ31镁合金组织与力学性能分析

针对镁合金在轻量化结构件领域的应用前景,采用基于MIG焊的电弧增材制造工艺开展了两组不同路径的AZ31镁合金增材实验,并对其微观组织和力学性能进行了分析。结果表明：增材构件相较于原始焊丝的化学成分无较大变化;单道次多层往复堆积路径相较于多道次多层堆积路径,更易制得表面更为平整,内部更为致密的构件,其屈服强度为77.3 MPa,抗拉强度为235 MPa,达到原始焊丝75%的力学性能水平,平均显微硬度为52.7HV,断后伸长率最高达到了27%;增材构件拉伸断裂方式为韧性断裂,并在多道次多层往复堆积构件断口处发现其内部存在气孔。验证了电弧增材制造AZ31镁合金工艺的可行性。     

电弧增材制造航空钛合金构件组织及力学性能研究现状

航空航天领域通常将钛合金作为承力结构件使用,对其性能和可靠性都有很高的要求,大型结构件的整体化制造是实现这些需求的有效途径。电弧增材制造技术因效率高、成本低、致密度高,在制备大型结构件方面具有一定优势。综述了国内外电弧增材制造钛合金组织的研究现状,介绍了改变形核条件以及引入轧制、超声等外场辅助技术调控后所得的电弧增材制造钛合金组织。对电弧增材制造钛合金的拉伸性能和疲劳性能进行了综述,总结了拉伸性能和疲劳性能的特点及断裂的原因。最后,对航空航天用钛合金电弧增材制造的组织及力学性能的关系进行了分析,并且对两者的调控前景进行了展望。     

Damage Features of Bolts Connecting Main-Shaft with Turbo-Disk Used in a Locomotive Turbocharger

The connection between the turbo-disk and the main-shaft was found to be loose when performing a regular inspection on a locomotive turbocharger. When the bolts connecting main-shaft with the turbo-disk were disassembled, it was found that smeared threads and contact pressing deformation, etc., severe plastic deformation took place on the three connecting bolts. Metallurgical examinations indicate that the core microstructure is composed of the banded ferrite and pearlite along the axial other than the sorbite by the standard toughening process (quenching and tempering). Abnormal microstructure leads to low the hardness and strength so that various damages appeared on the flanks when twisting bolts with nuts (loosening or tightening). The plastic deformation took place on the bolts under the axial load due to low hardness, which resulted in loosening of the connection between the turbo-disk and the main-shaft. Not standardized fabrication process of the bolts is responsible for the failure of the bolts.     

Effects of Butt Design and Gap Width on the Microstructure and Mechanical Properties of Magnesium Alloy Welds

The effects of parallel and angular butt designs with different gap widths on the microstructure and mechanical properties of the resultant magnesium joints in gas tungsten arc welding were investigated.The experimental results displayed that the tensile strength of the joint made with angular butt joint and 0.3 mm-wide gap reached 266 MPa,the joint fractured through the heat-affected zone or base metal during tensile testing,and the microstructure examination showed that no macropore appeared in the fusion zone.However,macropores in diameter of larger than 200 micron occurred for other joint designs,the tensile strength of the resultant joints decreased,and the joints fractured through the fusion zone during tensile testing.The fracture surfaces were examined by scanning electron microscopy and the microhardness distribution in the joints was measured by using a Vickers microhardness tester.