Metallurgical Failure Analysis of Cracked Forward Cylinder Head of Advanced Linear Actuator for Aircraft

Precipitation-hardened stainless steel of 15-5PH grade was used for the fabrication of forward cylinder used in advanced linear actuator of aircraft. The component had undergone a series of qualification tests. During one of the accelerated fatigue tests, an external leak was observed after experiencing more than 1,50,000 cycles of fluctuating stress. Subsequent to disassembly, the leak was found to be because of the hair line cracks in the forward cylinder. The cracked component was subjected to a detailed metallurgical investigation to understand the cause of failure. The failure analysis revealed that the fracture initiated at the hard chrome layer and propagated under cyclic loading. This premature failure of the component was attributed to the surface roughness and the presence of the hard chrome layer at the corner contour of the component. This article brings out the details of the investigation carried out and the results achieved.     

发动机气缸盖开裂分析

某发动机在进行台架循环负荷试验过程中出现漏油现象,拆机检查后发现气缸盖上有裂纹。通过断口分析、金相检验及化学成分分析等方法对气缸盖开裂的原因进行了分析。结果表明:气缸盖为脆性开裂,根本原因是缸盖固溶处理温度过高产生了过烧。建议加强气缸盖固溶处理温度的控制,防止过烧产生,保证产品质量。     

发动机缸体缸盖的铸造

对国内外发动机缸体、缸盖铸造生产进行了总结,其材质主要以C(质量分数,下同):3.15%～3.3%,CE:3.95%～4.05%,Si/C:0.6%～0.7%的灰铸铁为主。一般选择冲天炉-有芯工频电炉进行熔炼,孕育剂仍普遍采用75SiFe,立浇底注式浇注系统和保温冒口有利于获得优质缸体、缸盖,冷芯制芯工艺已逐渐取代热芯工艺。通过提高浇注温度、型砂紧实率等措施可减少缸体、缸盖常见缺陷渗漏的出现。     

Analysis of a Cracked Diesel Engine Camshaft

A truck diesel engine camshaft was found cracked following a straightening operation after the camshaft was carburized. The camshaft is made of 16MnCrS5 steel and was required to be surface-carburized. The cracking occurred just at the transition fillet root of a cam at a near middle position on the camshaft. This location bore the maximum tensile stress in the straightening process, a process involving three-point bending. The crack surfaces once exposed exhibit cleavage morphology, indicative of brittle fracture as the failure mechanism. Microstructural observation revealed an intergranular network of carbides and intergranular microcracks present in the carburized layer and a banded structure consisting of ferrite and pearlite in the core. These metallurgical defects decreased the deformation capacity of the carburized camshaft, in fact creating a ??metallurgical stress concentration?? that promoted the cracking of camshaft.     

氢气压缩机缸盖螺栓断裂失效分析

某石化公司氢气压缩机缸盖螺栓发生断裂,采用宏、微观断口分析、化学成分分析、金相检验以及力学性能测试等试验手段分析了该氢气压缩机缸盖螺栓的断裂原因。结果表明：断裂起源于螺杆与螺栓头部连接螺纹末端的应力集中处,由于应力集中,加之螺栓强度等级偏低,使螺栓在长期工作过程中的往复交变应力和扭转应力共同作用下发生了低应力高周疲劳断裂。     

Failure Investigation on the Cracked Diesel Engine Ring Gears

Many of the teeth on two ring gears used in a truck fractured in service. The fractographic investigation and metallurgical analysis of the fractured teeth showed that the cracks initiated at the root on the side of the loaded face. The fractured teeth surfaces reveal similar macrofractographic and microfractographic features typical of cleavage and quasi-cleavage and reflective of a low toughness in the material. The microstructure of the core of failed ring gears showed a coarse grain size and a continuous network of Widmannstätten ferrite. The defective microstructure was retained in the hardened layer of the groove, the root fillet, and half tooth height regions, decreasing the strength and the impact toughness of the ring gears and promoting the brittle fracture. Unsuitable forging and normalizing processes are responsible for the defective microstructure.     

Analysis of a Brake Cylinder Failure

A gray cast iron inlet boss was observed to have fractured following an accident in which a three-wheeled parking enforcement vehicle reportedly lost its braking ability and crashed on a hill in San Francisco, CA. Metallurgical and fractographic examination as well as exemplar testing conclusively determined that the inlet boss broke before the accident, and that the fracture occurred by overtorquing of the inlet hydraulic line during the vehicle servicing that occurred just prior to the incident.     

大功率柴油机气缸盖的残余应力

以解决气缸盖漏水问题为切入点,采用盲孔法研究了热时效退火工艺、机加工制造以及使用过程对气缸盖残余应力的影响。结果表明：采用较高保温温度、较长保温时间、较低降温速率、并保持较低出炉温度的热时效退火工艺可以使气缸盖毛坯件的残余应力保持在较低水平;机加工后气缸盖的残余应力有所增大;由于对气缸盖热时效退火工艺的优化,有效地控制了气缸盖底面的残余应力,故经过1 000h的台架试验后,未再发生气缸盖鼻梁区开裂现象,说明通过热时效退火工艺的优化来降低气缸盖底面的残余应力,能有效地解决气缸盖漏水的问题。     

Failure Analysis of Cracked Gear Shafts

Two gear shafts cracked during the fabrication. The cracks are through the grooves associated with the gear teeth and propagated completely across the shaft. Fractography on the cracked teeth indicates that the crack origins are about 3.5 to 4.0 mm from the groove and exhibit pointlike features. Fracture surfaces near the crack origin regions were intergranular, and the fracture appeared to have occurred instantaneously. Metallurgical analysis indicates that decarburization had occurred on the surfaces of the gear shafts, but occurrence of decarburization has no relation to the cracking. Cracking occurred during the carburization-quenching process and originated at a cluster of Al₂O₂ complex inclusions. The thermal stress produced by quenching drove the cracking process.     

Failure Analysis of Cracked Reducer Flange

A cracked reducer flange was analyzed for the cause of the failure. The flange was carefully cut to obtain samples for metallographic, X-ray, and scanning electron microscopy (SEM) Examinations. The examinations revealed that the introduction of chloride ions in the operational service led to pitting corrosion in the inner surface of the flange. Chloride ion inclusions were probably the result of chemical contaminations, i.e., cleaning chemicals’ contamination during shutdown of the operation. The introduction of corrosion pits caused unexpected load stress intensification and cracking of the flange. Consequently, stress corrosion cracking emanated from the pits under the influence of chloride attack and operational pressure. Then the cracks propagated in a transgranular manner, in the radial direction of the flange, until the final failure occurred.     

铝合金发动机缸盖的再制造技术研究

针对废旧汽车铝合金发动机缸盖的再制造需求,对发动机缸盖进行了失效分析,并研究了高速电弧喷涂Al-RE涂层的再制造工艺。实验结果显示,缸盖的失效主要表现为表面环形压痕和平面度超差,压痕区附近出现较多白色条状的Si相,硬度有所升高;高速电弧喷涂Al-RE涂层的组织致密,硬度略低于LM25TF铝合金基体,热压变形实验后涂层与铝合金基体结合良好,未出现裂纹和剥离。综合实验结果和典型零件的应用情况表明,高速电弧喷涂Al-RE涂层技术可实现铝合金缸盖的再制造。     

Failure Analyses of Six Cylinder Aircraft Engine Crankshafts

Results of failure analyses of two aircraft crankshafts are described. These crankshafts were forged from AMS 6414 (similar composition to AISI 4340) vacuum arc remelted steels with sulfur contents of 0.003% (low sulfur) and 0.0005% (ultra-low sulfur). A grain boundary sulfide precipitate was caused by overheat of the low sulfur steel, and an incipient melting of grain boundary junctions was caused by overheat of the ultra-low sulfur steel. The precipitates and incipient melting in these two failed crankshafts were observed during the examination. As expected, impact fractures from the low sulfur steel crankshaft contained planar dimpled facets (PDF) along separated grain boundaries with a small spherical manganese sulfide precipitates within each dimple. In contrast, PDFs along separated grain boundaries of impact fractures from the ultra-low sulfur crankshaft steel contained a majority of small spherical particles consisting of nitrogen, boron, iron, carbon, and a small amount of oxygen. Some other dimples contained manganese sulfide precipitates. Fatigue samples machined from the ultra-low sulfur steel crankshaft failed internally at planar grain boundary facets. Some of the facets were covered with nitrogen, boron, iron, and carbon film, while other facets were relatively free of such coverage. Results of experimental forging studies defined the times and temperatures required to produce incipient melting overheat and facets at grain boundary junctions of ultra-low sulfur AMS 6414 steels.     

汽油机燃用轻油基燃料的缸内过程参数分析

介绍作开发的一种汽油机用新型燃料,为检验其品质,对发动机缺内过程参数进行了全面测试分析。结果表明：汽油机燃用轻油基混合燃料时无爆震,动力性稍低于燃用汽油,缺内过程参数标准偏差明显低于燃用汽油。这充分说明,所开发的轻油基混合燃料品质优良,适于汽油机使用。     

基于EMD和STFT柴油机缸盖振动信号时频分析

以柴油机缸盖振动信号为研究对象,提出经验模式分解（EMD）和短时傅里叶变换（STFT）相结合的时频分析法。该方法首先利用EMD对缸盖振动信号进行自适应分解,得到一系列本征模态分量（IMF）。根据各分量的特点有针对性地选择高斯窗和汉明窗分别进行STFT,并以时频聚集性指标为目标函数计算各分量的最佳时频分布,最后叠加得到原始信号的最佳时频分布。经对比分析,该方法解决了窗函数及窗宽的选择问题,有效提高STFT的时频分辨率,准确描述柴油机缸盖振动信号的时频分布。     

某型柴油机缸盖罩噪声预测及降噪研究

车辆发动机噪声过大会影响驾驶舒适性,故有必要对其进行降噪。针对某型柴油机缸盖罩噪声突出的问题,采用仿真分析与结构振动声辐射理论相结合的方法,对缸盖罩的振动响应和辐射噪声进行预测。通过识别振动薄弱环节,确定优化方向,提出结构改进方案。根据改进方案得出的研究结果表明,缸盖罩顶面振动响应较大,改进后其总声功率级降低了5.81 dB(A),降噪效果明显。     

汽车发动机气缸盖与气缸垫组合结构的有限元分析

研究和探讨了有限元分析理论和方法在汽车发动机气缸盖与气缸垫组合结构强度计算中的应用.以通用有限元分析软件为平台,建立了三维CAD/CAE应用软件集成系统,实现了从预紧工况到爆发工况的结构分析、稳态热分析和热-结构耦合分析.     

Failure of a Cast Aluminum Engine Head in a High Performance Application

The investigation described in this article examined a cast aluminum head used in a high performance (drag racing) application. These engine heads had been failing within a few seconds of startup. A sample head was examined to determine the cause. Visual inspection found two cracks on the exhaust side of the head. One crack initiated at the bottom of a spark plug well. The head was then sectioned for metallographic examination and fracture analysis. Metallographic examination revealed a large number of shrinkage pores throughout the casting. The fracture surface intersected many of these shrinkage pores. A significantly sized pore was located near the bottom of the suspect spark plug well near a stress-concentrating feature. This pore acted as the crack-initiating defect, which caused the engine head to fail.     

柴油机气缸盖开裂分析

采用宏观和微观分析、化学成分分析、力学性能测试、金相检验和断口分析等方法对失效气缸盖进行了分析。结果表明,该失效件为疲劳开裂。材料组织不均匀、严重的组织疏松、热处理过程中出现的过烧现象使材料疲劳强度大幅度降低,导致其疲劳失效。     

疲劳累积损伤对发动机缸盖再制造的影响

通过对新、旧发动机缸盖取样进行疲劳试验,对比了其SN曲线,结果发现：应力幅较大时,新、旧缸盖材料的疲劳寿命相差明显;应力幅较小时,新、旧缸盖材料的疲劳寿命相差不大,表明旧发动机缸盖材料的损伤程度随着应力幅的增加而增大。因此当发动机经过再制造后,有可能因为累积损伤的原因存在安全隐患。     

大型齿轮箱输出轴断裂失效分析

利用金相检查、SEM分析和X射线衍射物相分析等方法对材料为42CrMo的开裂大型齿轮箱输出轴进行失效分析,结果表明残余拉应力和位于中心的严重的冶金缺陷是输出轴失效的原因.