铝合金用ASSAB 8407钢压铸模早期断裂失效分析

对铝合金用ASSAB 8407钢压铸模断口进行宏观观察,初步推断断口的特征形貌,确定裂纹源的位置以及裂纹扩展的状态特征。采用金相显微镜及扫描电子显微镜,对失效件断口及组织进一步检测,分析断口裂纹形成和扩展机理。检测结果表明,由于基体中存在大量沿晶裂纹,材料强度大幅度降低,脆性显著增大,模具在服役过程中发生早期断裂失效。沿晶裂纹的产生是由于模具锻造过热后慢冷析出片状碳化物,材料强度显著降低。在热处理过程中,热应力及组织应力的影响,使材料基体组织产生沿晶的应力开裂裂纹。同时锻造过热使得材料强度大幅度降低,进一步促使模具表面裂纹的萌生和扩展。     

波纹不锈钢换热板腐蚀开裂失效分析

对失效开裂的波纹不锈钢板进行了外观检查，用电子显微镜观察了开裂断口和裂纹断口的形貌与特征；能谱分析得到了断口腐蚀产物和不锈钢板上沉积物的元素组成与含量；红外光谱分析结果表明使用在波纹不锈钢板端之间的三元乙丙橡胶密封垫片用粘合剂为氯丁橡胶，分析推断氯离子来源于氯丁橡胶的高温分解.综合分析结果表明，波纹不锈钢换热板失效开裂是由氯离子导致的应力腐蚀开裂所致；以穿晶型开裂为主，同时也存在沿晶型开裂.      

片层Ti-55531合金的拉伸和扭转断裂失效行为

通过室温静态拉伸和扭转试验,结合TEM、SEM等分析检测方法,系统研究了片层Ti-55531合金在拉伸和扭转载荷下的断裂失效行为。结果表明,片层Ti-55531合金在拉伸和扭转载荷下的断裂失效有显著的不同:拉伸变形受滑移、次生α_s的孪生及剪切共同控制,扭转变形主要受滑移和剪切控制,未发现有孪晶;拉伸断口较扭转断口陡峭,失效以微孔聚集为主,含少量穿晶解理和沿晶开裂的混合断裂机制;扭转断裂失效则以微孔聚集和剪切开裂为主,含部分穿晶解理的混合断裂机制。无论在拉伸还是扭转载荷下,片层Ti-55531合金的断裂失效面均由最大剪切应力产生,剪切力比正应力更易使片层Ti-55531合金损伤破坏。     

690合金腐蚀疲劳裂纹扩展研究

基于DCPD方法测量了690合金在室温空气、325℃空气和325℃除氧超纯水中的疲劳裂纹扩展速率。试验结果采用Priddle模型进行拟合分析,得到690合金在3种条件下疲劳裂纹扩展的门槛应力强度因子幅值ΔK_(th)和失稳断裂应力强度因子Kc。结果表明,高温水环境下,疲劳裂纹萌生和扩展加快,这可以用滑移-溶解机理解释;高温下,材料强度下降,ΔK_(th)和Kc也下降,高温加速了材料的疲劳断裂。SEM断口形貌表明,空气中的疲劳断口观察到明显的滑移台阶,疲劳破坏形式为穿晶断裂;高温水下的疲劳断口则同时出现穿晶和沿晶开裂,为混合型断口特征。     

低纬度热带岛礁大气环境中高强度不锈钢螺栓断裂分析

高强度不锈钢05Cr17Ni4Cu4Nb螺栓在低纬度岛礁大气环境中短期服役后出现延迟断裂，通过成分测试、金相观察、力学性能、硬度测试、断口观察等方法进行断裂分析。结果表明:螺栓金相组织无异常，化学成分、力学性能、硬度符合标准要求；断口形貌表明螺栓具有呈现树枝状裂纹、沿晶断口等典型应力腐蚀断裂特征；结合螺栓的装配结构、受力情况和服役环境，可以判断螺栓的环境致脆机理为应力腐蚀开裂。该螺栓在我国海南及其他热带、亚热带、温带等沿海海洋环境长期服役后未发生延迟断裂，这说明干湿交替频繁、盐雾与温湿度较高的低纬度热带岛礁大气环境促进05Cr17Ni4Cu4Nb螺栓在装配结构中应力腐蚀开裂行为的发生。     

电连接器4J29合金接触体断裂分析

采用体视显微镜、扫描电子显微镜、能谱分析仪、光学显微镜,对断裂的电连接器接触体进行表面和断口的宏微观形貌观察,断口表面化学成分及显微组织分析。结果表明:电连接器接触体在含Cl~-介质中发生应力腐蚀开裂,最终在工作振动应力作用下发生疲劳断裂;裂纹萌生阶段主要特征为腐蚀坑和沿晶,扩展阶段的主要特征为裂纹穿晶、分叉和疲劳条带。酸洗后腐蚀性Cl~-的残留与接触体表面镀层开裂导致接触体断裂。     

S136钢塑料模零件开裂失效分析

S136钢塑料模零件发生早期开裂,为此对失效零件的化学成分、裂纹断口形貌及显微组织进行检测和分析,推断其失效原因和过程。结果显示,在裂纹断口上镶嵌大量细小颗粒状碳化物,并存在沿晶开裂的二次裂纹,断口剖面的显微组织中密集分布颗粒状碳化物,基体组织中存在大量粗大网状碳化物及晶间裂纹。表明材料淬火后回火不充分,奥氏体化不完全,碳化物未能充分固溶到基体组织中,使材料基体强度不足,造成材料组织应力增大,使用过程中应力作用下,最终造成模具零件早期开裂。     

商用Cu-Mg合金吊弦应力腐蚀断裂分析

商用Cu-Mg合金吊弦在西南路段隧道内频繁出现断裂。采用扫描电镜分析、能谱分析、断口金相检查、显微硬度检测等分析方法发现，断裂以吊弦线外侧为源，断面呈现沿晶脆性断裂及腐蚀特征，覆盖大量氯化物，且在断面形成多条垂直于扩展方向的二次沿晶裂纹。金相检查显示吊弦线纵向组织呈纤维状，加工变形量大；硬度检查显示加工硬化明显；环境监测发现隧道潮湿，空气流动性差，含有一定量的Cl-。结合失效机理，降低吊弦残余应力，表面进行喷漆防护，可有效防止类似故障的再次发生。     

8418钢铝合金压铸模早期开裂失效分析

采用直读光谱仪、数显洛氏硬度计、光学显微镜、扫描电镜及能谱仪,对8418钢铝合金压铸模具失效件的化学成分、表面缺陷形貌特征及显微组织进行检测和分析。结果表明,经过理化检测,基体的晶粒组织粗大,粗大晶粒的平均直径达1647 μm,材料过热程度异常严重。原始粗大晶粒带来最终热处理淬火应力的增大,因而产生解理特征的裂纹以及沿晶韧窝的断口形貌。在后期缓冷过程中,碳化物沿晶析出,使得晶间强度显著降低,在淬火组织应力的影响下,形成沿晶开裂的内裂纹。铝合金压铸模早期开裂的原因,是由于过热粗大组织及沿晶开裂的内裂纹,大幅度降低材料强度韧性。模具在使用过程中难以承受压铸加工过程的工作应力,最终在模具模腔表面圆角最小处,即应力集中倾向最大的部位产生开裂。     

核电厂溢流阀螺栓断裂的原因

某核电厂溢流阀螺栓发生断裂。通过分析和观察断裂螺栓的化学成分、硬度、显微组织及宏微观形貌,探讨了螺栓断裂的原因,并提出了相应的建议。结果表明:失效螺栓的化学成分及组织正常,螺栓等级为12.9级,硬度接近标准上限;螺栓的断口具有沿晶特征形貌,断裂性质是氢致应力腐蚀开裂,为环境促进开裂机制作用的结果。     

Failure Analysis of Weld Cracking in a Thick-Walled 2.25Cr-1Mo Steel Pressure Vessel

A crack in thick-walled 2.25Cr-1Mo steel pressure vessel girth weld was found during manufacturing. To investigate the cause of failure, optical microscopy, scanning electron microscopy, energy dispersive spectrometer, transmission electron microscopy, and microhardness tester were used in this study. According to test results, the fracture is classified as reheat cracking with multiple origins. The cracking occurred during surfacing or final post-weld heat treatment process. Coarse-grains in the weld and bulk-carbides precipitated along grain boundaries induced by multiple heating are main causes of the fracture from material aspect, while high level of the hoop stress component and excess localized deformation in stress relief procedure are mechanical aspect causes of the cracking. The fracture surfaces present major intergranular feature with a small fraction of transgranular morphologies. Large numbers of M₃C and M₂₃C₆ carbides particles were found on the fracture surface, these carbides mainly precipitated on prior austenite-grain boundaries, columnar-grain boundaries, and sub-grain boundaries. Additionally, several proposals were also offered to reduce weld cracking of 2.25Cr-1Mo steel pressure vessels.     

Tribotesting system for hardmetal mechanical characterization

Performance of a material in a tribomechanical system can only be determined by a carefully designed simulation in which mechanical elements undergo processes similar to those in a complex engineering environment. In this paper a simple and reliable integrated testing method, in which conjoin actions involving fracture and abrasion of hardmetals together with abradant comminution, is carefully monitored and analysed. The presented system approach to tribotesting enables the mechanical characterization of hardmetals in attrition contact with granular minerals which takes place in e.g. mineral processing or in drilling action. The results from this testing show that by using one apparatus and test specimen shape it is possible to obtain a reliable rating of hardmetals according to both criteria: their fracture toughness as well as their resistance to abrasion in rubbing contact with the abradant. The integrated testing method required a theoretical or empirical model which describes the relationship between fracture toughness, other mechanical properties and the test's fracture indicator. For the range of materials tested, the best correlation received was for the empirical model for the removal of brittle material by lateral cracking.     

2Cr13Mn9Ni4不锈钢卡箍腐蚀开裂分析

2Cr13Mn9Ni4不锈钢卡箍在服役条件下大量出现开裂、断裂问题，为飞行安全埋下较大的隐患。对大量卡箍裂纹进行统计分析，发现其断裂位置、形貌有相似性，进行2Cr13Mn9Ni4不锈钢卡箍腐蚀开裂分析，对卡箍的安全应用有重要意义。对故障件外观进行观察，对断口进行宏微观特征观察、能谱分析，对焊缝附近进行金相组织检查、硬度分布测试，在故障件上取样进行晶间腐蚀试验、热模拟试验，最后对卡箍应力进行模拟仿真。结果表明:卡箍残余应力、环境腐蚀、焊接影响的晶间敏化共同作用导致卡箍开裂。     

撞针体开裂分析

材料为30CrMnSiA的撞针体零件在交付之后发现部分零件外表面存在裂纹，与其同批次的原材料表面亦在热处理工序之后发现疑似裂纹痕迹。通过对撞针体进行断口宏观及微观观察、能谱成分分析、金相组织检查和显微硬度测试，并与其原材料进行对比检查分析，最终确定了撞针体的开裂原因。结果表明:在热处理工序之前撞针体原材料已经存在初始折叠缺陷，该初始折叠缺陷是导致撞针体开裂的主要原因；建议在原材料进行热处理之前增加无损检测工序，及时排除初始缺陷（折叠）隐患。     

The influence of simulated fuel-grade ethanol on fatigue crack propagation in pipeline and storage-tank steels

This study presents an evaluation of fatigue crack propagation in three steels (A36, X52, and X70) in a simulated fuel-grade ethanol environment. A fracture mechanics testing approach was used to determine crack propagation rates as a function of the stress-intensity-factor amplitude (ΔK). Results of this testing and the fracture analysis indicate that all three materials are susceptible to enhanced fatigue damage in fuel-grade ethanol environments. We show that the damage mechanism is attributed to susceptibility of each material to ethanol stress-corrosion cracking under fatigue loading conditions and propose a model for determining crack growth rates in ethanol fuel.     

7A04铝合金壳体失效分析

7A04铝合金壳体在使用过程中发生开裂失效，通过对失效件的外观、断口宏微观形貌观察，以及对失效件和库存件材质、显微组织、力学性能和表面应力等方面的测试，对壳体的开裂原因进行了分析。结果表明，失效壳体为沿纵向脆性断裂，壳体横向力学性能较低，使用过程中承受较大的工作应力是导致开裂的主要原因。     

发动机气缸体泄漏原因分析

发动机气缸体在南海试验时发生泄漏故障。对气缸体气道表面形貌、裂纹形态及裂纹断口进行了宏微观观察、能谱成分分析,对气缸体的金相组织及硬度进行了检查。结果表明:气缸体的裂纹性质为应力腐蚀开裂;由于气道表面残留高浓度的Cl-以及材料具有较高的应力腐蚀敏感性,使得气缸体在残余应力作用下发生应力腐蚀开裂。要预防气缸体发生应力腐蚀失效,可采取加强清洗以减少Cl-的残留、改善材质状态或更换材料以降低其应力腐蚀敏感性等措施。     

AZ31镁合金热轧边裂预判模型研究

采用等压法,通过等温热压缩实验获得了AZ31镁合金变形温度和应变速率分别在473~673 K和0.005~5 s-1条件下对临界断裂应变的影响规律,以及Zener-Hollomon表达式,据此针对AZ31建立了临界断裂应变与变形温度和应变速率间的基本模型;在此基础上,基于镁合金轧制边裂的基本机理,引入CockcroftLatham断裂准则,建立了含有材料变形激活能和基本轧制工艺参数的AZ31镁合金轧制边裂预判模型;并通过相同条件下有限元模拟和热轧试验分别得到沿板宽方向损伤值和边部裂纹深度,以此对所建立的边裂预判模型进行验证,结果显示所建立边裂预判模型的预测值和实测值平均误差为11.3%。     

Effect of Strain Rate on Stress Corrosion Cracking of 316L Austenitic Stainless Steel in Boiling MgCl2 Environment

Stress corrosion cracking behaviors of AISI 316L austenitic stainless steel at slow strain rates in two environments of air and MgCl₂ at ambient temperature and 154 °C were investigated. The results revealed that a decrease in strain rate, during testing in boiling MgCl₂ environment, led to a rigorous deterioration of the mechanical properties of the material, causing brittleness of the steel. The results obtained from fractography indicated that the samples tested in air had typical ductile fracture surface appearances, while the fracture surfaces of the samples tested in a corrosive environment showed a combination of intergranular and transgranular fracture modes, having a brittle macroscopic appearance. The transgranular mode became predominant as strain rate decreased. The results suggested that the presence of deformation bands in front of crack tips were responsible for transgranular cracking caused by stress corrosion.     

COMPUTER-AID ANALYSIS FOR CRACKING OF 280 CONNECTING ROD OF 16V280ZJ DIESEL LOCOMOTIVE ON QUENCHING PROCESS

280 connecting rod is one of the most important parts for 16V280ZJ diesel locomotive,so it needs much better mechanical performance.However.the crack is often generated at the middle section of connecting rod body on heat treatment process.A temperature-phase transformation-stress coupled 3D non-linear mathematical model has been developed to analyze the reasons of cracking.The simulation and experimental results show that the compressive stress on the cracking position is close to the fracture stress of connecting rod and the reasons of crack are mainly contributed to the carbon content excess of original material or the aging of quenchant.