Computational Modeling of Intergranular Crack Propagation in an Intermetallic Compound Layer

A micromechanical model is presented to study the initiation and propagation of microcracks of intermetallic compounds(IMCs)in solder joints.The effects of the grain aggregate morphology,the grain boundary defects and the sensitivity of the various cohesive zone parameters in predicting the overall mechanical response are investigated.The overall strength is predominantly determined by the weak grain interfaces;both the grain aggregate morphology and the weak grain interfaces control the crack configuration;the different normal and tangential strengths of grain interfaces result in different intergranular cracking behaviors and play a critical role in determining the macroscopic mechanical response of the system.

INFLUENCE OF RARE EARTH ON INTERGRANULAR CRACKING OF 9% Ni CAST STEEL

The intergranular cracking of 9% Ni cast steel seems mainly to relate to the segregation of contaminates H,S and P along boundaries.An addition of rare earth may eliminate the seg- regation of these contaminates along grain boundaries and improve the binding force among boundaries,so as to reduce remarkably the intergranular cracking.

The effects of sulfide stress cracking on the mechanical properties and intergranular cracking of P110 casing steel in sour environments

Variation and degradation of P-110 casing steel mechanical properties, due to sulfide stress cracking （SSC） in sour environments, was investigated using tensile and impact tests. These tests were carried out on specimens, which were pretreated under the following conditions for 168 hours： temperature, 60 ℃; pressure, 10 MPa; H2S partial pressure, 1 MPa and CO2 partial pressure, 1 MPa; preload stress, 80% of the yield strength （os）; medium, simulated formation water. The reduction in tensile and impact strengths for P-110 casing specimens in corrosive environments were 28% and 54%, respectively. The surface morphology analysis indicated that surface damage and uniform plastic deformation occurred as a result of strain aging. Impact toughness of the casing decreased significantly and intergranular cracking occurred when specimens were maintained at a high stress level of 85% %.

Intergranular Fatigue Cracking Behaviour in Aluminium

In order to understand the basic mechanism of intergranular cracking in pure metals during fatigue, stress-controlled push-pull fatigue tests were carried out with high purity aluminium. Tests were interrupted frequently so as to study the grain boundary (GB) cracking behaviour by the surface observation. The results show that crack initiation at GB was a process controlled by multi-factors, such as boundary structure, GB-slip interaction, GB sliding and so on. If these factors are varied so that the incompati- bility at a GB increased, the possibility of cyacking at the boundary will be raised. Some inteygranular cracking phenomena are not able to be explained by the GB stepping mechanism.

Improvement of Intergranular Stress Corrosion Crack Susceptibility of Austenite Stainless Steel through Grain Boundary Engineering

Intergranular stress corrosion crack susceptibility of austenite stainless steel was evaluated through threepoint bending test conducted in high temperature water. The experimental results showed that the frequent and efficient introduction of low energy coincidence site lattice boundaries through grain boundary engineering resulted in an apparent improvement of the intergranular stress corrosion crack resistance of austenite stainless steel.

Effects of Aging Treatment on Intergranular Corrosion and Stress Corrosion Cracking Behavior of AA7003

In order to study the effects of aging treatment on the intergranular corrosion（IGC） and stress corrosion cracking（SCC） of 7003 aluminum alloy（AA7003）, the intergranular corrosion test, electrochemical test and slow strain rate test（SSRT）, combined with optical microscopy（OM） and scanning electron microscopy（SEM） as well as transmission electron microscopy（TEM） observations have been carried out. The IGC and electrochemical test results showed that the IGC resistance of AA7003 for peak aged（PA） temper is the lowest, with double peak aged（DPA） the moderate, and retrogression and re-aging（RRA） the highest among three tempers, which is attributed to the continuous feature of precipitation on grain boundary of PA temper and the interrupted feature of precipitation on grain boundary of DPA and RRA tempers, as well as the wide precipitation free zones（PFZ） of RRA temper. In addition, the SSRT results indicated that all three tempers AA7003 are susceptible to SCC in IGC solution, and the change tendency of SCC susceptibility（ISCC） of AA7003 for three tempers follows the order： ISCC（RRA）

Mechanism of hydrogen-induced intergranular cracking of tubular steels

Hydrogen could be enriched on grain boundaries by stress induction and other traps.The enriched hydrogen facilitated emission and movement of dislocations and lowered plastic work.Consequently,fracture occurred easily along grain boundary.A normalized threshold value of hydrogen-induced cracking (HIC) along grain boundary is given as (KIH/KIC)2=1-0.162×10-3 βHCc(H)GH/(2γs-γb-0.16×103∑β CiGi),where βH and βi are the enrichment factors of hydrogen and other elements on grain boundaries,respectively;Cc(H) is hydrogen concentration induced by stress;C,is the average concentration of an element in the steel;GH and Gi are factors of fracture work along grain boundary with hydrogen and other elements,and γs and γb are surface free energy and grain boundary energy of Fe.For tubular steel,the calculated KH/KIC is 0.23 while the determined value is 0.26.The theoretical threshold value is well consistent with the experimental one.

核电厂海水泵用镀锌高强度螺栓的开裂原因

某核电厂海水泵用双头镀锌高强度螺栓发生开裂失效。通过材料性能(化学成分、显微组织和力学性能等)检测及断口和裂纹形貌观察,分析了螺栓的失效原因。结果表明:镀锌螺栓开裂的原因为氢致开裂。锌镀层破损的高强度螺栓与海水接触时,表面锌镀层和螺栓基体形成电偶对,基体作为阴极析出氢,部分原子氢扩散进入螺栓基体导致氢致损伤,螺栓在预紧力的持续作用下发生氢致开裂。

紧固管接件螺母开裂原因

某紧固管接件螺母发生开裂现象。采用宏观观察、化学成分分析、金相检验、扫描电镜及能谱分析等方法对螺母开裂的原因进行分析。结果表明:螺母显微组织可见富铬碳化物沿晶界析出,使材料的抗晶间腐蚀能力降低,腐蚀性介质渗入螺母的内壁,在腐蚀性介质与紧固力的共同作用下,螺母发生应力腐蚀开裂。

氧化铝厂压溶区弯管开裂原因分析

针对某氧化铝厂压溶区弯管的短时间开裂问题,对裂纹进行了宏观和微观形貌分析,对弯管材料进行了化学成分分析、微观组织观察、硬度以及拉伸性能试验。结果表明,弯管的短时间开裂裂纹主要沿晶扩展,断口表面腐蚀痕迹明显,是比较典型的碱脆开裂;弯管管体化学成分符合Q345E成分要求,微观组织为回火马氏体;失效弯管供货状态不合理是其力学性能、应力状态异常及管体短时间碱脆破裂的主要原因。

770 MPa级调质铸钢液态金属致脆失效原因及相应防护对策

采用金相显微镜、电子扫描显微镜等对某大型截面铸钢工件裂纹成因进行了分析。结果表明:开裂高强度调质铸钢,在热浸锌过程中,基体产生了垂直于表面的裂纹,裂纹长度约为3 mm,且裂纹沿晶开裂。能谱结果表明,裂纹中富含大量锌,为典型的液态金属致脆现象。针对此情况,在热浸锌工艺、焊接工艺、成型工艺、涂装体系等方面给出了避免产生液态金属致脆的防护对策。

42CrMo钢长轴感应淬火裂纹原因分析

观察了感应淬火后42CrMo钢长轴开裂的微观组织、裂纹形貌并测试其化学成分和力学性能,分析了裂纹的形成原因。结果表明:裂纹沿轴向分布且仅存在于大直径区域的淬硬层,与心部相比,该区域晶粒异常粗大且部分晶界熔化,裂纹沿晶扩展,裂纹内存在氧化物,因此判断裂纹产生的主要原因是感应淬火工艺参数不当,导致在长轴直径增大区域的淬火加热温度过高,从而造成淬硬层发生过烧、晶界弱化甚至烧蚀,从而出现裂纹。相比小直径区域,大直径区域的淬硬层较厚、硬度偏低,也佐证了该论断。

敏化处理对AA5083铝合金的腐蚀和应力腐蚀开裂行为的影响

铝镁系合金被广泛用于船舶建造和海洋结构,但是在服役过程中会在晶界析出低腐蚀电位的富镁相,引起环境断裂问题。研究了175℃/100 h敏化处理对铝合金AA5083的Al-Mg相析出行为、晶间腐蚀、电化学行为和应力腐蚀开裂性能的影响,并采用慢应变速率拉伸试验研究敏化前后的应力腐蚀开裂敏感性。结果表明,原始态材料不发生晶间腐蚀和应力腐蚀开裂,而经过敏化处理后,100 nm厚的β'-Al_(3)Mg_(2)或β-Al_(3)Mg_(2)相在晶界析出,导致材料发生严重的晶间腐蚀、电化学腐蚀以及应力腐蚀开裂。铝合金的应力腐蚀开裂行为是在裂纹尖端析出相的阳极溶解以及溶液酸化引发氢脆的协同作用下发生的。

0Cr12Mn5Ni4Mo3Al不锈钢自锁螺母的开裂原因

某0Cr12Mn5Ni4Mo3Al不锈钢自锁螺母在服役过程中出现开裂。对开裂螺母进行了宏观和微观断口形貌观察、表面能谱分析、显微组织及力学性能分析与测试等。结果表明:该螺母开裂原因为应力腐蚀开裂(SCC)。裂纹起源于法兰面与螺母体交接处,螺母体与法兰面呈直角此处易积液,螺母表面处理方式为钝化和涂MoS_(2),其耐蚀能较差。在腐蚀性潮湿大气环境中,干湿交替作用下螺母产生局部腐蚀点,在腐蚀点处萌生裂纹,裂纹在螺母圆周切向拉应力作用下扩展直至螺母开裂。

超超临界锅炉捞渣机链条断裂原因

某超超临界锅炉捞渣机链条多次出现断裂问题,采用宏观观察、化学成分分析、力学性能测试、金相检验、扫描电镜分析等方法研究了链条断裂的原因。结果表明:链条中Cr、Mo元素含量偏低,且未对链条进行回火处理,导致材料脆性过大,受外力冲击时易发生脆性断裂;长期服役后链条表面淬硬层沿晶开裂现象严重,链条内部夹杂物过多,使裂纹向内扩展,最终导致链条断裂。

20CrMoH钢齿轮开裂原因

在某20CrMoH钢齿轮渗碳处理过程中,其齿轮根部出现裂纹。采用宏观观察、化学成分分析、金相检验、扫描电镜及能谱分析等方法分析了齿轮开裂的原因。结果表明:材料晶界处分布枝晶状的硫化锰,该类夹杂物属于Ⅱ类硫化物,形成于第二脆性区的温度区间,由稳定的共晶反应生成,硫化锰夹杂物的形成与S、O元素含量及冷却速率有关;夹杂物破坏了基体的连续性,应力集中处萌生微裂纹并不断扩展,最终导致20CrMoH钢齿轮发生开裂。

镍铜合金类零件渗透检测方法分析

镍铜合金以其耐高温、耐腐蚀、耐磨损、易加工等优势在设备零件制造领域得到广泛应用。为保证镍铜合金类零件应用的可靠性、安全性、持续性,需要做好其质量与性能检测,以及时发现缺陷零件,做好相关处理工作。渗透检测是镍铜合金类零件质量检测重要手段,但在实践中经常发生晶间裂纹漏检问题,降低渗透检测结果的准确性、可靠性。现从理论与实践两个角度探寻镍铜合金类零件渗透检测问题,以获取较好镍铜合金类零件渗透检测方法。理论角度简要对镍铜合金类零件质量检测、渗透检测原理与方法进行介绍;实践角度主要对镍铜合金类零件渗透检测结果影响因素、改进方法及其应用效果进行探究。结果表明,镍铜合金类零件表面状态对渗透检测结果影响较大,利用三氯化铁溶液对镍铜合金类零件进行预处理,能够在一定程度上降低表面状态影响,减少晶间裂纹漏检率,提高渗透检测结果的准确性、可靠性。

玻璃幕墙固定用索锚具开裂原因

某玻璃幕墙固定用索锚具在使用过程中发生开裂现象。采用宏观观察、化学成分分析、力学性能测试、扫描电镜及能谱分析、金相检验等方法对索锚具开裂原因进行分析。结果表明:索锚具的开裂形式为晶间腐蚀开裂,索锚具固溶处理效果不佳,沿晶界析出了大量M_(23)C_(6)型碳化物,降低了索锚具的抗晶间腐蚀性能,在腐蚀性元素S、Cl的作用下,索锚具外表面萌生微裂纹,并沿晶界向内部扩展,最终导致索锚具发生开裂。

奥氏体不锈钢铸件晶间腐蚀弯曲裂纹改善研究

通过对奥氏体不锈钢铸件材料的晶间腐蚀弯曲裂纹的检测研究,对照相关检测标准,发现非金属间夹杂物和缩松是导致晶间腐蚀弯曲样产生裂纹的根本原因,而试样加工检测不规范会进一步降低晶间腐蚀弯曲符合率。为此提出实际生产中要提高铸件纯净度减少非金属夹杂物,提高铸件致密度减少缩松并规范弯曲试样的加工质量和加工尺寸、检测弯曲压头直径的正确选择,提高了奥氏体晶间腐蚀弯曲裂纹符合率,改善了铸件质量。

1000MW机组锅炉水冷壁焊接接头失效分析

通过观察并分析裂纹宏观和微观形态、金相、硬度、扫描电镜、能谱及接头结构,研究了1000 MW超超临界机组锅炉水冷壁安装焊缝裂纹失效原因。结果表明:焊缝根部间隙、焊缝根部凸出及其硬度超标;焊缝组织粗大及低熔点碳化物相偏析,弱化晶界;焊缝存在微裂纹,根据焊缝内部微裂纹氧化物相分析可确定为焊接热裂纹。在运行高温高压应力作用下,裂纹扩展最终导致开裂失效,焊接热裂纹是造成接头失效的主要原因。