热媒输送管线弯头开裂原因分析

某化纤厂CP车间的酯交换反应釜进行热媒试验时,热媒管线发生了泄漏事故。泄漏点在45°弯头侧壁近焊缝部位。对开裂弯头进行了宏、微观检验、化学分析及力学性能测定。结果表明,裂纹已贯穿弯头的内外表面,材质中的非金属夹杂物超标是裂纹形成的原因之一;而焊缝部位过热组织的存在降低了材料的力学性能;同时氰化三联苯介质具有极强的渗透力,在诸多因素的影响下最终造成弯头出现裂纹泄漏。     

预加氢单元管道弯头破裂原因

某石化公司催化重整装置预加氢单元管道弯头发生了破裂。采用宏观观察、壁厚测量、化学成分分析、力学性能试验、金相检验、断口形貌分析、X射线衍射分析及厚度计算等方法对弯头的破裂原因进行了分析。结果表明:该预加氢单元弯头在酸性HCl-H_2S-NH_3-H_2O腐蚀环境下,不仅发生环境腐蚀,亦受到物料及脱落腐蚀产物冲刷,腐蚀与冲刷相互促进,造成弯头外弧侧壁厚减薄严重,最终导致该部位承压能力不足而出现局部破裂。     

某超临界机组锅炉末级过热器钢管泄漏原因分析

某电厂超临界机组锅炉末级过热器钢管在弯头处发生泄漏。采用宏观检查、化学成分分析、金相检验、扫描电镜及能谱分析、力学性能试验等方法对钢管泄漏原因进行了分析。结果表明:钢管弯头在弯制中变形过大且其外弧侧因环境温度过高造成应力集中,随着钢管显微组织的变形和老化,晶界附近产生贫铬区,晶界强度降低,当应力集中达到晶界开裂的程度时,晶粒交界处产生楔型裂纹,裂纹迅速扩展最终导致钢管在运行时发生泄漏。     

A335P91管道焊接技术

A335P91管道属于马氏体型热强钢材质,焊接工艺要求难度大。本文针对该管道焊接工艺开展研究,以规格为Φ335×34mm的A335P91管道为试件,合理设计焊接工艺,介绍了采用手工钨极氩弧焊打底和手工电弧焊填充盖面氩电联焊工艺的焊前预热、充氩保护、焊后热处理等工艺措施和参数优化,通过理化试验,对焊接接头性能进行检测分析,取得了稳定可靠的A335P91管道焊接工艺,可为该种材质的焊接提供借鉴。     

铜管焊接弯头开裂分析

利用光学金相显微镜、扫描电子显微镜、电子能谱仪等方法对开裂铜管焊接弯头进行失效分析,发现该铜管焊接弯头久置于腐蚀性介质中,当在腐蚀坑处应力集中,便产生了开裂失效.同时,还发现该工件在弯制时易形成微裂纹.     

甲醇洗涤塔管线用弯头开裂原因

某甲醇洗涤塔管线用304不锈钢弯头在使用过程中发生开裂。采用宏观观察、化学成分分析、力学性能测试、断口分析、金相检验等方法,分析了弯头开裂的原因。结果表明：弯头发生了应力腐蚀开裂,弯头的焊缝焊接质量较差、残余应力较大以及在含硫元素的腐蚀环境中服役等因素共同导致了该弯头发生开裂。     

某超临界锅炉高温过热器出口弯头爆管原因分析

采用爆口宏观形貌分析、化学成分分析、显微组织观察、常温力学性能试验分析方法,对某电厂600MW超临界燃煤锅炉高温过热器爆管事故原因进行了分析,结果表明:长期超温造成T91组织性能劣化,在钢管弯头背弧薄弱位置发生爆管。推测超温原因是异物堵塞,建议对该管子下弯头及进出口联箱进行全面检查,是否存在异物堵塞。     

304不锈钢弯头开裂失效分析

某公司304不锈钢弯头在使用5a(年)后出现周向裂纹而失效,通过宏观观察、金相检验、扫描电镜分析、化学成分分析、力学性能试验和晶间腐蚀试验等方法,对弯头失效原因进行了分析。结果表明:该304不锈钢弯头裂纹为应力腐蚀形成的沿晶裂纹。弯头暴露在空气中并附着有长期因雨水、盐分产生的弱腐蚀介质,再加上管道输送的氧气对管壁外表面产生的拉应力以及弯头材料本身抗晶间腐蚀性差,最终弯头发生开裂失效。     

某油田地面集输管道弯头腐蚀的原因

某油田地面集输管道弯头腐蚀相当严重,穿孔事故频发.通过宏观观察、化学成分分析、金相检验、扫描电镜观察、能谱分析以及高温高压模拟腐蚀试验等方法,分析了管道弯头穿孔的原因.结果表明:管道弯头的化学成分符合技术要求,且显微组织无异常;管道减薄及穿孔主要是CO2和H2 S的协同腐蚀作用及流体冲刷所致.     

循环水管道弯头焊接裂纹的分析及对策

通过对焊接工艺、化学元素、金相组织诸方面的分析,揭示了循环水管道弯头侧焊接裂纹产生的根本原因,进而制定出了新的焊接工艺。经过实践,具体落实了消除延迟裂纹的措施,取得了满意的效果。     

Tensile deformation behaviour of a dissimilar metal weldment of P91 and 347H steels

Deformation of a weldment is governed by the mechanical properties of its base metals and fusion zone. In a weldment, the base metals and fusion zone exhibit changing microstructural features with various phases present along the weldment. Specifically, the heat affected zone of a base metal exhibits a heterogeneous microstructure generated during weld thermal cycles and by post-weld heat treatment. As a result, the mechanical properties in a weldment are often non-uniformly distributed. In this study, tensile tests combined with digital image correlation were performed to obtain the non-uniform distributions of the mechanical properties of a weldment composed of P91 and 347H steels. From the experimental tensile tests, it was found that the 347H base metal had significantly distinct mechanical properties compared to the other zones of the weldment. Furthermore, the 347H base metal had the lowest yield stress but the highest strain hardening exponent. Because of its lowest yield stress, the 347H base metal had the highest plastic strain accumulation at any stage of global deformation. However, the strain hardening rate of the P91 base metal enabled it to accumulate the necessary plastic strain to activate its necking first. Therefore, the failure location of the P91-347H weldment was expected to occur at the P91 base metal. A 3D finite element simulation of the tensile deformation of P91-347H weldment also suggested the same. However, from the present experimental observations, one weldment out of three was found to fail unexpectedly at the heat affected zone of the P91 base metal. The reason for this unexpected failure was determined by microscopic analysis to be the presence of a large defect.     

Thermal-mechanical fatigue simulation of a P91 steel in a temperature range of 400–600°C

Abstract

This paper deals with the identification of material constants to simulate the effect of cyclic mechanical loading and temperatures. A Chaboche viscoplasticity model was used in this study to model the thermal-mechanical behaviour of a P91 martensitic steel. A fully-reversed cyclic mechanical testing programme was conducted isothermally between 400 and 600°C with a strain amplitude of 0.5%, to identify the model constants using a thermo-mechanical fatigue (TMF) test machine. Thermo-mechanical tests of P91 steel were conducted for two temperature ranges of 400 – 500°C and 400 – 600°C. From the test results, it can be seen that the P91 steel exhibits cyclic softening throughout the life of the specimens, for both isothermal and thermal-mechanical loading and this effect can be modelled by the set of viscoplasticity constants obtained. Finite element simulations of the test specimens show good comparison to isothermal and TMF experimental data.     

Crack initiation and crack propagation under thermal cyclic loading

Theoretical and experimental investigations of crack initiation and crack propagation under thermal cyclic loading are presented. For the experimental investigation a special thermal fatigue test rig has been constructed in which a small circular cylindrical specimen is heated up to a homogeneous temperature and cyclically cooled down under well defined thermal and mechanical boundary conditions by a jet of cold water. At the end of the cooling phase the specimen is reheated to the initial temperature and the following cycle begins. The experiments are performed with uncracked and mechanically precracked specimens of the German austenitic stainless steel X6CrNi 1811.

In the crack initiation part of the investigation the number of load cycles to initiate cracks under thermal cyclic load is compared to the number of load cycles to initiate cracks under uniaxial mechanical fatigue loading at the same strain range as in the cyclic thermal experiment. The development of initiated cracks under thermal cyclic load is compared with the development of cracks under uniaxial mechanical cyclic load.

In the crack propagation part of the investigation crack growth rates of semi-elliptical surface cracks under thermal cyclic loading are determined and compared to suitable mechanical fatigue tests made on compact-tension and four-point bending specimens with semi-elliptical surface cracks. The effect of environment, frequency, load shape and temperature on the crack growth rate is determined for the material in mechanical fatigue tests.

The theoretical investigations are based on the temperature distribution in the specimen, which is calculated using finite element programs and compared to experimental results. From the temperature distribution, elastic and elastic-plastic stress distributions are determined taking into account the temperature dependence of the material properties. The prediction of crack propagation relies on linear-elastic fracture mechanics. Stress intensity factors are calculated with the weight function method and crack propagation is determined using the Paris relation.

To demonstrate the quality of the crack growth analysis the experimental results are compared to the prediction of crack propagation under thermal cyclic load.     

日本潜艇用钢及焊接材料的焊接性能综述

本文综述了日本的潜艇用钢及其焊接材料的各项焊接性能试验情况。对NS63和NS80钢的大量试验结果表明:为了防止产生焊接裂纹,应根据钢种强度级别、板厚和拘束应力的不同来选择不同的预热和道间温度,通常在50~150℃之间变化;相应的焊接热输入也宜在16~24 kJ/cm之内选配,以便得到与焊材相适应的临界冷却速度,使之满足焊缝力学性能的要求。还要限制施焊环境的水蒸气分压(≤25mmHg),焊材的吸潮量要≤0.20%。另外,对NS110钢的低强匹配焊接接头也进行了大量的试验工作,结果表明:在焊缝强度比母材降低15%的条件下仍具有良好的综合性能,将其应用到潜艇壳体上是可以信赖的。     

铝合金-纯铝-钢复合过渡接头性能试验研究

对厚度为10mm的铝合金-纯铝-钢复合过渡接头进行了界面剪切、界面粘结、侧向弯曲、系列温度等模拟试验,对焊后界面剪切、界面粘结等性能进行相关测试.结果表明,该规格的复合过渡接头常规力学性能和焊后界面性能均能满足相关标准要求.根据系列温度模拟试验,建议在工程焊接应用时,纯铝-钢界面温度应不超过300℃.     

Wöhlerversuche an bauteilähnlichen Proben aus MRI 153M

Woehler tests on component‐like specimen of MRI 153M The alloy MRI 153M shall increase the application of Magnesium because of its comparable properties to AZ 91 but better static strength properties at temperatures up to 150° C. Constant amplitude tests were performed with component‐like specimen under variable environment conditions. The modified parameters were the notch radius (r=3 mm or r=12 mm), the stress ratio (R=0 or R=‐1), the surrounding temperature (room temperature, 120° C or 150° C) and the cast skin. At room temperature the Woehler curves of MRI 153M trend to higher cycles to crack initiation than AZ 91 and AM 60. If the local stresses are compared, the specimen with r=12 mm show earlier crack initiation because of the influence of size. The decrease of fatigue under higher temperature is about one scatter range and can be valuated as moderate. Between 120° C and 150° C the fatigue values do not show any significant differences. The removal of the cast skin has no influence on the lifetime of the component.     

A572 Gr.50厚板对接焊缝断裂性能研究

A572 Gr.50厚板常用于锅炉钢结构大板梁的加工制作,其对接焊缝易存在焊接裂纹缺陷。为评定含缺陷的锅炉钢结构大板梁的安全性,通过焊接接头模拟制作实际大板梁下翼缘的A572 Gr.50厚板对接焊缝,分别对其母材、焊缝金属及热影响区材料进行了系列单轴静力拉伸试验、冲击韧性试验和直三点弯曲断裂韧度试验,并结合有限元分析对A572 Gr.50厚板对接焊缝存在裂纹缺陷时的断裂性能进行了研究。研究结果表明:厚板对接焊缝母材、焊缝和热影响区材料基本力学性能均能满足规范要求,均具有良好的塑性变形能力;随温度的降低,母材、热影响区及焊缝处的夏比冲击功减少,但均具有良好的冲击韧性;比较而言,母材的抗低温冷脆性能最好,焊缝最差;母材、焊缝和热影响区3个区域中焊缝的断裂韧度最差;厚板对接焊缝接头的焊缝区是大板梁焊接缺陷安全性评估的重点控制区域;基于断裂力学,可以运用有限元软件方便的对带裂缝的工作状态下工作的钢结构构件的断裂性能进行分析,保证其安全性。     

Crack path prediction of thermal cracks arising in self-stressed two-phase brittle compounds

Conclusions The crack path prediction of thermal cracks in self-stressed two-phase compounds subjected to uniform and nonuniform temperature distributions, respectively, has been studied by using the finite element method as well as by applying an appropriate crack growth criterion. A series of different self-stressed two-phase solids subjected to uniform as well as nonuniform temperature distributions, respectively, have been investigated. A comparison of the numerically obtained results in case of the existence of a uniform temperature distribution in the cross sections of two-phase composite structures for both the thermal crack paths and the corresponding fracture mechanical data at the crack tips with the experimental results gained from associated cooling experiments showed a good agreement.In addition, the influence of additional local temperature changes onto the prospective thermal crack paths has been investigated by means of the crack growth criterion already mentioned. Thereby some remarkable effects of interference between the centers of local temperature changes located in the vicinity of the tips of thermal cracks and their further crack paths could be stated. This interference is also observable concerning the corresponding fracture mechanical parameters at the tips of the arising curvilinear thermal cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 28, No. 1, pp. 85–91, January–February, 1992.     

Thermomechanical Processing and Superplasticity of AZ91 Magnesium Alloy

The effect of extrusion on grain refinement has been studied in the AZ91 cast ingots. It is found that grain sizesmaller than 10μm can be obtained by the extrusion processing. Vickers hardness measurements were also carriedout to evaluate the effect of these processes on the room temperature mechanical properties. The experimentalresults of high temperature tensile tests revealed that the stress was inversely proportional to the square of the grainsize and that the activation energy for superplastic flow was higher than that for grain boundary diffusion.     

Thoughts about drying shrinkage: Experimental results and quantification of structural drying creep

In this second article, we examine in greater detail the influence of concrete skin micro-cracking, linked with the non-uniformity of water content within the specimen. This cracking is responsible for the drying creep which results from a structural effect, also called “microcracking effect”, and which we prefer to call “structural creep” here. It is defined as the difference between the potential drying shrinkage of a specimen which does not crack and the shrinkage measured experimentally. We propose a simple experimental method making use of experimental curves of drying shrinkage as a function of weight loss and allowing the flanking and specification of certain properties of structural drying creep. We shall see that this method must also deal, in the interpretation of results, with the choice of the mechanical constitutive model allowing the processing of nonlinearities induced by concrete skin micro-cracking. Finally, to validate the hypotheses we have made, we shall base our approach on a probabilistic model of the cracking of concrete resulting from the work of Rossi, “coupled” with consideration of the drying of the concrete material in a sense that we shall specify. We shall see that this model is in satisfactory agreement with the curves of shrinkage as a function of weight loss. In addition, the results of simulations are very instructive, as regards the spacing and the width as well as the depth of cracks which appear in the drying shrinkage and creep tests.