海洋平台E36钢热处理前后断裂韧性研究

工程上普遍认为,通过焊后热处理（PWHT）,如果方法得当,可以提高焊缝金属的断裂韧性值。但是,近年来诸多研究发现,焊后热处理对断裂韧性也并不是都有好的影响,比如,热处理之后,焊接接头热影响区的断裂韧性就可能有恶化的现象。本研究设计了对比CTOD（裂纹尖端张开位移）试验,发现热处理之后热影响区组织变粗大,碳化物大量析出并聚集长大,使得断裂韧性大大降低。     

Effect of hydrogen environment on the notched tensile properties of T-250 maraging steel annealed by laser treatment

In the present work, slow displacement rate tensile tests were performed to find out the influence of ageing condition and hydrogen-charging on the notched tensile strength and fracture characteristics of T-250 maraging steel aged at various conditions. The influence of embrittling species in the environment on the notched tensile strength was accessed by comparing the measured properties in air, gaseous hydrogen and H₂S-saturated solution. The hydrogen diffusivity, permeation flux and apparent solubility of various specimens determined by electrochemical permeation method, were correlated well with the microstructures and mechanical property. The results indicated that the peak-aged (H900) specimen was highly sensitive to hydrogen embrittlement even in gaseous hydrogen. In contrast, the microstructures of over-aged (H1100) specimen comprising of reverted austenite and incoherent precipitates could trap large amount of hydrogen atoms, resulting in decreased hydrogen permeability and hydrogen embrittlement susceptibility. The solution-annealed specimen had the highest diffusion coefficient and the lowest quantity of trapped hydrogen among the specimens, showing high susceptibility to sulfide stress corrosion cracking. In the presence of notches, hydrogen atoms were prone to segregate and trap at grain boundaries, resulting in the formation of intergranular fracture.     

结构钢焊缝金属在低温和动载条件下的脆断本质

研究了结构钢焊缝金属在低温和高加载速率条件下的断裂韧度与断口形貌变化规律,讨论了沿晶脆性断裂的本质。研究指出,降低温度和提高加载速率都导致断裂韧度降低和沿晶脆性断裂倾向增加,表现出一定的低温效应和高加载速率效应,降低试验温度和提高加载速率的作用是相互促进的。焊缝金属沿晶脆性断裂是特定的材料因素、结构因素和环境因素共同作用的结果,沿晶脆性断裂倾向决定于晶界协调相邻晶粒塑性变形的能力,在晶界不足以协调其两侧晶粒的变形时便出现沿晶断裂。     

Prevention of liquid metal embrittlement cracks in resistance spot welds by adaption of electrode geometry

ABSTRACT

Advanced high strength steels are usually coated by a zinc layer for an increased resistance against corrosion. During the resistance spot welding of zinc coated steel grades, liquid metal embrittlement (LME) may occur. As a result, cracking inside and around the spot weld indentation is observable. The extent of LME cracks is influenced by a variety of different factors. In this study, the impact of the used electrode geometry is investigated over a stepwise varied weld time. A spot welding finite element simulation is used to analyse and explain the observed effects. Results show significant differences especially for highly increased weld times. Based on identical overall dimensions, electrode geometries with a larger working plane allow for longer weld times, while still preventing LME within the investigated material and maintaining accessibility.     

Effects of chemical compositions and microstructure on hydrogen embrittlement of austenitic stainless steel weld metal in high-pressure gaseous hydrogen environment

The effect of chemical compositions and microstructures on hydrogen embrittlement of austenitic stainless steel weld metals in high-pressure hydrogen gas was surveyed by using the slow strain rate test (SSRT). As a result, hydrogen embrittlement of the weld metal was hardly influenced by δ ferrite in the weld metal, but by stability of austenite phase, which was estimated by Md30 value or Ni equivalent. In the weld metal with poor stability of austenite, α′-martensite was formed near a crack induced by SSRT. In addition, although the crystal structure of α′-martensite is as same as δ ferrite, susceptibility of hydrogen embrittlement became higher with the increase in α′-martensite. The mechanism to explain the difference between δ ferrite and α′-martensite was considered as following. The hardness, which increases the hydrogen embrittlement susceptibility in bcc structure, is higher in α′-martensite than in δ ferrite. In addition, α′-martensite might be formed continuously with propagation of a crack. Therefore, the effect of α′-martensite on hydrogen embrittlement could be larger compared with δ ferrite.     

不同伸长率多晶Be室温拉伸断口分析

通过扫描电镜系统分析了伸长率5%和零伸长率多晶Be室温拉伸断口形貌。发现多晶Be无论伸长率高低,拉伸断口均平整、无颈缩;断口宏观形貌分纤维区和放射区,微观形貌既有裂纹沿一定结晶学表面扩展形成的解理断裂特征,又有一定塑性变形产生的撕裂棱,属准解理断口。但是,伸长率5%的多晶Be断口纤维区和放射区界限不清晰,放射花样细小且走向多变,断口没有明显的主裂纹源,断裂是由多个裂纹源汇合所致。而零伸长率多晶Be断口纤维区和放射区界限清晰,放射花样粗大且走向单一,并且纤维区占整个断口比例极小,放射花样则几乎遍布断口通区,断口上可见明显的主裂纹源,主裂纹源中心往往存在某种组织缺陷,断裂主要是单一裂纹扩展所致。这表明多晶Be的伸长率主要来自于裂纹形核阶段,微观组织缺陷造成裂纹过早地达到临界裂纹扩展尺寸,是导致多晶Be材室温伸长率降低的主要原因。     

Hydrogen embrittlement of a Ti-strengthened 250 grade maraging steel

Constant extension-rate tensile tests are performed to investigate the effects of strain rate and environmental hydrogen concentration on the tensile properties of various aged T-250 specimens. The 426 °C (800 °F) underaged specimens are very sensitive to strain rate; the 482 °C (900 °F) peak-aged specimens exhibit a reduced ductility under low strain rates; and the 593 °C (1100 °F) overaged specimens are insensitive to strain rate when tested in air. The excellent resistance to embrittlement of the overaged specimens in gaseous hydrogen could be associated with the extensive formation of reverted austenite and the incoherent Ni₃Ti precipitates. The tensile-fractured surfaces of such specimens reveal a ductile dimple fracture. However, the peak-aged specimens are susceptible to gaseous hydrogen embrittlement, and the embrittled region shows a primary fracture mode of quasi-cleavage. The least resistant to hydrogen embrittlement of the underaged specimens is characterized by a more brittle fracture appearance, that is, intergranular fracture, under a low strain rate or in the gaseous hydrogen environment.     

SAF2205双相不锈钢焊接热影响区在尿素介质中的腐蚀研究

研究了SAF2205双相不锈钢和尿素级316不锈钢对接焊热影响区在工业尿素合成介质中的耐蚀性.实验结果表明,焊接线能量对热影响区的耐蚀性有重要影响,腐蚀最重处是在距离熔合线3-6mm区域,局部腐蚀深度随线能量的增加而增大.SAF2205不锈钢焊接线能量对热影响区腐蚀的敏感程度比316L不锈钢大.     

A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.     

Special features of the formation of the microstructure and chemical heterogeneity in welded joints in alloys of the Al–Zn–Mg–Cu system alloyed with scandium

Special features of the formation of the microstructure and the development of chemical heterogeneity in the weld metal and heat-affected zone (HAZ) in arc welding of a complex alloyed alloy of the Al–Zn–Mg–Cu system are discussed. It is shown that the effect of welding heating results in melting of the phase components of the alloy in the HAZ with the formation of structures of the eutectic origin in the form of long interlayers at the boundaries of recrystallized grains, causing brittleness of the metal. Alloying the alloy with scandium reduces the intensity of the processes of recrystallization and the extent of liquation at the grain boundaries and also localizes the melting of the grains without the formation of thick eutectic interlayers.     

An experimental study of the fracture properties of oil pipeline girth welds

The application of fracture mechanics to the "Fitness For Purpose" assessment of the fracture properties of pipeline girth welds is generally essential for the safe operation of oil pipeline systems. Based on the analysis of the fracture toughness requirements of the oil pipeline girth weld, a systematic study has been conducted on the fracture behavior of the API 5L X65 steel pipelin egirth weld made by two welding materials. The critical COD values of the pipe line girth weld under the different temperatures are tested. In the paper other influential factors on fracture behavior are also discussed, such as the welding materials, the welding positions (flat welding, vertical welding, overhead welding), the welded joint area (weld, heat-affected zone, fusion line and base metal) and the experiment temperature. The result shows that the welding material, the welded joint area and the experiment temperature have considerable influence on the fracture behavior of pipeline girth welds. It is also concluded that t he effect of the welding position is related to the welding process. The study provides a scientific basis for the welding of oil pipelines.     

INFLUENCE OF LOCAL BRITTLE ZONE ON THE FRACTURE TOUGHNESS OF HIGH-STRENGTH LOW-ALLOYED MULTIPASS WELD METALS

1.IntroductionManyillvestigatiolls[1--5]havefOllndthattilelocalbrittlezone(LBZ)playsanimportantroleinfyacturetoughnessofheat--affectedzoneinthebasemetalfornormalizedandthermomechanicallycontrolledprocessingsteels,aswellasdirectquellchedandtemperedsteels.Therearefewreportsaboutthisphellomellainhigh-strengtllmllltipassweldmetals.Inmultipasswelds,thereareprimaryweldmetalandweldmetalreheatedtovarioustemperaturesbythesubsequentbeads[6'71.Thereheatedregionfinderabeadcanbedividedintoacoarsegrainedz…     

热影响区连续孔隙状裂纹的表征及产生机理

以凝固温度范围极宽的合金为对象,研究焊接热影响区的液化现象,表征该区域中出现的连续孔隙状裂纹,并分析其产生机理. 结果表明,连续孔隙状裂纹的产生与母材中残余第二相颗粒的连续分布、热影响区中出现的严重液化现象及液相的流失密切相关. 其产生机理在于:焊接加热时,第二相颗粒加剧了晶界液化的程度,并形成连接热影响区和焊接熔池的液相通道;冷却过程中,由于熔池区熔体体积收缩促使热影响区中的液相流向熔池;凝固末期,通道因胞状晶生长而被堵塞,导致液相无法从熔池回填,从而在热影响区形成连续孔隙状裂纹.     

Application of heat treatment technology for thermit welds at Da-qin railway

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Oxidation behavior of microstructurally-Different regions in the weldment of 9Cr-1Mo steel

Regions with different microstructures have been identified in the weldments of 9Cr-1 Mo steel. Weldments comprising threthree regions, i.e., weld metal, heat-affected zone (HAZ), and base metal, were oxidized in air at 923 K for different durations up to 500 hr. The crown area of the weld metal was found to form a thicker oxide scale than the other regions of the weldment. When the oxidation kinetics of different regions were compared (by separating out the coupons of the HAZ and the crown and root portions of the weld metal and then oxidizing them), the crown area of the weld was found to oxidize at a much higher rate than the others. Scanning electron microscopy (SEM) was carried out to assess the morphological variations in the different regions of the weldment. The compositional variations in the scales over the different regions have been characterized by the energy-dispersive analyses of X-rays (EDX), and the results thereof have also been corroborated by secondary ion mass spectrometry (SIMS).     

Atom probe investigations on temper embrittlement and reversible temper embrittlement in S 690 steel weld metal

Severe embrittlement was observed in weld material of a brand new penstock of a huge hydro power plant. Temper embrittlement (TE) was found as root case of embrittlement. Reversible temper embrittlement (RTE) treatment characterised by a short-time heating at about 600°C, by which the toughness of embrittled weld material can significantly be recovered, was qualified and successfully applied in the plant. Basic investigations were performed to explain the embrittlement as well as the de-embrittlement effect. By the application of high resolution analytics as Atom Probe Tomography (APT) applied on TE as well as on the RTE-treated material, revealed phosphorus segregation in the grain boundaries as root cause of embrittlement. By application of RTE treatment the APT results revealed, that the phosphorus segregation in the grain boundaries disappeared. The mechanism of this behaviour can be explained by referring the McLean [Grain boundaries in metals. Oxford: Clarendon Press; 1957] based grain boundary equilibrium segregation of phosphorous. During RTE treatment, which occur at higher temperatures (600°C) that segregation (which starts during cooling at about 550°C), desegregation occurs. During this higher temperature, the diffusion is much faster than segregation producing the fast recovery of toughness.     

The effects of cold rolling and sensitisation on hydrogen embrittlement of AISI 304L welds

The notched tensile strength (NTS) of 304L stainless steel welds subjected to cold rolling (20% thickness reduction) and sensitisation treatment (600 °C/10 h) was measured in a MgCl₂ (40 wt.%) solution at 80 °C. The NTS loss, which is used for comparing the susceptibility to hydrogen embrittlement (HE), was consistent with the extent of embrittled area on the fracture surface of various welds. The HE susceptibility of the weld decreased after cold working but increased with the additional sensitisation treatment of the cold-rolled welds. Besides, cracks tended to propagate along the skeletal boundaries for all welds tested in the MgCl₂ solution.