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1、大型发电厂离相封闭母线的全位置焊接钱放华（中国水利水电第十二工程局机电安装分局 浙江 丽水323000）摘要：简要介绍大型发电厂离相封闭母线的焊接技术，重点分析了纯铝在施工现场全位置MIG焊接遇到的质量问题以及气孔、未熔合、裂纹等焊接缺陷产生的原因，并讨论了相应的MIG全位置焊接工艺。关键词：焊接工艺；MIG焊；全位置；纯铝1060（L2）；发电厂；离相封闭母线相对于碳弧焊、手工TIG焊，手工氩气保护MIG焊具有可全位置焊接、焊接效率高、成本低、变形小，焊缝成形美观等诸多优点，在铝及铝合金焊接中获得广泛的应用。我国东部某大型水电站的离相封闭式铝母线的现场安装就是采用全位置MIG焊接工艺的成功实
2、例。该水电站建于20世纪60年代初，共安装有9台水轮发电机组，总装机容量为697.5 MW，是我国东部电网重要的调频调峰电厂。根据电网的要求，该水电站将增容到990MW。电气主接线为三机一变的扩大单元接线方式。发电机电压仍为13.8KV.改造后通过扩大单元汇流母线的工作电流为13.2KA，短时最大电流为13.9KA，汇流母线采用全连式离相封闭母线，连至主变压器低压侧，分别经03、02、01号主变压器升压后送至220KV开关站。1号封闭母线总长171m，2号封闭母线总长179m，因在施工现场60斜坡上安装、故全位置MIG焊接的确存在一定的难度。中国水利水电第十二工程局机电安装分局在该电厂改扩机项
3、目中承接了QZFM-15/14000型离相封闭母线的现场安装工程。该母线是由纯铝1060（L2）制成，其作用是将大型水轮发电机组输出的额定电流为14kA、额定电压15.75kV的电力输送至主变压器，经主变压器升压至220kV高压后输入东部电网。1 离相封闭母线主焊缝的质量要求三相母线的导体分别密封于各自的铝制外壳内，导体同一断面采用3个缘子呈120夹角与导体支撑。绝缘子一端开有凹孔，内装橡胶弹性块及蘑菇形金具，金具顶端与母线导体接触，另一端固定于支承板上，支承板与外壳固定，主回路封闭母线导体尺寸为530 mm16mm，外壳尺寸为980 mm10 mm，分支回路母线导体尺寸100 mm10mm，
4、外壳尺寸为550 mm5mm，母线导体最热点允许温升50K。根据业主及JB/T5615.8-1999封闭母线产品质量分等的规定，现场安装焊缝经外观检查合格后，需抽取25%焊缝进行X射线探伤，达到级标准为合格。为确保焊接质量，针对现场施工的具体条件和可能出现的困难认真进行分析，拟定了焊接工艺与作业程序，并经焊接工艺评定试验，筛选相应的焊接工艺参数。同时，在施工现场对施焊的持证焊工按新工艺进行强化培训，以满足实际工程的质量要求。2 1060（L2）的焊接工艺特点1060（L2）为工业纯铝1，其主要杂质是Fe和Si（Fe）=0.35%、（Si）=0.25%），严格讲，应把纯铝视为Al-Fe-Si三元
5、合金，其焊接性特点：（1）Al在空气中及焊接时极易氧化，表面易形成0.1-0.2m厚的氧化物薄膜（Al2O3）2，其熔点高，性能稳定，会阻碍焊接过程的进行。同时，Al2O3密度大，在焊接熔池中容易沉入底部而难以去除，造成焊缝夹渣。另外，氧化膜还会吸附较多的水份，焊接时会促使焊缝产生气孔。（2）Al的导热性约为低碳钢的2倍，在焊接时应采用大功率且能量集中的焊接热源，必要时要适当预热。焊接时焊接变形大，如果工艺措施不当还易促使其产生焊接裂纹。（3）Al金属焊接熔池中的气泡行为是形成常见的焊缝气孔的重要因素3，熔池中的气泡主要是氢气泡。弧柱气氛中的水份、焊接材料及母材所吸附（主要是物理吸附）的水份，
6、都是焊接熔池中氢气泡的重要来源。另外，铝金属焊接时之所以容易形成氢气孔，主要与H2在Al中的溶解度变化特性有关。氢在液态铝中的溶解度为0.69-2ml/g，在凝固时突降到0.036-2 ml/g，相差近20倍，而且Al的导热性很强，熔池冷却速度很快，加之铝的密度小，这些都不利于气泡逸出，更容易形成气孔。同时，MIG焊时，焊丝是以细小的熔滴形式通过弧柱进入熔池，由于弧柱温度最高，而熔滴表面积很大，有利于熔滴金属吸收弧柱氛围中的氢，所以在焊接时应尽可能减少电弧周围氢的含量。（4）Al由固态转变为液态时，没有明显的颜色变化，同时Al在高温时的强度很低，焊接时容易烧穿，因此在焊接薄板时要特别注意避免焊
7、漏、烧穿等缺陷。3 现场安装、焊接的主要技术措施3.1 设备及材料的配置纯铝母线及外壳焊接时，宜选用纯度比母材纯度稍高的焊丝，在此选用SAL-3焊丝、1.6mm。氩弧焊时使用的氩气（Ar）99.96%，且氩气中水含量50mg/m3。焊机选用北京时代集团公司生产的ZP7-400D焊接电源及FD20-150送丝机。3.2 焊前准备Al及铝合金的切割和坡口加工采用机械加工方法，但不宜用砂轮打磨，防止砂粒镶在金属表面，焊接时会产生缺陷。切割后的坡口表面应进行清理检查，对影响焊接质量的局部凹凸不平处应修整。为了尽可能地减少氢的来源，坡口、焊丝及坡口两侧50mm范围内应进行清理，清理油污可采用丙酮或四氯化
8、碳（后者有毒要慎用）等有机溶剂，Al2O3薄膜可采用不锈钢丝轮、不锈钢丝刷或刮刀去除，清理后用丙酮擦净焊接部位。清理好的焊件经干燥处理后应在8h内施焊，最长不应超过24h，否则应再次处理（搁置时间过长，不但会产生新的Al2O3薄膜，而且随时间增加还要增厚）。组对焊时，对口应做到外壁平齐，错边量不应超过筒壁厚度的20%，且不大于2.5mm。筒体不能强力组装，以减小应力。3.3 焊接工艺要点（1）MIG焊应采用直流反接法施焊，以求产生“阴极破碎”的作用，选用的焊接电流一般希望超过临界电流值，以得到稳定的射流过渡形式，这样不但能使焊缝成形美观，且飞溅极少，而且减少污染；但电流也不可过大。当焊接电流3
9、00A时，焊缝表面易产生“皱皮”现象，其原因是高速的等离子流破坏了氩气的保护，产生紊流而使空气卷入，致使焊缝及其附近发生氧化和氮化，因此出现皱皮。（2）在MIG焊接条件下，焊丝氧化膜的影响更为主要，减少熔池存在时间，难以有效地防止焊丝氧化膜分解出来的氢向侵入熔池。因此，一般希望增加溶池存在时间以利气泡逸出。必要时，可采取预热办法(预热温度为150250)来降低接头冷却速度，以利气体逸出，对减少焊缝的气孔生成倾向有一定好处。（3）焊接工艺参数的选定，主要根据接头尺寸、形状、焊接位置、焊缝成形要求，也必须考虑对气孔、裂纹的影响。表1 焊接工艺参数表壁厚/mm焊丝直径/mm喷嘴直径/mm焊接电流/
10、A电弧电压/ V氩 气流量/L.min-15-101.620160-18025.5-272510-161.620225-25026-27.525（4）施焊前，必须对焊接设备进行检查，并确认其工作性能稳定可靠。同时，按工艺规程调整焊接规范（电流、电压、喷嘴、焊丝、氩气流量）并应预先在试板上调整完好，最后方可正式施焊。（5）焊件组对点固焊时，选用的焊接材料及工艺措施，应与正式焊接要求相同，点固焊缝如发现裂纹等缺陷应及时清除重焊。点焊长度一般为1520mm，厚度35mm，焊接进行中点焊处开裂或产生较大的裂口时应立即停止焊接。（6）采用双人对称同步焊接，并且尽可能以相同的焊接参数施焊，以其减少焊接应力
11、。（7）多层焊时尽量减少焊接层数，降低焊接速度以利熔池中氢气孔的浮出。层间温度尽可能低，以不高于150为宜，层间存在的杂物应用不锈钢丝刷清理干净。焊接时，稍有不慎，就会烧毁导电嘴，或使含铜物等杂质熔入焊缝，严重影响焊缝质量。必须将该部位焊缝全部铲除后，方可继续施焊。4 焊后质量检验根据业主和封闭母线产品质量分等的有关规定，封闭母线导体焊接完成后，经外观检查，焊缝外观整洁、美观；抽取25%焊缝进行X射线探伤，全部达到级合格标准。同时，符合国外标准4相关条款的要求。封闭母线导体检验合格后，才能焊接外壳的连接件，连接前应将内部母线表面、绝缘瓷件及外壳内部清洗和擦拭干净，清除所有遗留工具或杂物，外壳的
12、焊缝质量要求与母线导体相同。通过上述焊接工艺及技术措施，焊接质量得到了有效的保证，工程质量和进度都受到了业主和设备制造厂家的高度赞扬。5 结论(1)在正确的工艺措施和焊接工艺条件下，施工现场1060（L2）的全位置MIG焊接完全可以获得性能良好的焊接接头。 (2)选用优良品质的焊接材料，有效清理油污和Al2O3薄膜，1060（L2）全位置MIG焊接可减少气孔的产生。(3)根据工程实际情况进行焊接工艺试验和持证上岗的焊工强化培训是确保工程质量的重要技术措施。参考文献：1 中国机械工程学会焊接学会、焊接手册（第二卷）M。北京：机械工业出版社。19922 李本端 、管型铝母线手工氩弧焊J.水电机电安
13、装技术、1980，（4）.3 FujiiH.Convection in Molten Pool under MicrogravityA. W Doc.212-1001-01 C.Ljubljana:s.n.,2001.4 American Welding Society ,Structural Welding Code- Aluminum. AWS D1.2/D1. 2MS.2003. 作者简介:钱放华(1965-),男,焊接技师、工程师,主要从事焊接工艺制定和焊工培训工作，中国水利水电第十二工程局 机电安装分局 （13575392980）Editors note: Judson Jones
14、is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radio and through the crackling static from spac
15、e hear the faint beeps of the worlds first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug
16、on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. I
17、m anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from the edge of space - live.The (lack of) air u
18、p there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a vertical straight line we would be go for launc
19、h.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a level I wil
20、l never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I
21、knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communications), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the di
22、sappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with
23、capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilomete
24、rs) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I would assu
25、me, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell be traveling fast, so despite the distance, it will not be like diving into the deep end of a pool.
26、 It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he approaches the more dense air closer to Earth. B
27、ut this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Instead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around
28、5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness at mach speeds.Even if everything goes as planned, it wont. Baumgartner still will free fall at a s
29、peed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.
