| Back to Back Issues Page | |
 |
|
|
PWL#051 - t8/5, Welding 1/2" plates, Exothermal Additions, Orbital Welding, Metals Knowledge, LHW November 01, 2007 |
|
| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#051 - t8/5, Welding Parameters for 1/2" plates, Filler Metal with Exothermal Additions, Orbital Welding, Metals Knowledge, Progress with Laser Hybrid Welding and more... This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers. You are urged to pass-along this publication to your friends, if you like it, and if you want to help them. If you received this from a friend and if you like what you read, please subscribe free of charge and you will also receive a bonus book on Practical HARDNESS TESTING Made Simple.
1 - Introduction 2 - Article: t8/5 3 - How to do it well: Welding Parameters for 1/2 inch Plates 4 - Filler Metals with Exothermal Additions 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Orbital Welding 8 - Site Updating: Metals Knowledge 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Progress with Laser Hybrid Welding 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Back from the 50th Issue Celebrations... we continue our daily work providing our faithful readers with new and updated information likely of being of help, at least sometimes. We start the present Issue of Practical Welding Letter for November 2007 with a technical issue (t8/5) that should be or become familiar to all involved in welding of alloy steels. In presence of cracks or of low impact properties, this subject should be given thorough consideration. An example of how should one approach the quest for the right welding parameters in a given practical case is presented as an answer to a specific question from one of our readers. This exercise shows the kind of details that should be taken into account. A recent research is reported introducing some new twists to the practice of developing filler metals. It could spur some different ideas for other applications. A review of Orbital Welding follows, now a mature and successful technology, that ought to be used for all suitable applications. The results would assure improved quality with less dependence from skilled welders. Laser Hybrid Welding, that was reported earlier as a promising technique, continues to be sought for its significant advantages but still needs considerable investment in developing suitable parameters. A review is given of recent applications. It is not known if the equipment suppliers provide this assistance, but it would seem that this kind of practical development could encourage many shops to introduce the combined process in their activities. Regular columns appear at their usual place. It would be nice to receive your comments and feedback. Use either Welding Talk or Contact Us. Please visit our new Page on Metals Knowledge: it might be just what you need for your benefit and gain. Only a few days left... 2 - Article: t8/5 You may have met the expression t8/5 and, unless you know what it is, you may have wondered on its meaning. It is simply a shorthand way of saying how long it takes, for a given weld bead including nearby material, to cool down from 800 0C to 500 0C after welding. The reason why it is so important to know how much it is and if and which means should be taken to either increase or decrease it, depends on the great influence that this parameter has on certain mechanical properties of the finished weld, especially with high strength steels. It is also a practical way of concentrating the influence of different factors in a single parameter for appreciating the relationship between welding conditions and mechanical properties. In particular the Hardness of the Heat Affected Zone (HAZ) bordering on the weld will be higher the shorter is this specific cooling time. This condition might promote the formation of cold cracks. If the cooling time is extended, possibly by increasing the Preheating temperature, the HAZ hardness will decrease, but also the impact properties of the weld may be impaired. If those are specification requirements, one has to make sure that they will not fall below the minimum acceptable value for the case in question. The factors that influence the selection of suitable welding parameters for given situations are:
The chemical composition is evaluated by calculating the Carbon Equivalent, a composite number that takes into account weighted proportions of the alloying elements present. A few common formulas are known, usually selected according to their ability to provide more accurate simulations in typical cases. The welding processes suitable to weld large thicknesses in a limited number of passes are also those who provide the highest heat input. Therefore they should be scrutinized to determine if their influence on the weld outcome is not going to impair the mechanical properties required. Empirical relations and formulas were proposed by various sources and their predictions were confirmed by actual testing. In general the applicability is limited to a range of validity based on the cases where basic data are quite similar to those that were used to develop the expressions in the first place. The calculation of acceptable welding parameters can be based on the methods presented in the European Standard EN 1011-2. See the following additional information: Software support for plate welders: ProWeld Version 5.0 See also, downloadable from the University of Cambridge: Program MAP_Neural_NNWork and Program MAP_Steel Weldsoft To readers wishing to exercise their patience and solve specific problems by applying these principles to practical cases we would suggest to look for 3 - How to do it well: Welding Parameters for 1/2 inch plates Q: I've been set a task to find the welding parameters for welding two mild steel plates together using Manual Metal Arc Welding [...] A: We will assume that the plates are to be welded in the flat position, edge to edge with groove joint. The given thickness cannot be welded in a single pass with the process specified, so that the edges are to be machined by selecting one of the two most common configurations. We will refer to the recommendations of a document readily available online, In page 5 of that presentation, beveled butt joints are described:
The joint proposed above is therefore on the high limit of single V joint or on the low limit of the double V joint. U joints will not be considered in this case. What is the difference? Single V joint is welded from one side only. For plates it may make not much of a difference. For a tube or vessel, especially if of limited dimensions, the internal side may not be easily reachable for welding. Double V joint requires more expensive machining, possibly including a new positioning and set-up in the milling machine used. If a flame or plasma machine is available, bevels could be also cut with this equipment. It requires much less weld metal though, about half of it, for purely geometrical consideration. This has remarkable consequences, much less heat input is required and less deformations are generated. For welding from both faces the construction has to be turned upside down (if positioned flat), which may be an expensive exercise for bulky and heavy constructions. The electrode to be used, if special requirements are not imposed, can be E-6011 per AWS classification, that is suitable for AC. The power source for AC is a simple transformer, a quite economic piece of equipment. If other electrodes were selected, welded by DC with more expensive equipment, then Arc Blow (deflection of the arc due to magnetic forces) could become an issue. First, with the plates securely clamped to the weld table, tack welding should be performed. This means short stretches of weld bead, about 13 mm (1/2") long, at some distance from each other, say 100 to 150 mm (4 to 6"). These should be deposited in proper sequence, at distant places to avoid localized shrink stresses. Then tack welds should be cleaned from slag and ground to permit smooth blending of the coming new weld beads. Electrode size depends on operator's skill and preference. It could be 3/32" (2.4 mm) at the manufacturer's recommended current, about 60-80 Amp or 1/8" (3.2 mm) at about 80-100 Amp. The root pass could be performed with backing, if advisable, that should normally be removed after welding. For best quality the root pass should be gouged from the back side and a new pass should be added. Slag of each pass has to be thoroughly removed before applying subsequent passes. Welding speed is strongly operator dependent and should be
measured. The total number of passes needed to fill the groove depends on the operator technique, if depositing thin parallel beads, or wide beads obtained by weaving the electrode from side to side. Larger size electrodes could be used but at the risk of higher heat input and increased deformation. Three types of deformation should be foreseen and prevented. The first deformation to be prevented is the overlapping of the joint ends as shrinkage tends to close the gap. As mentioned elsewhere, the first weld will start at one end of the joint for a length of about 150 mm (6 inches) only. Then the upcoming weld will start at the other end of the joint and proceed towards the already welded short stretch. The second type of deformation affects the single V groove joint. As the weld metal in the groove shrinks upon cooling it tends to pull in a direction transversal to the joint. The two plates lying flat tend to decrease the angle between them (from 1800 to 170 - 1600 and even less). To prevent this movement one should increase the angle even more before welding, laying the plates not flat but like a roof (say at 2000). With the correct starting angle, welding is going to straighten the plates flat. The third is the longitudinal shrink along the joint which may cause the joint to curve following an arch. This can be minimized by welding short stretches at a time in a proper sequence, that will avoid excessive localized heating, and by peening with a hammer the weld bead while still hot, to introduce compressive stresses to counter the shrinking stresses. Note: The articles of this section are regularly collected in our page on Weld FAQ. 4 - Filler Metals with Exothermal Additions Additions of exothermal mixtures, like magnesium powders and hematite, to the flux of cored filler wires have been studied with the purpose of determining if electric power requirements could be eased and manual skill reduced for field repair procedures. Although enough heating can be produced by thermite process mixtures, it is difficult to retain the reaction to the tip of the welding rod. Therefore the research concentrated on the exothermic-assisted Flux Cored Arc Welding. It was determined that an upper limit in the melting rate for such an electrode should be around 200 in/min (85 mm/sec). It was concluded that exothermic additions to the flux resulted in measurable increases in the arc process efficiency, considering that the chemical heat evolved is accompanied by a reduction in the electrical energy consumed. Although not yet usable in practical ways this research illustrates the kind of avenues pursued and the concepts explored to advance the efficiency of welding processes. The full article on 5 - Online Press: recent Welding related Articles Performance Improvement of Rotary Ultrasonic Testers Hot-Wire TIG: Not new but gaining
appeal Super-MIG™ to debut in Chicago From AWS Inspection Trends: Long Range Guided Wave Ultrasonics
6 - Terms and Definitions Reminder Constant Current power source is characterized by a small current variation even as a consequence of a remarkable voltage change. Melting Rate is the weight of filler metal (electrode, wire, rod, powder) melted in a welding process per unit time. Power Source is the device that supplies electric current suitable to feed the arc used in a welding system. In a Root Bend Test specimen of given dimensions the weld root is oriented to lay in the maximum stress convex surface of a specified bend radius. Seam Weld made on overlapping sheet metal elements may be continuous (if made by laser or electron beam) or consist of a succession of overlapping spot welds (in resistance welding). Strongback is a temporary, auxiliary element used to keep aligned in place the members of a joint during welding. V-groove weld is a joint design where two plates presenting a bevel on part or all of their thickness are assembled so that the shape of the joint reminds the letter V. Welding Symbol is a graphical representation of the welded joint required. The accepted document explaining and specifying the meaning of the
symbols is ANSI/AWS A2.4:2007 7 - Article - Orbital Welding We briefly dealt with Orbital Tube Welding in Section 9.2 of our PWL#005 for January 2004. Orbital Welding is generally performed by adapting Gas Tungsten Arc Welding (GTAW) to specially designed equipment where a special torch is made to circle a tube joint in manually assisted mechanical welding or fully automated process. This process has now a history of successful applications in many different industries for small, thin wall tubes, either without or with added filler metal. It is used to weld tube to tube, tube to fittings, tube to tube-sheets. It is performed automatically after clamping a weld head around the joint, and having it move around to perform either a single pass (for thin tubes) or multiple passes (for thicker tubes). Welding heads have a range of tube sizes to which they fit, and power sources are suitable to the jobs involved. The joint fit up should be consistent and tolerances should be tight. Most heads have limited dimensions, permitting them to move around in the limited spaces found generally in multiple tube applications. It is amazing to find that the best machines boast being able to duplicate the work of skilled manual welders, while other manufacturers insist that their equipment will outdo skilled welders. The first of the following articles summarizes nicely the An Article discussing "Orbital Welding in Compliance with the An Article introducing GMAW/FCAW (as opposed to GTAW) for Orbital Welding of large pipes with thick walls, outlines the main options to consider. See it at An Orbital Welding Machines Buyer's Guide can be found at
8 - Site Updating: Metal Knowledge Now you can easily assemble your Encyclopedia Online! The Page of this Month is somewhat different from what you may have become accustomed to. It includes an offer providing to interested readers lots of valuable links to pages and pages readily available through the Internet on essential information on Metals. This first installment covers Steel, Stainless Steel, Cast Iron, Aluminum and Magnesium. Lots of carefully selected links are live from a document in pdf format that can be downloaded for a nominal fee (a discounted price is still available for a limited time). By following the proposed links, and then digging deeper to the available internal links, one can reach essential building blocks of significant knowledge to assemble one's own Encyclopedia Online. You have only to save the downloaded pages in well ordered Folders in your computer and you will have available at your fingertips a vast repository of useful metallurgical basic and advanced wisdom ready to help you for professional advancement. You should grab this offer as long as it is valid. You have still a few days... You will thus enjoy a huge advantage not available to those unaware of this opportunity. See the page on Metals Knowledge. 9 - Short Items 9.1 - Biodegradable matter is generally organic material such as plant and animal matter and other substances originating from living organisms, that can be treated by microorganisms which have the capability to degrade, transform or accumulate a huge range of compounds. Plastic materials, due to their long life span, need special processing to become biodegradable. 9.2 - Ceramic Armor is a class of special materials that provide ballistic protection combined with light weight. The properties sought to defeat high velocity projectiles are high hardness, compressive strength and elastic modulus. The systems are either personal, to protect individuals, or designed to protect vehicles. 9.3 - Exothermic Reactions are chemical combinations that release heat in addition to the product generated. Thermite welding exploits such a reaction generating a mass of molten metal to be "cast in place" to provide welding. A novel application can be seen in section 4 above. 9.4 - Fusion welding is the melting together of the base metal with or without added filler metal to produce a weld. It is opposite to solid state welding where joining is obtained without fusion. 9.5 - Sonic Shape is a temporary, simplified shape given to forgings to ease the examination by ultrasonic testing, before final machining. The purpose is to eliminate internal reflexions due to geometric features of the finished part. 9.6 - Spintronics. Spin is a quantum property of the electron, closely related to magnetism. Spintronics describes an emerging technology trying to exploit the ability to change or influence the spin state of electrons. Devices that rely on an electron's spin to perform their functions form the foundation of spintronics (short for spin-based electronics), also known as magnetoelectronics. 10 - Explorations: beyond the Welder Explore the sky with Google Earth News and Articles from Scientific American NASA Missions Career Development: (let's not pretend that jealousy, rage, frustration, and competition don't happen in science). Jobs for Students 11 - Contribution: Progress with Laser Hybrid Welding We presented Laser Hybrid Welding in a short article (7) in issue 16 of Practical Welding Letter for December 2004. See: PWL#016. We came back to this subject in an article published in November 2006 on Promising Welding Innovations. The main problem with this successful process that combines a powerful Laser Beam with conventional Gas Metal Arc Welding (GMAW), is that it needs development of all parameters involved if its promised advantages are to be materialized in practical applications. Therefore the challenge has been picked up mainly by large organizations who could devote substantial resources to the preparatory work in view of reaping the potential benefits of high productivity, flexibility and quality. If equipment suppliers provided assistance with practical development of welding parameters, many more welding shops would probably adopt the process. Two articles published in the October 2007 Issue of the Welding Journal describe recent achievements. The first (page 36) reports on progress in automotive industry applications where the gap bridging capability of GMAW combined with the limited heat input of high energy density laser beam, its welding penetration and high speed, contribute to rapid and economic solutions. When the hybrid process is incorporated in a robotic cell, it is easy to shut down temporarily one of the two processes for those joints best served by only one of them, in the same set-up, and then again operate both of them for the next joint as needed. Case histories are illustrated and application of tandem (two wires) GMAW is presented with explanations on the improved stability of the arcs and of achievable flexibility. The second article, starting at page 47, presents once more the potential advantages of the combined process and briefly comments on demonstrated applications for Shipyards, Bridges, Rail Cars and Pipelines. Interested readers are urged to seek the original articles mentioned here. 12 - Testimonials Date: 01 Oct 2007, 02:01:22 AM Dear Elia, I have been a faithful reader of your newsletter for a long time, and I just wanted to congratulate you on achieving 50 issues! [...] I am a mechanical engineer and greatly appreciate the summarized information you provide about new and old techniques, tools, and materials. Your newsletter is a valuable resource for me, as it teaches me more about the techniques that are available and how to design for manufacturability when welding processes are present, and I will continue to enjoy it as long as you produce it! Yours,
Date: 31 Oct 2007, 06:24:51 AM 13 - Correspondence: a few Comments 13.1 - Every now and then the reply
address given by those who write us would not work. We may try to correct it if we have an idea of the possible error. But most often we are at loss. We generally answer all letters. Therefore if you miss our answer please check once more your own address and write again, possibly providing an alternative e-mail. 13.2 - The most usual shortcoming of the correspondence received is that the case is not presented with all details. Therefore no useful suggestion can be supplied unless the missing data are provided later. It is not difficult to see if essential information is lacking. The writer should assume that we know nothing of the case involved, even if work went on it for months. So please, ask yourself if there is some more to say. 13.3 - We were asked to provide parameters for a practical case: in fact it was for a school exercise. After the short answer we sent to the inquirer we expanded it to the full article presented above in section 3. We feel that this kind of educative information might help our readers. 14 - Bulletin Board 14.1 - Engineered Surfaces for Aerospace & Defense 14.2 - 32nd International Conference and Expo on 14.3 - Grab the Opportunity! Accept our Offer! Visit our new Page on Metals Knowledge. 14.4 - Leave your Mark. Add your comments. Visit Welding Talk.
POWERED BY:
Click on this Logo NOW! Copyright (©) 2007, See you next time... |
|
| Back to Back Issues Page |
